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COLLEGE OF THE VIRGIN ISLANDS Caribbean Research Institute WATER j } RESOURCES > RESEARCH WAoawy CENTER ‘usituatianting CAPSULE REPORT NO. 3 AUGUST 1985 RECONNAISSANCE OF GROUNDWATER QUALITY IN THE U.S. VIRGIN ISLANDS July | 1984 Rene’ Garcia P| Michael Canoy INTRODUCTION The demand for groundwater in the U.S. Virgin Islands, especially for domestic use, has increased markedly in recent years. In 1982, groundwater withdrawals for public and do- mestic use in the U.S. Virgin Islands were about 1.1 million gallons per day. Recent investigations have shown that addi- tiona] withdrawals might be possible. In spite of increasing demands for groundwater, data on its quality is very scarce, The lack of data constitutes a major gap in the knowledge of the U.S. Virgin Islands’ hydrologic environ- ment, There are no active water-quality monitoring networks, and the most recent reconnaissance was conducted nearly two decades ago. In July 1984, the U.S. Geological Survey, Water Resources Division and the Water Resources Research Institute of the College of the Virgin Islands conducted a reconnaissance of the groundwater quality at selected wells throughout the U.S. Virgin Islands. Wells representing the principal aquifers were sampled for physical, chemical and bacteriological characteristics. Selected wells were also sampled to determine potential con- tamination with priority pollutants as designated by the U.S. Environmental Protection Agency (EPA). The data collected is summarized i in this report and will provide a base line for future gations and an of water use versus quality. METHODS AND PROCEDURES Nineteen wells on St. Croix, St. Thomas and St. John, rep- resenting the most important pumping centers, were sampled. The samples were collected as closely as possible to the well casings, and standard methods were used in the collection pro- cess. The physical characteristics of temperature, specific con- ductance, pH and total alkalinity of the samples were measured at each site immediately upon collection of the samples. The depth of the water level below the land surface was also deter- mined. Samples for chemical analysis of trace elements, com mon ions and nutrients were collected, filtered and preserved in accordance with standard procedures. Samples for the deter mination of priority pollutants were collected at five wells and preserved. The samples were then shipped to the U.S. Geologi- cal Survey National Water Quality Laboratory in Doraville, Georgia, for chemical analysis. Determinations of fecal coli- form and fecal streptococci bacteria were completed from raw water samples using the membrane filter method. Processing and incubation of the samples were begun in the field shortly after collection. RESULTS The principal findings were as follows: 1. Specific conductance values normally ranged from 1,400 to 3,000 micromhos per centimeter. Two wells had values which surpassed 4,500 micromhos per centimeter. . Fecal coliform and fecal streptococci bacteria were detect- ed in samples from 18 of the wells sampled. Colony counts of fecal streptococci bacteria were as high as 5,800 colonies per 100 milliliters of raw water sampled. . Nitrate concentrations exceeded 1.0 milligrams per liter at ten of the wells sampled. . Chloride concentrations generally ranged from 180 to 600 milligrams per liter. At 13 wells, chloride concentrations exceeded the EPA drinking water standard of 250 milli- grams per liter. Sulfate concentrations for most wells were less than the drinking water standard of 250 milligrams per liter. All wells sampled had total dissolved solids exceeding the EPA secondary drinking water standard of 500 milligrams per liter. 6. Concentrations of organic compounds and most trace elements at selected sites were less than detection limits. Nn w > w Condensed by Yahay Bello from: “Reconnaissance of Ground- vater Quality in the U.S. Virgin Islands, July 1984” by Rene’ Jarcia and Michael Canoy, Technical Report No. 20, Water Resaurces Research Center, Caribbean Research Institute, St. Thomas, V.I. July 1984. For further information on this report contact: Water Resources Research Center College of the Virgin Islands St. Thomas, Virgin Islands 00802 (809) 774-9200 The research on which this report is based was financed in part by the United States Department of the Interior, Geological Survey, through the Water Resources Research Institute. Contents of this publication do not necessarily reflect the views and policies of the United States Department of the Interior, nor does mention of trade names or com- mercial products constitute their endorsement by the United States Government.