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Geology and Ground-Water Virgin Islands Hy D. J. CEDERSTROM GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1067 Ground-water resources V a moderately dry Caribbean island This e6py is PUBLIC PROPERTY and is not to be removed from the official files. PRIVATE POSSESSION IS UNLAWFUL (R- S. Sup. Vol. 2, pp. 380, Sec. 749) "NITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1950 UNITED STATES DEPARTMENT OF THE INTERIOR Oscar L. Chapman, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price $1.25 (paper cover) CONTENTS Page Abstract .................................... . . - 1 Introduction ............................................................ ...........-....-...... ... . 2 Purpose and scope of investigation........................................-. ... .. 2 Acknowledgments................ .. . .. -. 3 Location .................. . ........................... . .... ................ 3 Topography and drainage............................................................................ 4 Historical sketch ............................. ....................................................... 6 Population ............ .................................................................................... 7 Agriculture and industry. . . 7 Flora and fauna . . .. . - . . . 9 Climate ..................... ............. ..... .. ...... . .......................... 9 Rainfall ..... .................... .............. ... . .................................. 9 Accumulated departure of rainfall from normal.......................... 12 Relation of rainfall to sugar production........................................ 12 Geology......................................................................-..-...--...-...--.. ............ 12 Work of previous investigators..............-.... . . 12 Summary of geologic history... .. . ..... .. 14 Geologic formations .................... .......... ................... ... ..... .............. 16 Mount Eagle volcanics.............. . ......... .................................. 16 Diorite intruded into Mount Eagle volcanics . ..... 17 Jealousy formation .. ........................... ................................... 19 Kingshill marl ............... ........ ..................................................... 20 Description and distribution................. ............. 20 Geologic age ................................. ....... ................................ 22 Factors influencing deposition of Jealousy formation and Kingshill marl .. .. ............ 26 Alluvium ................................................................................................ 27 Structure ..... .................... ..... ................... ... ....... ............... .......... 29 Physiography ................. . . . .. ................... ........................................... 34 Early Tertiary erosion cycles..................... .. ............... 34 Drainage pattern .......................................................................................... 35 Land forms ................................ .. ......... ... .......................... 36 Submerged coral reef ..... .......... ..................................................... 36 Coastal terraces and buried valleys..-......... . ......... 37 Emerged coral reef and beaches... 38 Sources of water supply and use of water......... .... . . 38 Rain water ...................................................................................................... 38 Stream flow ...................... . ............................................. 39 Wells .-- .-.----...---...----...--. . . . .................... 39 Springs ........................- ............................................ 40 Water supplies at Central Factory............................................................ 40 Water supplies at Bethlehem Factory.. ....... ................ 40 Summary ................. ............. ........... .............................................. 41 Well construction .................................................................................................. 41 Dug wells ........ .. .. . . .. ..................................................... 41 Drilled wells ................................ ............................................................... 41 ill IV CONTENTS Page Occurrence of ground water................................................................................ 43 Previous investigations of ground water.................................................. 43 General conditions ...............~.. .................................................................. 45 Ground water in the Mount Eagle volcanics............................................ 47 Ground water in the diorite... .................................................................. 50 Ground water in the Kingshill marl.......................................................... 51 Ground water in the alluvium.................................................................... 56 North Side ............................................................................................ 56 East End ................................................................................................ 56 Central plain .......................................................................................... 58 Ground water in beach sand........................................................................ 64 Test-drilling program .................................................................................. 64 Quality of water.................................................................................................... 69 Comparison of surface water with ground water.................................... 69 Contamination by sea water.....................-...-.......-..................-.- .............. 69 Chloride contamination resulting from organic pollution.................... 73 Saline minerals associated with limestone of Tertiary age.................... 74 Effects of alkali soils................................................................... 75 Effects of salt spray..... .... ..... .. ........... . . . 77 Base exchange .. . ... ........... .. ....... .. . .................. 77 Summary ............. ..................................... ......................................... 78 Quality of water in the various formations............................................ 78 Mount Eagle volcanics................. .................................................... 78 Dioritic rocks .................. ........ .... . ... . ............... ........ 79 Kingshill marl ........ .. ............ ......................................................... 79 Alluvium . ........... .................................................................... 82 Wells from alluvium within areas of Mount Eagle volcanics -. . .. 82 Wells from alluvium within Tertiary limestone areas. . 85 Beach sand .............................................................................................. 86 Suitability of water for various uses.. . ..................................................... 86 Human consumption ... .... ................................................ 86 Cattle - .-----.-...--.-...--...-..-. ...... ..____ ....................... 87 Irrigation ........................................................................................................ 87 Sugar and rum production........... ...................................................... 89 Summary of ground-water resources................................................................ 90 Water-bearing formations .......................................................................... 90 Adequacy of present supply.. .................................................. 90 Quality of water....................................... .................................................. 91 Records of wells and springs . ............................................._..... 91 Recommendations .. .... ..... .................. ................................................... 91 Appendix, National Park Service drilling project ....................................... 103 Introduction . ....................................... .......................... ........ .......... 103 Selection of well and test-hole sites . . . .. . ...... 104 Summary of results.. ... .. ....... .............................................. 104 Description of wells and test-holes............................................................ 105 Whim homesteads .................. ....................................................... 105 Paradise .................................................................................................. 106 Proposed underflow dam at Fair Plain............................................ 106 Fair Plain ................. ........................................................................ 107 CONTENTS v Appendix Continued Description of wells and test-hole sites Continued Page Golden Grove......................................................................... ...---.. 107 Fredensborg ................. ....... ....................... .. . 108 Colquhoun-Mount Pleasant homesteads ................. ............ ... . 108 Rattan ...... ........ . ........ .. ......... ......... ............. . .. . 109 St. John-Princess homesteads........................................ . .. . 109 Central Factory ........ .. .... ............. ...._................ .. ...... HO Christiansted .......................................................................................... Ill Cotton Valley ........................................................................................ Ill Cottongarden..... .... ............. .................. ..... . .... . Ill Resume of National Park Service drilling project..... .. . H2 Index ...................................................................................................................... 117 ILLUSTRATIONS Page PLATE 1. Geologic map of St. Croix............................................. ...... In pocket 2. A, Basinal area in vicinity of Fountain estate; B, Central alluvial plain. View toward Mount Eagle. . . 10 3. A, View across inner lowland; B, View across narrow alluvium-filled valley . . . 34 4. A, View across western plain; B, Emergent coral reef at Canebay, north side... . . 35 5. A, Artificial catchment basin at Mary's Fancy; B, Municipal well, Christiansted.. 50 6. A, Old well tower at Strawberry; B, Rotary rig at test well near Jealousy... ....... ......... .... . . ..... . 51 FIGURE 1. Map showing location of St. Croix.............................................. 4 2. Chart showing annual rainfall and accumulated departure from normal at St. Croix, 1852 to 1938 .. 11 3. Generalized geologic section showing structure in the eastern part of the Tertiary marl plain.. 31 4. Cross section of central St. Croix showing location of test wells with reference to geology.................................... 32 5. Section showing development of early Tertiary erosion surfaces along east edge of marl plain...................... 35 6. Diagrammatic section showing the zone of saturation and the three belts of the zone of aeration.............................. 46 7. Diagrammatic sections showing the movement of ground water: A, In times of normal or exces- sive rainfall; B, In times of deficient rainfall ................... 46 8. Diagrammatic section showing artesian conditions. .. ... 47 9. Diagram showing results of pump test on the developed well at Fair Plain.......................................... 62 10. Graphic representations of analyses of some St. Croix ground waters and surface waters ........ 71 11. Diagrams showing relation between salt water and fresh water in homogeneous water-bearing sands making up a small island: A, Under nonpumping conditions; B, Where pumping is taking place...................... 72 VI CONTENTS TABLES Page TABLE 1. Annual mean rainfall on St. Croix from 1852 to 1938............ 10 2. Average monthly rainfall on St. Croix, 1852-1935.................. 10 3. Logs of test wells drilled in St. Croix, 1938-39........................ 68 4. Analyses of ground water in Leper Asylum we1!, St. Croix, showing chloride contamination and analyses of ocean water and of water from the Tertiary limestone area for comparison.................................. 72 5. Analyses of waters from wells showing organic pollution...... 74 6. Mean monthly evaporation on St. Croix from 1920 to 1935.... 75 7. Analyses of water from well at Kobel in dioritic rock.............. 79 8. Analyses of waters from wells in the Kingshill marl.............. 80 9. Analyses of waters from wells in the alluvium within areas of Mount Eagle volcanics..... ... .................... 83 10. Analyses of waters from wells in the alluvium within Tertiary limestone areas................................................ 84 11. Records of wells and springs in St. Croix.................................. 94 12. Logs of wells and test holes drilled by the National Park Service in 1940-41............................................ 112 13. Records of wells and test holes drilled by the National Park Service in 1940-41............................................ 115 Geology and ground-water resources of St. Croix Virgin Islands By D. J. CEDERSTROM ABSTRACT Field work in the island of St. Croix, V. I., was carried on from December 1938 to April 1939 in connection with a test-drilling program for water sup- plies. The island is 21 miles long and has a maximum width of 6 miles. Its western part consists of a range of mountains flanked on the south by a rolling plain; its narrower eastern part is entirely mountainous. There are only a few small streams. The rolling and flat lands are cultivated or are in grass, and the mountainous areas are either wooded or in grass. The average rain- fall of the island is 46.34 inches, but severe droughts and periods of excess precipitation are not uncommon. The island is made up of rocks of Upper Cretaceous age, mostly voTcanic tuffs and limestones known as the Mount Eagle volcanics; diorite intruded into the cretaceous rocks; and Oligocene to Miocene blue clays and yellow marls (the Jealousy formation and Kingshill marl, respectively). Alluvium is widely distributed. The Mount Eagle rocks were strongly folded in early Tertiary time and the Kingshill strata gently folded in post Lower-Miocene time along an east-northeast axis. Three early Tertiary cycles of erosion are recognized. After the folding of the Kingshill marl, streams followed the strike of the folded rocks in a westerly direction, but they gradually asrumed southward courses across the marl plain and as a result a western ar^a of old-age topography, a central area of late-mature topography, and an erstern area of early-mature topography have been created. Submerged reefs and emergent reefs and beaches indicate several fairly recent stands of the sea. Water for human consumption is obtained by collecting rain water in cis- terns, but water for other purposes is almost entirely supplied by wells which are generally less than 100 feet deep. Many dug wells are used, but in recent years drilled wells have been constructed. Most of them are discharged by wind-powered pumps of small capacity. Wells are developed in all the rocks mentioned (except coral reef), but the best yields are obtained from the alluvium. A maximum yield of 80 gallons a minute was obtained from'a gravel-packed well in the alluvial valley at Fair Plain. Further exploration of the alluvium is recommended. The weathered diorite also appears tc be a fairly good water-bearing formation. Test drilling showed that deep v^ater- bearing formations should probably not be expected beneath the Tertiary rocks. Most of the ground waters of St. Croix contain a moderately high mineral content owing to the solution of rock-forming minerals and the deposition of alkali and salt spray in the soil. Only a few wells are contaminated ty sea water. The low hardness of some highly mineralized waters is believe! due to base exchange. The most highly mineralized waters are found in the allu- vium in areas with alkali soil and in some places in the Tertiary limestones where presumably soluble salts were deposited in those strata. The least 2 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS mineralized waters are found in shallow wells in the alluvium: near the foot of the mountains and in the areas of dioritic rock. Many well waters in St. Croix, if properly protected from contamination might be entirely suited to human consumption. Although many waters are hard, th^y are used for domestic purposes. Most waters, even those high in chloride, are reported to be excellent for cattle consumption. Most ground waters in St. Croix cannot be used for boiler feed without treatment but are used for other purposes in the manufacture of sugar and rum. A brief discussion of the results of test drilling by the National Park Service in 1940-41 is also given. INTRODUCTION PURPOSE AND SCOPE1OFJINVESTIGATION A program of test drilling for additional water supplies on lands owned by the United States Government, in the island of St. Croix, V. I., was carried out in the period of December 1938 to April 1939 through an appropriation made by the Public Works Administration. These lands have been put to economic use by the Virgin Islands Co., a quasi-government corporation. The serv- ices of the Geological Survey were requested for technical advice, and the writer was accordingly assigned to the project. Sites for drilling were selected, samples of rock penetrated by the test wells were collected, and other duties attendant upon the drilling pro- gram were carried out. In the remaining time available the geology and ground-water resources of the island were studied and a geo- logic map of the island was prepared. (See pi. 1.) An account of the physiography of St. Croix, accompanied by a geologic map, was published in 1941,1 but in that publication much of the detailed geology and most of the hydrologic data were not considered. The investigation was made under the general direction of O. E. Meinzer, then geologist-in-eharge of the Division of Ground Water. M. D. Foster, Nathaniel Fuchs, E. W. Lohr, L. W. Miller, and W. N. Noble made the analyses of water samples. C. S. Ross made petrologic examinations of thin sections of the rocks. T. W. Vaughan and L. G. Henbest made determinations of Foraminifera and W. P. Woodring and T. W. Vaughan made determinations of macrofossils. Cushmanla has published a description of the smaller microfossils found in drill cuttings from the test wells described in this report. During a drilling project carried on by the National Park Serv- ice on St. Croix in 1940 and 1941, data on wells and test holes 1 Cederstrom, D. J., Notes on the physiography of St. Croix, V. I.: Am. Jour. Sci.. vol. 239, pp. 553-576, 1941. lm Cushman, J. A., Tertiary foraminifera from St. Croix, V. I.: U. S. Geol. Survey Prof. Paper 210-A, 1946. INTRODUCTION O were compiled by Donald G. Hazlett working under the direction of H. E. Rothrock. These data are discussed in an appendix to this report. ACKNOWLEDGMENTS The field work was made possible through the kind offices of Mr. Boyd Brown, president, Virgin Islands Co., and willing assist- ance was given by Messrs. C. Hunt, A. N. Gray, and James Tily of that company. Mr. Jensen and Mr. B. Nelthropp supplied much valuable information on wells throughout the island. Mr. Paul Schweitzer, of the Layne-Atlantic Co., who was in charge of the test drilling, supplied equipment for making pumping te?ts on existing wells and later donated several photographs, which are included in this report. A great number of local residents con- tributed information and in other ways helped to make the work in St. Croix a pleasant task. Thanks are due Brent S. Drane of the National Resources Plan- ning Board and his associates for their efforts in making available much data on the Park Service wells. LOCATION The island of St. Croix (or Santa Cruz) is about 95 miles south- southeast of San Juan, P. R., 1,400 miles southeast of New York, 1,000 miles east of Key West, and lies between latitude 17° 41' and 17° 46' and between longitude 64° 34' and 64° 54'. (See fig. 1.) It is one of the Virgin Islands group, of which St. Thomas, St. John, and St. Croix belong to the United States. St. Croix is the largest of these three islands. It is about 21 miles lorg and 6 miles wide at the center and has an area of 84 square miles. St. Thomas, 40 miles north-northwest of St. Croix, has an area of 28 square miles, and St. John, a few miles east of St. TJ omas, has an area of 20 square miles. The town of Frederiksted, with a population of about 2,000, is located on the west end of St. Croix, and Christiansted, with a population of about 3,000, is situated on the north coast in the eastern part of the island. The island has semiweekly steamship service from San Juan, P. R., by which passengers and freight are accommodated. Much freight is also carried by small sailing vessels. Frederiksted is a regular port of call for a passenger steamship line between New York and South America, and irregular calls are made by other merchant ships. A large number of vessels from many parts of the world call at St. Thomas. Weekly airplane service connects St. Thomas and Miami, and more frequent airplane service is avail- able at San Juan. GROUND WATER OP ST. CROIX, VIRGIN ISLANDS TOPOGRAPHY AND DRAINAGE The northwesteni part of the island is a mountainous area flanked on the south by a rolling plain. The mountains are broken by many narrow steep-sided valleys, through which intermittent streams flow and continue south or southeastward across the plain. A few deeply incised streams flow directly westward in the area between Frederiksted and Ham Bluff. The highest peaks on the island, Blue Mountain and Mount Eagle, are 1,090 and 1,165 feet above sea level, respectively. Other peak? attain eleva- INTRODUCTION 5 tions of 700 to 900 feet above sea level. Near Annaly is a very small area of flat upland and near Mount Eagle and Blue Moun- tain streams have carved out two small basinlike depressions. (See pi. 2 A.) Elsewhere high peaks and ridges and steep-sided valleys predominate. On the west near Frederiksted a gently undulating lowland surface borders the sea. In the central part of the islard the lowland is wider and is characterized by rounded hills. West of Christiansted the lowland is submountainous. High hilh and elongated ridges are common, and near Christiansted, where ero- sion has been least effective, a north-south belt of high limestone hills cut by narrow valleys appears superficially to be part of the mountain rather than of the lowland area. That part of the island, extending eastward from Christiansted, locally referred to as "East End," is also mountainous. Here, however, the maximum elevations are less, the stream valleys are not so sharply incised, and intermontane alluvial areas are of more importance. Two lowland areas separated from one another by a low narrow ridge extend inland from Southgate Pond and Great Pond. The northern range of mountains is drained by streams that flow to the west and south. With the exception of Salt River the streams flowing northward are very short and unimportant. Most of the streams discharging from the northern rar<?e of mountains cross at least a narrow strip of coastal lowland before entering the Caribbean. Few of the streams are permanent. Rec- ords on stream flow are not available, but local reports seem to indicate that in times of drought or slightly subnormal rainfall flowing water is found only in the upper reaches of the streams. In seasons of normal rainfall many streams pass underground shortly after reaching the coastal lowland. Some of these rer.ppear at the surface near the coast, and others discharge into the sea as underground streams. Except in time of, or immediately following, heavy rainr most stream beds do not carry flowing water throughout their course. The coastal lowland is drained largely by streams which origi- nate in the northern range of mountains and flow west or south. The stream flowing from Anna's Hope to Jerusalem, however, originates in the eastern area, and a few lesser intermittent streams originate in the high hills of the eastern part of the coastal lowland. Salt River, in contrast to other rivers, flows northeastward. As indicated above, practically none of these streams are perennial. The short streams in the more arid East End area flow only in seasons of heavy rainfall. 6 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS HISTORICAL SKETCH2 The Virgin Islands group was discovered by Christopher Colum- bus on his second voyage to America, in 1493, anc1 according to records he landed on the island of St. Croix on November 14 of that year. The island was inhabited by the Carib Indians at the time of Columbus' visit, but by the time of the first settlement by the English and Dutch in 1625 the Indians had been either killed, driven out, or enslaved and carried away. In 1645, following the death of the governor of the English colony, there was a bitter struggle between the Dutcl' and English, which resulted in the departure of the Dutch for otl er islands. In 1650 the Spanish governor of the island of Pu°.rto Rico sent an expedition to attack the English colony on St. Croix. During the attack many of the English were killed and of the remainder some were taken prisoner and the others permitted to leave the island. Thus St. Croix was claimed for Spain. Within the year the French captured the island from St. Christopher, and in 1651 sold it to the Knights of Malta. In 1665 it was sold back to the French West India Co., and in 1674 King Louis XIV took over the island as a crown colony. The colonization of the island was not a success, and in 1695 it was abandoned by the French and for the following 37 years was uninhabited. In the meantime the island of St. Thomas had remained prac- tically uninhabitated. But in 1671 two vessels from Denmark brought colonists to St. Thomas, and to forestall British occupa- tion of St. Croix the Danish West India and Guinea Co. in 1733 purchased that island from the French. In 1800, as a result of Denmark's identity with French inter- ests in a war between England and France, St. Thomas sur- rendered to the English, who held the island for 10 months, after which the Virgin Islands again passed back to Denmark. On December 22, 1807, during the Napoleonic Wars, the Danish West Indies again surrendered to the British, but in 1815 the islands were restored to Denmark in return for the Danish island of Heligoland. Several minor slave insurrections took place at various times in St. Croix but were quickly stilled. In 1847 a Dr.nish law was enacted proclaiming that at the end of 12 years slavery in the Danish West Indies should cease entirely and that all children born of slaves in bondage should be immediately set free. How- ever, this proclamation only created unrest and on July 2, 1848, an insurrection broke out and the slaves forced the Governor 2 Evans, Waldo, The Virgin Islands of the United States, a general report by the Governor: U. S. Navy Dept., Washington, D. C., pp. 22-49, 1929. INTRODUCTION 7 General to proclaim freedom for all slaves in the Danish West Indies. During the Civil War, the United States recognized its need for a naval station in the West Indies. Accordingly, in 1867 an at- tempt to purchase St. Thomas and St. John was made b;r the United States but the treaty that was drawn up was not ratified by the Senate. During the Spanish-American War another attempt was made by the United States to purchase the islands, but the treaty failed to be ratified in the Danish upper house. Finally in 1917 the islands were purchased by the United States for $25,000,000, and the formal transfer took place on March 31 of that year. The islands were at first administered by the Navy Department. In 1930 when the West India Sugar Factory Co., Ltd., went into bankruptcy the American Red Cross took over the responsibility of caring for the people. In March 1931 the President transferred the Virgin Islands from the supervision of the Navy Department to the Interior Department, and Congress appropriated funds for a program of economic rehabilitation. POPULATION In 1940 St. Croix had 11,413 inhabitants, of whom 95 percent were classified as either Negro or mixed. For many years the chief elements in the population of St. Croix were Scotch and Irish planters, Danish Government officials, and Negro slaves and their descendants. In recent years several thousand enter- prising Puerto Rican laborers have come to the island. There has also been a slight increase in the white population, cHefly owing to an increase of Government employees from the United States. With the exception of a large proportion of the Puerto Ricans the entire population is English speaking, a condition which prevailed even before the island became a possession of the United States. The native Cruzians speak an easily under- stood dialect in which the English broad a, and the Danish rising inflection are dominant elements. AGRICULTURE AND INDUSTRY St. Croix has been described as a one-crop sugar-cane island.. All other agricultural ventures have failed. Attempts to grow cotton were thwarted by the pink bollworm. Ventures in tomatoes, limes, sisal, onions, and the like proved to be without economic success and were discontinued. Except for the relatively small cattle business, which can employ but a few people, sugar and! 8 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS rum manufacture barely survive as the only industries left to which people can turn in search of employment and are the industries to which the local government must look for tax revenues.3 In 1934 a charter was granted to the Virgin Islands Co., a quasi-government agency, to operate agricultural lands and in- dustries in the interest of the government. In November of that year lands comprising several old estates, most of which had been operated by the West Indies Sugar Co. before the Bethlehem Sugar Factory was closed, were purchased by the Federal Govern- ment and turned over to the newly formed Virgir Islands Co. This company is making great advances in reviving agriculture on the island. Progress has been made in the economic rehabilitation of the island by reviving the growing of sugar cane, by homesteading individual families, and by establishing the Virgin Islands Co., which not only mills the cane of small growers, but grows cane itself. In 1939 it grew 1,300 acres of cane. The success of this crop and the general economy still suffer from a tax on sugar and a recent increase in the costs of labor and the natural recur- rent adversities of hurricanes and severe droughts. Some fruits are grown on the island, chiefly plaintains, bananas, guavas, papayas, soursops, sugar-apples, coconuts limes, and melons. The most common vegetables are sweet potatoes, yams, tanyas, tomatoes, and peppers. Practically all fruits and vege- tables are consumed locally. The growing of sugar cane and processing of sugar are the basis of the most important commercial activity of St. Croix. Even though about 41,000 or 80 percent, of the island's 51,000 acres of land are now given over to grazing, the raising of cattle for export employs only 3 percent of the island's labor. The economic life of the island therefore is almost totally dependent upon the success or failure of the sugar crop. In the prosperous days before the development of regions better adapted, because of greater rainfall and more fertile soil, to the cultivation of sugar cane, the island was controlled by wealthy planters who operated their large estates with slave labor. The ruins of their estates overlook the old slave villages, which, in spite of their dilapidated condition, are still used by the native field laborers. (See pi. 2B.) Of the 20,000 acres of arable land on the island, as much as 18,000 acres were grown to cane in one year. With the decline of sugar prices, St. Croix prosperity 3 Brown, Boyd J., Report on St. Croix economic conditions: Mimeographed at Chris- tiansted, St. Croix, V. I., 1939. INTRODUCTION also declined. Acreages grew smaller, landholders moved away, and finally in 1930 the largest sugar factory closed down and the acreage devoted to cane fell to a minimum of a few thousand acres.4 In addition to the chief business of growing sugar care and processing sugar, the Virgin Islands Co. manufactures rum, a special distillate used in the manufacture of bitters, and flcohol for use in the Government hospitals. Other adjuncts include the operation of a poultry farm and the crushing of rock for road construction. i ^ Besides the two sugar mills of the Virgin Islands Co., the La Grange Sugar Factory is also an important producer. Ir 1939 only two of the four distilleries on the island were in operation. An Army air base was built on St. Croix in 1940. Very little has been done on St. Croix to attract tourist tr°de. FLOBAfANDjFAUNA "" ~ 1 The island of St. Croix has a tropical vegetation, but owing to the comparatively light rainfall the plant life is not as rich and luxuriant as in other places in the Tropics where the rainfall is greater. , ^ The island was probably at one time entirely covered with thick forests, but in the early stages of development the forest cover was almost entirely removed. In many places, traces of contoured furrows reaching to the highest summits indicate that can« was grown to the very mountaintops. Owing to the decline in prices of sugar and other deterrent factors, land was taken from cultiva- tion and forest growth began to regain its lost ground. At present forests are characteristic of the northern mountains, the central plain is under cultivation, and the east end of the island is covered in large part with a dense growth of thorny brush. Grasslands, however, are found in all these areas. Birds and animals are not numerous on the island, but firh are plentiful in the surrounding waters and form an important p.rticle of diet of the islanders. CLIMATE A record of the rainfall at St. Croix has been kept since 1852. Figures giving the monthly and average annual precipitation at 4 Johnson, Arthur F., Preliminary report on the water supplies for properties man- aged by the Virgin Islands Co., St. Croix, V. I. : U. S. Dept. Interior, Bur. Reclamation manuscript rept., p. 5, May 1, 1937. 10 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS Christiansted, Frederiksted, and Kings Hill from 1852 to 1935, inclusive, have been assembled in a report of the United States Bureau of Reclamation.5 A graphic representation of the annual rainfall and the accumu- lated departure from the normal or average rainfrll from 1852 to 1938 is shown by figure 2. TABLE 1. Annual mean rainfall on St, Croix from 1852 to 1938 Year 1852-------- 1853.. ..-..--- 1854. __ ..... 1855 ___ -.--- 1856 -------- 1857-.--.---.. 1858 ------- 1859 ____ ---. I860-.-.--- ... 1861.. -------- 1862... ......_ 1863.-..-.-.-. 1864.--. -.---- 1865- ____ -. 1866---.------ 1867 - 1868 __ _----_ 1869--------- 1870-.. ------ 1871 1872 -.-- Inches 47.58 55.05 53.55 50.74 37.49 50.92 49.02 44.08 43.27 58.75 41.27 34.05 39.53 49.52 46.41 46.59 V7 7Q 47.74 51.24 35.90 29.36 Year 1873 . 1874------.- 1875--__ __ 1876 1877-------- 1878. 1879 - 1880- __ ... 1881------ 1882-. ---_.. 1883_-______ 1884_--_--__ 1885 1886------ 1887_--- _ . 1888------. 1889 1890 .--- 1891 _ ----- 1892 _ ----- 1893-.-_---_ Inches 29.62 48.21 30.72 35.80 48.95 48.97 55.16 41.45 57.99 44.28 48.29 44.45 45.71 54 O9 ATI Q9 54.82 54.66 30.38 50.36 34.20 56.12 Year 1894 1895. ------- 1896-------- 1897 _ ----- 1898 1899-- _ -.- 1900. _ --.- 1901------- 1902... _ .- 1903----. ... 1904-. _ -- 1905-------. 1906 ____ . 1907 ____ - 1908-------- 1909 ____ . 1910 ___ ... 1911 ___ ... 1912-. _ ... 1913--.;--.- 1914-. _ .-. Inches 44.12 55.87 56.13 54.93 50.50 34.09 45.52 65.98 61.39 45.31 37.37 53.05 55.79 38.31 45.09 51.83 43.79 44.20 37.16 38.98 37.53 Year 1975- 1916 ____ -. 1917 ____ - 1918 1919 ____ . 1930 1931 __ . ... 1932 __ . 1933- - 1934- ____ . 1935 ____ . 19.36- 19.37 ____ . 19.38-- ____ 19.39 __ ---. 1930 - 1931 ___ -.. 1932 - 1933 _ - .... 1934 ........ 1935 Inches 65.64 58.92 39.49 46.08 51.52 34.55 35.39 29.10 33.59 51.92 38.43 35.93 55.15 48.43 41.45 40.13 66.11 56.26 71.44 40.98 42.78 From 1852 to 1935, inclusive, the average annual rainfall in St. Croix was 46.34 inches. The average monthly rainfall was as follows: TABLE 2. Average monthly rainfall on St. Croix, 18Z2-1935 Month Inches 2.41 1.90 1.72 Month Inches 2.67 4.57 3.87 Month July.- __ ... Inches 3.68 4.46 6.02 Month Inches 6.52 5.11 3.39 The maximum rainfall occurs in May and from August to November; the minimum in February and March. The lowest recorded annual rainfall was 29.10 inches in 1921, but less than 30 inches of rain was also recorded in 1872 and in 1873. The highest recorded annual rainfall was 71.44 inches in 1933, and more than 65 inches was recorded in 1901, 1915, and 1931. A severe drought occurred from 1871 to 1876, inclusive. In that period of 6 years the annual rainfall was only once slightly above normal, and the average of those years was 34.94 inches, 8 Johnson, Arthur P., op. cit., pp. 55-58. GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1067 PLATE 2 A. BASIJNAL AREA Ii\ VICINITY OF FOUNTAIN E3T4TE. This Htceri-walled canyon is carved out of less resistanl diorilic rock. B. VIEW NORTHWESTWARD ACROSS CENTRAL ALLLVIAL PLAIN. Mount Eajrle is in background; Estate Slob is in middle ground. INTRODUCTION 11' low ooooo T w ni * 10 S3HON1 Nl 'TVWMON HlObJ 842663° SO 2 0261 0061 : 0381 5991 ;098I 5581 O O N S3HONI Nl ' ; 0-L8I a 12 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS or 11.40 inches below normal. Another severe drought is recorded from 1920 to 1926, inclusive, in which rainfall was again above normal only once and the average was 35.56 inches. Other droughts, which were less intense or which lasted only a few years or a single year, are recorded. However, periods of greater-than-normal rainfall are also evi- dent. From 1886 to 1889 the average rainfall was 52.76 inches, or 6.42 inches above normal; from 1895 to 1898 the average annual rainfall was 54.37 inches; and from 1931 to 1933, in- clusive, the average annual rainfall was 64.94 inches, or 18.60 inches above normal. ACCUMULATED DEPARTURE OF RAINFALL FROM NORMAL The accumulated departure curve (fig. 2) shows that cycles of subnormal precipitation occurred from 1861 to 1876, 1909 to 1914, and 1919 to 1926, inclusive. These periods of subnormal rainfall were followed by periods of greater-than-average rainfall, the longest of which was from 1877 to 1906, inclusive. RELATIONfOF RAINFALL TO^SUGAR PRODUCTION A study of the average yield of land in tons of sugar per acre and of the annual rainfall and accumulated departure indicates that the yield in any year is dependent upon the rainfall of that year or the preceding year or both and is not noticeably affected by the accumulated departure. A high yield was obtained follow- ing the heavy rains of 1915 and 1916, by the end of which time the accumulated departure was +21.91 inches, but a comparable high yield was also obtained in 1925 following one year of above- average precipitation that occurred in a dry cycle, when the ac- cumulated departure was 27.15 inches. GEOLOGY WORK OF PREVIOUS INVEST! GATORS** The geology of St. Croix has been a subject of study by Danish, German, Swedish, and American geologists since 1817. In that year Maclure wrote briefly on St. Croix7 and noted the distribu- tion of the older volcanics and younger limy rocks. In 1839 Hovey8 first used the term "indurated clay" for the older rocks and re- 6 Previous work relating' strictly to ground water is discussed in a later chapter. 7 Maclure, Wiliam, Observations on the geology of the West India Islands from Barbados to Santa Cruz, inclusive: Acad. Sci. Philadelphia Jour., vol. 1, pp. 134-149, 1817. 8 Hovey, S., Geology of St. Croix: Am. Jour. Sci., 1st ser., vol. 35, pp. 64-74, January 1839. GEOLOGY 13 ferred to the marl and limestone as of Tertiary age. Chve,9 a Swedish naturalist, studied the Virgin Islands in detail. His pub- lication, which appeared in 1871, describes the older highly folded strata of St. Croix upon which rest flat beds of coralliferous lime- stone. The origin of the older rocks is given as igneous and igneo- sedimentary, and the presence of gray limestones near Judith's Fancy with poorly preserved fossils is mentioned. A number of fossil shells found in the limestone of Tertiary age are listed. On the basis of the fossils found in St. Thomas, Cleve concluded that the basal strata of the Virgin Islands are of Cretaceous age and stated that "in St. Croix the white marl also seems to belong to Miocene time." In 1907 John T. Quin10 of Christiansted, inspector of schools under the Danish government of the islands, had privately printed a popular account of the geology of St. Croix. This publicrtion is accompanied by a geologic map in which the distribution of the older (Cretaceous) rocks and the younger (Tertiary) rocks js sketched. He explained geological processes in a simple fashion in order to interest his neighbors in the rocks and hills about them. However, Quin, an amateur, did not presume to g've the whole story of the geology of St. Croix but pointed out that his findings should be amplified and "corrected where necessary." Boggild11 observes that at its east end and along its north side, St. Croix is geologically much like the islands of St. Thomas and St. John. There are many mountains of cretaceous volcanic rocks, but the volcanic breccia is less abundant than the stratified varie- ties of volcanic rocks, such as tuffs. A gray limestone contains a few poorly preserved fossils. Dikes are uncommon. The structure is so complex, and the rocks strike in so many directions that it seems impossible to arrange the strata in an orderly sequence. Following a paper in which submarine terraces of several West Indian islands are described,12 Vaughan presented13 a discussion in which the biogeographic relationship of St. Croix and Puerto Rico is stressed. In this paper the precipitous north shore of St. Croix is referred to as a fault line. 9 Cleve, Per Teadore, On the geology of the Northeastern West India Islands: K. svenska vetensk. akad. Handl., Bandet IX, No. 12, Stockholm, 1871. 10 Quin, John T., The building of an island: Printed in New York by Chau^cy Holt and published by the author in Christiansted, St. Croix, 1907. 11 Boggild, O. B., On the geology of the Danish West Indies : Geog. Jour., publishing the transactions of the Royal Danish Geographical Society, vol. XIX, pp. 6-11, 1907. 12 Vaughan, T. W., Some littoral and sublittoral physiographic features of tl 3 Virgin and northern Leeward Islands and their bearing on the coral reef problem: Washing- ton Acad. Sci. Jour., vol. 6, no. 3, pp. 53-66, 1916. 13 Vaughan, T. W., Some features of the Virgin Islands of the United Stater*: Assoc. Am. Geographers Annals, vol. 9, pp. 78-82, 1919. 14 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS In a report to the Governor of the Virgin Islands in 1923, Kemp14 referred to the Upper Cretaceous rocks as the Mount Eagle series and the Tertiary rocks as the Kingshill series. Kemp's terms were adopted but altered to Mount Eagle volcanics and Kingshill marl in the writer's previous report, published in 1941.15 Kemp states that "there is some evidence that these [Kingshill] beds, in the central belt of the island . . . , as noted by the late John T. Quin, Esq., ... lie in a broad flat trough or syrcline whose axis pitches at a very low angle downward to the southwest." An important paper by Vaughan16 was published in 1923. Fos- sils found in the Cretaceous rocks of St. Croix and St. Thomas and fossils found in the limestone of Tertiary age at several localities in St. Croix are listed. The fossils from the Tertiary rocks range in age from middle Oligocene to lower Miocene. SUMMARY OF GEOLOGIC HISTORY Vaughan's summary of the geologic history of the islands, in which the place of St. Croix with respect to other VTest Indian Islands is clearly and concisely set forth, is outlined Nlow: (1) The presence of shoal-water deposits of Upper Cretaceous age in St. Croix and in other islands on the Virgin Bank show that the major tectonic axis of this part of the West Indies antedates Upper Cretaceous time. These major trends may be even as old as late Paleozoic. (2) During Upper Cretaceous time most or all of th? areas now occupied by land were under water; volcanic activity at that time is proved by the water-laid tuffs and lava flows, which are inter- bedded with the shoal-water calcareous sediments. (3) In early Tertiary, probably Eocene, time mountain making by folding occurred and was accompanied by igneous intrusion and probably extrusion of some volcanic rocks. (4) Early Tertiary mountain making was followed by pro- longed subaerial erosion. (5) In middle Oligocene time a large part of the island was submerged and, with slight fluctuations, remained under water until sometime during the Miocene. (6) Following the deposition of early Miocene seliments an uplift occurred that brought Miocene and older Tertiary sedi- ments, in places where they are present, above sea level. The 14 Kemp, James F., Report to H. H. Hough, Captain, U. S. N., Governor, Virgin Islands, printed at the Naval Station, St. Thomas, 1923. 15 Cederstrom, D. J., op. cit., pp. 556-557. 16 Vaughan, T. W., Stratigraphy of the Virgin Islands of the United States and of Culebra and Vieques Islands, and notes on eastern Porto Rico: Washington Acad. Sci. Jour., vol. 13, no. 14, pp. 303-317, 1923. GEOLOGY 15 Tertiary sediments are tilted and gently flexed, but they have not been so much deformed as the Upper Cretaceous deposits. About this time land connections permitting migration of land animals from Anguilla to Puerto Rico, Haiti, and Cuba probably existed. St. Croix seems to have been connected with Anguilla, St. Martin, and St. Bartholomew. (7) The period of high stand of land was followed by faulting and folding. Anegada Passage between the Virgin Bank and Anguilla was produced, and the islands assumed very nearly their present outlines. (8) Following the episode of faulting there was emergence of the land and terracing of the margins of the Virgin Bank, followed by submergence. At places in and near Puerto Rico there has been local emergence owing to differential crusts! move- ment. (9) The living coral reefs on the Virgin Bank are growing on a recently submerged extensive flat. This flat is geologically an old feature that orginated in large part at least in the lonjr period of erosion following the early Tertiary mountain making. Kemp17 in 1927 reviewed in detail the literature pertaining to St. Croix. Davis,18 after a brief visit to St. Croix, described the physi- ography of the island. He classified St. Croix as an island of the second cycle, of which lavas and pyroclastics forming the nucleus of the island are relatively old (pre-Tertiary?) and have been deformed and the island itself is now in the stage of topo- graphic maturity. First-cycle islands are made up largely of younger undisturbed volcanics. The Tertiary limestones of St. Croix are interpreted as lagoon deposits laid down within an encircling coral reef. The absence of early Tertiary sea-c^t cliffs is stressed. A detailed account of the physiography of St. Croix has been published by Meyerhoff19 and is referred to many times in the text. MeyerhofFs thorough discussion has been of considerable help in providing basic data and in clearly defining important problems. 17 Kemp, James F., Introduction and review of the literature on the geology of the Virgin Islands: New York Acad. Sci., Scientific survey of Porto Rico and the Virgin Islands : vol. 4, pt. 1, pp. 48-50, 1926. 18 Davis, W. M., The Lesser Antilles: Am. Geog. Soc. Pub. no. 2, Map of Hispanic America series, 1926. 19 Meyerhoff, H A., The physiography of the Virgin Islands, Culebra and Vieques: New York Acad. Sci., Scientific survey of Porto Rico and the Virgin Islands, vol. 4, pt. 2, pp. 145-184, 1927. 16 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS GEOLOGIC FORMATIONS MOUNT EAGLE VOLCANICS The oldest rocks of the island, which form the high mountain terrain, were named the Mount Eagle series by Kemp,20 and are designated the mount Eagle volcanics. (See p. 14.) Cleve sug- gested that they are of Cretaceous age, 21 and since then Upper Cretaceous fossils have been found in them.22 These rocks have been variously described by different authors. All agree, however, that a large part of the material is volcanic in origin, that much of it is stratified, and that some limestone beds are interbedded with the volcanics. Dark fine-grained massive, laminated, or slaty rocks, hard thin- to thick-bedded limestone, and spotted or por- phyritic rocks are most common. In some places masses of vol- canic breccia made up of angular pieces of dark porphyry or felsite and rounded scoriaceous stones cemented by a fine-grained greenish felsitic material, are found. The writer's brief study of the Mount Eagle rockr is sufficient to justify only a very general statement concerning the rock types and their areal distribution. Many of the rocks are strongly metamorphosed and difficult to identify correctly in the field, and in many areas outcrops are almost entirely lacking. It may be said with a fair degree of certainty, however, that metamorphosed bedded tuffs are the most widespread rocks in the Mount Eagle volcanics. Two types of these tuffs are present, bcth of which have wide distribution. One type, a massive fine-grained homo- geneous black rock that resembles slate, makes up much of the west end of St. Croix. This rock is exposed in outcrops at Jolly Hill and is quarried as road material near Creque Dam. The other type of metamorphosed tuff, a bedded rock somewhat resembling a volcanic flow, is believed to make up most of the eastern part of St. Croix. It also is fine-grained but much less so than the slaty rock and on weathered surfaces many of the flowlike rocks have a felsitic texture. Unmetamorphosed tuffs were found at a few places. A bed of yellow tuff about 20 feet thick is exposed along the coast between the mouth of Salt River and Judith's Fancy. Elsewhere beds of un- metamorphosed tuff are only a few inches to a foot thick. Volcanic flows are present at some places but are thought to be 20 Kemp, James P., Introduction and review of the literature on the geology of the Virgin Islands: New York Acad. Sci., Scientific survey of Porto Rico and the Virgin Islands, vol. 4, pt. 1, p. 49, 1926. K Kemp, James F., idem, p. 19. ^Vaughan, T. W., Stratigraphy of the Virgin Islands of the United States and of Culebra and Vieques Islands, and notes on eastern Porto Rico: Washington Acad. Sci. Jour., vol. 13, no. 14, pp. 303-317, 1923. GEOLOGY 17 less abundant than metamorphosed tuff. Volcanic breccia is, how- ever, common in the Mount Eagle volcanics. Breccia is particularly well developed in the vicinity of Barons Bluff and is interbedded with tuff and limestone at many other places. Mention should be made of the "blue beach." This term was originally used by Cleve23 in 1871 to apply to a conglomerate or breccia having angular pieces of dark porphyry or fels'te and rounded scoriaceous stones cemented together by a greenish mat- rix, probably derived from decomposed hornblende which is widely distributed in St. Thomas*. The term has since been used loosely and now is used by the natives of St. Croix to apply to all the older dark indurated rocks of the island. Thick-bedded fine-grained black crystalline limestone forms a large part of the section along the south flank of the northern range of mountains. Thinner bedded limestone of grayish hue, with which thin to thick beds of dark tuffaceous material are intercalated, is present at many places between Christiansted and Vagthus Point (Waiters Point, also known as Watch Eo) but appears to be much less widely distributed elsewhere. Cleve24 reported poorly preserved fossils from thin-bedded limestones at Judith's Fancy, northwest of Christiansted. His determinable Cretaceous fossils were found in limestone inter- bedded with volcanics on the island of St. Thomas. Vaughan25 determined that fossils found in the limestone strata of the Mount Eagle near Vagthus Point were of Upper Cretaceous age. DIORITE INTRUDED INTO MOUNT EAGLE VOLCANICS Meyerhoff26 pointed out that basin-shaped valleys within the mountainous area between Fountain and River estates (pi. 3) and the broad depression extending from Southgate Pond across the island to Great Pond are underlain by dioritic rock, which is intru- sive into the Mount Eagle volcanics. They are differentiated from the volcanics on the geologic map. (See pi. 1.) The most common rock in the Fountain and Hermitage Basins is a light-colored coarsely crystalline diorite. In most exposures it is deeply weathered and crumbles in the hand. Fresh material taken from a boulder indicates that this rock consists largely of plagioclase feldspar with minor augite altered to chlorite. 23 Cleve, Per Teadore. On the geology of the Northeastern West India Is'ands: K. svenska vetensk akad. Handl., Bandet IX, No. 12, Stockholm, 1871. 24 Cleve, Per Teadore, op. cit. 28 Vaughan, T. W., Stratigraphy of the Virgin Islands of the United States and of Culebra and Vteques Island, and notes on Eastern Puerto Rico: Washington Acad. Sci. Jour., vol. 13, p. 305, 1923. 28 Meyerhoff, H. A., op. cit., p. 158. 18 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS A less common type of diorite has a texture similar to that described above but contains a larger proportion of dark minerals, and the feldspar itself is darker. The feldspar is andesine. Augite constitutes more than 25 percent of the rock; little horn- blende and magnetite are present. All the minerals are quite fresh, and only minor sericitization of the plagioclase has oc- curred. This rock was seen in place as an orbicular unweathered mass within the common deeply weathered more feldspathie diorite. A hard black fine-grained crystalline rock is present as a selvage bordering the diorite west of Fountain estate and makes up the larger part of the narrow southeast dikelike extension of the diorite underlying that basin. This rock shows l;ttle evidence of weathering. It consists of 50 percent augite, 40 percent plagioclase, and 10 percent magnetite, and is properly termed a gabbro. The feldspar is excessively calcic and contains from 90 to 92 percent anorthite. Augite is scattered uniformly throughout the slide as small blebs and fewer larger fragments. It seems likely that this rock is an early differentiation product resulting from the chilling of the intrusive magma by the wall rock. Absorption of limy sediments may account for the excessive lir^e content of the feldspars. A sample of light-colored diorite similar in appearance to the diorite of Fountain and Hermitage Basins collected near South- gate, East End, was found to be made up of andesine feldspar and hornblende. The latter mineral makes up about 25 percent of the rock. Augite appears to be entirely absent. The rock also differs from the diorite found near Fountain in that the feldspars are strongly zoned. Gabbro was not found in place nor as float in the area between Southgate Pond and Great Pond. Outcrops are exceedingly poor, however, and it cannot be said with certainty that gabbro is absent. Dikes are rare in the rocks of St. Croix. One dike that intrudes the diorite near Colquhoun is about a foot wide and trends N. 50° E. It was composed originally of plagioclase feldspar and hornblende but is now badly altered. The northern British and American Virgin Islands are reported to be made up almost entirely of rocks similar to those described above. Some differences between the northern islands and St. Croix are apparent from the descriptions given by Cleve,27 27 Cleve, Per Teadore, op. cit. GEOLOGY 19 Hogbom28, and Earle29. In the northern islands the intrusives may be of smaller extent and have a wider range in composition, dikes are more common, bosses are present on the British Islands, and contact metamorphism of the sediments surrounding the intrusives is pronounced in some places. JEALOUSY FORMATION It was formerly thought that the white and cream-colored lime- stones and marls, similar to those exposed throughout the central plain, made up the entire Tertiary section of the island. When dark clay was encountered in the Castle Burke well30 at depths of 90 to 260 feet it was assigned to the Kingshill marl f.nd the top of the basement rocks was estimated to be at a slightly greater depth. The drilling of the deep test wells showed, however, that in the central part of the plain the Kingshill marl is underlain by a dark clayey formation that is known to be a minimum of 1,398 feet thick. The maximum thickness is not known because the base has not been reached. In the test hole drilled near Bethlehem (well 41, table 3, and pi. 1), in which 1,398 feet of dark clayey strata lying below the Kingshill marl were penetrated, the lowest stratum consists of 305 feet of gray clay in which a few streaks of limestone, not more than a few inches in thickness, are present. Five feet of calcareous conglomerate overlies the gray clay. The conglomerate is made up of rounded and subangular boulders of Mount Eagle volcanics (Cretaceous) in a limy matrix. Eighty-five feet cf gray clay overlies the conglomerate, and above that 16 feet of calcareous conglomerate is present. A core of the conglomerate showed that soft clayey partings were present. Above the upper conglomerate stratum 987 feet of greenish gray clay is present. As in the deepest clay stratum, thin streaks of limestone are found, but in this stratum the limestone streaks do not occur throughout the thickness but are localized within a zone 293 to 387 feet from the bottom of the stratum. The sediments described in the preceding paragraph hav* been named the Jealousy formation, and the section penetrated in the test well near Bethlehem, as described, is the type section. 28 HOgbom, A. G., Zur Petrographie den kleinen Antillen [a contributior to the petrography of the Lesser Antilles] : Geol. Inst. Univ. of Upsala Bull., vol. 6, pp. 214- 232, 1905. 29 Earle, K. W., The geology of the British Virgin Islands: Geol. Mag., vol. 61, pp. 339-351, 1924. 30 Johnson, Arthur F., Preliminary report on the water supplies for properties man- aged by the Virgin Islands Co., St. Croix, V. I.: U. S. Dept. Interior, Bur. Reclamation, manuscript rept, p. 29, May 1, 1937. 20 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS Near Jealousy estate the Jealousy formation is much thinner, 368 feet, but as determined in test drilling (well 39, table 3, and pi. 1), 61 feet of hard calcareous conglomerate lies immediately above the basement of the Mount Eagle volcanics. The remainder of the section is dark clay, but the clay here contains much more sandy material than at Bethlehem. It seems likely that at places the sand and boulders may be present as thin strata free of clay, but this was not apparent in drilling by the rotary method. A calcareous conglomerate underlying the Kingshill marl is exposed in stream beds west and northeast of Jealousy. It is be- lieved that this conglomerate is the basal conglomerate of the Jealousy formation and that it may be correlated with the calcareous conglomerate underlying the dark clays in well 39. With a marked change in the conditions favoring th e deposition of the Jealousy formation, the soft clays of that formation were easily eroded and overlapped by the younger Kingshill marl; the basal conglomerate of the Jealousy formation was more resistant, however, and the overlap was not complete where th^ basal con- glomerate had been laid down at a relatively high elevation along the sloping sides of the depositional basin. It may b? noted that in most places along the contact of Tertiary and Cretaceous rocks the overlap was complete, and the Kingshill marl rests directly upon Mount Eagle volcanics. Samples of drill cuttings of the Jealousy formation were sub- mitted for the study of the larger microfossils. According to the findings of T. W. Vaughan and L. G. Henbest, the Jealousy forma- tion is of Oligocene age. The large forms determined by Vaughan and Henbest are given on page 25. The small Foraminifera found in the well cuttings collected on St. Croix were g :ven special study by Cushman.31 The results of Cushman's study have been published recently. Cushman states that the age of sr.mples from test wells at Jealousy and Fairplain and from the upper part of the test well at Bethlehem "seems to be definitely Miocene" and in the lower part of the test well at Bethlehem the Oligocene was probably penetrated. KINGSHILL MARL DESCRIPTION AND DISTRIBUTION The rocks forming the plain between Christiansted and Fred- eriksted were named the Kingshill series by Kemp/2 They are 31 Cushman, J. A., The Tertiary Foraminifera of St. Croix, V. I.: U. S. Geol. Survey Prof. Paper 210-A, 1946. 82 Kemp, James F., Introduction and review of the literature on the geology of the Virgin Islands: New York Acad. Sci., Scientific survey of Porto Rico and the Virgin Islands, vol. 4, pt. 1, p. 28. GEOLOGY 21 now designated the Kingshill marl. (See p. 14.) The results of drilling show that the marl making up much of the surface of the coastal lowland forms only a small part of the total section above the Mount Eagle volcanics. The Kingshill marl consists of buff-to-white moderately thick- bedded limestone, alternating with soft cream or white marl. The limestone is hard and rather brittle and forms fairV good cliffs; the marl is softer and does not stand well. These strata are well exposed along the south coast line and in many of the small quarries formerly operated for building stone. At many places the limestone has a structureless appearance, and where made up of corals and coral debris the bedding planes of tr° lime- stone are obscure. Below Enfield Green the limestone is riddled by small solution holes. Similar solution holes have been seen elsewhere, and it is suggested that perforated limestone of this kind may yield water to some wells drilled into the Kingshill marl. At Work-and-Rest hill and Dolby Hill a small proportion of sub- angular detrital material is included in the basal part of the formation. The pebbles are less than an inch in diameter, and the average size is considerably smaller. The thickness of the Kingshill marl, where exposed just west of Christiansted along the scarp extending from Beeston Hill to Dolby Hill, may be more than 600 feet. At Fair Plain, wh^re the Kingshill marl forms hills that attain an elevation of about 200 feet above sea-level, the marl extends to about 180 feet below sea level (well 45b, table 3), making the thickness here about 400 feet. Elsewhere the known thicknesses are less. The subsurface thick- ness of the Kingshill marl in the wells drilled is as follows: Jealousy (well 39), 91 feet; Castle Burke (well 34), 90 feet; Bethlehem (well 41), 108 feet; Colquhoun (C-15), 77 feet; and St. John (C-20), 87 feet. East End area. The geologic map prepared by the writer shows mid-Tertiary strata present only west of a north-south line drawn through Christiansted. Meyerhoff,33 however, has found" what appears to be evidence that these strata are represented in tl ?, area to the east. "In the east half of the island, coastal-plain deposits were found in situ at only one point a small patch above Milord Point, on the south coast; but a few weathered Tertiary corals were picked up in the alluvium in two of the larger valleys fronting the Caribbean (north of Longford estate and in Cottongrove Valley)." 33 Meyerhoff, H. A., The Physiography of the Virgin Islands, Culebra and Vieques: New York Acad. Sci., Scientific survey of Porto Rico and the Virgin Island"' vol. 4, pt. 2, p. 161, 1927. 22 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS The writer has not seen the outcrop at Milord Point. However, on the north coast, just east of Cottongarden, a deposit of calca- reous conglomerate fills a narrow reentrant in the ITount Eagle volca'nics. The base of this deposit lies a few feet above sea level. Inasmuch as remnants of a raised sand beach of Recent age are found nearby, the conglomerate is interpreted to be a facies of that deposit. GEOIOGIC AGE Vaughan34 has written an account of studies of fossils from outcrop samples of the Kingshill marl along the base of the northeasterly facing scarp between Christiansted and St. John. The strata at Evening Hill (near the Leper Asylum) and at Montpelier (near St. John) were found to be of probable upper Oligocene age although there is a possibility that they may be lower Miocene. At Anna's Hope, several hundred feet higher stratigraphically, the horizon was determined to be lower Miocene. Fossils collected from the Kingshill marl at a Iocalit7 two-tenths of a mile southwest of Wheel of Fortune estate house (near Fred- eriksted) are assigned to the middle Oligocene. The following list of Tertiary fossils collected by T. W. Vaughan is given here for comparison with fossils found at other localities: Station 8647. One and four-tenths sea miles in a straight line from Chris- tiansted lighthouse, on the south side of North Shore road at Evening Hill. Probably upper Oligocene. Foraminif era: Amphistegina sp. Heterostegina antillea Cushman. Also Antigua and northeastern Mexico. Heterosteginoides sp. cf. H. antillea Cushman. Also AnguiFa. Station 6850. Montpelier (east). Collected by John T. Quin. Foraminif era identified by J. A. Cushman. Probably upper Oligocene. Foraminif era: Alveolina sp. Also Saint Martin. Orbitolites duplex Carpenter? Also Saint Martin. Spiroloculina sp. Also Saint Martin. Also indeterminable species of Quinqueloculina, Triloculina, Globigerina, and Amphistegina. Mollusk: Orthaulax sp. cf. 0. aguadillensis Maury. Station 8648. North Shore road, Montpelier (east). Probably upper Oli- gocene. Foraminif era: Amphistegina sp. Compare above, station 8647. Peneroplis sp. Gypsina globulus (Reuss). Also Anguilla; Recent. 34 Vaughan, T. W., Stratigraphy of the Virgin Islands of the United States and of Culebra and Vieques Island, and notes on eastern Puerto Rico: Washington Acad. Sci. Jour., vol. 13, pp. 309-311, 1923. GEOLOGY 23 Station 6851. Anna's Hope estate, along the road. Foraminifera identified by J. A. Cushman. Miocene, possibly very low Miocene. Foraminifera: Clavulina sp. cf. C. parisiensis d'Orbigny. Also Culebra formation of Panama Canal Zone. Clavulina sp. cf. C. communis d'Orbigny. Also Culebra formation of Pan- ama Canal Zone. Nodosaria sp. cf. N. insecta Schwager. Also Culebra formation of Pan- ama Canal Zone. Uvigerina sp. (?) Orbulina sp. Globigerina sp. Truncatulina wuellerstorfi Schwager. Also Culebra formation of Panama Canal Zone, also Recent. Siphonina sp. cf. species from Oligocene of the United States. Pulvinulina sp. cf. species from Oligocene of the United States. Asterigerina sp. cf. species from Oligocene of the United States. Amphistegina sp. Ellipsoidina sp. Corals: Obicella sp. cf. a species from the lower Miocene of Trinidad and Vieques Island. Psammocora sp. cf. a species from the Miocene of Trinidad. Other species of this genus are known from the lower Miocene of Trinidad, Vieques Island, and the Dominican Republic. Station 8649. Two-tenths of a mile southwest of Wheel of Fortune estate house; collection made on north slope of a low hill. Middle Oligocene. Foraminifera: Rotalia sp. Abundant. Amphistegina sp. compare with station 8648. Lepidocyclina morgani Lemoine and R. Douville: Also Cuba. Carpenteria americana Cushman. Also Cuba. Corals: Astrocoenia decaturensis Vaughan. Also Antigua; Bainbridge, Ga. Gomiastrea reussi (Duncan). Also Antigua. Cyathomorpha tennis (Duncan). Also Antigua; Hill's "Pepino forma- tion" of Porto Rico and other places. Goniopora microscopica (Duncan). Also Antigua. The writer collected fossils from limestone in the Kingshill marl capping the hill overlooking and one-half mile southv-'est of Judith's Fancy, 3 miles northwest of Christiansted. The strata in which these fossils occur are within 30 feet of the base of the Kingshill marl in this area, but as the Kingshill was laid down here upon an uneven surface of Mount Eagle volcanics the horizon represented may be stratigraphically much higher than the lowest beds of the Kingshill, where they rest upon blue clay of the Jealousy formation. The Tertiary macrofossils from this locality 24 GROUND WATER OF ST. CROIX, VIRGIN ISL/NDS were determined by W. P. Woodring and T. W. Vaughan and are listed below: Foraminifera: Sorites? sp. Coral: Stylophora sp. Mollusks: Gastropods: Natica? sp. Cerithium? sp. Turritella cf. T. crocus Cooke Orthaulax? sp. Olivella sp. "Marginella" sp. Pelecypods: Sacella sp. Area? sp. Anadara sp. Glycymerls sp. Lithophaga cf. L. nigra d'Orbigny sp. Aequipecten sp. Ostrea cf. O. cahobasensis Pilsbry and Brown Glans? sp. Lucina? sp. Semele? sp. Tracycardium sp. Laevicardium sp. With reference to the mollusks, Woodring reports that: "Others are represented, but even generic identification is uncertain. The age is Miocene, probably lower Miocene, and probably of the same age as the Quebradillas limestone of Puerto Rico." Samples of cuttings taken at various depths while the test- drilling program of 1938-39 was being conducted wer? submitted to F. G. Henbest and T. W. Vaughan for determination of Forami- nifera. Nothing older than upper (?) Oligocene appears to be represented by the fossils found. Further, as the boreholes were drilled by the rotary clay-seal method and muds from higher horizons were later recirculated when the test hole had attained greater depth, much mixing of material has doubtless taken place. In samples from test well 41, at Bethlehem (see the following list), Oligocene Foraminif era were first found at a depth of 580 feet, or 497 feet below the base of the Kingshill marl. In samples from test well 39, at Jealousy, Oligocene forms were encountered at a depth of 90 feet, or 1 foot above the base of the Kingshill marl. In test well 45a, at Fair Plain, Oligocene Foraminifera were found in cuttings taken at 100 feet, or 108 feet above the base of the marl. It would appear that the most important fact to be GEOLOGY 25 gained relative to the age of the strata is that the Oligocene- Miocene contact falls within the Kingshill marl and is st least 108 feet above the base of that formation at Fair Plain. The microfossils that have been identified by Vaughan and Henbest are listed below. With a few exceptions, which are noted, they are Foraminifera and were collected from test holes at Bethlehem, Jealousy, and Fair Plain. Genera or species mot pre- viously known from St. Croix are marked by an asterisk. Well 41, Bethlehem (contact of Kingshill marl and Jealousy formatio^ at 108 feet). Miocene (22 to 1,450 feet): Globigerina sp. *Siphogenerina lamellata Cushman. Oligocene (580 to 1,450 feet) : Miogypsina sp. cf. M. cushmani Vaughan. Heterostegina sp. *Heterostegina iraelskyi? Gravell and Hanna. Carpenteria? (fragment). Amphistegina sp. *Nullipores (calcareous algae). *0perculinoides sp. Adeonelopsis (a bryozoan). *Archaias sp. cf. A. aduncus (Fichtel and Moll). *Spiroclypeus? sp. (resembles a species found -in Martinique and Trinidad). *Lepidocyclina sp. cf. L. forresti Vaughan. Well 39, Jealousy (contact of Kingshill marl and Jealousy formation at 91 feet). Miocene (10 to 360 feet) : Globigerinids. Siphogenerina lamellata Cushman. Polymorphinid. Oligocene (90 to 390 feet): Miogypsina, spp. probably 2 species. Archaias? (fragment). Gypsina sp., the form called G. globulus (Reuss). Amphistegina sp. *Robulus sp. *Lithothamnium (calcareous algae). Carpenteria sp. *Borelis sp. *Lepidoeyclina sp., aff. L. waylandvaughani Cole. *0perculinoides, spp. probably 3 species. Well 45a, Fair Plain (contact of Kingshill marl and Jealousy forrration at 208 feet). Miocene (at 90 feet): Globigerina sp. 26 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS Oligocene (100-130 feet) : Amphistegina 2 spp. *Operculinoides sp. Carpenteria sp. Gypsina sp. cf. G. globulus (Reuss). FACTORS INFLUENCING DEPOSITION OF JEALOUSY FORMATION AND KINGSHILL MARL The manner of formation of the Tertiary sediments is a matter of some interest. It is thought that the basal calcareous con- glomerate of the Jealousy formation may have been formed within the very narrow fringing reef which grew up the ^ope as the encroaching sea slowly rose to higher and higher elevations. The coarser material brought out of the highlands by the streams was trapped and the boulders and pebbles incorporated in limy lagoonal muds. Calcareous conglomerate is forming in this man- ner along the shore at Judith's Fancy at present. Although coarse material was trapped in muds close to the shore of the encroaching sea, suspended matter was carried out beyond the narrow reef and was deposited in the deeper waters. The quantity of fine clay thus deposited was great, rnd it seems evident that streams emptying into the deep trough underlying the central plain must have drained a land mass of much larger extent than the -present island of St. Croix. It likewise seems evident that if the deep channel in which the Jealous:T formation was deposited is an erosional channel, it could hardly have been created by any drainage system confined to as small an area as the present island of St. Croix. Vaughan's statement35 that St. Croix seems to have been connected with Anguilla', St. Martin, and St. Bartholemew subsequent to early Miocene time may be recalled at this point. The trough that received the sediments may have been partly landlocked. Certainly the predominant!/' fine sedi- ments indicate absence of strong ocean currents. Not all the coarse material was trapped by the encircling reefs, however, for near the old shore line several strata in the clay of the Jealousy formation contain a moderate amourt of sand, pebbles, and boulders. The test well at Bethlehem showed that at two horizons the clayey section is interrupted by thin calcareous conglomerate. The widespread deposition of limy strata might be attributed to a temporary cessation of, or decreasing rate in, the submergence during which time coral growth flourished or deposition of lime was brought about on a warm shallow-water 3B Vaughan, T. W., Stratigraphy of the Virgin Islands of the United States and of Culebra and Vieques Island, and notes on eastern Puerto Rico: Washington Acad. Sci Jour., vol. XIII, p. 316, 1923. GEOLOGY 27 shelf. However, the occurrence of boulders of the Cretaceous rock at a distance of perhaps half a mile or more from the shore is not as easily explained. Wave action during storms or proximity to the mouth of a major tributary may help to account for this distribution. Following the deposition of sediments of the Jealousy forma- tion, a marked change in conditions took place and the limy F.ings- hill rocks were next laid down. The dark clays of the Jealousy formation neither interbed with nor grade into the Kingshill marl; and, where known, the contact between the two formations is abrupt. It is suggested that the rate of submergence was much slower at the time the Kingshill marl was laid down than when the Jealousy formation was deposited, and therefore corals fring- ing the shore were enabled to grow outward and encircle the island and contribute to the formation by reef growth and crea- tion of lagoons in whose warm waters limy mud collected. At the same time erosion of the land areas proceeded at a much rlower rate, and the moderate amount of fine suspended material carried out into marine waters was incorporated with limy mud to form a light-colored marl. AULUV1UM Alluvium covers large parts of the area underlain by mid- Tertiary rocks, but its distribution in the mountainous regions of the island is limited. In the mountain areas bordering the sea alluvium fills the valley mouths but generally extends only a very short distance inland and is a corollary of the more youthful stage of erosional develop- ment of the hard-rock areas. South of Christiansted a somewhat larger area of alluvial fill extends from Catherine's Rest tc Sur- laine Point. Two other larger areas of alluvium are seen in the vicinity of Great Pond and Southgate Pond, where wide valleys have been carved from less resistant rock. Within the northern mountain range at Fountain and Hermitage the erosion of similar rocks has resulted in two long valleys floored by alluvium. Within the Hermitage Basin a maximum thickness of 40 feet of sandy and bouldery alluvial fill is exposed. In the East End region the alluvium is made up of sandy and gravelly strata but also contains important amounts of clayey material. According to Mr. B. Nelthropp, who has drilled many wells in St. Croix, the alluvial cover in the East End region is clayey to the extent that it acts as an impermeable cap rock over the fractured Mount Eagle strata, and water is generally present 842683° SO 3 28 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS in these strata under artesian rather than water-tab^e conditions. The maximum thickness of the alluvium is not known, but near the head of Cotton Valley (well C-28) it is 69 feet thick and at Cottongarden (well C-29) it is 83 feet thick. Slightly greater thicknesses may be present in other places. The Tertiary limestone plain, however, shows the most impor- tant development of alluvium. At the foot of the mountains from Grove Place to Morningstar, streams have spread their load across a previously eroded lowland skirting the range. (See pis. 2B and 3A.) Southward, alluvium-filled valleys thread a range of hills (pi. 3J?), but near the south coast the alluvial vr.lleys coalesce to form a broad low terrace along the coast line. Between Enfield Green and Whim to the west a broad alluvial area extends from the base of the mountains to the sea (pi. 4B), but in the eastern- most Tertiary limestone belt, alluvium partly fills young valleys which do not extend to the mountains but head in the high lime- stone hills. Alluvium also covers much of the broad flat extending from Frederiksted to Spratt Hall. Within the area underlain by limestones, the fill contains sand and boulders derived from the older rocks of the highlands, but marly fill derived from the limy rocks of the Kingshill marl covers large areas or may alternate with sandy material in the valleys. In the test well at Fair Plain (45a) the following alluvial material was encountered: 33 feet of fine sand becoming coarser with depth, 24 feet of clay containing some sand and gravel, 5 feet of coarse sand and gravel, 8 feet of clay, and 3 feet of sand and gravel. Near Bethlehem (well 41) the alluvial material consisted of 18 feet of yellow marl overlying 7 feet of coarse sand and gravel. In the Hope and Ruan Bay areas sur- face exposures show that the fill is more marly tr.an sandy, al- though many scattered subangular pebbles and cobbles are present. In well C-8, at Fair Plain, 83 feet of alluvial fill was pene- trated. It should be noted that an equal thickness of alluvium is present filling the short valley at Cottongarden. Hence it does not necessarily follow that in seeking sandy alluvial water-bearing formations exploration must be confined to valleys beading inland several miles; steep-sided short valleys may also contain favor- able thicknesses of alluvium. The small catchment and storage area of the short valleys, however, should be considered in rela- tion to the quantity of water desired. GROUND WATER OF ST. CROIX, VIRGIN ISLANDS 29 STRUCTURE The Mount Eagle rocks of Upper Cretaceous age have been strongly folded and are everywhere steeply inclined. The folding has been complex and the delineation of specific structures is a difficult task. Certain generalizations can be made, however, on the basis of limited observations by the writer, which give some picture of the problem presented by these rocks. In the extreme eastern part of the island most of the rocl-s dip to the south or southwest. At Mount Fancy there are subordinate local folds whose axes trend northwest-southeastward. South of Christiansted a syncline trending northwest-southeastward is present, and another synclinal axis of similar trend extends from Rustoptwist to Kirkegaard Hill. In northwestern St. Croix the attitude of the older strata was determined in a number of places, as noted on the geologic map (pi. 1), but it is not possible to interpret the structure of the region as a whole on the basis of the limited data at hand. Both Vaughan36 and Meyerhoff37 point out that the north shore of St. Croix is characterized by steep cliffs extending to great depths and attribute this feature to faulting in Miocene or post-Miocene time. Other structures are probably present, but it is felt that data at hand an too meagre to define them properly. Quin, in his popular account of St. Croix,38 recognized the rorth- west-southeastly trend of the folds in the Mount Eagle rocks. However, his plotting of many structural axes is founded on what seem to be scattered observations. It can be shown that erough of his observations are incorrect to cast doubt on the advisiMlity of accepting all his structural interpretations. Detailed and painstaking work is needed to establish the geology of the old rock areas more precisely. Quin39 has written at length upon the structure of the younger (Kingshill) rocks which make up the marl plain. He states that these rocks dip seaward from their point of contact with the Mount Eagle rocks at the foot of the mountains. The seaward dip, however, is not maintained across the marl plain, for the rocks are folded along various axes, which are as follows: A north- 88 Vaughan, T. W., Some littoral and sublittoral physiographic features of the Virgin and nothern Leeward Islands and their bearing on the coral reef problem : Washington Acad. Sci. Jour., vol. 6, no. 3, p. 56, 1916. 87 Meyerhoff, H. A., The physiography of the Virgin Islands, Culebra and Vi?ques: New York Acad. Sci., Scientific survey of Porto Rico and the Virgin Islands, vol. 4, pt. 2, p. 181, 1927. 38 Quin, John T., The building of an island : Published by the author in Christiansted, V. I., pp. 44-52, 1907. 39 Idem, pp. 20-33. 30 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS east-southwest synclinal axis passing through Rr.ttan in the northeast, a syncline passing northwest-southeast through Golden Grove and another parallel to it through Adventure in the central part of the island, and an anticline passing northeast-southwest through Hannah's Rest, southeast of Frederiksted. These inter- pretations have been checked by the writer, who found that the Rattan syncline is present as stated, synclines are present at Golden Grove and Adventure, but their axes do not trend north- east-sou thwestward, and there is no evidence for an anticline passing through Hannah's Rest. A structural interpretation of the Tertiary limestone area which is based upon field observations made by the author and upon data obtained in the drilling of three test holes is given below. Positive data are not everywhere available, bnt a sufficient number of exposures do exist to delineate the main structural features and establish the structural habit of the limestone rocks. The Tertiary rocks rest unconforrnably upon an eroded surface of the Mount Eagle rocks. On the north the contact of these rocks lies along the foot of the mountains from Two Williams to the mouth of Salt River, but owing to the alluvial cov-^r the contact is sharply defined in only a few places. In the stream beds west of Jealousy, northeast of Jealousy, and at St. George the boundary is well defined, but in no one of these three localities was the actual contact of the basal calcareous conglomerate of the Jealousy formation upon the Mount Eagle rocks observed. At St. George the attitude of the basal conglomerate could not b? determined, but at the other two localities mentioned the dip varied from 15° to almost 40° to the southeast near the contact. The slope of the underlying rock surface was not exposed. The data brought to light by the test well at Jealousy (well 39) shows that there the older basement rock floor lies 4 60 feet below the surface. (See fig. 4.) The surface of the hasement rock therefore slopes south-southeastward at an angle of more than 24° in this vicinity. The contact of the Tertiary rock with the older rock is seen also 2!/i> miles northeast of Jealousy in the stream bed at the crossroads between Morningstar and Concordia. A small ex- posure opposite the abandoned pumping station about two-tenths of a mile northeast of the crossroads may be of considerable significance. Here, in a small pit, is a succession of buff lime- stone, overlain by. 3 feet of limy conglomerate carrying pebbles 2 inches in diameter, and 1 foot of clayey sandstone, which in turn is overlain by white limestone. These strata strike east and STRUCTURE 31 dip 85°N. The significance of this outcrop is not understood, but it certainly suggests that more deformation has taken place, locally at least, than is generally apparent. From the mouth of Salt River to St. John, limestone of the Kingshill marl lies upon a warped plane that slopes gently to the southwest. As far as could be determined, north of Dolby Hill the plane slopes about 400 feet per mile, or slightly more tran 4°. Between St. John and the Leper Asylum the limestone descends below sea level, but from the Leper Asylum to Cane Garden on the south coast the contact lies above sea level and its location was determined with fair accuracy in a number of places. From the Leper Asylum to Beeston Hill the line of contact rises to an elevation of 350 feet above sea level and lies at a conn: arable elevation on Work-and-Rest hill to the southeast. Exposures on the north side of Work-and-Rest hill indicate that here the plane of contact slopes westward about 530 feet per mile, or 5Va°. Gentle inclination to the southwest of the surface of the base- ment rock cannot account for the low elevation of the lirrestone between St. John and the Leper Asylum, and it is apparent that either the basement rock surface was channeled by erosion or diastrophically deformed before the deposition of the limestones, or the basement rock and the limestones were strongly folded after the deposition of the limestone (Kingshill marl). Data on the elevation of the base of the limestones of the Kingshill in the central part of the island (fig. 3) indicate that these rocks are -250- FIGURE 3. Generalized geologic section showing structure in the eastern part of the Tertiary marl plain. not greatly infolded, and it is therefore believed that they were laid down upon a highly irregular surface. Exposures of limestones of the Kingshill within the central plain are few. Except along the coast line, where the action of waves has produced fine cliff faces, along roads and trails, and in the several small quarries dotting the hillsides naturally occurring outcrops are very scarce indeed. Even there many of the rocks exposed are marly or massive, and determination of their attitude is impossible. Enough data have been obtained to deliner.te the main structural trends in different parts of the limestone plain, 32 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS and the data are considered sufficient to make possible a charac- terization of the plain as a whole. The two small hills east of Golden Grove have been quarried for their blocky limestones, and a more or less continuous series of outcrops girdles each hill a short distance below its crest. These two hills are synclinal and the breach between them, in which New Works lies, is an anticline. (See fig.4.) The axes of N. NW. MT. EAGLE -500' -1000 BLUE MOUNTAIN JEALOUSY FR£DENSBOR6 NEW WORKS FAfR PLAIN EXPLANATION Alluvium Kinqshill marl Jealousy formation FIGURE 4. Cross section of central St. Croix showing location of test wells with reference to geology. the folds appear to extend in a west-northwesterly to east-south- easterly direction. The two hills at Adventure and Fox have a similar origin. The strata are even more gently folded, but synclinal structures are certainly present and form the hills. The axes of folding trend more nearly in a northeast-southwest direction parallel with the Center Line road. At Kings Hill police station the strata dip southeastward. There are not sufficient outcrops along the hill extending to the south to state definitely that the rocks are folded rs to the two localities discussed above, but a reversal of dip (dip to the north- west) near Anguilla suggests folding. Sion Hill to the northeast is also a synclinal hill. TTie structural axis trends northeast-southwestward. A more or less continuous series of observations taken along the road from Bellevue through Rattan to Dolby Fill indicates that Rattan lies within a much broader syncline whose axis trends north-northeastward to south-southwestward. As noted above, this structure was mapped by Quin. Other field observations strongly suggest folded structures but do not define them as conclusively as those already described be- cause exposures are lacking at critical points. The fine series of exposures along Cane Garden Bay show that the strata dip to STRUCTURE 33 the northwest, and it is thought that these rocks form part of a synclinal limb. The prominence on which Cane Garden estate is located may mark the axis of the syncline. Other scattered obser- vations suggest that Work-and-Rest hill and Anna's Hope are on synclinal axes and that the course of the stream flowing be- tween them follows an anticlinal crest. The structure of the area underlain by limestones of the Kings- hill may therefore be characterized as follows: The rocks in the central and eastern part of the area have been compressed into a series of closely spaced open folds, the axes of which generally trend northeast-southwestward, but departures from this trend are seen at Rattan and at New Works. In the eastern and east- central areas the folds plunge from 6° to 14° to the southwest. The axes of the folds at New Works and at Adventure appear to be horizontal. Very little of the structure of the underlying rocks extending from Spratt Hall through Fredricksburg to En- field Green could be determined. The writer agrees with Meyer- hoff,40 who questions the presence of the anticline passing south- west through Hannah's Rest, which was mapped by Quin. In the deeper portions of the Oligocene sedimentation are?., more than 1,400 feet of clays accumulated. The basin gradually filled, and as waters shallowed an extensive growth of coral covered the area of clayey sedimentation. The contact between the clay of the Jealousy formation and the calcareous Kingshill marl is abrupt and appears to denote a marked change in conditions of sedimentation. The line of contact between the limy rocks and the Mount Eagle volcanics from the mouth of Salt River to Cane Garden indicates that the basin was troughlike. The de^th to bedrock is unknown at Little Princess, where the limestone extends below sea level. The trough probably extends west-southwertward under the plain and near Bethlehem. In the central part of the island the base of the Kingshill marl lies 60 feet above sea level at Jealousy, 10 feet above sea level at Castle Burke and Bethlehem, and 180 feet below sea level at Fair Plain. These facts may be taken to indicate that the rocks have not been greatly deformed but were only gently folded in the manner already described. It is believed that if the trough had resulted from deformation after sedimentation the base of the limestones would lie at least several hundred feet lower at Bethlehem and Castle Burke than at Jealousy and Fair Plain. The basal conglomerate of the under- lying Jealousy formation is 60 feet thick, and some of the clay strata higher in the series contain imbedded pebbles and boulders. 40 Meyerhoff, op. cit., p. 164. 34 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS This fact seems to indicate that steep slopes were already present at the time of deposition of the Jealousy formation. The gentle folding of the sediments along west-southwest to east-northeast axes has already been described. Mild compressive forces threw the Tertiary strata into a series of folds in most of which the dips are less than 12°. Steeply dipping recks along the foot of the mountains may be the result of a low dip due to de- formation and a high initial dip. That component of the dip due to deformation may be greater than usual along the foot of the mountains, where the Tertiary strata were forced against a resistant old land mass. It has been noted that the direction of plunge of the axes of folds varies. It has also been brought out that at-Work-and-Rest hill and Anna's Hope the folds plunge to the west-southwest at a minimum of 5°, or 400 feet per mile. If this plunge were unin- terrupted the base of the Kingshill marl at Fair Plain should lie 2,500 feet below its present position. Therefore, the plunge to the west has been interrupted by faulting or cross folding. There are no data at hand that tend to show which of thes? possibilities is more probable. There is very little evidence of faulting, although Quin figures some normal faults, of not more than a foot or two of displacement, which are exposed in the cliffs along Cane Garden Bay, and the writer has noted a small normal fault north of Rattan which trends north-southward and is inclined 65° to the west. However, evidence of the young scarp topography along the north coast shows that faulting has profoundly affected the island, and as a working hypothesis the eastern limestone area is re- garded as a fault block that has been tilted toward the south- southwest. The central and western parts of the island may be relatively undisturbed or may plunge gently in the sr.me direction. PHYSIO GR APH Y The physiography of St. Croix has been dealt with in a previous publication41 and need be only summarized in this paper. EARLY TERTIARY EROSIONfCYCLES The present land surface of St. Croix began to be shaped shortly after the accumulation and deformation of the Mount Eagle rocks of Upper Cretaceous age. In early Tertiary time a land mass much greater in size than the present island of St. Croix was beveled to a rolling plain. Remnants of this surface are seen in tt Cederstrom, D. J., Notes on the physiography of St. Croix, V. I.: Am. Jour. Sci., vol. 239, pp. 553-576, 1941. GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1067 PLATE 3 A. VIEW NORTH-NORTHEASTWARD ACROSS INNER LOWLAND FROM FOX ESTATE. Blue Mountain is in left background. This lowland is largely veneered with alluvium, and here the courses of more deeply buried valleys leading out of the mountains can be determined only by shallow test drilling. B. VIEW ACROSS NARROW ALLUVIUM-FILLED VALLEY SOUTH OF BETHLEHEM. Narrow valleys of this type are excellent areas for wells of moderate yield. GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1067 PLATE 4 A. VIEW SOUTHWARD ACROSS WESTERN PLAIN SOVTHEAST OF FREDERIKSTED. Topographic relief is low, and the locations of more deeply filled valleys leading from the mountains, if present, are not apparent. IT" B. EMERGENT CORAL REEF AT CANE BAY, NORTH SIDE. PHYSIOGRAPHY 35 the high upland at Annaly.42 Eocene (?) uplift and subsequent erosion resulted in a mountainous topography characterized by high relief and moderate ruggedness. The gently rolling basin floors near Fountain (pi. 2A) were created at this time.43 The beveled spurs at elevations less than 250 feet above sea level from Little Fountain at Windsor may likewise have been formed during this cycle of erosion. In figure 5 part of this surface of the SE - DIAMOND BEESTOM HILL WORK AND REST KETURAH SECOND CYCL_C^L-r,'y*!EPL. f //V ~- ?- " VALLEY Of THIRD CYCLE KingshHlmarl FIGURE 5. Section showing development of early Tertiary erosion surfa.ces along eastern edge of marl plain. second cycle is shown to be covered by Tertiary sediments. A still later uplift initiated a third cycle of erosion. This erosional cycle is represented by the channel in the Cretaceous rocks between St. John and the Leper Asylum. The records of wells indicate that the channel extends westward through the central part of the limestone plain (fig. 4). From geological similarities to Puerto Rico, where a more com- plete record is available, Meyerhoff44 postulated that the peneplain of the first cycle was developed in Paleocene-Eocene(?) time, the second partial peneplain in Eocene time, and the third-cycle partial peneplain in lower Oligocene time. In middle or upper Oligocene time submergence occurred, and in the deepest valleys clayey sediments accumulated. As the waters cleared and shal- lowed, great reefs girdled the island, and extensive limy sediments were deposited above the clays. The Oligocene and Miocere sedi- ments thus formed were then gently folded and faulted, ?,nd the island was uplifted. DRAINAGE PATTERN Streams flowing southward from the mountains tended to follow the old valleys, but upon entering the area of folded lime- stone they were deflected to the west-southwest. This west-south- 42 Meyerhoff, H. A., The physiography of the Virgin Islands, Culebra and Vieques: New York Acad. Sci., Scientific survey of Porto Rico and the Virgin Islands, vol. 4, pt. 2, pp. 152-156, 1927. 43 Meyerhoff, H. A., op. cit., p. 100. 44 Idem. 36 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS westward drainage was encroached upon by streams originating along the south coast, and by a series of stream captures the drainage was deflected generally southward.45 Vestiges of the early drainage pattern are present in the eastern part of the limestone plain. LAND FORMS As a result of the drainage pattern outlined above three types of topography have been developed in the area underlair by the lime- stone of Tertiary age. The western area of old-age topography is almost featureless. The central area of late-mature topography is characterized by synclinal valleys and rounded anticlinal hills and ridges, and the eastern area of early-mature topography, is predominantly a region of ridges and valleys trending southwest- ward or south-southwestward. The hard-rock mountain high- lands have resisted erosion and are only a little more advanced in erosional stage than at the beginning of Miocene tirre. Erosion of the gently folded strata which plunge gently south- westward has created steep northeastward-facing scarps ex- tending from Cane Garden to St. John. The distinct terracing seen in places along this scarp has been ascribed to th« alternation of strata of unequal hardness, but possibly some of these high terraces may be due to higher stands of the sea46 for which other evidence has not been recognized. SUBMERGED CORAL REEF The island of St. Croix is almost entirely surrounded by a fringing reef, 2 miles in width along the southeast but generally narrower elsewhere. From Ham Bluff to the mouth of Salt River the reef is very poorly developed along a series of young fault scarps.47 Submerged reefs or banks are also present and h?.ve been dis- cussed by Vaughan48 and Meyerhoff.49 A bank generally from 6 to 10 fathoms deep and about 1 mile wide skirts the present reef 46 Cederstrom, D. J., op. cit, p. 567-571. 46 Meinzer, O. B., Geologic reconnaissance of a region adjacent to G^antanamo Bay, Cuba: Washing-ton Acad. Sci. Jour., vol. 23, pp. 256-258, 1933. Woodring, W. P., and others, Geology of the Republic of Haiti: Dept. Public Works, p. 374, 1924. Stearns, H. T., Ancient shore lines on the island of Lanai, Hawaii: Geol. Soc. America Bull., vol. 49, pp. 615-628, 1938. 47 Meyerhoff, H. A., op. cit, p. 181. 48 Vaughan, T. W., Some littoral and sublittoral physiographic features of the Virgin and northern Leeward Islands and their bearing on the coral reef problem: Washington Acad. Sci. Jour., vol. 6, p. 56, 1916. "Meyerhoff, H. A., op. cit, pp. 170-175. PHYSIOGRAPHY 37 along the south coast. For 10 miles east of St. Croix this platform broadens and deepens and lies generally 11 to 16 fathoms below the surface. On the eastern extermity of this submarine plat- form Lang Bank forms a higher elongate crescent, the terminals of which point westward and enclose East Point, the eastern extremity of St. Croix. Lang Bank lies 8 to 10 fathoms below the surface and appears to have been a barrier reef at one stage in the development of the island. A still lower reef is also present. It is very narrow and lies generally 12 to 20 fathoms below the surface and rims the next higher bank. The narrowness of this bank suggests that it is covered in part by the higher and younger bank, which is quite well developed. Meyerhoff has suggested that the higher bank was largely formed by subaerial erosion.50 Beyond the reefs the sea floor drops steeply. Along tY<?. south coast, a descent from 27 to 123 fathoms in one-half mile is typical. East of Lang Bank there is a descent from 13 to 435 fathoms in about a mile. Even more precipitous slopes characterize tH north side, and at Ham Bluff the bottom drops from 40 to 818 fathoms, over 4,600 feet, in 1 mile. A depth of 2,412 fathoms, 14,472 feet, is recorded about 8 miles north of Ham Bluff. COASTAL TERRACES AND BURIED VALLEYS The post-lower Miocene erosion of the area underlair by the Kingshill marl resulted in a gently rolling to moderately rugged land surface, across which a number of streams had cut relatively deep V-shaped channels. The sea stood at different levels 40 feet or more below the present sea level. The 6- to 10-fathom sub- marine bank is correlated with one of these levels. Later there was submergence of the island and the sea stood at an elevation of about 30 feet above present sea lev^l. The streams became overloaded, and alluvium spread out c^er the inner lowland. Along the coast the waves and currents cut cliffs in the limestone hills, and on the rock-cut benches a series of deltas radiating from the valley mouths built up broad, flat terraces. The coastal terrace is best observed encircling Krause Lagoon. Here it extends coastwise for more than 4 miles and in places is as much as three-fourths of a mile wide. At Jerusalem, Hope, Anguilla, and elsewhere the limestone hills rise abruptly above the terrace and present toward it a rather definite clifflike face- In the Ruan Bay area the coastal terrace is likewise well devel- oped, but mountainward it merges with the alluvial apron extend- 60 Meyerhoff, H. A., op. cit., p. 183. 38 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS ing from the canyon mouths. (See pi. 4A.) A similar terrace that extends from Fredricksburg to Spatt Hall merges with, and may be partly covered by, the alluvial fans of the canyon mouths. The low rolling land bordering the sea from Christiansted to St. John may also have been beveled at this time. Remnants of these terraces of small areal extent are recognized in several localities in other parts of the island. Flood plains of streams extend mountainward from the level of the coastal terraces. In many places wide, flat valleys between low, rounded hills are well developed. They attain th<ur greatest development and coalesce laterally to form a continuous plain for over 5 miles parallel to the foot of the mountains along the inner lowland. (See pi. 3A.) Insofar as the ground-water resources of St. Croix are concerned, the coastal terraces and inland alluvial plains present special problems since these deposits effectively con- ceal the axes of the deeply channeled buried valleys in which are intercalated the better graded sandy strata that yield Tvater freely to wells. EMERGED CORAL REEF AND BEACHES At the time of a still more recent 10-foot stand of the sea, beaches and coral reefs were formed which are novr emergent. The beaches are generally of small extent and occupy reentrants between the hard-rock spurs which extend to the coast and form headlands, as at Cottongarden, Turner Hole, and alorg the shore at La Grande Princess, northwest of Christiansted. Meyerhoff51 has pointed out that Southwest Cape, built up of limy sand by the action of ocean currents, has been recently uplifted. Emergent coral reefs of this stand of the sea have been recog- nized only at very low elevations. At Ham Bluff a platform of coral limestone about 30 feet wide lies not more than 3 feet above sea level. Emergent reefs are also particularly well displayed at Cane Bay (pi. 4B.) Elsewhere they are present only in very small patches. Plate 9. Emergent coral reef at Cane Bay, north side. SOURCES OF WATER SUPPLY AND USE OF WATER RAIN WATER Practically all water used for human consumption en the island of St. Croix is rain water which has been stored in cisterns. In most places this water is caught on the roofs of buildings and piped to a storage tank, but in places (pl.,5A) concrete catchment basins have been constructed. "Meyerhoff, H. A., op. cit., p. 177. SOURCES OF WATER SUPPLY AND USE OF WATE^ 39 Villages to house labor for the Virgin Islands Co. have been recently built or rebuilt or are planned at Adventure, Bethlehem, Castle Burke, Fredensborg, Golden Grove, Jealousy, Lower Love, New Works, Peter's Rest, Sion Farm, and Slob. In these villages each home is provided with its individual cistern for drinking and cooking Water. Complete plans also call for a mucl larger community cistern of 10,500 gallons at each village with provi- sions for catching the overflow from private cisterns and from a community house. In times of severe drought a great many cisterns become empty, and wells supplying water for domestic purposes may go dry. At such times an entire village supply may be brought in by barrel and donkey cart and carried in in oil tins on the heads of villagers. STREAM! FLOW Stream flow is used in only a few places. The city of Frederick- sted has dammed the stream flowing into Spratt Hall, about U/ij miles east of the coast, at an elevation of 700 feet above s^a level. This water is piped to the city, but it is used for purposes other than human consumption. The water is reported to be turbid at times, and the reservoir goes dry in periods of drought either through leakage of the reservoir, reduced stream flow, or both. Remains of an old dam are present at Orange Grove, above Frederiksted. Water from this reservoir was piped to La Grange in years past. Details of the amount of water furnished or why the dam was abandoned are not known. A small dam is still standing in River Gut a short distance below the estate houses, and although apparently in good repair it is now badly silted and not in use. Water from this reservoir was formerly piped to the West India Co. sugar factory at Bethlehem. Central Factory, previous to 1924, was supplied with water from Salt River, but the amount furnished is believed to have been very small. Use of Salt River was discontinued because of its poor quality for boiler feed. WELLS Wells supply the remainder of water used on the island. Throughout most of the island water from wells is generally used for domestic purposes other than for human consumption. In a very few places the water from wells is occasionally used for drinking purposes. Cattle are nearly everywhere supplied with water from wells. There are a few manually operated or power-driven pumps for discharging water. In most places the water is discharged by 40 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS wind-powered pumps either into storage tanks, or irto troughs of small capacity for drinking water for cattle. At Fredricksted, Christiansted (see pi. 5B), and other places water may be drawn by bucket. Large masonry tanks are used in many places to store well water, but some old cane mills have been sealed and also put to use as reservoirs. Many of the wells, both dug and drilled, are poorly constructed, are not sufficiently deep, or are improperly located with respect to geological conditions and therefore yield very meager supplies and fail in times of drought. SPRINGS There are only a few small springs in St. Croix. One spring, which issues from the canyon wall opposite Little Grarge, is used to supply cattle with drinking water. The spring at Envy, which issues from strata of the Kingshill marl, is put to similar use. Another small spring is said to issue from the Kingshill marl strata below Beeston Hill. The sump at Castle Burke is fed by seeps from surface gravels along River Gut as well as from surface flow. WATER SUPPLIES AT CENTRAL FACTORY At Central Factory, where both sugar and rum' are produced, some water is used in the maceration of the cane when the juice is extracted. Additional water is used to bring the juices up to a constant value when the molasses is run into fermentation vats for the production of rum. The water requirements at Central Factory when rum or alcohol is being produced vary somewhat with operat:on, but in August 1940 it was estimated that nearly 49,000 gallons a day was needed, of which 7,000 gallons a day was furnished by wells 8la and 81b. In normal seasons an 800,000-gallon cistern supplies a reasonably large part of the needed water, but in periods of drought the cistern supply is depleted. In the drought of 1940 some well water and rain water were pumped from Bethlehem Factory. WATERfSUPPLIES'AT BETHLEHEM FACTORY At the recently modernized Bethlehem Factory, where only sugar is produced, much of the maceration water is recovered. However, there is demand for boiler make-up water to replace or dilute steam condensate, which amounts to over 38,000 gallons per day, or 5,720,000 gallons for a full-capacity working season of about 31/2 months. WELL CONSTRUCTION 41 In 1939 a cistern of 1,000,000 gallons capacity was built. This cistern and wells at Fair Plain and Jealousy and sumps at Castle Burke and Bethlehem are the sources of water supply for the factory and for villages near Bethlehem. SUMMARY The present facilities for the recovery of rainfall water, stream flow, and ground water are generally sufficient in periods of normal or above-normal rainfall to supply most needs. However, the island is subject to frequent and severe droughts, <,nd in periods of drought the lack of water becomes acute. Many wells go dry or decrease in yield, stream flow diminishes or disappears, and vital cistern supplies may become totally depleted. At such times, the normal tempo of the island is badly deranged anc1 great personal inconvenience to all classes of persons, loss of stock and crops, and shutdown of distilleries and cane mills may occur. WELL CONSTRUCTION DUG WELLS The majority of wells on St. Croix are dug wells. Most of those located either in the alluvium or in the Mount Eagle, rocks are less than 40 feet deep, but those developed in the soft Tertiary rocks may be of greater depth. Eleven or more dug wells in Tertiary rock are 90 feet deep or deeper. The deepest of these is at Constitution Hill, where it is reported that the total depth of the well was 225 feet. Dug wells in the alluvium are prevented from caving by masonry walls generally constructed of hard limestone blocks quarried from the Kingshill marl or boulders of the Mount Eagle rocks. Below the alluvial cover, wells are not cased. Many of the dug wells, particularly the older ones, have stone towers erected over them (pi. 6A) to support wind vanes for the operation of the pump. The individualism and architectural beauty of the diverse types of well towers are features of the island. Other dug wells are generally protected at the surface by a masonry or concrete curb. DRILLED WELLS In recent years many 8-inch wells have been drilled in St, Croix with a small cable-tool rig. All the producing wells are less than 100 feet deep, but the abandoned well at Peter's Rest School was carried to a depth of 200 feet. These wells are generally finished without screens. Where water is obtained from sandy alluvium 42 GROUND WATER OF ST. CROIX, VIRGIN ISL/NDS it is to be expected that the maximum yield is not obtained and that the wells may be short-lived because of clogging. Wells drawing water from sandy formations should be equipped with modern high-grade manufactured well screens, the openings of which are of such size that the coarser one-third of the sand stratum is retained. Upon agitation the finer two-thirds of the sand is drawn into the well and removed by bailing. By these means a permeable coarse sand or gravelly envelope is built up around the screen and the flow of water into the well is at a maximum. Well screens are ordinarily used in wells in sandy formations which are intended to develop a maximum amount of water. A casing is driven to the top of the sand, and an open hole is made through or into the sand. The screen is then lowered inside the casing until it is opposite the water-bearing stratum, after which it is sealed to the casing with a lead packer. Where it proves impossible to drill open hole into the sand, casing must be driven through or into the sand to the proper depth before the screen is placed. After the strainer is placed the casing is jacked back to uncover the screen, and the screen is sealed. Development of more than one stratum may be more complicated. After the screen is set and sealed, the well should be developed by strong intermittent pumping for a few hours or a few days, depending upon the conditions encountered. Intermittent pump- ing draws out the fine-grained material, which tends to restrict the flow of water into the well. The backflow from tl'e pump line when the pump is shut off agitates the sand in the vicinity of the well screen and thereby loosens it so that it will be removed when the pump is again started up. By these means the maximum amount of water is obtained from the well. A properly constructed well in the alluvium can only be con- structed by a driller who has a reasonable amount of equipment with which to work. Screens cost anywhere from $3 or $4 a foot to 3 20 or $30 a foot depending on the diameter of the screen, the type of manu- facture, and the metal of which the screen is made. The cost of a well in alluvial formations may be increased from $50 to $500 by the installation of such a screen. However, the increased water supply resulting from proper well construction and from the use of proper well equipment cannot be overestimated in St. Croix, where good wells are few and where water in reasonably large quantities is obtainable in only a few places. As far as is known, no benefit would be derived from installing OCCURRENCE OF GROUND WATER 43 screens where weathered Mount Eagle volcanics, diorite, or Fings- hill marl is penetrated. The importance of intermittent pumping at a maximum rate when a hole is completed in these formations should be emphasized since it is believed that this is not commonly practiced. So far as well construction is concerned, most of the wells in the Mount Eagle rocks and the diorite are relatively shallow, and the exploration for an increased supply at depths of from 100 to 200 feet has not been attempted in most places. Most of the drilled wells and some recently constructed dug wells are pumped by wind power provided by the ordinary steel- girder type of tower and vanes. A well was drilled by the rotary method at Fair Plain in 1939 by the Layne-Atlantic Co. for the Virgin Islands Co. (See pi. 65.) An uncased hole was drilled to 225 feet, an operation made possible by circulating a drilling fluid the consistency of a thin mud. The fluid was pumped down inside the drill rods, out through openings in the bit, and up along the space between the rods and the hole. This fluid plasters the walls of the hole and prevents the caving of the walls and at the same time carries out the drill cuttings. The hole was reamed to 36 inches to a depth of 73 feet (the bottom of the lowest water-bearing stratum), and a 36-inch casing was set to the top of the water-bearing strata. A string of 10-inch casing and 8-inch strainer was then set in the hole. Gravel was filled in the space outside the string of casing and strainer and inside the 36-inch hole and casing. Very few data on the drilling operations of the National Park Service (CCC) project in 1940-41 are at hand. Drilling was done by a light rotary rig, presumably in the usual manner. Casing set was 8 or 6 inches in diameter. It is believed that modern manu- factured well screens were used where sand formations were developed but elsewhere, as in weathered Mount Eagle volcanics, perforated pipe was used for well screens. OCCURRENCE OF GROUNDftWATER PREVIOUS INVESTIGATIONS OF GROUND WATER Prior to 1817 Maclure52 visited St. Croix and in addition to publishing the first brief description of the geology of the island noted that "The west end and middle of the island are low and covered with a shell limestone and madrepore rock. The founda- 52 Maclure, William, Observations on the geology of the West India Islands from Barbados to Santa Cruz, inclusive: Acad. Sci. Philadelphia Jour., vol. 1, pp. 134-149, 1817. 842683 50 4 44 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS tion on which this rock reposes is a stratum that retains water » Although many other geologists visited St. Croix in the follow- ing years53 no attention seems to have been given to the ground water. After the American occupation of these islands in 1917 the desire to provide an adequate water supply for military estab- lishments and later for sugar mills and for irrigation focused attention on the possibility of obtaining larger and riore depend- able supplies of water than that previously furnished by streams and cisterns. T. W. Vaughan, of the Geological Survey, was the f rst geologist to make a study of the ground-water resources of th^ Virgin Is- lands, and in his report to the Governor of the Virgin Islands, June 20, 1919, he made the following statement: There is in no one of the Virgin Islands of the United States nor in eastern Puerto Rico any arrangement of rocks that would make possib1'? the presence of artesian water in any appreciable quantity. No water-bearing formation extends from Puerto Rico under the Virgin Bank. In general the rocks of St. John, St. Thomas, Culebra, Vieques, and eastern Puerto Rico are com- posed of sedimentary rocks that have been tilted and folded so that the angle of inclination of the beds in many places exceeds 45° or even 60°. In places the beds are practically vertical. Furthermore, these sediments are cut by igneous rocks that came into their present position while in a molten state. These intrusive rocks would interrupt the underground movement of waters, were other conditions favorable to the presence of artesian supplies. Water from artesian sources in any appreciable quantity in the Virgin Islands or in eastern Puerto Rico is physically impossible, and any money expended on test wells would be wasted. The above report was written with St. Thomas in mind, but it applies to those areas in St. Croix that are made up of the Mount Eagle rocks. In connection with Vaughan's work on St. Croix in 1919 a number of water samples were collected by H. Rydeen and ana- lyzed in the laboratories of the Geological Survey.54 All of these have been included in the present report. Kemp in 192355 reviewed the ground-water prospects of St. Croix and summarized the water-bearing characteristics of the rock formations. The alluvial flats opposite valleys and amphi- theaters in the mountains and hills were suggested as good sites for small wells. 83 Kemp, James P., Introduction and review of the literature on th?. geology of the Virgin Islands: New York Acad. Sci., Scientific survey of Porto Rice and the Virgin Islands, vol. 4, pt. 1, pp. 1-69, 1926. 54 Vaughan, T. W., Ground waters of St. Croix, V. I.: Unpublished Manuscript, U. S. Geol. Survey, 1919. 88 Kemp, James F., Report to H. H. Hough, Captain, U. S. N., Governor, Virgin Islands: Printed at the Naval Station, St. Thomas, Apr. 6, 1923. OCCURRENCE OF GROUND WATER 45 In "A report on St. Croix's water prospects,"56 H. A. Meyerhoff presented more detailed data on the water-bearing character of the formations. With regard to the surficial alluvium Meyerhoff points out that below River and Fountain estates the presence of wash derived from the diorite renders the material more permeable ths.n in most other places and that the difficulty of impounding vater immediately west of Jealousy and the intermittent character of the stream near River are good evidence of the permeability of this material. The limy rocks of the Kingshill marl are described as b<nng exceedingly unfavorable. The basal conglomerate (of the Jealousy formation) was sug- gested as a possible aquifer, and exploration of that stratum was recommended. In a report on the various possible sources of additional vater supply for the properties owned by the Virgin Islands Co., John- son,57 of the United States Bureau of Reclamation, discussed the possibility of increasing the supply by wells. He added but little to what was already known of the geology and hydrology, relied in large part on Meyerhoffs findings, and made further recom- mendations on the basis of Meyerhoff's report discussed above. He recognized the better-than-average yields of the New Works and Fair Plain (45b) wells and recommended developing the alluvial valleys west of Kings Hill and west of Golden Grove. GENERAL CONDITIONS5" Ground water occurs in voids, cavities, fissures, and pore spaces in various types of rocks and saturates the ground up to a variable height known as the water table. If the material is fine-grained, the earth above the water table is more or less moistenei by water drawn up from the zone of saturation by the action of capillarity, whereby small communicating interstices form irr-egu- lar capillary tubes through which water is drawn up by molecular attraction, acting against gravity, and is held in this suspended position at a height above the water table where the two opposing forces are in equilibrium. This is known as the capillary fringe. In clean gravels the fringe may be practically absent, but in silt or clay loam it may be as much as 8 feet thick (fig. 6). 56 Quoted by Johnson, Arthur F., Preliminary report on the water supplies for properties managed by the Virgin Islands Co., St. Croix, V. I.: U. S. Dept. Ir terior, Bur. Reclamation, manuscript rept., pp. 65-67, 1937. « Op. cit., pp. 48-49. *» Meinzer, O. E., The occurrence of ground water in the United States: U. S Geol. Survey, Water-Supply Paper 489, 1923. 46 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS ZONE OF SATURATION Belt of r soil water . ~L Intermediate belt Capillary fringe V Water.; table FIGURE 6. Diagrammatic section showing the zone of saturation and the three belts of the zone of aeration. Between the capillary fringe and the belt of soil moisture the earth contains only a very little water. This is cp.lled the zone of aeration. At and near the surface the earth generally contains a somewhat greater amount of water, and this zone is known as the zone of soil moisture. Where the zone of saturation lies close to the surface, the zone of aeration may be absent. Some of the water falling upon the earth percolates into the ground and tends to accumulate in the zone of saturation. Where the water table is high the ground water tends to move toward lower elevations. It may continue to move underground until it reaches the sea, or it may discharge as springs at the surface and contribute to the flow of streams (fig. 7, A), evaporate, or be transpired by vegetation. When in periods of drought, the water table declines and falls below the levels of the springs and streams, the springs and streams become dry. (See fig. 7, B.) In and near the mountains in St. Croix ground water flows into the streams, Ihit across the Arrow shows direction of movement of water FIGURE 7. Diagrammatic sections showing the movement of ground water: A, In times of normal or excessive rainfall; B, In times of deficient rainfall. OCCURRENCE OF GROUND WATER 47 central plains the water table generally is below the stream beds and many surface streams are "lost" in crossing this plain. Near the sea, at very low elevations the stream beds are again lower than the water table and the streams reappear. Water falling as rain at Fountain estate, therefore, may at first be aHorbed by the soil but may move underground a short distance and pass into River Gut. After flowing at the surface for a short distance it may become lost in sands and gravels below Jealousy, move underground for several miles, and finally reappear at the surface below Fair Plain. Elsewhere, spring-fed streams from the moun- tains disappear before they reach the gravelly plain and discharge into the sea without ever coming to the surface, as in the valley between Jolly Hill and Little Grange. Under artesian conditions water occurs in strata sealed off from the surface by overlying beds of impermeable material, such as clay. The water-bearing stratum fills with water back to the intake on higher ground and hence is under pressure. Be- low the intake area water will rise in wells which penetrate this saturated stratum. If the ground is low enough the well may flow. Much of the water in the deeper alluvium-filled channels in the central plain and elsewhere occurs under artesian conditions, but almost nowhere is the artesian head sufficient to 'cause the wells to flow. The diagram, figure 8, illustrates this phenomenon. Height to which water will rise in wells penetrating artesian water-bearing formation WELL !:=S=~r--~iri'~" I -J.N Sea level '. v?=r-^-E£Dnst sand - '-~~r = TT!\ Basement rock FIGURE 8. Diagrammatic section showing' artesian conditions. GROUND WATER IN THE MOUNT EAGLE VOLCANICS A few wells within the mountainous area in western S't. Croix obtain water from the hard-rock formations. Little Grange lies along the wall of a narrow valley draining a relatively large part of western St. Croix and is located above the canyon mouth at an elevation of about 130 feet above sea level. A drilled well (6, table 11) at this estate penetrated 30 feet of "gravel" and 20 feet of hard rock. The water level in the well is not known, but the well yields an adequate amount of 48 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS water for cattle and is pumped about 30 hours a week by a power- driven deep-well pump, The stream flow just above Jolly Hill was measured on January 11, 1939, and found to be about 180 gallons a minute. This flow, in addition to a flow of perhaps 25 gallons a minute entering the main gut at Jolly Hill, disappears entirely before reaching Little Grange, one-half mile downstream. It seems indicated therefore that exploration of the hardrock valley floor east of Frederiksted by wells might prove profitable. A drilled well (16) 621/4 feet deep is located in a canyon mouth at Robe Hill. The water level was 7 feet below the stream bed. At Valbygaard, near the mouth of the next canyon to the east, the water level in a 32-foot well (18) was about 18 feet below the stream bed, and at Cane Valley estate, higher in the same valley, the water level in well 17 was about l*/2 fegt below the surface of the stream bed. Similar relations prevail at the River (36) and Grove Place (26) estates. These wells, including those at Little Grange, take advantage of the underflow moving down the valleys from the higher collecting grounds in relatively re- stricted channels in the decomposed rock of the stream beds. The well at Grove Place extends to a depth of 11 feet below the stream bed. It goes dry in periods of low precipitation. The well at Robe Hill is probably the only one drilled deep enough to take real advantage of the underflow in periods of drought. It is thought that water-bearing fissures might be encountered down to a depth of 100 or even 200 feet. The chloride content of waters from these shallow wells in hard rock is relatively low, and in three wells the ws.ter contained less than 150 parts per million of chloride (6, 1C, 17, 18, 26, table 11). In the vicinity of Christiansted are several wells that may ob- tain water from the weathered upper part of the older rock formation. In the city itself are several public wells from which water is drawn by buckets (pi. 5.5). Water from well 91, located in Water Gut, is highly mineralized. Well 90, at the foot of Strand Street, yields water in which the total mineral cortent is more than twice as great (table 9). This difference is considered to be due to the fact that the former well is located along a minor drainage line where a greater volume of underflow and surface flow has tended to prevent the accumulation of salts in the ground. Several wells at Hermon Hill supply water for cattle and a small distillery. One well (85) is located in the valley between Alderville Hill and Hermon Hill at an elevation of about 175 OCCURRENCE OF GROUND WATER 49 feet above sea level. The well is 90 feet deep, and the water level was 58 feet below the surface on January 22, 1939. This well is reported to yield 1,200 gallons a day even in periods of drought. In well 86, in the valley head south of Hermon Hill at about the same elevation,, the water level was 61 feet below the surface, but it is reported that during and following periods of heavy rains the water level rises to within 20 feet of the surface. This latter well penetrated 2 feet of clay, 58 feet of sandy rotter rock, and 12 feet of solid rock. Although this well does not fail in periods of drought, a third well (87) on the estate located at a lower elevation one-half mile to the south does fail in periods of drought because the well is not sufficiently deep. A drilled well (116) at Granard, 2*4 miles south-southeast of Christiansted, encountered rock beneath 18 feet of alluvium. The water level is reported to be 10 feet below the surface. At Long- ford, about li/2 miles to the east of Granard and likewise located about 50 feet above sea level on a low plain sloping gently toward the sea, a well (114) 83 feet deep is reported to have flowed when it was drilled in 1930. When measured in 1939 the water level was 38 feet below the surface. Water is delivered from the well by a gasoline-powered pump having a capacity of 15 to 20 gallons a minute, and the well is reported always to yield an ample supply of water for stock and domestic use. This well may penetrate bedrock within a short distance of the surface. The well (112) 45 feet deep at Nugent estate penetrated 44 feet of clayey alluvium and 1 foot of rock. Mr. B. Nelthropp, who has drilled many wells on the island, states that in the East End area in general the slightly consolidated clayey alluvium is an imper- vious formation and water under low artesian head is obtained from the weathered bedrock beneath the alluvium. This state- ment is of interest when the soil map of St. Croix is studied.59 This map shows that the greater part of the level or gently rolling intermontane area in the East End is surfaced by clays or clay loams, whereas the gravelly soils are confined to narrow valleys or occur as fringes on the steeper slopes above the clay areas. These facts may indicate that in general most of the alluvial material contains notable proportions of clay and that the allu- vium might be expected to be relatively impervious. At Hartman (well 111, table 11) the "rotten rock" W/T.S en- countered at 20 feet below the surface. At the time the well was visited the wind-powered pump was delivering about 2 gallons 59 Thorp, James, Soil survey (reconnaissance) of St. Croix Island, V. I.: U. S. Dept. Agr. Tech. Bull. 315, 1932. 50 , GROUND WATER OF ST. CROIX, VIRGIN ISLANDS a minute and the water level in the well was 39 feet below the surface. No data are available on the well at Petronella (110). Wells 89 and 88 lie in the alluvium-filled lowlands between the hills bordering Altoona lagoon. Well 89, north of St. Peter estate, encountered "rotten rock" at about 40 feet. Water stands 16V<j feet below the surface in this well. At Mount Welcome water is probably also derived from weathered bedrock as the well is 82 feet deep. The water level is reported to be 20 feet below the surface. The chloride content of water from wells in the East End sec- tion of St. Croix differs considerably. Wells at St. Peter (93), Catherine's Rest (117) and Hermon Hill (85) yield water containing less than 300 parts per million of chloride. The other wells discussed have a higher chloride con- tent, according to field tests; at Granard (116), Nugent (112), and Hartman (111) the water contained 780, 645, and 1275 parts per million, respectively. The reason for these differences is not everywhere apparent. A moderate chloride content, as in water from well 85 at Hermon Hill, may be the result of restively great underflow in a restricted rock-cut valley or of water flowing to the well only through hard-rock formations. A high chloride content appears to be due to the fact that some or much of the water which enters the well has percolated through alluvium or other material in which alkali salts have been mor? or less con- centrated60 by high evaporation, low rainfall, and poor under- ground drainage. It is not believed that the water in any of the wells referred to above has been contaminated by sea water. GROUND WATER IN THE DIORITE Well 38, known locally as the Kobel well, three-fourths of a mile north of Jealousy estate, is 63 feet deep and ends in dioritic rock. Here about 10 feet of fill overlies the deeply weathered rock. Water enters the well below the casing, which extends to a depth of 57 feet. On February 3, 1939, the wat3r level stood 10.90 feet below the top of the casing, which is about 11 feet above the stream bed on the banks of which the well is situated. A pumping test was made on the well to determine the water- bearing characteristics of the deeply weathered diorite. The well yielded 191/2 gallons a minute with 15 feet of drawdown at the end of a 6-hour pumping period. More water could probably be w The soil map of St. Croix referred to above shows that alkali soils commonly occur at the lower elevations within the alluvial areas in East Enc1 , Other areas are probably less alkaline. GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1067 PLATE 5 rv> "mZWliim « « « * A. ARTIFICIAL CATCHMENT BASIN AT MARY'S FANCY. Windmill pumps water from underground cistern. Old wind-powered cane-mill tower and stack of more recent steam-powered cane mill is at right. Photograph by Paul Schweitzer. B. MUNICIPAL WELL, WATER GUT, CHR1STIANSTED. GEOLOGICAL SURVEY A. OLD WELL TOWER AT STRAWBERRY. Modern wind vanes have been installed. Photograph hy Paul Schweitzer. WATER-SUPPLY PAPER 1067 PLATE 6 B. ROTARY RIG AT TEST WELL NO. 2 NEAR JEALOUSY. Note proximity of well to mountain at left. Photograph by Paul Schweitzer. OCCURRENCE OF GROUND WATER 51 obtained at this place by drilling additional wells or by construct- ing a well in such a" way as to provide a greater intake area. If the weathered diorite is too rotten to allow firm seating of the casing at a depth of about 20 feet, it might be advantageous to install perforated or slotted pipe of a smaller diameter than the hole below 20 feet and fill the space between the hole and casing with sized small gravel. It is also possible that fissures and cracks which would further increase the capacity of wells developed in the dioritic rock might be encountered at greater depths, and additional wells in this formation should be carried to a de^th of about 300 feet unless it is found by careful testing that increases in yield do not occur with increases in depth of hole or that water from depth is highly mineralized. It is reported that in the drought of 1940 this well continued to yield about 2,000 gallons a day. In view of the pumping test referred to above it would seem likely that the installation of a low-capacity power pump on this well and substitution of a 3- or 4-inch pipe line for the existing ^-inch line would be profitable. By this means the supply at Bethlehem Factory might be increased from 5,000 to 10,000 gallons a day. At Southgate a drilled well (100) 80 feet deep probably draws upon weathered diorite. It is believed that the diorite here is as good a water-bearing formation as the rock penetrated by Kobel well, but no data on the capacity of the well are at hand. How- ever, it is reported to have furnished the wind-powered pump with ample water in the 1940 drought. There is good reason to believe, considering the record of the Kobel well and the performance of wells in coarsely crystalline rock areas in general, that a moderate quantity of water could be developed by properly constructed wells in the Fountain Basin and to a lesser extent in the Hermitage Basin to the east. Like- wise it may be expected that water in moderate quantity could be obtained from properly constructed wells penetrating the dioritic rock in the region around Southgate and southeast of Sally's Fancy in East End. [GROUND WATER IN THEfKINGSHILL MARL About 25 wells in St. Croix obtain water from the limestone of Tertiary age which makes up the plain between Frederiksted and Christiansted. The wells vary considerably in depth. A few wells less than 20 feet deep may obtain water from the limy rod's, but most of the wells are deeper and at least two wells are mor? than 200 feet in depth. Most of the wells are dug wells and eight of the 52 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS dug wells are more than 100 feet in depth. The dug well at Con- stitution Hill is more than 225 feet deep. The wells are protected near the surface by natural stone masonry but are not cased below the surficial alluvium. In recent years a few 8-inch open-end wells have been drilled. The limy Kingshill rocks lie at lower elevations n^ar the coast than they do at the foot of the mountains, but details on the structure of the central plain are incomplete and rlthough it is believed that only one to two specific strata yield the water en- countered in wells, this cannot positively be determined. Water in wells in the Tertiary limestones stands from a few feet to 20 feet above sea level near the south coast. Northward, water generally stands at higher elevations, and along th^ foot of the mountains the water level in times of normal rainfall may lie within relatively few feet of the surface. At Mary's Fancy, in the higher area in the eastern part of the plain, the static water level is as much as 250 feet above sea level. Water-bearing limestones of the Kingshill marl are character- ized by small solution holes at Paradise (well C-5). Rock of this type has not been reported in other wells. Strata with small solu- tion holes have been noted in a very few places in road cuts and stream banks. It is possible also that in many wells water is derived from openings along bedding planes and fissures which have resulted from structural deformation. The water-bearing limestones of the Kingshill marl may be largely replenished along the foot of the mountairs by surface streams and by underflow from the mountains. Recharge across the limestone plain is probably hindered in many places by a cover of clayey soil, but elsewhere direct downward percolation of rain- water may take place. A detailed description of representative wells is g:ven below. At Frederiksted seven dug wells (9a-9g) supply the greater number of the residents with water for some domestic uses. The wells along the shore are only 10 feet deep, but those on higher ground are deeper. Along the coast the water level is within one- half foot of sea level, but at a distance of 800 feet east of the seashore the water level may be as much as 4 or 5 feet above sea level. As the water here is a body of fresh water floating on sea water it might be expected that there would be danger of salt- water contamination, particularly during those times when the water-bearing formations receive little fresh-water recharge or when relatively large amounts of water are taken from the well. OCCURRENCE OF GROUND WATER 53 An analysis of water from well 9g (table 5) shows that more magnesium is present than calcium and suggests that the mineral- ization is in part due to the mixing of fresh water with sea water. However, the high nitrate content of this water indicates con- siderable pollution, and it is more likely that the chloride is the result of pollution rather than mixing with sea water. The water level in the dug well (12) at Camporico, on the limestone "plain southeast of Frederiksted, stands a few feet above sea level. The well is reported to be a dependable sourc? of a moderate amount of water even in periods of severe drought. At Whim (14), Concordia (13), and Hope (15), shallow wells in which the water level stands within a few feet of the surface in periods of normal rainfall are developed in the limestone area. However, these wells are too shallow to be dependable in times of drought. Thus far, according to local reports, no efforts have been made to deepen these wells at such times of low water levels. It should be noted that an attempt to secure an adequate water supply by a deeper drilled well, made in the vicinity of Hop% was unsuccessful. It is reported that the well was carried to a depth of more than 100 feet, but no water-bearing strata were en- countered. A drilled well 42 feet deep at Cain (22) obtained an entirely adequate supply of water for stock. The water level stood within 2 feet of the surface when the well was visited by the writer. The well isjocated at an elevation of 90 feet above sea level within one-half mile of the foot of the northern range of mountains. This well is reported to have been abandoned in 1940, but it is not known whether clogging or excessive lowering of the water table was the cause of failure. The well at Diamond (24), 90 feet deep, lies at a slightly greater distance from the mountains than the well at Cain. Water rises in this well to within 32 feet of the surface or about 53 feet above sea level. At Enfield Green, which is Jocated about midway between Dia- mond and the south coast, water rises to within 31 feet of the surface, or probably less than 10 feet above sea level. With respect to location and water level, it is similar to the well at Camporico. Data on the maximum yield of these wells is not available. The well at Diamond (24), however, is reported to continue tc yield an adequate supply even in periods of drought. The amount dis- charged from the well is limited by the capacity of the wind- powered pump and probably at no time exceeds 3 gallons a minute. The dug well 37 feet deep at Paradise (29) is reported to yield less than 2 gallons a minute. 54 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS A spring issuing from limestones at Envy (31) along the south coast has a flow estimated to be 30 or 40 gallons a minute. The occurrence of this spring suggests the presence of channels in the limestones capable of yielding moderately large quantities of water. The well (32) at Manning Bay, less than one-half mile north- east of Envy, is probably dug in alluvium. However, the presence of the spring at Envy suggests that the alluvium here may be receiving water from the limestone rocks rather than from the underflow passing down the valley at Fair Plain. Wells in the limestone rocks are also present along the south coast at Jerusalem (61) and Cane Garden (62). The water level in these wells is approximately 221/2 to 18J/2 feet atove sea level, respectively. The former well is reported to yield an ample supply in periods of drought. A number of wells in the more easterly part of the plain obtain water from the Tertiary limestone for domestic use and for stock. Well 54 at Mary's Fancy extends to an elevation of 190 feet above sea level, and water rises in the well to 93 feet below the surface or to about 245 feet above sea level. At other places the wells reach strata nearer to sea level and water does not rise as high, generally less than 100 feet above sea level, and, as indicated above, as the coast is approached the water may stand less than 20 feet above sea level. At Sion Farm it is reported that, with the present installation, well 68 yields 2,000 gallons a day even in periods of severe drought. Mr. Robert Skeoch reports that for the past 30 years the well at Diamond (55) has always yielded an ample supply, estimated as at least 2,000 gallons a day. The well extends nearly to sea level, and the water stands probably as much as 35 feet above sea level in the well in seasons of abundant rainfall. The Ruby well (57), one-half mile north of Diamond, is 103 feet deep and extends to about 100 feet above sea level. Water stood at 94^/2 feet below the surface (110 feet above sea level) when the we1! was visited by the writer. If the wells at Diamond and Ruby are developed in the same ground-water body, the slope of the water table is more than 110 feet per mile. A similar apparently high gradient is present in the vicinity of Enfield Green and Cain. Certainly this is a very high gradient, and it seems possible that parched bodies of ground water exist above the body of fresh water occurring a short distance above sea level. OCCURRENCE OF GROUND WATER 55 There is reason to believe that in wells located some distance from the coast, where the water level is normally from: 50 to 100 feet above sea level or higher, the water level varies widely from season to season and year to year. Measurements or est*mates of depth to water in a number of wells were made by Mr. H. Rydeen on June 4 and 5, 1919, in conjunction with a report on the water supply of the island by T. W. Vaughan. At Strawberry, where the water level stood at 116 feet above sea level when measured by the writer in December 1939, the water level vras re- ported by Rydeen to have been 27i/2 feet higher in June 1919. The water level at Ruby stood about 116 feet above sea level in 1939 and was reported by Rydeen to have been 201^ feet higher in June 1919. Rydeen also notes that at Constitution Hill the water level was 190 feet from the surface or 110 feet above sea level. In 1939 this well was dry at 225 feet below the surface. On the other hand, only slight differences in the depths to water in coastal wells at Jerusalem and Cane Garden are noted. Ry- deen's figures indicate that the water levels in these wells border- ing the coast were 3i/> and 7% feet higher in 1919 than in 1939. It is further reported by Mr. Hazlett, Associate Geologist of the National Park Service, that on January 1, 1941, the water levels at Jerusalem and Diamond (55) were 3i/^ feet lower than when measured in December 1939, on which date water levels stood at 221/2 and 35 feet above sea level, respectively. Measurements by Hazlett were taken during a period of severe drought. It is re- ported that at Spanish Town the water level declines only a foot or so in times of drought. The water level is normally only a few feet above sea level. The chloride content of the waters in these wells range? from 150 to 950 parts per million except for two wells, which ar^ even higher in chloride content. The quality of water will be discussed in detail in another chapter. A few wells (72, 74) are developed in the Tertiary limestone rocks along the coast northwest of Christiansted. The drilled well at La Grande Princess may encounter limestone beneath a shallow alluvial cover. It is reported that in 1936 the water level was 3 feet below the surface, or about 37 feet above sea level. The dug well at La Grande Princess is located at a somewhat higher elevation. This well is 33 feet deep, and water stood at 28 feet below the surface, or about 27 feet above sea level. The well had not been pumped in 3 months. In the unused dug well at St. John the water level is 321/2 feet below the surface, or about 4 feet above sea level. 56 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS The wells at La Grande Princess yield highly mineralized water. It appears probable that the mineralization is due tc salts in the water-bearing formations rather than to contamination by sea water since the water levels are quite high above sea level. In general, the Kingshill marl is a poor water-bearing forma- tion, although several wells with very low yields are of consider- able value in furnishing water to livestock. GROUND WATER IN THE ALMJVIUM NORTH SIDE Only a few wells from the mouth of Salt River westward are developed in the Quaternary deposits. In one well (77), located just south of Greig Hill on low swampy land, water stands slightly above sea level. This well is pumped every day by a low-capacity gasoline-powered pump and furnishes ample water for cattle. The water is highly mineralized. A second shallow well (78), pumped by a wind-powered pump, is located on the coast north- west of Greig Hill. As closely as could be determined, water stood about 1 foot above sea level in this well. The chloride content of the water is relatively low 378 parts per million. EAST END Several shallow wells (95-97) are located at the east end of Altoona lagoon at slight elevations above sea level. Water levels stand as high as 6 or 7 feet above sea level in these wells. When these wells were visited the chloride content of the water from two wells was relatively low, but the chloride content of the water from a third well was about 950 parts per million. The reason for this difference is not understood, but it is thought that since the windmill pump was operating in the third well and not in the first two, more highly mineralized water was being drawn into this well from the immediately adjacent salt pan bordering Altoona lagoon. Another well (98) in this general area, just south of Darby Hill, yields water containing more than 1,000 parts per million of chloride. In a dug well (99) at All-For-The-Better, at the head of the broad valley carved in dioritic rock embracing Southgate Pond, water stood 29y2 feet below the surface, or about 60 feet above sea level. This well is probably developed in the alluvium but may reach the weathered bedrock beneath. OCCURRENCE OF GROUND WATER 57 Two wells are present in a similar topographic and geologic situation at the head of the broad southward-facing valley em- bracing Great Pond. In well 109, 35 feet deep, the water stands 33 feet below the surface, and in periods of drought the water level falls below the bottom of the well. Well 108 is a drilled well 76 feet deep and obtains water from sandy strata about that depth. In constructing this well, much of the sand was cas^d off to prevent sand clogging. The chloride content is somewhat higher in the shallower well water, being 544 parts per million. In a shallow well (101) at Green Cay, along the north coast, water stands about 2 feet above sea level. This well supplies more than 300 cattle with water even during periods of drought. The chloride content is relatively low. This well may be in the path of greatest underflow down the vflley, otherwise it might be ex- pected that the mineralization would be higher. The well (102) at Mary's Fancy is poorly situated on th« east side of Cotton Valley and yields a water containing abort 600 parts per million of chloride. If it were located more westerly in the path of greatest underflow the water might be lower in chloride. Three wells are located at Great Pond estate. Well 105 is de- veloped in a narrow strip of beach sand just west of Mount Fancy. The well near the great house (106) is developed in alluvium and encounters rock at 24 feet below the surface. The water here has a chloride content of more than 1,700 parts per millior. The water level had declined at the time the well was visited to within a foot of the bottom of the well, and the yield of the well was de- creasing. A third well (107), 40 feet deep, is in the broad valley one-half mile northwest of the great house. Water stood 6% feet from the surface at the time the well was visited. The well is not used and is reported to furnish only a limited amount of water. In summary, small supplies of rather mineralized water are obtained from shallow wells developed in alluvium along the. coast in the East End area. In some of these waters (wells 78, 95, 96, 97) contamination or the danger of contamination by sea water is apparent. Other well waters (wells 70, 71, 77, 98) are highly mineralized because of salts in the sediments from which the water is taken. Wells (93, 94, 101) which are located on or near the main channels of underflow from large drainage areas yield more dependable supplies of water, and the water obtained is relatively low in mineral content. Water levels fluctuate more widely in response to local seasonal rainfall in the wells on higher 58 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS ground. Wells that are too shallow (109) go dry in periods of drought. CENTRAL PLAIN The alluvium that occupies the valleys and broac1 depressions in the central marl plain supplies fair amounts of water of rela- tively moderate mineral content to a number of shallow dug wells and to a few deeper drilled wells. In most of these wells the water level is within 15 feet of the surface. Further, most of the wells are so shallow that in times of drought the water level may fall below the bottom of the well and the well go dry. The deeper drilled wells are generally depend- able sources of water even in times of drought. North of Frederiksted three wells take advantage of the under- flow coming down from the large drainage area in the mountain- ous western part of St. Croix. The wells at Willir.ms (4) and Wheel of Fortune (10) supply water for domestic use and for cattle. The chloride content of the water in the well at Williams is only 112 parts per million but is 410 parts in the veil at Wheel of Fortune. In the dug well just south of La Grange factory (8) the water level was 8 feet below the surface, or about 30 feet above sea level. This well, 15 feet deep, is reported to yield 200 gallons a minute for periods of several hours. It seems likely that this area just north of Frederiksted, which is at the mouth of a valley carrying a fairly large urderflow, is a particularly favorable locality for the development of moderate to large supplies of water. The underflow coming down the valley varies, of course, but except in periods of protracted droughts it may be as much as several hundreds of gallons a minute. If moderately large amounts of water are to be pumped in this area, as for a municipal supply for Frederiksted, care should be taken not to draw the water level down to sea level, as a general lower- ing of water levels will tend to bring in salt water and endanger the quality of the supply. For maximum yield and efficiency and to lessen the danger of chloride contamination, any moderately large quantity of water discharged should be pumped from sev- eral widely spaced wells rather than that the total amount be taken from one well. It seems evident that a fairly large supply of water might be also obtained between Spratt Hall and Williams sir^.e this area too receives underflow from a drainage area extending eastward to Annaly. From Whim to Mount Pleasant very little is know:^ concerning OCCURRENCE OF GROUND WATER 59 the thickness of the alluvium and its quality as a water-bearing formation. Most of the wells obtaining water from the alluvium here are shallow and fail in periods of drought. However, if favor- able thicknesses of alluvium are found in this area, it seems likely that supplies of water may be obtained which would be an im- provement over many of the existing supplies. Dependable sup- plies of small quantities of water are reported to be obtained from wells at Cain (22), Diamond (24), and Enfield (23). Although these wells are said to penetrate the marls and limestone of the Kingshill it is conceivable that much of the water available to them has been carried down from the mountains as underflow in the alluvial cover. It seems possible that deep alluvium-filled channels in these underlying rocks from which somewhat larger amounts of water could be obtained might be located. Areas re- ceiving the greater part of the underflow from the mourtain valleys terminating at Grove Place, St. George, and below Cane Valley will probably be the most favorable places to prospect. The fact that streams from the mountains become dry ever be- fore they reach the alluvium but reappear as the coast is ap- proached indicates that subsurface flow is normally of importance in this area. The inner lowland from Plessen to Mon Bijou and the valleys extending southward from the inner lowland to the sea are filled with a variable thickness of alluvium. In this area three shallow wells, four deeper wells, and two springs furnish water for several villages and for cattle. The streams discharging across the alluvial plain from the mountains are lost beneath the surface a short distance below the mouth of the valleys. Water appears in their courses from place to place; for example, the stream near Fair Plain flows on the surface. A short distance below the South Side road the flow was roughly measured at 50 gallons a minute. This measurement was taken late in March in the dry season. A short distance below the place at which the above mentioned measurement was taken the surface flow increased considerably. The discharge south of the South Side road represents only a small part of the water taken into the ground at the foot of the mountains inasmuch as important losses are sustained all along the stream courses by evaporation where the water level is at or close to the surface. Further, it is known from logs of wells that deeper sands and gravels, separated from the surface sands by relatively impervious clays, are present in the major valleys traversing the central plain, and these undoubtedly carry off much of the drainage from the mountain area. 842683° SO S 60 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS At Plessen (28b) and near Castle Burke (34b) rrater is pumped from springs issuing from the surface sands. The spring at Plessen yields a water containing 316 parts per million of chloride. It is said never to go dry. At the Castle Burke sump from 10 to 20 gallons a minute is pumped continuously in seasons of nor- mal rainfall, but in periods of drought the yield is considerably less. However, since water (in somewhat smaller quantities than normal) was available from the Castle Burke sump in the 1940 drought, it seems indicated that the water levels may not fall very greatly in this vicinity in periods of deficient rainfall and that moderate quantities of water should be available to properly located shallow wells in this area at all times. As the water from Castle Burke sump is of very low mineral content (see table 10) and as ground water occurs near the surface e^en in times of drought, this area should be explored to determine the location of the deepest part of the ancient-filled valley leading southward from Upper Love. Here the greatest thickness of coarse sands will be found, which should provide an excellent supply of water to properly screened wells. A dug well, not more than 20 feet deep, is located along the stream bed near Fredensborg. Normally this well yields about 7 gallons a minute per 8-hour day. In times of drought, how- ever, it is practicable to pump the well only about 3 hours a day. The chloride content of the water is high but not excessive, being 452 parts per million. The extent of variation in water-level is illustrated by several measurements made on the Golden Grove dug well. Rydeen noted that on June 2, 1919, the water level was 10 feet below the surface. On December 9,1938, the water level war 14i/2 feet below the surface. This was in a season of normal light rains. Dur- ing the following dry period, water levels fell, and on March 25, 1939, the water level was about ISVs feet below the surface. Hazlett reports that in January 1941 the water level in the well had declined to about 24 feet below the surface. It seems apparent that since the level of ne&r-surface water is subject to wide variations in this locality (and perhaps in other localities some distance from the foot of the mountains) every effort should be made to continue shallow wells through the water-bearing sands to the'underlying impervious formation, either clay or marl, in order that water be available to pumps at all times. This can be accomplished easily by drilled wells using casing and screen but may be impossible with dug wells. OCCURRENCE OF GROUND WATER 61 Deeper sand strata are developed in the axial part of the valley between Fredensborg and Fair Plain. Drilled wells at New Works (44) and at Fair Plain (45b) encounter coarse gravel about 58 feet below the surface. Water is present under artesian head and rises to within 14 feet of the surface in the latter well. These wells are not screened, nor are they developed to the fullest ex- tent possible. Yet the latter well, at least, yields about 3 gallons per minute per foot of drawdown, and both wells continued to yield ample water to low-capacity windmill pumps during the drought of 1940. Plate SB is a view across the alluvium-fillec" valley south of Bethlehem. A test well (45a) was drilled within 100 feet of the well at Fair Plain discussed above. The log of this well shows that in addition to near-surface water occurring under water-table conditions, two strata of artesian water-bearing gravels and sands occur from 57 to 62 feet and from 70 to 73 feet below the surface, respec- tively. The near-surface water, which occurs in fine-to-coarse sand extending from the surface to a depth of 33 feet, stood at an elevation of 21 feet below the surface (about 2 feet above the level of the nearby stream), but the water from the deeper strata rose to within 17 feet of the surface. A pumping test was made on well 45a after it had been fully developed. The results of this test are shown in figure 9 ard may be briefly summarized as follows: Before pumping began the static water1 level was 16.83 feet below the top of the casing; at the end of 231/2 hours of pumping at 61 gallons a minute, the water level was 30.02 feet below the top of the casing. Tl ° rate of pumping was increased to 70 gallons a minute, and pumping at this rate was continued for 2414 hours, at the end of which tirne the water level was 32.31 feet below the top of the casing. The rate of pumping was increased to 80 gallons a minute and continued at that rate for 2i/2 hours. The water level fell to 35.33 feet below the surface. The rate of pumping was decreased to 53 gallons a minute and maintained at that rate for 17y» hours, at the end of which time the water level was 26.S9 feet below the surface. The specific capacity of the well (gallons a minute per foot of drawdown) was indicated to be 5.09, 4.52, and 5.55, respectively, in the several pumping tests. The chloride content of the water fell from 515 parts per million to 435 parts per million during the first 23% hours of pumping. When the rate of pumping was increased to 70 gallons 62 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS ^ // 1 / //\ 1 <g * 2 1s ^" (H 8»2 z ! o 1 « 3 T ,1, 5 § 5! j-> s tsa -t-> A on S ^ Its of pi " § a> R> 03 6 1s * t o> S tl0£ 03. IO £ 5 U33J) ONISVO JO dOl M0138 Oi Hld3Q o o o 00 * O '109) oo dO 3JLtfb 1N31NOO IQlbOlHO OCCURRENCE OF GROUND WATER 63 a minute llhe chloride content rose to 660 parts per million, de- clined to 565 parts per million, and then increased to 710 par^s per million at the end of 48 hours of continuous pumping. The chloride content was still about 700 parts per million at the con- clusion of the 70-hour test. It seems likely that, as a cone of depression is created around the well, water from other sources, either from the limestones or from the near-surface alluvium, is brought into the well causing an increase in the chloride content of the water. Analysis of a sample (table 10) collected in April 1940 shows that the water contained 500 parts per million of chloride at that time. The dug well (32) at Manning Bay is 10 feet deep. According to a field test the water contained 685 parts of chloride on December 8, 1938. In 1939 Manning Bay was taken over by the United States Army. The well was used as a source of water supply, but it was found that when the withdrawal increased, the chloride content increased from 500 to 960 parts per million. It is stated that for several months, at least, the well was pumped at a rate of 30 gallons a minute, a remarkable yield for a dug well in St. Croix. Very few wells are developed in the alluvial valley system discharging at Hope and Blessing. A well (50) at Lareine 42 feet deep supplies water to a wind-driven pump, as does a well 28^ feet deep at Hope (60). It is of interest to note that the chloride content of the water from the well at Hope was relatively low, about 110 parts per million. The Castle Coakley well (63) was thought to be developed in limestone, but the low chloride content (79 parts per million) of the water suggests that it may be drawing water from the alluvium. The drilled well at Anna's Hope (64) is 180 feet deep anc1 may penetrate the weathered bedrock. However, it seems probable that the overlying alluvium is contributing a large part cf the water available to the well and the well should be considered as being developed in Quaternary material. The well is said to yield ample water to a windmill pump even in times of protracted drought. Several wells are located along the coast northwest of Christian- sted. Two shallow wells at Little Princess are reported to yield small amounts of water at all times. In normal seasons water stands about 5 or 6 feet from the surface. One of the wells (69) yields water containing about 700 parts of chloride, but the 64 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS other (70) yields water containing over 1,800 parts of chloride. The water from well 70 is used for drinking purposes occasionally by some of the native Cruzians and is described by them as "heavy" but quite palatable. It seems established from the record of wells in the alluvium, particularly the Fair Plain developed well (45a), that there exists in St. Croix a potential supply of moderately large quantities of water which has hitherto been barely touched. It seems likely that the storage capacity of the alluvial sands and gravels may be relatively large and that with proper development several hun- dreds of gallons of water a minute might be easily obtained from the alluvial sands and gravels in the valley north and northeast of Fair Plain and in similar narrow alluvial valleys. Further, one or both of the deeper water-bearing strata may be present in relatively deep channels leading seaward from the mountain gorges across the alluvial lowland. Here the mireral content of ground water is lower than at localities nearer the sea. The location of these channels is not so apparent wher^ the valleys are broad and in the general area north of Center Line road (pi. 3A), and it is not possible to predict their location, but their presence may be established by inexpensive test drilling with a light rotary or jetting rig. GROUND WATER IN BEACH SAND A well at Cane Bay (80) and another at Great Pond (105) obtain water from beach sand. Beach sand, including emergent beaches, is of limited extent in St. Croix except at Southwest Cape and cannot be counted on for more than a limited quantity of water. A well in beach sand generally yields a rather highly mineralized water, owing in large part to excessive salt spray and possible contamination by sea water resulting- from over- pumping. At Cane Bay water stands 1^2 feet above sea level. Although containing more than 1,000 parts of chloride the water is satis- factorily used in a cattle dip. At Great Pond water from beach sand contains over 5,000 parts of chloride but it said to be entirely satisfactory as a source of drinking water for cattle. TEST-DRILLING PROGRAM Through funds made available by the Public Works Administra- tion, a program of test drilling on lands controlled by the Virgin Islands Co. was initiated in the fall of 1938 to determine the presence or absence of deep water-bearing strata in the area OCCURRENCE OF GROUND WATER 65 underlain by mid-Tertiary limestone. The wells were to be drilled on the property of the Virgin Islands Co., and a depend- able supply of water for use at the Bethlehem sugar mill was to be obtained, if possible. The writer was assigned to the project and selected the sites for drilling. The drilling itself (pi. 65) was done by the rotary clay seal method by the Layne Atlantic Co., of Norfolk, Va. A study of the geology in the area north and northwest of Bethlehem Factory indicated that the marl and limestone of the Tingshill rested upon the older Mount Eagle volcanics, as had b^en described in the literature. The actual contact between the two formations was not displayed, and the dip of the surface of older rock plane was not known. However, it was thought that the plane nrobably sloped more or less gradually seaward in a mariner analogous to the basement rock beneath the Atlantic Coastal Plain. A site was selected for test drilling just east of Bethlehem Factory and slightly less than 1 mile south of the contact be- tween the Kingshill and Mount Eagle rocks. It was hoped that information on the following points might be forthcoming upon the completion of the test well: 1. The presence or absence of water-bearing beds in the Ter- tiary limestones at greater depths than had been previously attained by wells in St. Croix. 2. The determination of the water-bearing characteristic? of the basal conglomerate. 3. The presence or absence of water-bearing alluvial material or weathered bedrock beneath the basal conglomerate. In the first test well (41, table 3, and pi. 1) 18 feet of soft yellow marl was first penetrated, beneath which lay 7 feet of s^.nd and gravel. The next 83 feet of material consisted of soft (but probably indurated) yellow marl of the Kingshill exactly similar to that seen outcropping in many places on the island. However, at a depth of 108 feet a greenish-gray clay (blue when wet) was encountered, and at a depth of 1,508 feet from the surface the drill was still in this same material. Hard, thin layer? of limestone, not more than 1 foot thick, occurred between 728 and 802 feet and between 1,291 and 1,448 feet, and a stratum of hard limestone conglomerate 16 feet thick was penetrated between 1,095 and 1,111 feet below the surface, a similar stratum occur- ring between 1,196 and 1,201 feet below the surface. The near-surface sand and gravel was water-bearing and was being drawn upon by a nearby well, and hence no attempt was made to develop this stratum. The underlying limestone and clays were entirely barren of water-bearing beds. 66 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS It was apparent that the bedrock surface did not slope gradually seaward but plunged sharply downward from the contact near Jealousy, and, accordingly, to obtain the information sought in this program it was necessary to select a site nearer the foot of the mountains. The location for the second test was one-eighth of a mile west of estate Jealousy and 1,000 feet soMth of the con- tact of the Kingshill and Mount Eagle rocks. In the second test well (39, table 3, and pi. 1) yellow marl ex- tended from the surface to a depth of 91 feet. Hard layers of limestone, each one only a few inches in thickness, were encoun- tered in the yellow marl. Below the marl gray clay of the Jealousy formation was penetrated which extended to a depth of 398 feet. The gray clay here contained much more detrital material than was found in the section at the first test well. A stratum con- taining boulders was encountered between 224 and 228 feet. Between 245 and 255 feet the clay contained many pebbles and small boulders, and from 255 to 398 feet below the surface the clay contained a variable but small amount of sard and pebbles. Cores of this material showed that the inclination of the bedding planes was not constant and the dip of individual strata ranged from 12° to 35° within a foot o'f core. The presence of thin cross-bedded sandy strata suggests that these are not true dips but are the result of cross bedding on a larger scaK Hard limestone conglomerate extended from a c1c*pth of 398 to 459 feet below the surface. It is exactly similar to the rock seen cropping out in the stream bed a short distance northwest of the well. The conglomerate was well cemented and, r.s far as could be determined from cores, it was not characterized by solution cavities or fractures. Weathered bedrock was encountered beneath the limestone con- glomerate and cored for 11 feet, making the total depth of the test well 470 feet. The weathered bedrock was fractured but in the samples brought up the fractures were sealec1 by secondary deposits of calcite (lime). Water was found only in the basal part of the limestone ex- tending to a depth of 91 feet below the surface. The yield was less than 16 gallons a minute, and the water contained more than 1,600 parts per million of chloride. No water was yielded by the conglomerate or by the basement rock. The information obtained by these test wells may be sum- marized as follows: 1. The limestones and marls of the Kingshill, which are seen at the surface in many places, are of limited thickness and OCCURRENCE OF GROUND WATER 67 are underlain by a grayish clay of unknown maximum thickness. The limestones and marl rocks penetrated in these wells contained no stratum which* under any conditions might be considered a particularly good water-bes ring formation. The clay, as might be expected, yields no water. 2. The basal conglomerate, where not weathered, is a tight and impervious formation and will probably not yield water anywhere. 3. Bedrock lies at relatively great depths a short distance from the base of the northern range of mountains. Although fractured and open where weathered, it may become im- permeable through the deposition of mineral matter by percolating ground waters. The weathered bedrock has proven to be an excellent water- bearing formation in several places, as at Little Grange, Fobel (north of Jealousy), Hermon Hill, Longford, and Southgate. Whether fissures and cracks tend to be filled with secondary mineral matter at depth in most places or only in very few places is not known, but in any event the weathered bedrock, and pos- sibly also the fresh harder rock beneath if encountered in wells at shallow depths, should be considered as possible sources of at least small supplies of water. This point has been previously discussed, but is repeated here to avoid the implication that the evidence gained in the second test well is applicable to all r.reas underlain by hard rocks in St. Croix. It was decided to develop a well at a selected location wher^ the alluvium would most likely be of maximum or nearly maximum thickness and subject to continuous and ample recharge. A site was selected (45a) near the existing drilled well (45b) at Fair Plain. In this place the surface and subsurface drainage of a wide area extending up into the mountains is concentrated in a narrow channel. The existing well demonstrated the presence of other than near-surface water-bearing formations. In addition it was felt that the location was far enough removed from the sea to avoid salt-water contamination. In addition to near-surface water-bearing sands, water-bearing strata were encountered from 57 to 62 feet and from 70 to 73 feet below the surface. The relatively large yield of water obtained from these strata as compared to-yields of other wells on the island has been discussed in a previous chapter. Yellow marl lay below the alluvial sands and clays and extended to a c'epth of 162 feet, and alternating harder limestones and soft narls extended to a depth of 208 feet below the surface, at which c^pth 68 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS gray clay was encountered. The well was continued to a depth of 225 feet. No water-bearing strata were encountered in the limy rocks. Tiie test well was converted into a gravel-packed well and at present is supplying water to the Bethlehem Factory. It is reported to have furnished 18,000 gallons a day during the severe drought of 1940 and was pumped continuously at a rate of 40 (?) gallons a minute in 1943. TABLE 3. Logs of test holes drilled in St. Croix, 1938-39 Well 41, Virgin Islands Co. at Fredensborg [Altitude, 90 feet] Alluvium (Quaternary) : Kingshill marl (Miocene-up- (?) Oligocene): Greenish-gray clay (thin, hard layers of limy rock at 728, 732, 762, 770, 778, and 802 feet).-.... Thickness (feet) 18 4 3 83 987 Depth (feet) 18 22 25 108 1,095 mentedbylime. Clayey Gray clay with hard streaks at 1,291, 1,305 to 1,307, and 1,448 Thickness (feet) 16 85 5 305 Depth (feet) 1,111 1,196 1,201 1,506 Well 39, Virgin Islands Co. at Jealousy [Altitude, 150 feet] Kingshill marl (Miocene-up- per (?) Oligocene): Yellow marl (contains hard, limy layers, each a at 81, 83, 84, 85, 88 feet); Jealousy formation (upper (?) Oligocene) : Gray and black clay con- 91 133 4 17 91 224 228 245 Jealousy formation Cort. taining a considerable amount of rounded pebbles and sms.ll Gray clay containing a amount of sand and Hard basement rock.___- 10 143 61 11 255 398 459 470 Well 45a, Virgin Islands Co. at Fair Plain [Altitude, 30 feet] Alluvium (Quaternary) : Fine sand gradually be- Clay somewhat sandv or Clay. - .. Sand and gravel, water.. .. 33 24 5 8 3 33 57 62 70 73 Kingshill marl (Miocere- upper (?) Oligocene): Yellowish to white marl . 89 5 8 10 13 17 162 167 185 195 208 225 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS 69 QUALITY OF WATER COMPARISON OF SURFACE WATER WITH GROUND WATER Samples of surface waters that were taken at Two Friends and at Jolly Hill may be characterized as moderately hard bicarbonate waters containing less than 400 parts per million of total dis- solved solids, in which sulfate and chloride are low. So far as the two available analyses may be used as bases for characteriza- tion of St. Croix surface waters, the waters are rather simple chemically and about what would be expected from the solution of limestone rocks by percolating waters. Sodium and chloride may have originated in greater part as ocean spray carried over the islands by the constant winds. The ground waters are in part derived from the infiltration of surface streams but are more highly mineralized than the surface waters. Most of the ground waters of St. Croix thf.t .were analyzed contained from 400 to 2,400 parts per million of total dissolved solids, and a few waters had more than 4,000 parts per million. The higher mineral content of the ground waters may be due to solution of some of the constituents of the materials through which the waters percolate. A brief study of the r.nalyses of the ground waters shows, however, that the increase is not a simple proportionate increase of the elements found in 8. simple calcium and magnesium bicarbonate water, but, as shown graphi- cally in figure 10, in proportions of constituents present, the ground waters generally contain a high, sometimes very high, content of sodium chloride and may also contain moderate amounts of sodium sulfate. Analysis C in figure 10, a sample of surface water taken near the mouth of Salt River appears more like some of the ground waters analyzed than like either of the two other surface-water samples. Analyses from Castle Burke sump and Kobel well con- tain less sodium salts than most of the other ground waters analyzed because these waters have not traveled any considerable distance underground and have passed through relatively in- soluble rock material only. CONTAMINATION BY SEA WATER One of the first questions which arises when the quality of the underground waters of an oceanic island is considered is the relation of the fresh water to the salt sea water. There- fore, this relation will be considered in detail. It has been common knowledge for many years that fresh 70 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS water falling upon a sandy coast or upon a small sandy island will percolate into the sand and move laterally toward the shore to mingle ultimately with the sea. It might be supposed that in such places the salt water surrounding the island or adjoining the coast would penetrate the sand to mean sea level and immedi- ately absorb all the fresh water that might percolate downward to its surface. For several physical reasons this does not happen. Such islands are found, in reality, to contain a dome-shaped lens of fresh water floating upon a concave surface of salt water, as shown in figure 11 A. The fresh water is enabled to float upon the salt water because the fresh water has a considerably lower density. This principle was first applied to the hydrology of seacoasts by Baden Ghyben61 in 1887 as a result of investigations made in Holland and was also noted by Herzberg62 of Berlin in 1900, and in more recent years has been applied in the United States.63 In figure 11, A, the weight of the higher column of fresh water, H, should be equal to the weight of the smaller column of salt water displaced, h. Thus it has been found that in places where the law is applicable, for every foot which fresh wrter extends above sea level it also extends approximately 40 feet below sea level. Thus, in a well along the coast at St. Croix where the water level in wells stands 3 feet above sea level, salt water should be encountered in that well at 120 feet below sea level. However, a corollary of this principle is that if the height of fresh water above sea level (t, in fig. 11, A) is reduced 1 foot, then the salt-water boundary moves upward 40 feet, as shown in figure 11, B. Consequently, if wells along the coast are heavily pumped, water levels in the immediate vicinity of the wells may be lowered to sea level or below sea level and cause an upward intrusion of salt water into the wells. In most wells in St. Croix the water level stands more than a few feet above sea level and the danger of salt-water con- tamination appears to be negligible. There are a few places along the coast where ground waters have a high chloride con- 81 Baden Ghyben, W., Nota in verband met de voorgenomen put. boring nabig Amsterdam: K. Inst. Ing. Tydschv., p. 21, 1888-1889, The Hague, 1889. 42 Herzberg, Baiirat, Die Wasserversooging einigir Nordseebader: Jour. Gasbelench- tung und Wasser versorging, Jahrg. 44, Munich, 1901. 83 Stearns, H. T., Clark, W. O., and Meinzer, O. E., Geology and water resources of the Kau district, Hawaii: U. S. Geol. Survey Water-Supply Paper 616, pp. 10-21, 1930. Stearns, H. T., and Vaksvik, K. N., Geology and ground-water resources of the Island of Oahu, Hawaii: Div. Hydrography, T. H., Bull. 1, pp. 346-364, 1935. Barksdale, H. C., Sundstrom, R. W., and Brunstein, M. S., Supplementary report on the ground-water supplies of the Atlantic City region: New Jersey Water Policy Commission, Special Kept. No. 6, pp. 10-26, 1936. EQUIVALENTS PER MILLION \ \ \ mm:^t m 10.- Graphic representations of analyses of some St. Croix grrouna waters and surface waters. 72 GROUND WATER OF ST. CROIX, VIRGIN ISL/NDS Nonpumping well \ Water table Pumping well Water table water B FIGURE 11. Diagram showing- relation between salt water and fresh water in homo- geneous water-bearing sands making up a small island: A, Under norpumping condi- tions ; B, Where pumping is taking place. tent; these places are (1) in the city of Frederiksted, and (2) along the coast northwest of Christiansted. A few wells else- where may also be contaminated by sea water. TABLE 4. Analyses of ground waters in Leper Asylum well, St. Croix, show- ing chloride contamination and analyses of ocean water and of water from the Tertiary limestone area for comparison. [Results in parts per million] Silica (SiO2)_---__ ____-_-,______-----__-_- Sulfate (SOO-- ---------------------------- Chloride (Cl)_ ______ ----------------------- Range of composition of ground waters in Tertiary limestone area 1 21 to 49 4 .3 to 1.7* 10 to 133* 6.9 to 101 4 615 to 971 02 to 500 84 to 1,985 Tr to 25 0 to 1 .44 48 to 747 812 to 4,4864 6 6 Lep?r Asylum3 12 12 38 64 / 1,639 I 4.4 712 638 1,895 1.3 1.0 357 4,708 EWL 7 July 7, 1936 Ocean water3 /fl Q 1,300 10,710 390 150 2,690 i Q ^3 cn 6,380 35,000 1 Wells are Bethlehem, Bonne Esperanee, Constitution Hill, Sion Farm, Profit, and spring at Envy. (See table 11.) 2 Well 81a. 3 Mean of 77 analyses of ocean water collected by Challenger Expedition. 4 Not determined in some analyses of wells in Tertiary marl. 5 Sodium and potassium content of waters from wells at Sion Farm and Bonne Esperanee (1940 sample) were calculated. « See table 8. ' EWL, E. W. Lohr. QUALITY OF WATER 73 In Table 4 ground water from the Leper Asylum well (81a), believed to be contaminated by sea water, is compared to ocean water and to water from wells in the Tertiary limestone area. In this water magnesium is high in relation to calcium, in addi- tion to having a high chloride and fairly high sulfate contert. The analyses of water samples collected in 1919 from wells 77 and 80, located near the mouth of Salt River and at Cane Bay, respectively, suggest that little if any contamination by sea water had taken place at that time. However, field determinations made in 1939 (see table 11) show that the chloride content of the water from these wells is now much higher than it was in the samples collected for T. W. Vaughan in 1919. Well 77 yielded water containing 344 parts per million of chloride in 1919, but in December 1938 the chloride content was almost 2,000 parts per million. Likewise, the chloride content of well 80 at Cane Bay increased from 303 to over 1,300 parts per million. Well 77 discharged daily by a low-capacity gasoline pump. Well 80 supplies water to a cattle clip and is discharged somewhat less heavily by a hand pump. Both these wells are located at low elevations near the sea. As complete analyses of the wr.ters yielded by these two wells in 1939 are lacking it is not possible to determine definitely the cause of the increased chloride con- tent, but it is suggested that they may have been contaminated by sea water as a result of moderate discharge by pumping. Wells 105 and 106 at Great Pond are located at low elevations near the sea and are pumped more or less continuously at a low rate. These wells yielded water containing 5,150 and 1,740 parts per million of chloride, respectively, in January 1939. It is suggested that here too the wells are probably somewhat con- taminated by sea water. The other ground waters of St. Croix for which analyses are at hand either show reasonably low chloride content or are of such chemical character or so located that contamination by sea water is most unlikely. Therefore, since it has been shown that prob- ably only a few wells in St. Croix are contaminated by sea'water, the source of the relatively high mineral content of waters from many wells calls for some other explanation. CHLORIDE CONTAMINATION BESULTING FBOM ORGANIC POLLUTION The analysis of the water from well 9c (table 5) in FrederiJ-sted shows a chloride content of 3,549 parts per million and, consider- ing the location of the well near the coast, it might be thought that this represents mixing with sea water in part. However, 74 GROUND WATER OP ST. CROIX, VIRGIN ISLANDS the analysis also shows that the water has a ver;r high nitrate content, generally an index of organic pollution, and it is probable that the very high chloride content has been derived from the same source. Further, the proportion of magnesium to calcium is lower than would be expected if sea-water contamination had taken place. Well 9g at Frederiksted is located on higher ground some distance from the sea. When the analysis is examined, it is seen that the total mineralization is only about half that of the water from well 9c, near the shore, although it is still high. Chloride is less than half that in the water from well 9c, 1,609 parts per million. The nitrate content, 57 parts per million, is less, and it seems likely that here the high chloride content may be "ascribed partly to organic pollution but the high ratio of magnesium to calcium and the very high sulphate content indicate sea water contamination. It may be added that the water in well 90 at No. 1 Strand Street, Christiansted, which contains 507 parts per million of chloride as compared to 204 parts per million of chloride in well 91 in Water Gut, also shows high organic pollution since the analysis shows a nitrate content of 305 parts per million. TABLE 5. Analyses of waters from, wells showing organic pollution [Results in parts per million] Silica (SiO2) ___ ...... _ . _ - ___ ---- -- Sodium (Na) ____ - _____ ... ___ ... ------ Sulfate (SO4) ------------ _ ------------ Chloride (Cl) ___ - __ ..-- __..- ____ .- Fredericksted Well 9c 26 .60 401 123 } 2,119 0 390 273 3,550 291 1,507 7,443 NF June 6, 1919 Well 9g 23 .67 83 115 1,099 0 57i3 63.^ 1,609 57 68^ 3,64,5 NF June 6, 1919 Christiansted Well 90 25 .17 86 65 552 0 656 175 507 305 482 2,060 MDF June 4, 1919 1 NF, Nathaniel Fuchs; MDF, M. D. Foster. SALINE MINERALS ASSOCIATED WITH LIMESTONE OF TERTIARY AGE It has been found in St. Croix that in the area near Jealousy and Bethlehem the limy rocks yield a salty water. An analysis (43, table 8) of water from the 245-foot well drilled near Bethlehem in 1934 (?), shows that the water had a total mineral QUALITY OF WATER 75 content of 4,486 parts per million with 1,985 parts per million of chloride. A 105-foot well drilled one-fourth mile southwest of Jealousy yielded water containing over 2,400 parts per million of chloride. This water was obtained from a cased weF in the marl (Tertiary) at a depth of 90 feet below the surface. Another water with high chloride content, 1,700 parts per million, was obtained from well 39 nearby at a comparable depth below the surface, and it seems evident that in this area the marl contains salt. Marly strata containing saline minerals may be responsible for the high mineral content of water obtained from wells at Bonne Esperance and Paradise and are undoubtedly a contribut- ing factor to the generally high mineral content of water from wells in many other places in the limestone plain. EFFE C TSZOFjALKALIfS OILS It is believed that the solution of alkali salts which have been concentrated at the surface in practically all low-lying parts of the island accounts in large part for the mineral content of many of the ground waters of St. Croix. In the discussion of the occurrence of ground water it was noted that where the zone of saturation lies close to the surface, the capillary fringe may extend to the surface. Where this condition exists, ground water is continually being brought up to the surface and evaporated. Evaporation is undoubtedly high as cool ocean breezes are warmed in their passage across the sunny island, and their drying effect, coupled with periods of low rainfall, is evi- denced by the type and degree of verdure of the vegetation. The mean evaporation at St. Croix from 1920 to 193564 was as follows: TABLE 6. Mean monthly evaporation at St. Croix from 1920 tc 1935 Month laches 4.81 5.05 6.52 7.09 7.06 , .. ,., Month July....... _ ..... Inches 6.74 7.01 6.96 6.12 5.66 Month Total _ ___ .. Inches 4.64 4.65 72.31 As evaporation proceeds, the salts carried in solutior by the ground waters are deposited at or near the surface and in time appreciable concentrations of mineral matter are built up. The concentration of salts at the surface varies inversely Y"ith the rainfall. As rain falls upon the surface and percolates into the 64 Johnson, Arthur P., op. cit., p. 64. 842683° SO 6 76 , GROUND WATER OF ST. CROIX, VIRGIN ISLANDS ground it tends to dissolve these salts out of the earth and decrease their concentration at the surface, but in so doing th?. mineral content of the water in the zone of saturation is increased. Thus, under conditions of high evaporation and intermittent rainfall a process of concentration of salts at the surface and Ir.ter trans- ference of these salts to the zone of saturation may go on progres- sively where the ground-water levels are close to the surface. In the evaporation of ground water the water itself passes off into the air, but the greatest part of the dissolved chemical con- .tent remains in the surface soil and is deposited in lari;e part as less soluble calcium and magnesium carbonates and sulfates, and as highly soluble sodium carbonates, bicarbonates, sulfates, and chlorides. The concentration of these salts increases with time. During periods of heavy rain, the water percolating into the soil dissolves some of the salts deposited there and transports them downward into the zone of saturation in relatively concentrated solutions. The amount of calcium and magnesium corbonates and sulfates dissolved is probably not great and is limited by the amount of free carbon dioxide available. However, the sodium (and potassium) salts are freely soluble, and water passing through alkali soils containing these salts becomes heavily charged with them. By this process, the ground water maintains its calcium bicarbonate content relatively unchanged but becomes more highly mineralized in soaium (and potassium) snlfate and chloride. In a reconnaissance soil map of St. Croix, Thorp65 har delimited areas of alkali soils. Many alkali-soil areas are found at low elevations along the coast; for example, from Fredricksted to Spratt Hall on the west coast, many places along the south coast from Hope to Cotton Grove, along the north coast at Rustoptwist, and in patches from the mouth of Salt River to Cottongarden. The alkali soils are largely confined to areas below canyon mouths, or surrounding mouths of rivers, or small embayments of the sea. However, they are not entirely confined to the coast. Areas of alkali soils also border several streams and extend as much as 2 miles inland, as from Enfield Green to St. George, from Fair Plain to New Works, from the mouth of Salt River to Glynn, from Surlaine Point nearly to Catherine's Rest, and surrounding Great Pond and Southgate Pond and extending inland to Mount Wash- ington and All-For-The-Better. Other areas of alkali soil occur inland as isolated patches sur- rounded by unaffected or alkali-free soil. Those which have been 65 Thorp, James, op. cit. QUALITY OF WATER 77 plotted are in the vicinity of the spring at Plessen, between Castle Burke and Lower Love, and along the streams west and north of Bethlehem. EFFECTS OF SALT SPRAY In a great many places the concentration of salts at the surface by evaporation of ground water will be augmented by salt spray blown for considerable distances inland. Evidence of the im- portance of salt spray is seen in the fact that cistern water, which had been collected from the roof at Central Factory near Chris- tiansted, was found to contain 26 parts per million of chloride. Legget66 states that "as. evidence of what rainwater may contain may be mentioned the fact that for England the average chloride content is about 2.2 ppm . . ." and "rain falling at Land'? End during strong southwest winds, blowing in from the sea, har been found to contain one hundred times this amount." Vaksvik67 notes that on Oahu, Hawaii, all natural waters contain salt. "As no point on the island is more than 12 miles from the ocean, all of it is reached by fine ocean spray or minute particles of salt that remain suspended in the air when the spray evaporates. The material is blown over the land, and some of it is brought down by rains." A table is given showing the rainwater collected dur- ing showers at various points on Oahu contained from 6 to 42 parts per million of chloride. Thorp68 notes that white alkali, mostly sodium chloride and sodium sulfate, is "most concentrated near the numerous lagoons, and the salt concentration rapidly decreases toward the inland/' whereas the black alkali, sodium carbonate, and bicarbonate, "was found affecting a few fairly large areas near Great Pond, the Leper Asylum, Southgate, Bethlehem, Castle Burke, and Jerusalem." This appears to indicate that along the coast the effect of salt spray upon the soil is dominant, whereas at more inland localities the salts in the soils have largely resulted from the evaporation of ground water. BASE EXCHANGE Inasmuch as some highly mineralized soft waters found in the Tertiary limestones are sodium bicarbonate waters rather than 66 Legget, Robert F., Geology and engineering: McGraw-Hill Book Co., New York, p. 459, 1939. H7 Stearns, H. T., and Vaksvik, K. N., op cit, p. 345. 68 Thorp, James, op. cit., p. 19. 78 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS sodium chloride waters, the sodium may best be explained as having resulted from base exchange.69 By the process of base exchange, hard bicarbonate waters which come into contact with certain materials bearing exchange- able sodium, exchange their calcium and magnesium for equivalent amounts of sodium. No change in the total amount of dissolved solids occurs. There seems to be no reason to believe that the marine marls forming the greater part of the Tertiary marl plain could not furnish exchangeable sodium to hard bicarbonate waters passing over them. SUMMARY The mineral content of most St. Croix ground waters is de- rived from (1) the solution of rock-forming minerals, (2) salt spray blown inland from the ocean and carried down into the zone of saturation by rainwater, and (3) concentration by evaporation at the ground surface of salts in ground water, originating from (1) or (2) above, and subsequent solution and addition of these concentrated salts to the zone of saturation below by rainwater. A very few wells have been contaminated by sea water, and a few owe a major part of their mineral content to organic pollution. QUALITY OF WATER IN THE VARIOUS FORMATIONS MOUNT EAGLE VOLCANIC8 No analyses of waters from the few wells obtaining water from the Mount Eagle rocks are at hand. However, field tests for chloride were made of waters from a number of wells penetrat- ing these rocks and a brief discussion may be based on these determinations. In general, those wells which penetrate the Nount Eagle volcanics yield water with a moderate to high chic ride content. The lowest chloride content, 172 to 250 parts per million, was found in water from wells located at high elevations at Orange Grove (83), Hermon Hill (85), and Catherine's Rest (117), whereas wells encountering bedrock beneath alluvium at lower elevations generally have a somewhat higher chloride content. Drilled wells at Longford (114) and Nugent (112) yield water containing more than 600 parts per million of chloride, and the dug well at Hartman (111), likewise entering the zcne of weath- 69 Renick, B. C., Geology and ground-water resources of central an-? southern Rose- bud County, Mont.: U. S. Geol. Survey Water-Supply Paper 600, pp. 41-42, 1929. Foster, Margaret D., The chemical character of the ground waters of the South Atlan- tic Coastal .Plain: Washington Acad. Sci. Jour., vol. 27, No. 10, pp. 405-411, 1937. QUALITY OF WATER 79 ered rock, yields a water containing 1,275 parts per mill'on of chloride. In these wells at Longford, Nugent, and Hartman, how- ever, it is believed that the source of mineral content is in the overlying alkaline soils. A dug well on the mountainside at Barons Bluif (79) yields water containing 340 parts of chloride. Inas- much as these cliifs lie in the path of the prevailing wind* they are in a position to receive much salt spray, and the ground water here may be high in sodium chloride and relatively low in other constituents. DIORITIC ROCKS Only two wells in St. Croix are known to be developed in the dioritic rock. Water from the Kobel well (38), north of Jealousy estate, contains 647 parts of total dissolved solids, as shown in the following analysis made in the spring of 1940: TABLE 7. Analyses of water from well at Kobel in dioritic rock [Results in parts per million] Silica (SiOs) _ _ .. . ___ ...... Iron (Fe) .. . . ......... Sodium (Na). .... __ ......... Bicarbonate (HCOs) __ _ .. Carbonate (CDs). _ ...- _ . ... . 572.9 AQ 50 104 3.0 398 26 Sulfate (SOO . __ ... _ - _ Chloride (Cl)---.-. ...------- Nitrate (NO»)__ ---------- ___ 18 148 .05 .4 378 647 May 10, 1940 It is a hard bicarbonate water in which the magnesium con- tent in only slightly less than the calcium. Sodium and chloride are farly low as compared to most other St. Croix ground vraters. In a pumping test made on this well on February 3, 19f9, the chloride content had increased from 160 to 278 parts per million after pumping 20 gallons a minute for 6 hours. This ir crease may be due to the flow of more highly mineralized water from near the surface into the well, induced by the pumping. The well (100) at Southgate in eastern St. Croix also penetrates dioritic rock and yields water containing 280 parts per million of chloride. As the well is located in a "slightly alkalin0" soil area, it would seem that the water did not enter the well through the immediately overlying soil. KESTGSHTLL MARL The following table contains analyses of waters from wells developed in the Tertiary limestones of the plain extending from Frederiksted to Christiansted. An analysis of water from one spring issuing from the limestones is also included. These wells, with the exception of the well at Castle Coakley, are locr.ted in 00o TABLE 8. Analyses of waters from wells in the Kingshill marl [Results in parts per million] Silica (SiO2) ------- _ ---- ___ -- _ ---- _ -------- Sulfate (SO4) _ ------ _ .. _-__ -------- _ ------- Chloride (Cl) ....-...-_-__.-_----......-..----.--- Envy1 42 1.6 42 35 442 0 655 130 375 9.3 229 1,421 MDF June 4, 1919 Bethlehem2 21 1.7 133 101 1,385 0 615 442 1,985 .4 1.4 747 4,486 EWL July 7, 1936 Anguilla3 27 .12 61 39 426 0 551 139 428 45 312 1,472 MDF June 4, 1919 Profit4 *23 1.0 26 8.9 478 0 . 812 89 269 Tr 101 1,320 MDF June 4, 1919 Bonne Esperance5 49 .3 10 10 673 0 971 189 364 25 66 1,842 MDF June 4, 1919 Bonne Esperance5 91,391 890 10500 1,400 20 1.6 "216 MDF, WMN, LWM April 15, 1940 Castle Coakley6 755 180 79 21 .8 "26 MDF, WMN, LWM April 15, 1940 Constitution HilF 23 7t; 7t; 21 6 9 287 0 622 62 84 14 81 MDF 1919 Sion Farm8 9510 j 658 10170 97 z. 5.1 0 "48 MDF, WMN, LWM April 15, 1940 1 Spring. 2 Well 43. s Well 46. 4 Well 47. 'Well 53. 6 Well 63. 7 Well 67. " MDF, M. D. Foster; WMN, W. M. Noble; LWM, L. W. Miller; EWL, E. W. Lohr. 8 Well 68. 9 Calculated. 1 By turbidity. ^.^ u Determined. QUALITY OF WATER 81 places where seepage of water to the wells from Quaternary alluvial deposits seems unlikely, and hence the analyses are con- sidered as truly representative of water from the Tertiary lime- stones. Brief inspection of the analyses (table 8) shows that tvro rather well denned types of water are yielded by the Tertiary rocks. One type of water, yielded by a spring at Envy and by wells at Anguilla, Bethlehem, and Bonne Esperance (1940 sample), is a hard water in which the total hardness ranges from 216 to 747 parts per million and in which sodium is present in large part as sodium chloride and sodium bicarbonate. The bicarbonate con- tent falls within about the same range as that of the softer waters from the Kingshill, however. It is interesting to note that the sample taken from Bonne Esperance well in 1940 is much harder than the water yielded in 1919, but in the 1940 sample the bicar- bonate content is less and the additional hardness appears to be present as carbonate hardness. There is also much more sodium chloride. The other type of water is yielded by wells at Prof t, Bonne Esperance (1919 sample), Castle Coakley, Constitution Hill, and Sion Farm. It is relatively soft and contains generally less than 100 parts per million of total hardness and from 551 to 970 parts per million of bicarbonate. The Castle Coakley sample, contain- ing only 26 parts per million of total hardness, is soft IT almost any standard but may not be representative. This type may be classed, therefore, as soft to slightly hard sodium bicarbonate water. The sample from the Bethlehem "salt well" and the 1940 sample from Bonne Esperance are high in chloride 1,985 and 1,400 parts per million, respectively but the chloride content of other samples analyzed ranges from relatively low to moderate. The sulfate ranges from 62 to 500 parts per million and is highest in those samples containing a high chloride content. Botl sulfates and chlorides are thought to have their source in soluble salts in the limestone which were deposited syngenetically. The difference in mineral content of the samples collected from the Bonne Esperance well in 1919 and 1940 is believed due to the lack of recharge in a period of low rainfall and consequent inflow of highly mineralized water, induced by more or less continuous pumping at a low rate, from limestone areas contain- ing high concentrations of soluble salts. 82 GROUND WATER OF ST. CROIX, VIRGIN ISLArDS AULUV1UM WEIIS FROM AILUVIUM WITHIN AREAS OF MOUNT EAGIE VOICAWICS Analyses (table 9) indicate that the mineral content of waters from wells located in Quaternary alluvial deposits within areas of Mount Eagle rocks differs widely, those analyzed ranging from 659 to 2,329 parts per million total dissolved solids. It is thought that the difference in total mineral content may generally be ascribed to the location of wells with respect to drainage channels along which relatively large underflow takes place. It seems apparent from the analyses at hand and from field determinations of chloride content that in wells located along main drainage lines the waters are less highly mineralized than those from wells in other locations. It is believed that the relatively grea ter amount of water moving along such channels, both on the surface or underground, tends to reduce the concentrations of raits.in the alluvium along those channels, or that wells in such locations receive larger accretions of less highly mineralized surface waters than wells located elsewhere, or both. Wells at Anna's Hope (64) and Christiansted (91) are located along drainage lines. These wells have a mineral content of 659 and 896 parts per million total dissolved solids, respectively. Conversely, water from the Longford well, located in the more arid eastern part of the island and not along a main line of drainage, is nore highly mineralized and contains 2,329 parts per million of total dis- solved solids. Well 90 at Christiansted is located a short distance away from the minor drainage channel in which well 91 is situated. Water from the former well contains more than twice the amount of total dissolved solids found in water from the latter. The waters from these wells are quite hard with the exception of the water from the Orange Grove well, which has a hardness of 87 parts per million. The hardness ranges from 173 to 519 parts per million. The waters may be classed as hr.rd calcium and magnesium bicarbonate waters containing variable amounts of sodium salts. In a few waters sulfate is present as calcium sulfate. The chloride content of the waters mentioned above ranges from 56 to 794 parts per million. A field determination of chloride content shows that the water yielded by the well at Salt River may be more highly mineralized at times than is indicated by the analysis in table 8. The increase in dissolved constituents may be due to contamination by sea water as the well is pumped, but the greater part of the mineral content is thought to be derived locally from the adjacent "salt pan" or alkali-soil areas. TABLE 9. Analyses of waters from wells in the alluvium within areas of Mount Eagle volcanics [Eesults in parts per million] Silica (SiOi)-- ------------------- Chloride(Cl)--. ----_--------- - Nitrate (NOs) ____ . ____ _____ .. Anna's Hope1 65 .10 40 37 \ 134 502 0 20 56 38 252 659 MDF 1919 Anna's Hope2 35 .93 30 24 / 814 I 10 828 0 351 625 17 1.6 173 2,300 WMN April 15, 1940 Salt River' 35 .13 82 48 \ 234 449 0 50 3444.3 402 1,040 NF June 3, v 1919 . Cane Bay4 28 .25 106 62 253 672 77 303 11 519 1,218 NF 1919 Orange Grove5 "369 698 0 80 1652.1 1237 WLL March, 1933 Chris tian- sted6 25 .17 86 65 552 656 175 507 305 482 2,060 MDF 1919 Christian- sted7 24 .11 82 39 188 489 71 204 12 365 896 MDF 1919 Green Cay* 57 .62 54 41 319 606 77 283 24 303 1,176 NF 1919 Cotton- grove9 34 .76 93 46 410 499 108 544 27 421 1,520 MDF 1919 Long- ford10 32 .23 53 43 780 868 151 794 33 309 2,329 MDF 1919 'Well 64. 2 Well 65. s Well 77. * Well 80, « Well 82 or 83. « Well 90. ' Well 91. « Well 101. Well 104. 10 Well 115. "Calculated. 12 Determined. ls MDF, M. D. Foster; WMN, W. M. Noble; NF, Nathaniel Fuchs; WLL, W. L. Lamar. 00 CO CO TABLE 10. Analyses of waters from wells in the alluvium within Tertiary limestone areas [Results in parts per million] Silica (SiOa)-- -- -- - ----- - - Carbonate (CDs) __ --- __ ____ __ -. ____ Chloride (Cl).... ._______.....__.__-____.__...-___ Nitrate (NO8). ___ ._ _ . ..... .. __ ..... ... . Plessen1 362 20 2001.6 .8 "195 MDP, WMN, LWM April 15, 1940 Golden Grove2 47 .13 50 33 286 709 0 74 1529.6 260 1,031 MDF, * WMN', LWM June, 1919 Castle Burke3 46 .18 80 39 95 526 .6 10 86 .16 1.2 360 595 EWL July, 1936 Castle Burke3 530 3 92 .75 .8 207 April 13, 1940 Fredens- borg4 771 "60 3901.0 1.2 "273 MDF, WMN, LWM April 15, 1940 New Works5 50 .04 21 17 528 748 28 132 3358.8 1.6 122 1,497 EWL July 7, 1936 Fair Plain6 36 .22 42 29 435 714 123 318 12 .04 224 1,175 EWL July 7, 1936 Fair Plain' 40 102.3 64 41 f 514 \ 3.2 662 0 157 500 121.0 328 1,631 WMN April 15, 1940 Lareine8 688 9300 510 15 .4 »i!05 MDF, WMN, LWM April 15, 1940. 1 Sprins "WK 2 Well 33. s Sump 34b. * Well 42. » Well 44. 6 Well 45b. 7 Well 45a. 11 Determined. 1J EWL, E. W. Lohr; MDf, M. D. Foster; WMN, Y7. M. Noble; LWM, L. W. Miller. >> Well 50. By turbidity. 10 In sediment. QUALITY OF WATER 85 WELLS FROM ALLUVIUM WITHIN TERTIARY LIMESTONE AREAS The waters from wells and springs in the alluvium within the areas of Tertiary marls, for which analyses are shown below (table 10), generally contain more than 1,000 parts per million of total dissolved solids. The least highly mineralized water is obtained from springs in which water has traveled a relatively short distance underground through surficial gravels, whereas water obtained from somewhat deeper wells a greater distance from the foot of the northern range of mountains may contain a considerably greater amount of total dissolved solids. The analyses indicate that the waters are moderately hard or hard bicarbonate waters generally containing fairly high sodium bicarbonate, chlorides, and sulfates. The sample from the Castle Burke sump had a carbonate hard- ness of 360 parts per million and no noncarbonate hardness. The nonhardness-forming constituents are relatively low, and the total dissolved solids are only 595 parts per million. The mineral content of this water may be derived in large part through simple solution of the surficial sediments free of excessive alkr.li or other salts through which this water passed. The water from the spring at Plessen also has a relativel;r low mineral content; more than half the bicarbonate is present as calcium and magnesium bicarbonate, and the remainder is present as sodium bicarbonate. The chloride content is fairly high 200 parts per million. Inasmuch as the spring is located in an alkali- soil area, as noted by Thorp70 it is believed that much of the mineral content of the water is derived from the solution of those salts from the soil. The samples from New Works and Lareine have a relatively high content of total dissolved solids, yet the hardness is relatively low, only 122 and 105 parts per million, respectively. It is sug- gested that here too base exchange may have been an important process in creating a fairly soft high sodium bicarbonate water. However, the moderately high chloride and sulfate content of these waters suggests that there has been some solution of alkali salts from the soil. The proportions of sulfate and chloride in samples from the wells at New Works, Golden Grove, Fredensborg, Fair Flair, and Lareine suggests that the solution of salts from the alkal ; soil may have been effective. 70 Thorp, James, op cit., p. 19. 86 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS BEACHfSAND Water from well 105 at Great Pond contained over 5,000 parts per million of chloride when visited by the writer. It is thought that this well is overpumped and has been contaminated by sea water. (See fig. 11, B.) At Canebay a field determination in- dicated that the water from well 80 contained more than 1,000 parts of chloride, although an analysis of a sample collected in 1919 (included in table 9 with waters from wells in the alluvium, for convenience) contained only about 300 parts of cl loride. The increase may also be due to sea-water contamination. SUITABILITY OF WATER FOR VARIOUS USES HUMAN CONSUMPTION Vaksvik71 has pointed out that, although most people can detect the presence of salt when th-s chloride con- tent exceeds 200 parts per million, water containing considerably higher quantities of chloride can be consumed by human beings without harmful effect. People who are accustomed to drinking water containing as high as 350 parts per million of chloride have stated that they prefer their own water to that of the Honolulu municipal supply (chloride ccntent from 45 to 72 parts per million), which they consider tasteless. Several families of Japanese living on the west side of the entrance to Pearl Harbor use waters from a spring and several shallow dug wells that have a chloride content of 950 to 1,100 parts per million. In several places in Tidewater Virginia water containing from 200 to 400 parts per million is used for drinking purposes by municipalities or military posts. Water from one well near Nor- folk contains 1,080 parts per million of chloride and is said to be used from time to time by some individuals. In St. Croix one individual stated that water from well 69 at Little Princess was occasionally used as drinking water. This water contains more than 1,800 parts per million of chloride. It seems probable that the water from many wells in St. Croix would be quite satisfactory as drinking waters. In r. great many wells the chloride content is moderate, and these waters would probably not be unpalatable. It may be pointed out, however, that many wells may be polluted, as the nitrate content, which is derived from the de- composition of organic matter, is high in waters frorr many wells. Waters containing 1 or 2 to 45 parts per million of nitrate are generally from wells where cattle are watered, but nitrates in excess of 200 parts per million are found only in wells in the towns. Conversely, it is interesting to note that ritrate is ex- 71 Stearns, H. T., and Vaksvik, K. N., op. cit., p. 345. SUITABILITY OF WATER 87 tremely low in waters from wells located in cane-field areas, namely, Fredensborg, the sump north of Castle Burke, and the spring at Plessen. Only 0.05 part per million of nitrate was found in the water from the well at Kobel. CATTLE Well No. 105, developed in beach sand adjacent to the ocean at Great Pond, yields water containing more than 5,100 parts per million of chloride. This water is the sole source of drinking water for a herd of cattle. According to the owner of the cattle, this water is of excellent quality for this purpose and hs.s been used successfully for many years. IBBIGATION The question of suitability of ground waters in St. Croix for irrigation is a pertinent one. Large volumes of water ?,re not available for the purpose, but on the other hand smaller volumes which might be used to irrigate small tracts of sugar care, par- ticularly in seasons of severe drought, may be obtained in some places. Scofield72 has discussed the limits of permissible sulfste and chloride content in irrigation waters and has stated that 480 or more parts per million of sulfate or 355 or more parts per million of chloride in irrigation water are unsafe. In regard to sulfate content the water from only a fer? wells exceeds 480 parts per million, notably at Cottongrove and1 Long- ford in the East End area and at Bonne Esperance (1940 srmple). However, with respect to chloride the water from many yrells is not considered desirable, notably Cottongrove, Longford, Bonne Esperance (1919 and 1940 samples), Lareine, Fredensborr, Fair Plain (developed well), Enfield Green, Paradise, Castle Burke, and others in various parts of the island. In Hawaii73 water containing more than the permissible 355 parts per million set by Scofield is used to advantage although excessive chloride content of water has a bad effect on sug-ar cane, the amount that it will tolerate varies with the type of soil, the terrene, and the quantity of water applied to the fields. Several plantation officials have mentioned figures ranging from 700 to 900 parts per million of chloride as the maximum permissible without impairing the yield of sugar. On one plantation it was found that best results were obtained when the wter dis- charged from a group of wells containing as high as 850 parts per irillion of chloride was mixed with high-level spring and tunnel water very low in 72 Scofleld, C. S., Quality of irrigation waters: California Dept. Public Works, Div. Water Resources, Bull. 40, 1933. 78 Stearns, H. T., and Vaksvik, K. N., op. cit., p. 345 88 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS chloride. On another plantation brackish water from spring-fed sea-level ponds is used for the reason that for certain fields it is the only water available. Most of the waters from wells in the central plain, the sugar- producing area, contain less than 900 parts per million of chloride. Scofield has further given the limits of a permissible sodium to calcium and magnesium ratio. In terms of equivalents per million (not parts per million) it may be expressed as follows: Na X 100 = percent sodium Ca + Mg + Na When the percentage of sodium is 60 percent or more, the water is considered unsafe for irrigation. When the ratios for typical waters obtained from wells in central St. Croix are calculated, it is evident that nearly all waters must be classed as poor or harmful if used for irrigation. The per- centage of sodium, according to the formula given above, for waters from the well at Anguilla is about 75 percent; from the spring at Envy and the Bethlehem "salt well," about 80 percent; from the Bonne Esperance well, 96 percent and 92 percent (1940 and 1919 samples, respectively) ; and from Sion Farm well, 96 percent. Thus, it seems improbable that the Tertiary limestone will anywhere yield a good irrigation water. Wells drawing from deeper strata in the alluvium yield water within the same range. Both the developed well f.nd the older drilled well at Fair Plain yield water containing r.bout 81 per- cent sodium, and water from the New Works well has about 91 percent sodium. Water from the drilled well at Anna's Hope contains about 90 percent sodium. Shallow wells and springs in the central part of the island yield water containing smaller proportions of sodiim. The water from the Golden Grove dug well yields water containing more than 70 percent sodium in the Scofield formula, but water from the sump north of Castle Burke contains only about 38 percent sodium, an amount within the permissible limits by a wide margin. Water from the spring at Plessen is probably only slightly less favorable, as is water from the shallow well at Fredensborg. Water from the Kobel well north of Jealousy in the dioritic- rock area has a sodium content of about 38 percent in the Scofield formula and may be classed as a good water for irrigation. In summary then, it would appear that most ground waters of St. Croix are very poorly adapted to use as irrigation waters. Most of them contain excessive sodium in relation to total calcium and magnesium. Some waters also contain excessive quantities of SUITABILIY OF WATER 89 chlorides and sulfates, and a few may be classed as poor on the basis of total dissolved solids alone. Although this paper is primarily concerned with the ground- water supply, it may be pointed out that samples of water taken from streams at Two Friends, Jolly Hill, and Salt River at Con- cordia show that the sodium content according to the Scofield formula is about 33 percent, 23 percent, and 32 percent, respec- tively. These waters may, therefore, be classed as good irrigation waters if the proportion of sodium to calcium and magnesium does not greatly increase in some seasons. It seems logical that if irri- gation is ever to be carried out on a large scale in St. Croix, an effort should be made to impound surface water for the purpose rather than to develop ground water. However, it is reported that in times of severe drought very little surface water is avail- able, and at such times only ground water could be used. As practically all ground waters in St. Croix contain a mod- erately large amount of dissolved salts, which are harmful where concentrated in the soil, irrigation should be carried out by meth- ods insuring adequate natural or artificial drainage of the irri- gated tracts. Thorp74 has emphasized this point in discussing proper methods of irrigating certain acreage in St. Croix: In case a water supply of considerable volume is found, it will be necessary to take precautions to use the water carefully and to dispose of the waste water in such a manner as to prevent the excessive accumulation = of alkali and soluble salts. Alkali salts in small quantities were noted in many places. Most of these areas, except those close to the sea or to lagoons, contain only enough alkali at present to slightly affect the soil. Irrigation would tend to concentrate the salts in low places where subdrainage is poor and e.long areas where there is a change in slope from comparatively steep to mor? gentle. Thorp has further pointed out that soils affected by black alkali become puddled and impervious to water. Their naturally good granular structure breaks down, the soil runs together and it is impossible to make them productive without expensive chemical treatment combined with copious irrigation and carefully planned drainage. SUGAR AND BUM PRODUCTION It is probable that no ground water obtained on St. Croix should be used for boiler feed without treatment. Few of the waters can be made suitable for use merely by reduction of the hardness. Most of the waters contain undesirable amounts of sodium as bicarbonate and chloride, which would probably necessitate ex- pensive treatment to reduce corrosiveness, foaming, or other deleterious action. 74 Thorp, James, op. cit., p. 19. 90 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS Moderately mineralized ground waters are used as maceration waters. In the production of sugar the very small amount of salts which are precipitated with the sugar are not objectionable. In the production of rum practically all the salts are left after fermentation, and waters of high mineral content can be used. The water from well 8la containing about 1,900 parts per million of chloride and having a total dissolved solid content of 4,700 parts per million has been used successfully at Central Factory. SUMMARY OF GROUND-WATER RESOURCES WATER-BEARING FORMATIONS The existing wells on the island of St. Croix obtain water from the Mount Eagle volcanics, the diorite, the Kingshill marl, and the alluvium. The alluvium is the best water-bearing formation, and properly constructed drilled wells located in the larger alluvial valleys can supply moderate amounts of water. A modern gravel-packed well constructed during this investigation in such an alluvial valley produced from 50 to 80 gallons a minute in a 68-hour pumping test. Smaller amounts of water are also obtained by other wells developed in this formation. Wells in the alluvial valleys appear to be more dependable in times of drought than wells that obtain water from rocks. A smaller number of wells obtain water from the Kingshill marl. These wells are more likely to fail in times of drought and generally yield only a few gallons a minute under conditions of continuous pumping. Deep test drilling indicated thr.t Tertiary Kingshill strata are probably not underlain by water-bearing for- mations at depth. A few wells obtain water from the Mount Eagle rocks, and some of the areas underlain by these rocks are worthy of further exploration. There are two drilled wells in dioritic ro°.k, both of which furnish ample water to wind-powered pumps in times of drought. One of the wells yielded 20 gallons a minute with less than 15 feet of drawdown in a 71^-hour pumping test. Springs yield water in a few places. Small amounts of water are obtained from springs that issue from the alluvium not far from the base of the mountains in the central part of the island, and a somewhat stronger spring issues from the Kingshill marl near the seacoast. ADEQUACY OF PRESENT SUPPLY The existing wells, springs, streams, and cisterns on St. Croix are sufficient to supply most of the island's needs in periods of SUMMARY OF GROUND-WATER RESOURCES 91 normal rainfall. During periods of drought, however, streams go dry, cisterns become depleted, many wells that are poorly located or improperly constructed fail, and the lack of water may become acute. QUALITY OF WATER The quality of water obtained from wells varies within wide limits, but in general it may be said that the ground waters are hard and contain a moderate to high content of dissolved salts, which are present largely as sodium and calcium bicarbonates and chlorides and lesser amounts of sulfates. The water from Mount Eagle rocks may have only a moderate mineral content and, as far as is known, waters from the diorite contain a very mod- erate amount of dissolved solids. The Kingshill marl generally yields water containing a moderately high to high mineral con- tent, and the mineral content of waters from the alluvium varies from moderate to high. The mineral content of well water is thought to be derived from rock-forming minerals, from alkali in the soil, and from salt spray. In a few places contamination by sea water has apparently taken place. The water from only a few wells may be suitable for irrigation, but most well water is suitable for domestic ures and for cattle. Most ground waters would need treatment before they could be used for boiler feed, but existing supplies can and are used as maceration waters and in further diluting molasses prior to fermentation in rum manufacture. RECORDS OF WELLS AND SPRINGS Wells and springs in St. Croix which are listed in table 11 were visited by the writer in 1938-39 with the exception of wells 47 and 64. These are listed by Vaughan75 in an unpublished manu- script and are included here because chemical analyses wer^> made of their waters. RECOMMENDATIONS Although water is present in all the rocks of the island, the deposits that will yield water to wells are not as widely distrib- uted. It has been found from a study of old wells drilled by the islanders and from more recent drilling by Government agencies that moderately large quantities of water may be obtained from sandy beds filling old alluvial valleys. Several of these valleys may yield several hundreds of gallons of water a minute, in the aggre- 78 Vaughan, T. W., Ground waters of St. Croix, V. I.: Unpublished manuscript, U. S. Geol. Survey, 1919. 92 GROUND WATER OP ST. CROIX, VIRGIN ISLANDS gate, to properly constructed and located wells. Probably enough water to care for a large part of the needs of the island could be developed in these valleys. Discussion of whether the costs of construction of efficient modern wells, installation of pumps, pipe line to points of use, and maintainence can be met or justified by either private citizens or governmental agencies does not fall within the scope or purpose of this report. It is merely pointed out that in certain areas water in reasonable quantity can be ob- tained from wells that are properly located with respect to geo- logic conditions and constructed by an experienced driller using modern equipment. The locations favorable for the development of a few hundreds of gallons of water a minute are in the lower courses of larger streams in the western and southern parts of the island, as fol- lows: (a) North of Frederiksted, (b) from Fair Plair northward and northwestward, (c) north of Hope and Blessing, (d) north of Jerusalem, and (e) northeast of Jerusalem. Other valleys, wholly or partly filled with alluvium, may be productive of lesser quantities of water. These are: (f) Valleys west of Spratt Hall, (g) Salt River Valley, (h) the upper reaches of the valleys mentioned in the preceding paragraph, (i) the short valleys near Rustoptwist and LaValle on the North Side, and (j) the short valleys in the East End area. At areas (a) to (e) the largest yields will be obtained, but less mineralized water will be obtained from areas (f) to (j) and particularly (h), the upper reaches of the major valleys. In almost all these favorable areas exploratory drilling may have to be done before the optimum sites for wells are known. Most of the exploratory holes will be less than 100 feet deep and need be only very small in diameter. They need not cost much to drill, and drilling them would be good insurance against the possibility of installing expensive equipment and obtaining less than a maximum amount of water. Further, by exploring several areas it may be possible to make savings in pipe line or to get a better quality of water than if only one or two areas wore explored. The maximum amount of water obtainable from wells drilled into the dioritic rock or into the Mount Eagle volc^.nics is not known. The diorite appears promising as a source of moderate amounts of water, but only a few shallow holes have been drilled in it and confirmatory data are lacking. Further, this rock is limited in areal extent. It is thought that streams in the areas of the Mourt Eagle vol- canics may be localized along fracture or fault zoner. and hence RECOMMENDATIONS 93 along stream valleys wells that have been drilled from 100 to 200 feet in these rocks may strike crevices which will yield more than the minimum quantities now pumped from shallow wells developed in these rocks. This is problematical, however, but might be most worth while to determine where there is a possibility of intercept- ing a fairly large underflow of only slightly mineralized w^ter, as in the valleys above Spratt Hall, Little Grange, Grove Place, and Little Fountain. The Tertiary limestones, by and large, are poor water-bearing strata. Only one well, at Paradise, is known to yield more than an extremely meager supply. In St. Croix, as in many other places, it is difficult to construct wells by hand that will yield water in times of drought. St. Croix is subject to severe and frequent droughts, and many inhabitants who depend upon shallow wells find themselves greatly incon- venienced at times. This recurrent situation cannot, except by chance, be eliminated by digging more shallow wells or even by constructing drilled wells unless they are properly located and finished. It must be emphasized that accumulated data on St. Croix ground-water conditions show that reasonable quantities of water from wells may be obtained in relatively few places; wells must be located in these relatively limited favorable areas in order to provide an ample supply of water for the island in times of severe drought. All wells proposed to furnish a maximum amount of water should be located by a trained ground-water geologist. Selection of each well site is an individual geologic problem on St. Croix; the location and interpretation of results of test holes, the location and details of construction of wells, and testing of wells, carried out with an understanding of the quantity and quality of water needed or sought, cannot be performed with maximum efficiency by an engineer or geologist lacking ground-water experience. TABLE 11. Records of wells and springs in St. Croix CO No. 1 2 3 4 5 6 7 8 9a 9b 9c 9d 9e 9f 9g 10 11 12 13 14 15 16 Estate or location M mile north of Butlers Bay--. Sprat Hall - _ .......... William-...-.........-- . Jolly HilL ................ Hill and Strand Streets, Fred- eriksted King Cross and Strand Streets, Frederiksted King and King Cross Streets, Frederiksted Queen and King Cross Streets, Frederiksted Hill and Princess Streets, Fred- eriksted New and Queen Cross Streets, Frederiksted 30 Hospital Street, Frederiksted Wheel of fortune _ ___ . ... Whim....-.. ............ Robe Hill... Topographic situation Se coast ... do - Valley.... ...... Coastal terrace ... ... do.... ..... ... Coastal terrace ... ... do... .... ..... ...do... ......... Slope toward sea ...do ........ ...do............ ... do ... .... .. do... .... .-... Broad valley... .. Rolling plain _ .. do .- ... do .... . do Coastal terrace ... Mountain valley Approxi- mate altitude above sea level (feet) 26 26 35 12 25 40 40 10 10 18 15 21 46 25(?) 30 40 55 125 95 14 240 Type of well' Dg Dg Dg Dg Dg Dr Dg Dg Dg Dg Dg Dg Dg Dg Dg Dg Dr Dg Dg Dg Dg Dr Depth of well (feet) 24 40 35 12 25 15 12 11 15 16 18 43 26 12 90 15 16 7 62 Principal water-bearing material Character of material Weathered volcanics -.do ... do ...do... ..... Weathered volcanics do. ... do... ..... ... - ... do... ... ... ... ...do.... .... ... do ...do...... ... do ... ... ...do. ... do... ... ..... ... . do Marly alluvium ..... Weathered volcanics Geologic horizon Upper Cretaceous do. . ...do ... . ... do........ . Upper Cretaceous ...do.._. ..... Mid-Tertiary do.... .... . ...do......... do......... do ... do. .... Pleistocene... ... do- .... Mid-Tertiary _ do ....... ... do .. .... do. ..... Pleistocene.. . Upper Cretaceous Water level Feet below surface 13J* 26 15 3M 23(7) 8J4 9 9 13 15 17H 41H '24 3Ji 21J-2 48 4}i 5 025H Date of measurement Dec. 21, 1938 do .. ... do do Jan. 11, 1939 Dec. 20, 1938 ...do... ...... do......... ... do ._ do ...do... ...... June 6, 1919 Jan. 12, 1939 Dec. 20, 1938 ...do......... do .. ...do......... ...do ..... Dec. 19, 1938 Method of lift2 M M W W W* G G G« B B B B B B B W W W W W W Use of water* DS DS Ss Ss Ind. Ind. PS PS PS PS PS PS DS S Obs. S Ss s s Chloride content (parts per million)4 98 280 110 112 548 218 1,800 2,220 1,690 2,740 2,470 1,270 410 206 710 168 240 470 138 Remarks According to owner, well yields 50 gallons a minute for limited periods. Goes dry in severe drought. According to owner, well yields 200 gallons a min- ute for limited periods. Water level varies from H£ to 9H feet from surface. Yields an "ample" supply. Water level 2 feet below surface June 5, 1919.7 Water level about 7 feet be- low immediately adjacent stream bed. RECOMMENDATIONS 95 3-1*!!!! S .*S»".3: H*° ^ " fe° o cSz E^"^ <M^.^J o_ ,g' ' Q H" £ -H -«« NN ^cooco« =0 . ^ ^ £ 5 Q ,9 COCO CO CO 3L 3 co racocococo ooQ QQQcococo Q Q Q Q cococo co HO . » torn .. *»*»» 00 00 § g os oa 4; (J C C C C « VN*<N V r-f- r>> -i -i it? ib 11 11 Q ^H ^^H sill" i i : ! air^'e^'^Z- s*~"' © " ' o> llll Illl" c c c -c -c -c -" »' iO-^ COC^ i i d g a 5§ ^ "§ 5 .3 cLr^.SJ C S a^ 53*^ .2 1 > T3 IJ 1 II 1" * -i c O^N^O »^ ^.oo > tj) t»D t»D tn i* (_i tj)tj) bC t»D Q QQQQQ QQ QQ c? c1^ w oa -^f co ws os KO<M -§ ' 5? a > bD fe ^ «^ m*^ «^ M co ! oa s >> a I *^ ^"a & ! f .0 rt a J "S B 'S 4 § |4 S «! i O 2So s^ § I ii u^ 0000 00-3- II Y CO h if t! S t E^ .SE^ 3|.i » i oo oo S § 06" oT 1 1 \* ^r 1 1 -i o> aj § S § ill -^ 4i C o -c c t i O OO O U)U> bD bD tn QQ Q Q Q 1OO O O W5 j|I Ala ill -§ McoO i i* 'I 3'is-sj >< o o . sss * ? i I > a a ' 1 * < dl Q .M fli J ! i * % s § s CO' gj N 8 -g -g 8 Q &H &H Q ^ r W3 O W3 O i-H *-. i-H -i 4 § 1 b fe"S a H ° &5 g -d ^ ~ &^^ O u t3 W S .2 o > > 3 1 ^ > &B | T3 T3 a? a | g § J H "i 2 04 o oo ws co o w: QQQ Q Q Q 0*0*0 o o c i li"H (M <N i t »-H C a | J ^ fi c S 5 00 S? oo - 8 Q -itf ' T3 § I 1 > J3 Is QP 0 01 a -1 ^1 J bD a * § c e 'J i^ o-w -w | _S| § S ^i K COCO O 3 O 1-5 ,» >-S 'S *a 'S c, ° « | i 1 1 H III 1 1 3ih ^Q ja oo oa»-«Neo i^t' u3?ot*ooooosoT-tc^co -^ -^ ws?ot co o>o*-'*> ^ i 1 i IC3 <N N <M CQNNfNC^fNCOCOOOCO CO CO COCOCO CO CO 1^ 1^ 1^ TABLE 11. Records of wells and springs in St. Croix Continued No. 43 44 45a 45b 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Estate or location New Works ...do..... ~ - Profit... .._- Clifton Hill . . . ... Slob Lebanon .............. ... Topographic situation Rolling plain.. ... Broad valley..... ...do........... do . .. Coastal terrace ... Hilltop.- ..-. Rolling plain _ ~ ...do. ....... Broad valley __ . Rolling plain ... Valley... .... Hilltop.. .. .._ Valley ....... ... do Rolling plain __ . do... ......... do Approxi- mate altitude above sea level (feet) 100 55 30 30 20 150 100 100 105 150 200 275 340 190 150 200 140 100 Type of well1 Dr Dr Dr Dr Dg Dg Dg Dg Dg Dg Dg Dg Dg r>n. Dg Dg Dg Dg Depth of well (feet) 245 58 225 58 11 170 90 67 42 145 25 145 150 1«6 94 99 135 Principal water-bearing material Character of material Marl and clay _ _ . - do ------- do Alluvium. .-...-. ... ...do....... - - .-do-.--... Limestone.. -------- Weathered volcanics Limestone. - ... do do. . do do ... ...do .......... do... .... ........ Geologic horizon Mid-Tertiary do do do. Mid-Tertiary .do .. . do Pleistocene . Mid-Tertiary Upper Cretaceous Mid-Tertiary do do......... do do......... do .... . ...do .... Water level Feet below surface 28 17 14M 9 110 89 '40 36}3 53 y2 '60 9314 B3} 2 W'4. 94J£ 115^ Date of measurement 1936 . Mar. 23, 1939 Mar. 2 , 1939 Dec. 8, 1938 June 4, 1919 Dec. 8, 1938 June 5, 1919 Dec. 9, 1938 Dec. 8, 1938 June 4, 1919 Dec. 9, 1938 do do ..... Dec. 8, 1938 ...do. . Method of lift' W G W W» W W W» W5 W6 w« W6 w« W W Use of water3 Obs. D Ind. S S S S DS S Obs. DS DS Obs. Ohs. DS S DS SD Chloride content (parts per million)4 340 700 384 585 235 830 418 380 904 600± 254 145 214 250 Remarks Yield small. Water highly mineralized. Reported to yield more than 20 gallons a minute. Yields 70 gallons a minute with 15% feet of draw- down after pumping 52 hours. Yields about 10 gallons a minute with 5 feet of drawdown. Water level 9 L£ feet below surface June 4, 1919.7 Supply does not fai. in driest seasons. Water reported to be highly mineralized. Water level 55 feet below surface June 4, 1919.' Chloride content increased slightly after pumping 2 hours. Yield estimated to be about 7 gallons a minute. Water level 56 feet below surface June 3, 1919. 7 Yields from 3 to 5 gallons a minute. Supplied 50 people and 50 animals with water in severe drought. Water level 74 feet below surface June 4, 1919.' Water level 49 feet below surface June 3, 1919. 7 CO W H O S O RECOMMENDATIONS 97 "SS-gS ' >> s >.<=> >>=, 2 o .l-as-ess g JioJTS^ a's"' fe S "^ *2 0 . +* S~^ ^* "~ M* § 5 "5 JO O OO 5 oa ^ tf5 co H »O CO T » CO IN O2 OQQQ X H Q OQ QmraQ C = & * *»»* Ol Oi Oi t OC CO 1-H 1-H I Ci s «" -*"S » -= g | I"! & N \« \M 1 \ ^f\ TH\ M "1 C3 IO IO t-1 CO It ^H c< c T: c ^ c c c -i 4 l.| S-1§ 4 SSS £^ III Jl.i -^ *** O OOO OO O OO OO O OQ O 1** M5 OO M IO Oi -^ i-H 00 OO CO -^ CM CO t- **< i-H 00 *-* O »-l l>- IO -IN-' '-"-' ""* 02 02 .2.0 02 02 O2"2 "2 "2 02 Q O2 QO2O2 OO Q QO2 O2O2 Qw WO2O2 QI-H Q O3 * 6 S&& OOss CO M CO 05 "C 2 Q OO U3 CC M c -§ a § .1 5 i 4 1 -§ c T: c T: ^ i OO ^ CO 1 r-H O CN -<fl< r-H r-< oo CO §3 8 -i Q i. OO C^] c^ # 1 Hit -i § o .1 ^ CO CO -i 3 C1 c & bb * S& S&* &o S * 4 -i 3 W CO (M M 1 |fc i is § s is s S §§ 8 Q 1Q Q N^Nff^ Nrfi j i § ! 1 i11!! * | ! || | j j j| 1 = :J ^1 1 'i ll 11 * 5J" (N COt^ MO CO | 4 -i CO CO n? IN| 44 -i s -s s §j I S.r'j.a 0.5 c e S ! S 'i i '1 O I U "3 ! "3 &> ! > 1 i« 1 |'|| | -§ .-( lO W O5 ^p i Oi t W OM fct fcl] t- Ml 1- Ml M UlMbMMMb MMMMMb bbMMb b M Q Q QQQQ Q Q Q QQQ QQ Q QQ QQ QQ QQQ QQ Q Q M5 O MOO °SS s £ & S e= 'ti gi O5 cBo! Jo -§| §=3 o>- 3 OS Ui ! O =s "o S 1 &.M fe a g 5^ ^2 II 5 S -s 1^ of '"a fe I 111 S 1"8 -S s co^ 10 co r* oo 1 CO Tf ift J GO COCO oo oo TABLE 11. Records of wells and springs in St. Croix Continued No. 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 Estate or location Water Gut and West Streets, Christiansted Water Gut and MarketStreets, Christiansted No. 1, Strand Street, Chris- tiansted King and Princess Streets, Christiansted ...do... .... .............. ... do... ..... ... .... ... Darby Hill ............... Cotton Valley- .. __ ... 14 mile north of Great Pond-.. Topographic situation ... do. ... ..... do ... Coastal terrace ... ...do. ...... ...do ........ Valley. ...... Edge of coastal terrace Valley... ....... Coastal terrace _ do. ... Broad lowland... ...do ..... . ... do... .... ..... Approxi- mate altitude above sea level (feet) 60 20(?) 10(7) 20 30 30 75 9 12 8 35 90 45 9 11 25 30 10 20 30 60 50 Type of well1 Dg Dg Dg Dg Dr Dr Dg Dg Dg Dg Dg Dg Dr Dg Dg Dg Dg Dg Dg Dg Dr Dg Depth of well (feet) 14 9 82 80 33 18 12 10 80 10 10 12 30 10 24 40 76 35 Principal water-bearing material Character of material ... do- ... ... .. ...do ... ... do .... .... - ... do dO ... ... ... ... ... do ... .... ... do - ... do ... --- ... do . ... do ... ... ... - ... dO .... Weathered granitic rock? do. ... ... ..... do... ... ... ...... ... do... .... .... Sand Weathered volcanics ... do... .... ..... ... dO ..... ... .. Geologic horizon do do do ... do ... .. do do -.dO . ... do... ... ... do .... do ... do ..... Pleistocene. . . ... do ... do-. .... do Upper Cretaceous dO- ... - ... do .... - Water level Feet below surface 4 '5 10 19* 7 3M JflH 6* ne' 2 19* 6M 17 Date of measurement June 4,1919 1927 Jan. 25, 1939 Jan. 25, 1939 do June 3~, 1919 Jan. 25, 1939 do ... Jan. 25, 1939 Method of lift2 W5 B B B W W w« W6 W« W5 W5 W W5 W5 W W W5 W5 W5 Use of water5 D D D D S S Ss ss ss DS S s Obs. 0S Ss s s s Chloride content (parts per million)4 340 375 950 508 270 275 250 225 950 1,020 320 280 336 870 500 5,150 1,740 538 320 544 Remarks Water level reported by B. Nelthropp. Well may also develop wea- thered granitic bedrock. In nearby abandoned well more nearly in axial part of valley water level is 20 feet from surface. Furnished water to 300 cattle during a period of drought. Water level 5 feet below surface June 3, 1919. Supplies 2,500 gallons a day. Yield is small. Yields about 2 gallons a minute. Dried up in drought of 1924-25. CD OO § CQ H O » O M X 110 11? 1 1 Q 114 1 1 K I1fi 117 do- do -- - do- -- .- do do ... do ..... ... do ... ... ... Vallov 80 45 40 5 50 50 190 T> Dg F)r Dg F)r Dg Dr Dr 47 45 10 83 38 82 28 rock --do__--. --------- ... do do do... .......... .- --do.,.. . _-.dO ----- do. dO Cretaceous do. ... 39^ 4?4 38 723 Jan. 24, 1939 Jan. 24, 1939 do W W5 W5 M6 G« W W W S Ssss s DS S 455 1,280 645 830 608 780 250 Well reported to have flowed when drilled in dry season in 1930. 1 Dg, dug, Dr, drilled. 2 M, pump operated manually; W, wjndmill; G, pump powered by gasoline engine; B, bucket. 8 D, domestic; S, stock; Ind., industrial; PS, public supply; Obs., obsolete; T, test well. 4 As determined by field tests. 5 Well being pumped when visited. 6 Well not being pumped when visited. 7 Data from unpublished manuscript "Ground waters of St. Croix, Virgin Islands," by T. W. Vaughan. CO CO APPENDIX 101 NATIONAL PARK SERVICE DRILLING PROJECT INTRODUCTION A well-drilling project for the Virgin Islands was undertaken in the period December 1940 to July 1941 by the Civilian Con- servation Corps in response to a request made by Governor Law- rence W. Cramer, of the Virgin Islands, to Mr. Conrad L. Wirth, CCC and ERA Coordinator, National Park Service, United States Department of Interior. This request was prompted by severe local shortages of water that occurred as the result of drought condition in 1941, during which many existing wells and Creque Dam Reservoir went dry. It was subsequently arranged that the CCC would furnish a geologist to investigate the project, recommend the most favor- able drilling sites, and send its drilling crew and equipment to the islands to develop water supplies on Federal-owned or con- trolled land for Federal or Federal-sponsored projects. Benefiting agencies were required to furnish the well casing, screens, and production equipment. The project was carried out under the supervision of Dcnald C. Hazlett, a geologist of the National Park Service, working under Mr. H. E. Rothrock, of the same organization. The writer advised both Mr. Rothrock and Mr. Hazlett in detail on ground-water conditions in St. Croix before the project was initiated, recom- mended many sites for drilling, and made available copies of his geologic map and basic well data. The writer was not associated with the project, however, while it was being carried out. Subse- quently, largely through the efforts of Mr. Brent S. Drane, of the National Resources Planning Board, the basic data gained in the 103 104 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS CCC drilling project were made available to the writer and sub- mitted with the request that they be incorporated with the results of the writer's own studies and interpreted accordingly. The following section has been prepared mostly from a series of typewritten memoranda written by Donald C. Hazlett. Much of the data is particularly valuable in bringing out new geologic information. However, the lack of background detail concerning the various wells that were drilled, lack of data on quality of water obtained, and lack of information on the performance of wells after completion are keenly felt. For some wells basic data are incomplete. This report on the CCC project is not intended to be a full account of the various drilling or water-suppl;T activities engaged in by the CCC while in the Virgin Islands, but rather a discussion of drilling on St. Croix only. SELECTION OF WELL AND TEST-HOLE SITE" Wells were drilled in the Whim homesteads area southeast of Frederiksted, in the Colquhoun homesteads area at the south foot of Blue Mountain, and in various other locations in the central part of the limestone plain, particularly at Fair Plain and in the St. John Princess homesteads area northwest of Christiansted. A few wells were also drilled in the eastern part of the island. With few exceptions the location of well sites was restricted to the limited areas in which water was to be used; much drilling was done in areas known to be unfavorable, and the low yields obtained from many wells and failure of others wer^ to be ex- pected. By and large, however, the light rotary rig used seemed to be adapted to the drilling conditions encountered, and the small- yield wells that were developed in these areas seem tc have filled a real need. Wells of somewhat larger yield, which were sunk in a few places, were very worth while and confirmed the writer's conclusions regarding the occurrence of ground water in the island. A few of the "dry" wells or test holes showing negative results require attention because it is believed that some of these were not properly located, or, although located at a logical spot from a consideration of the geology and existing records, drilling showed that that particular location was unfavorable and that exploration for probable water-bearing strata should have been carried on elsewhere, in some instances only a few hundred feet away. SUMMARY OF RESULTS Of the wells discussed in this report on the CCC drill'ng project, five wells provided 3 gallons per minute or less, two others yielded WELLS AND TEST HOLES 105 respectively 4 and 5 gallons a minute, two yielded 8 gallons a minute, two yielded 15 gallons a minute, and two yielded 40 gal- lons a minute. Six wells were "dry" and two provided very small quantities of brackish water. The two wells of highest yield were developed in the alluvium- filled valley near Fair Plain. Fifteen gallons a minute was ob- tained from wells in alluvial fill at Golden Grove and near Chris- tiansted. A well that obtains water from the Tertiary limestones at Paradise yields 15 gallons a minute. A well at Whim that yielded 8 gallons a minute may obtain water from Tertiary lime- stones, and a well of similar capacity at St. John may also obtain water from these limestones. The data on the successful wells, together with informatics on wells of low yield and the dry holes, confirm the conclusions pre- viously drawn by the writer; namely, the alluvial-filled channels yield fairly good supplies to properly constructed wells, ard in places the Tertiary limestones yield moderate amounts of water. In many places where the alluvium is reasonably thick, as north- west of Christiansted, permeable water-bearing formations such as coarse clean sands are lacking. Apparently good water-bearing strata in the alluvium are confined to restricted channels leading back to well-defined gorges in the mountains or hills. Where broad areas are overlain by a mantle of alluvium, as in the vicinity of Bethlehem and Colquhoun, the present streams do not neces- sarily mark the course of the deep alluvium-filled valleys and a number of shallow test holes may have to be drilled before the ancient valleys are located. It is also shown conclusively that solution cavities in limestone, in St. Croix as elsewhere, are very irregular in occurrence, and in some places these rocks yieM a reasonable amount of water but at other nearby localities the fame strata may yield very little or no water to wells. DESCRIPTION OF WELLS AND TEST HOLES WHIM HOMESTEADS Four wells were drilled on Federal homestead lands north of Hope, about 2y% miles southeast of Frederiksted, for domestic consumption and livestock. Their locations, C-l, C-2, C-3, and C-4, are shown on the map (pi. 1). These wells are 114, 136, 123, and 90 feet deep, respectively, and presumably they penetrated the Kingshill marl. C-l and C-2 each yielded 1 gallon a minute of brackish water, C-4 yielded 2 gallons a minute of water, and C-3, 8 gallons a minute. Water from C-3 and C-4 was probably fresh. The available data are so meager that no comments or dis- cussions of the results can be made. 106 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS PARADISE Well C-5 was drilled to supply livestock at Paradise, in the narrow valley just east of well 29. The Kingshill mr.rl was struck at 34 feet, and strata of "very porous" white limestone were present from 40 to 45 and from 60 to 71 feet. Perforated 6-inch casing was set from 45 to 71 feet; upon bailing, th^ well yielded 15 gallons a minute. The length of the test or draw-down are not known. This well is presumably considerably better than well 29, at Paradise, which is dug to a depth of 37 feet and which is reported to yield about 100 gallons per hour. It is at a slightly higher eleva- tion than well C-5 and obviously does not reach the, porous lime- stone strata encountered in C-5. Further, water from well 29 contains 945 parts per million of chloride, according to the field test, whereas water from C-5 is reported to contain only 210 parts of chloride. It would appear that well 29 obtains water seeping in from the immediate sur- rounding area through material highly loaded with salts originat- ing as salt spray or otherwise, whereas C-5 reaches a stratum through which ground water has been moving relatively rapidly, perhaps fed from the alluvium to the north or east, in which the dissolved salt content is low. In connection with the relatively high yield of well C-5, atten- tion may be called to the limestone spring at Envy, about 1 mile to the southeast. This spring may issue fnom the strata encoun- tered in well C-5, or from similar strata. PROPOSED UNDERFLOW DAM AT FAIR PLAIN Tests were made at Fair Plain to determine the feasibility of creating an underground dam by grouting the water-bearing strata with cement through 2-inch wells. Test hole No. 1 (C-6), on the east side of the valley near the contact of alluvium with the Tertiary limestones, encountered clayey fill to a depth of 79 feet (see log, table 12). This area was considered undesirable for a grouting test. Test hole No. 2 (C-7), on the east bank of the present stream, struck permeable sands and gravels at 22 to 29 fe^t, at 48 to 51 feet, and at 72 to 78 feet. Two hundred gallons of cement grout was poured in the hole through 2-inch pipe, which was perforated opposite the water-bearing horizons. After 24 days a hole was drilled 2.6 feet north of the grouted hole in an effort to determine the effectiveness of the grout. Sands proved to be still open, and drilling mud was lost constantly and attempts to secure cores were unsuccessful. When cuttings were examined it was found WELLS AND TEST HOLES 107 that even in the most gravelly beds "evidence of penetration of the grout was so slight as to be negligible. It was estimated that the particles of grout found in these cuttings would constitute less than a teaspoonful. No grout was found adhering to the pebbles." It was concluded therefore that an underground dam could not be created by this method. The writer is of the opinion that an underground dam at this point, even if it were possible to build one, might be of doubtful value. It is thought that the largest amount of underflow passing Fair Plain and other restricted alluvial channels (at Hope, Bless- ing, Jerusalem, and other places) could be captured by pumping wells heavily enough to create a cone of depression in th^ channel- mouth area. Such pumping could be controlled to prevent back- flow from the sea, yet be great enough to withdraw from the ground most of the normal seaward underflow. FAIR PLAIN At the request of United States Army officials, test holes and wells were drilled to supply water to the airdrome located in the Manning Bay area. Test holes were drilled in the deer* alluvial- filled valley 14 to V& mile northwest of the Fair Plain vrell (45a). The results of drilling were about what would be expected from the record of the Fair Plain well; in two test wells (C-8 and C-9) three water-bearing sands were penetrated (see logs, table 13). The second test hole was converted into a 10-inch gravel-pack well, and when pumped with air it furnished about 45 gallons a minute. The chloride content is said to have "decreased from 450 parts per million to 245 parts per million after several months' pumping. To avoid overdraft on one well, two other wells (C-10 and C-ll) were drilled, 100 and 200 feet west of C-9, respectively. Data on these are not available except a statement that they yield about as much water as the first well. It was planned to pump the three wells by suction from a central point using only one pump. It was further anticipated that 30 gallons a minute w<rild be the normal total draft on these wells. GOLDEN GROVE A well was drilled on property of the Virgin Islands Co. at Golden Grove to provide fresh water for Adventure village. The record of this well (C-12) is particularly interesting in that it shows the presence of an excellent water-bearing stratum in the alluvium-filled valley about a mile upstream from Fair Plain. Below 27 feet of clayey fill, 21 feet of gravel and boulders were 108 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS encountered. (See log, table 12.) A screen (possibly perforated casing) was set from 38 to 50 feet. The well was bailed for 31/2 hours at a rate of 15 gallons a minute with very slight draw-down below the static level at 29 feet. It is apparent, therefore, that much more water than 15 gallons a minute could be made available by increasing the draw- down. The well was equipped with a wind-powered pump and in November 1943 was reported to be giving excellent service. The water contained 295 parts per million of chlorides. Ground-water conditions here are comparable to those found at Fair Plain. It is of interest to note that the alluvium is 48 feet thick at this place as compared to 80 feet at Fair Plain. It is sug- gested that the 48 feet of alluvium encountered here is not the maximum thickness present in this area and tl at a somewhat greater thickness of alluvium may be present 50 or 100 feet to the east or west of this well. FREDENSBORG Results of drilling on the Virgin Islands Co. property for water for the village at Fredensborg provides a' good example of the difficulty of locating the axis of the buried alluvial valley leading back to the mountains, where the present valley is very wide. A hole (C-13) was drilled just east of Bethlehem on the east bank of the present stream. Only 13 feet of fill was penetrated before reaching a limestone of Tertiary age. A secord hole (C-14), about 100 feet east of the first hole, struck the limestone at 15 feet. As the alluvial cover was very thin, it appeared that developed wells would produce very small quantities of water, and the test holes were not converted into wells. COLQUHOUN MOUNT PLEASANT HOMESTEADS Wells and test holes were drilled in the vicinity of Colquhoun Mount Pleasant to obtain water for the homesteads in that area. The first hole (C-15) was drilled in the southeast corner of the homesteads area along the course of an intermittent stream lead- ing from Little Fountain to Fredensborg. The scant data avail- able indicates that 77 feet of alluvium (clay?) was encountered, below which blue clay of the Jealousy formation was penetrated to 91 feet. The strata were not water bearing. The second hole (C-16) was drilled about 700 feet north of the first test hole at Colquhoun. No data on this well are at hand except that it yielded only 1 gallons minute. " . . WELLS AND TEST HOLES 109 A third hole (C-17) was drilled about half a mile west of the first hole. No data on this test hole are available except that the hole was drilled to 85 feet and that the results were negative. It is difficult to interpret the results of drilling these three holes when so few data are available for study. Certainly the area should be a reasonably favorable one, as it is in this are^». that much of the run-off from the mountains seeps into the alluvium. In the first test hole 77 feet of alluvium were penetrated, about as great a thickness as was found in wells at and near Fair Plain. It seems that here the alluvium may have been clayey, there may have been other factors that prevented the completion of a suc- cessful well at that site, or the data at hand may be incorrect. The fourth hole at Colquhoun Mount Pleasant (C-18) was drilled in or along the gut east of the village. Again data are meager, but it appears that the hole was drilled to a depth of 55 feet and yielded 4 gallons a minute. Whether this well obtains water from the underlying diorite or is drawing water from the surface alluvium is not known. RATTAN A well (C-19) 226 feet deep was drilled at Rattan about %. mile east-southeast of the old estate in a low valley about 300 feet above sea level. The water level was 108 feet below the sur- face. No other data are available except that the well apparently obtains some "fresh" water. ST. JOHN PRINCESS HOMESTEADS A number of wells were drilled in the St. John Princess Home- steads area northwest of Christiansted. Data on these wells are meager, but certain facts are definite and of interest. Well C-20 is located near the head of a small valley south of St. John. This well penetrated alluvium from the surface to a depth of 26 feet, Tertiary limestones from 26 to 87 feet, and blue clay from 87 to 100 feet. It is not known whether the limestone or the alluvium furnished the water, but in any event the well is reported to yield 8 gallons a minute. It is of geologic interest to note the thickness of the Tertiary limestones, which extend to about 13 feet above sea level, and the underlying blue clay here, close to the inner edge of the basin in which the Kingshill marl and Jealousy formation were de- posited. Geologic conditions here may be similar to or identical with those at the location at which the second test well was drilled in 1939 at Jealousy (39, table 3). It may also be recalled that 842683° BO 9 110 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS well 75, in a small valley immediately north of St. John, went through a cover of marl of Tertiary age and encountered weath- ered Mount Eagle volcanics at 21 feet (about 55 feet above sea level). Several wells were drilled in the area fronting the Caribbean Sea between St. John and Little Princess. One well south of St. John (C-20) yielded 8 gallons a minute but another (C-21), just oast of the road leading to Judith's Fancy, was dry. The third well (C-22) was about a quarter mile southeast of fie second and is reported to yield 3 gallons a minute. It is a minimum of 46 feet deep, but it is not known whether water is obtained from the alluvium or the underlying limestone. A fourth well (C-23), just southeast of La Grande Princess, was dry, and a fifth well (C-24), near Little Princess, obtained 3 gallons a minute. A well (C-25) 80 feet deep was also drilled at the head of a small valley below Bellevue at an elevation of about 150 feet. This well probably penetrated Tertiary limestone beneath a thin alluvial cover, but it is not known from which stratum water is obtained. Depth to water is reported to be 45 feet and the yield 5 gallons a minute. CENTRAL FACTORY A test hole (C-26) was sunk at Central Factory to determine conclusively whether a supply of fresh water could be obtained by drilling. Yellow clayey material containing minor sand and gravel was encountered down to a depth of 99 feet (about 60 feet below sea level), below which 7 feet of blue clay was found. A screen was set from 98!/2 to lOli/o feet, but the \vell proved to have a yield of less than 1/2 gallon a minute. It might be expected that the older bedrock would be en- countered here about sea level. As bedrock is not present it might be concluded that beneath a veneer of alluvium the material pene- trated was Kingshill marl underlain by blue clay of the Jealousy formation and that at this place the surface of the pre-Jealousy formation drops off sharply in a manner similar to that noted along the north edge of the basin. However, the yellow clay and boulders reported are more likely alluvium than Kingshill marl, and it is concluded that a filled post-Miocene valley is indicated. The depth below sea level to which this valley has been eroded is noteworthy. At Fair Plain, 1 mile from the south coast, fill extended.to about 50 feet below sea level. At Central Factory, on the north coast, a post-Miocene valley was apparently eroded to more.than 70 feet below sea level. WELLS AND TEST HOLES 111 CHRISTIANSTED A well (C-27) was constructed at the Parade Ground, *4 m^e east of Christiansted, to provide additional water for the Chris- tiansted Hospital. The well, near well 92 but seemingly better located with respect to the axis of the valley heading up into Mount Welcome, encountered sand and gravel lying upon weath- ered bedrock at 35 to 47 feet below the surface. (See log, table 13.) The well was continued to a depth of 59 feet. Perforated 6-inch casing was set from 41 to 59 feet, and, upon bailing, the well yielded 15 gallons a minute with very slight drawdown. This well is one of those encountering strata in which water moves freely to the sea. The water level (44 feet below thQ. sur- face) is 3 J/2 feet above sea level; pumping this well continuously at 40 or 50 gallons a minute might lower the head to su°.h an extent that sea water would move up the dip and enter the veil. COTTON VALLEY A well (C-28) was drilled at Cotton Valley, East End, to pro- vide water for picnickers to the area. The well is near the head of the valley and slightly west of its axis. Much coarse detrital material was encountered, but most of it is described as clayey. Weathered bedrock extended from 69 to 92 feet below the surface; the hole was continued in the unweathered bedrock to 93 feet. Ten feet of perforated casing was set from 70 to 80 feet. Upon bailing, the well yielded about 2 gallons a minute. The writer believes that for optimum results the well should have been located a little nearer the sea and more nearly along the axis of the valley, where detrital material would probably be better sorted and graded. However, for the purpose in view, ample water was obtained. It is interesting to note that he^e, as in the Christiansted Hospital well, water stood only a little above sea level. Static level in the Cotton Valley well was 51.2 feet below the surface, or 3.6 feet above sea level. COTTONGARDEN A test hole (C-29) was drilled in Cottongarden Valley near the easternmost end of St. Croix. The well was located in the axis of the valley about 200 yards from the sea. Alluvial fill was pene- trated to a depth of 83 feet (about 50 feet below sea level). The alluvium was very coarse, and gravel and boulders are reported at various depths (see log, table 12), but most of the strata, were clayey. Drilling was continued into the weathered bedrocl* to a depth of 111 feet. Although the well was not cased and tested it was suggested that a completed well might yield 1 or 2 gallons a minute. 112 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS RESUME OF NATIONAL PARK SERVICE DRILLING PROJECT Invaluable data have resulted from the CCC drilling project on St. Croix. Wells and test holes in various types of formations furnish a clearer picture of conditions than that which was available to the writer in 1939 based on only a few test holes and on fragmentary data. The ancient alluvium-filled valleys are the best sites for wells in many parts of the island, in the area underlain by the old rock as well as in the area underlain by Kingshill marl. The fill is now known to extend 50 feet or more below sea level in several places other than Fair Plain, and it appears safe to assume that favor- able thicknesses of fill may be found at widely scattered localities near the sea, not only at the mouths of large streams but under- lying the beds of many smaller streams as well. The necessity of shallow exploratory drilling in order to locate the axes of buried valleys is well shown; in places holes were drilled which encountered the underlying older rocks at very shal- low depths, as at Fredensborg. Elsewhere, although reasonable thicknesses of fill were located, predominantly sandy strata free of clay were not present, as at Central Factory and Colquhoun. The limestone of Tertiary age was a good water-bearing forma- tion in a few places but in general it may not be relied upon to furnish much water. The weathered surface of the Mount Eagle volcanics is much more reliable as a source of small quantities of water. TABLE 12. Logs of wells and test holes drilled by the Nation-it Park Service in 1940-41 Well C-5, Virgin Islands Co. at Paradise [Altitude, 80 feet] Alluvium (Pleistocene) : Brown and yellow clay...- Kingshill marl (Miocene- Thickness (feet) 2 13 8 11 6 Depth (feet) 2 15 * 23 34 40 Kingshill marl (Miocene- Oligocene) Continued: White (porous) lime- | Thickness (feet) 5 15 11 Depth (feet) 45 60 71 RESUME OF DRILLING PROJECT 113 Well C-6, Virgin Islands Co. at Fair Plain [Altitude, 20% feet] Alluvium (Pleistocene) : Brownish-yellow clay with Yellow clay with large Brownish-yellow clay with 8 26 39 6 8 34 73 79 Kingshill marl (Miocene- Oligocene) : White clay and soft lime- White limestone and clay 3 5 5 82 87 92 Well C-7, Virgin Islands Co. at Fair Plain [Altitude, 21% feet] Alluvium (Pleistocene) : 8 4 10 7 3 4 12 8 12 22 29 32 36 48 Alluvium (Pleistocene) Con. Oligocene) : White clay 3 21 6 2 51 72 78 80 Well C-8, U. S. Army at Fair Plain [Altitude, 32 feet] Alluvium (Pleistocene) : Sand and gravel, water. Sand and gravel, water___- 3 12 5 35 5 9 3 3 15 20 55 60 69 72 Alluvium (Pleistocene) Con. Kingshill marl (Miocene- white clay and marl__._. 3 3 5 19 75 78 83 102 Well C-9, U. S. Army at Fair Plain [Altitude, 31 feet] Alluvium (Pleistocene): Sand and gravel, water...- 3 8 3 8 9 11 8 9 3 11 14 22 31 42 50 59 Alluvium (Pleistocene) : Con. Oligocene) : Yellow clay 1 1 7 11 2 4 60 61 68 79 81 85 Well C-12, Virgin Islands Co. at Golden Grove [Altitude, 55 feet] Alluvium (Pleistocene) : Brownish-yellow clay.-.-- Yellow clay, gravel, and Gravel and boulders, water 6 8 13 21 6 14 27 48 Kingshill marl (Miocene- Oligocene): White lime- 64 112 Well C-13, Virgin Islands Co. at Bethlehem [Altitude, 80 feet] Alluvium (Pleistocene) : 4 9 4 13 Kingshill marl (Miocene- Yellow and white clay 12 24 36 114 GROUND WATER OF ST. CROIX, VIRGIN ISLANDS Well C-14, Virgin Islands Co. at Bethlehem [Altitude, SO feet] Alluvium (Pleistocene) : Clay, gravel, and boulders. 7 3 8 15 Kingshill marl (Miocene- 38 KQ Well C-15, Homesteads at Colquhoun-Mount Pleasant [Altitude, 135 feet] Alluvium (Pleistocene) : Al- 77 77 Jealousy formation (Oligo- 14 91 Well C-20, Homesteads at St. John-Princess [Altitude, 100 feet] Alluvium (Pleistocene) : Al- Kingshill marl (Miocene- Oligocene) : Limestone- 26 61 26 87 Jealousy formation (Oligo- 13 100 Well C-26, Virgin Islands Co. at Central Factory [Altitude, 30 feet] Alluvium (Pleistocene) : Yellow clay with sand and boulders Yellow clay with boulders 8 23 26 8 31 57 Alluvium (Pleistocene) : Con. Yellow clay with some Jealousy (?) formation (Oli- 42 7 99 106 Well C-27, Christiansted Hospital at Parade Ground, Christiansted [Altitude, 47 ^ feet] Alluvium (Pleistocene): Yellow clay with gravel- __ Gravel and sand with a 3 32 12 3 35 47 Mount Eagle volcanics (Upper Cretaceous) : 12 59 Well C-28, Municipality of St. Croix at Cotton Valley [Altitude, 53% feet] Alluvium (Pleistocene) : Brown clay and gravel -.. Yellow clay and graveL-.- 8 18 40 3 8 26 66 69 Mount Eagle volcanics (Upper Cretaceous): Gray weathered rock, 23 1 92 93 Well C-29, Municipality of St. Croix at Cottongarden [Altitude, 30 feet] Alluvium (Pleistocene) : Yellow clay with boulders 15 2 3 13 4o 35 15 17 20 33 37 39 48 83 Mount Eagle volcanics Weathered dark-gray clay 21 1 6 1 104 105 111 112 RESUME OF DRILLING PROJECT 115 15 fi O .« | ij- c-3 se-a ^42 -g -g « « 2 1 * 0 r m « ° « i2 tS g^, m g 00 -M ca S K 4! 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Valley.. ... ...do ....... ...do... .... do ...... ...do ....... Valley........ ...do... .... do 80 135 140 155 190 300 100 50 25 55 65 150 30 47^ 54.8 111 53 91 85 55 100 46 44 80 106 59 93 30 8 6 6 8 do Marl........... do ... ...do........... ...do........... do. ... ... .. ... do (?) ...do.... ............. ... do..... ...... ...... do. . ...do.. ............ 0 89 60 80 0 108 45 44 51.2 28 1 Dry 1 Dry 4 8 Dry 3 Dry 3 5 H 15 2 Ab Ab D Ab D D D Ab D Ab D D D T of C-13. See log. formation encoun- tered at 77 feet. 101 feet 6-inch casing and strainer replaced by 3-inch casing. Lower end perforated. pump test "slight." Casing perforated from 41 to 59 feet. 70 tn 80 fppt. SPP W. See log. 1 Ab, abandoned; D, domestic; S, stock; T, test well. * Chloride content in parts per million. INDEX Page Acknowledgments .................... 3 Agriculture in the area....... 7-9 ; pi. 2B Alluvium, distribution of........... 27-28 Analyses of water 71, 72, 74, 79, 80, 83, 84 Beaches, emerged ................... 38 Buried valleys................ 38 ; pi. 3 A Coral reef, emerged........... 38 ; pi. 4B submerged ....................36-37 Cushman, J. A., fossils identified by 22, 23 Drainage pattern ................. 35-36 Drilled wells, construction of 41-42 ; pi. 6B Dug wells, construction of............ 41 Erosion cycles, early Tertiary ...... 34-35 ; pi. 2A Flora and fauna of the area........... 9 Fossil lists. ............. 22-23, 24, 25-26 Geologic history, summary of...... 14-15 Ground water, general condition of. 45-47 in alluvium. ........... 56-64 ; pi. 3.4. in beach sand ................... 64 in diorite..................... 50-51 in Kingshill marl ............. 51-55 in Mount Eagle volcanics .... 47-50; pi. 5B previous investigations of...... 43-45 Henbest, F. G., fossils identified by. 25, 26 Industry in the area.................. 9 Jealousy formation ........ 19-20 ; 26-27 Kingshill marl, deposition of....... 26-27 description and distribution of. 20-22 geologic age of............... 22-26 Land forms ......... Location of the area. 36 3-4 Mount Eagle volcanics, description of ....... 16-17 diorite intruded into..... 17-19 ; pi. 1 Population of the area............. Previous investigators, work of.... Purpose and scope of investigation., Quality of water, contamination by sea water ..... contamination from organic pollution ......... ... 7 12-14 . 2-3 69-73 73-74 Page Quality of Water Continued effect of alkali soils........... 75-77 effect of base exchange on .... 77-78 effect of salt spray ............. 77 in alluvium .................. 82-85 in beach sand ................... 86 in dioritic rocks ................. 79 in Kingshill marl ............. 79-81 in Mount Eagle volcanics ...... 78-79 saline minerals and Tertiary limestone ........... 74-7.3 surface water compared with ground water ..... 69 Quin, J. T., fossils collected by. ....... 22 Rain water, use of................ 38-39 Rainfall, accumulated departure from normal of ........ 12 relation to sugar production. ..... 12 yearly and monthly averages. . . 9-12 Recommendations ............... 91-93 Springs, use of water from........... 40 Stream flow, use of.................. 39 Structure in the area. ....... 29-34 ; pi. 1 Suitability of water, for cattle. ....... 87 for humans .................. 86-87 for irrigation................. 87-89 for sugar and rum production. . 89-90 Terraces, coastal. ......... 37-38 ; pi. 4A Test-drilling program........ 64-68 ; pi. 1 Topography and drainage. . . . 4-7 ; pi. 2A Vaughan, T. W., fossils collected l^ 22, 25 fossils identified by. ....... 24, 25-26 Water supplies, at Bethlehem Factory ............ 40-41 at Central Factory .............. 40 Well logs................... 68, 112-114 Well records ................... 115-116 Wells, use of Avater from. . . 39-4C ; pi. 5B Wells and springs, records of...... 94-99 Wells and test holes, at Central Factory .............. 110 at Christiansted ............... Ill at Colquhoun-Mount Pleasant homesteads ....... 108-109 at Cotton Valley .............. Ill at Cottongarden ............... Ill at Fair Plain ................. 107 at Fredensborg ................ 108 at Golden Grove ........... 107-108 at Paradise ................... 106 at Rattan ..................... 109 at St. John-Princess homesteads ....... 109-110 at Whim homesteads .......... 105 Woodring, W. P., fossils identified by.. 24 117 &U.S. GOVERNMENT PRINTING OFFICE: 195O 842683 o cj 55 & s- s- o 8" Co - s. CO I <B -I ' 3 \ <B A, If Early Tertiary or Upper Cretaceous late Cretaceous fill! fifii| <D ^ s;§ o 3^1 2-1 S- P OK? - o^ 'a. n" 2 « ^ 3 CO S.^o * o*p 2 cj T-i-l 3 i? . Oligocene ____A____ spflt »I"! §-"2. 3 o fill Hi -S'°S o CRETACEOUS Miocene Recent § §.§ B |" 2.3'fr. IIS 2 5^3 Qo.2. S^S.^ g ^ " p 5 3 ll «B cT 3 » 3-0 So-2. i R "< B S' !-« C TERTIARY QUATERNARY m x "O O