USVI Public Records

Document text

od ~ THIRD INTERNATIONAL SYMPOSIU ~ s ON é TROPICAL HYDROLOGY Fifth Caribbean Islands fee Water oe Resources feeny po Congress et : A AW Sas <r ba tated wee TE. oe as = — a Sad 4 os 4 od ce oe — PROCEEDINGS ne ea (2 PROCEEDINGS TROPICAL HYDROLOGY AND CARIBBEAN WATER RESOURCES THIRD INTERNATIONAL SYMPOSIUM ON TROPICAL HYDROLOGY AND FirrH CARIBBEAN ISLANDS WATER RESOURCES CONGRESS Edited By Rafael I. Segarra-Garcia Associate Professor Department of Civil Engineering Mayagtiez Campu University of Puerto Rico, P.O. 9040 Mayagtiez, Puerto Rico 00680 Sponsored By The AMERICAN WATER RESOURCES ASSOCIATION Co-Sponsored By The WATER RESOURCES RESEARCH INSTITUTE UNIVERSITY OF PUERTO RICO U.S. GEOLOGICAL SURVEY WATER RESOURCES DIVISION WATER RESOURCES RESEARCH CENTER UNIVERSITY OF THE VIRGIN ISLANDS JULY 12-16, 1998 SAN JUAN, PUERTO RICO AMERICAN WATER RESOURCES ASSOCIATION 950 HERNDON PARKWAY, SUITE 300 HERNDON, VA 20170-5531 (703) 904-1225/ Fax: (703) 904-1228 Literature citation for this volume: Rafael 1. Segarra-Garcia (Editor), 1998. Tropical Hydrology and Caribbean Water Resources. Proceedings of Third International Symposium on Water Resources and Fifth Caribbean Islands Water Resources Congress. American Water Resources Association, Herndon, Virginia, TPS-98-2, 233 pp. AMERICAN WATER RESOURCES ASSOCIATION TECHNICAL PUBLICATION SERIES TPS-98-2 LIBRARY OF CONGRESS CATALOG CARD NUMBER: 98-71485 ISBN 1-882132-43-2 1998 COPYRIGHT BY THE AMERICAN WATER RESOURCES ASSOCIATION All rights reserved. No part of this book may be reproduced in any form or by any mechanical means without per- mission from the publisher. These proceedings were published by the American Water Resources Association, 950 Herndon Parkway, Suite 300, Herndon, Virginia 20170-5531. The views and statements advanced in this publication are solely those of the authors and do not represent official views or policies of the Editor or of the American Water Resources Association; the Water Resources Research Institute, University of Puerto Rico; the U.S. Geological Sur- vey, Water Resources Division; and the Water Resources Research Center, University of the Virgin Islands. Commu- nications in regard to this publication should be sent to the Circulation Department of the American Water Resources Association, 950 Herndon Parkway, Suite 300, Herndon, Virginia 20170-5531, (703) 904-1225, FOREWORD The tropics are the most hydrologically active regions of the world — the regions where the great rain forests are located. The moisture and heat circulation cycles of the tropics affect weather patterns on a global scale. Research in tropical hydrology is making evident the tremendous importance of this field to understanding the role of the hydrologic cycle in weather systems, ecosystem dynamics, and ecological effects of human activity. The wide range of topics presented in the symposium is representative of the breadth of dis- ciplines that have found application in the field of tropical hydrology. The fields of water -supply and distribution, climatic change, water resources planning, water quality, ecosystem studies, and water resources education are aptly represented by several symposium papers reflecting the diversity of current research efforts in the field of tropical hydrology. The papers are grouped into specific sessions according to the topics requested in a call for abstracts. First drafts of papers were peer-reviewed and returned to authors for preparation of final manuscripts. These are the papers presented in this Proceedings. The Proceedings will be useful to students, practitioners, and researchers alike due to the diverse number of contribu- tors to the publication. Contributions from universities, research institutes, government agen- cies, and consulting firms are indicative of the widespread involvement of a gamut of organiza- tions in the field of tropical hydrology. Rafael I. Segarra-Garcia Editor and Technical Program Chair University of Puerto Rico Mayagiiez, Puerto Rico ACKNOWLEDGMENTS The organization of the Third International Symposium on Tropical Hydrology and Fifth Caribbean Islands Water Resources Congress has been possible due to the contributions of many individuals who collaborated with no other interest than accomplishing the success of this event. We appreciate the help of Ms. Maria M. Irizarry from the Caribbean District of the U.S. Geological Survey who kindly participated in the technical peer review of these papers. We also are thankful to Eng. Fernando Vargas who cooperated in the reviewing of the paper for- mats. The Organizing Committee is also grateful to the many individuals who contributed their time and effort to the accomplishing of the symposium. We greatly thank the authors for their ~ cooperation during the revision process. ili ORGANIZING COMMITTEE GENERAL CHAIR Jorge Rivera-Santos Director Puerto Rico Water Resources Research Institute University of Puerto Rico, College of Engineering P.O. Box 9040 Mayagtiez, PR 00681-9040 . Co-CHaIrs Rafael Rodriguez Henry Smith District Chief Director U.S. Geological Survey U.S. Virgin Islands 651 Federal! Drive Water Resources Research Institute Guaynabo, PR 00965 No. 2 John Brewer's Bay St. Thomas, VI 00802-9990 TECHNICAL PROGRAM CHAIR Rafael I. Segarra-Garcia Department of Civil Engineering ~ Mayagtiez Campus University of Puerto Rico Mayagiiez, PR 00680 POSTER SESSION AND COMMERCIAL EXHIBITS COMMITTEE CHAIR ) Fernando Vargas Department of Civil Engineering . Mayagtiez Campus University of Puerto Rico Mayagiiez, PR 00680 TECHNICAL FIELD TRIPS ORGANIZING COMMITTEE CO-CHAIRS Ferdinand Quifiones Anibal Camacho-Delgado | CSA, Architects and Engineers Super Aqueduct of the North Coast Project — Mezzanine Suite, Mercantil Plaza PR Aqueduct and Sewer Authority ~ San Juan, PR 00918 . Mercantil Plaza ' Guaynabo, PR 00965 Matthew C. Larsen Fred Scatena U.S. Geological Survey international Institute of Tropical Forestry ' 651 Federal Drive — : USS. Forest Service Guaynabo, PR 00965 Call Box 25000 Rio Piedras, PR 00928 AMERICAN WATER RESOURCES ASSOCIATION 1998 BOARD OF DIRECTORS PRESIDENT N. EARL SPANGENBERG College of Natural Resources University of Wisconsin-Stevens Point Stevens Point, Wisconsin (715) 346-2372 / Fax: (715) 346-3624 PRESIDENT-ELECT JOHN J. WARWICK _ University of Nevada-Reno Renewable Resources Reno, Nevada (702) 784-6250 / Fax: (702) 784-1953 PAST PRESIDENT STEPHAN J. NIX Chair, Civil and Environmental Engineering College of Engineering and Technology Northern Arizona University Flagstaff, Arizona (520) 523-4339 / Fax: (520) 523-2300 DIRECTOR JOHN S. GROUNDS III Halff Associates, Inc. Houston, Texas (713) 523-7161 / Fax: (713) 523-4373 DIRECTOR DIANA L. WEIGMANN University & Community College System of NV Reno, Nevada (702) 784-4902 x239 / Fax: (702) 784-1127 DIRECTOR MICHAEL C. YUREWICZ U.S. Geological Survey Reston, Virginia (703) 648-5811 / Fax: (703) 648-4850 SECRETARY/TREASURER JANET L. BOWERS Chester County Water Resources Authority West Chester, Pennsylvania (610) 344-5400 / Fax: (610) 344-5401 DIRECTOR DENNIS H. BLOCK Assistant Director Water Resources Research Institute Auburn University Auburn, Alabama (334) 844-5075 / Fax: (334) 844-4462 DIRECTOR PATRICIA H. LODGE Consultant Coral Gables, Florida (954) 966-6300 x1121./ Fax: (305) 967-3489 DIRECTOR GREGORY J. WESTFALL USDA-Natural Resources Conservation Services Somerset, New Jersey (732) 246-1171 x-165 / Fax: (732) 246-2358 Ex OFFICIO KENNETH D. REID, CAE Executive Vice President American Water Resources Association . Herndon, Virginia (703) 904-1225 / Fax: (703) 904-1228 AWRA NATIONAL HEADQUARTERS STAFF Executive Vice President........ccccccccsscesssesceesees Director of Membership Services... Director Of Finance .............cccccsscccssssesecsserecseree ee OR reer Kenneth D. Reid, CAE coo bibszachanspii aaa ATRMasTi Tok fetheses ere Kerry L. Curtis nee eer ee ee Michael J. Kowalski WN sesiaailiasidung Sos ea paTRTR AG eaORE ye eon eee Patricia A. Reid errr ORY ert Charlene E. Young oer a sete tieate i TABLE OF CONTENTS PROCEEDINGS THIRD INTERNATIONAL SYMPOSIUM ON WATER RESOURCES AND . FIFTH CARIBBEAN ISLANDS WATER RESOURCES CONGRESS SURFACE AND GROUND WATER QUALITY Detection of Conduit-Controlled Ground-Water Flow at Selected Sites in Northwestern Puerto Rico — Jestis Rodriguez-Martinez and Ronald T. Richards ......s00000 Hint cowed eaves ss ovis pacivavondvubasupe ACLS SNES 3 Geochemical Constraints on Recharge and Groundwater Evolution: The Pleistocene Aquifer of Barbados ~ lan C. Jones, Jay L. Banner, and Bwalya ]. MWAnsa....... cscs ieteneeeees sanuatenagheaseatencvagecenaeaunetets 9 Nitrate Contamination of the Upper Aquifer in the Manati-Vega Baja Area, Puerto Rico —. Carlos Conde-Costas and Fernando Gormez-G6mez .....sscscsesessseresesesseccessnsssssesesenenensnensensaeneneney Ssasnianeeanee 15 Ground-Water Chemistry in the Valle de Yabucoa Alluvial Aquifer, Southeastern Puerto Rico cS JOSEPH Ve THES BE sede cx vcsyscezonswennvtanvsevsseamvseatsstosnssanessnverrapipessvaesslaljveiony ci vavenernueteaegrsiny aeonpiegheriairnthaedib ae 21 Comparison of the Nutrient Loads to Laguna Joyuda, Puerto Rico. +. -Kennetice. Sttelé, D, S, Hicks, anid Jost Mi. Lipeesvisisssassas saascvsstvesaveyautpensbineans araernn vans penavennsmenarneni: 27 Contaminants in Sediments Deposited in the San Juan Bay Estuary System (1925-95) — Richard M. T. Webb, Fernando Gomez-Gomez, and Sherwood C. McIntyre ......:.:ccsccecesecserecetenenesesese 33 Hydraulic Characterization of Sinkhole Protection Filters for Highway Drainage — Laura D. Carbo-Maldonado, Walter Silva-Araya, and Jorge Rivera-SAant0 «1... heer 39 Analysis of Organic Compounds and Trace Elements in Cario Tiburones — Michel Soto, Delysmar Pagan, Jeffrey de Jestis, Osvaldo Rosario, Ana M. Garcia, PTE IEA sc RAIS ccxpecoxtiapiceanvsnnysnntivbenfotkessishgininiihseunrs taeenrnatgs tna iiannliaaaongh testa tinsntbtaans4Ts scant 45 Total Phosphorus and Total Nitrogen Mass Balances for Lago de Cidra, Central Puerto Rico. tt OP ARAO RAROGAGINES ss .ccsicxccmmcnacnmimainnen tra anon noe Ree Aaa Saab EXTREME HYDROLOGIC EVENTS Determination of a Regional Skew Coefficient for Puerto Rico SH FOP RY AOU E sooctnesevonexnessinansanidbopbninadhensisd0SI510<Sak ISOLA RTO ate A acacia eae 59 Flood Frequency Estimation for Ungaged Catchments in Puerto Rico to. Rafael SEGA A=GAAe acissgustsevscsvencesesavessanscslsabiutvargslessovosdensestnonnsntdnydineletonelbtontin 2g aerasing ie hcaneneege 65 vii Ieee se Ee enna Su eedechne Soap el stern eRe Lerten Let ante An Intermittent Stream Investigation of a Tidally Influenced Watershed i ini ‘ Northern “Puerto Rico . = Ken a Ramon L. Calzada, and Gregg DelVecchio Bee Se eee ne Seer Sicwwsmuaweasee: 71 Active and Reactive Responses to Drought Induced Water Shortages 6 TRE ETD cxxcscevestayvernevcennavaenasranocnnsnaarcocentnexenstntivssasics inet igi ribo apt IE NN ii sdicnxg ies spas crasaseonasiibe 79 Minimizing the Impact of 1998 Drought in Western Pacific _ — Shahram Khosrowpanah, Leroy F. Heitz, and Charles Guard .2....ccccscccsscecsesssssessssseseeesneesstecceeecesegaceneeneeess 85 Drought Effects on Pool Morphology and Neotropical Stream Benthos -= A. P. Covich, T. A. Crowl, S. L. Johnson, and F. N. Scatent...cc.ccsccccssssssssesssssessssvessesessesssscacesseseeneviesseeess 91 WATER RESOURCES INFORMATION AND EDUCATION CATHALAC: Coordinated Research on Hydrological Processes in the Humid Tropics of Latin America and the Caribbean “Nicolaas de Groot and Antonius Bakkum ....c..c.cceccccceee0eee a a ee ee eee ae 99 . Incorporating Knowledge, Skill, and Experience in Water Resources Education 7 i PONE ERD GT WEIN S KIRN g cvcesincsevessaserepsesnsssmaipaigulststbeyastiouscigaie ui heasesiensaennainncleseesenivieniaceanies 105 Soil Erosion in Micronesia: Guam, A Case Study — Shahram Khosrowpanah and Peter-Paul G. Dumaliang vo... peonpelapicayereeiiapee: a nen ee 111 fhegae: a Watershed — Configuration Tree in Taiwan es = Sheng LIQNg nissesssssevsissecsesnsssnsnsssonsvnsosonssnsnesosensonsessnsonessansnaciocssunsssssnecosassesnensssecenneseeenn pi Gaseleceationalinegs 117 WATER RESOURCES PLANNING _ Development of a Monitoring Strategy for the Estuary of the Rio Grande de Arecibo ~ . Ferdinand Quifiones, Brenda Guzmdn, and Ernesto Rodriguez ....ssssssssssssecsssescssssecssssessssseeseesnseseee 125 The Vital Issues Process and the Puerto Rico Water Resources Initiative ~- Jorge Ivin Vélez-Arocho, William Frey, Dennis Engi, Nestor Ortiz, , SARE JOEL WLC TOERG schsainilestiinstonsoenesnnernciesenneathesssicotemrtenigtaomensncsttloweet Si anmemusintercteiiast aakgthen 129 Alternative On-Site Sewage Disposal Systems in the United States Virgin Islands = me Hodge, William F. McComb, and Henry H. Smith... Ce a es eee fo MANAGEMENT OF LIMITED FRESHWATER RESOURCES Distributed Demand Model for Estimation of Water Consumption in Pipe Systems =~" | Walter F. Silva-Araya and. Marcela Dundn-Sanvedt at sisisscassiccasssseessisssisatiastaenaacessaacsanetsansicavcnssonssivntione 143 Computer Control of Water Distribution in Irrigation Systems -~ Rati! Rivas-Pérez, Gerson Beauclhainip-Béiez, Domingo Rodriguez, ARE RR Tae Ee ox econ sues vsmsascctosa asthe adi cease aig Bader 149 viii Storage Innovations: Methods for Making Rainwater Harvesting More Attractive =: EL “Dap CroOler...nsccsvccssasserscssessaase waccdcamwsavrcaavwaentirersoessacens od Mhemanegegecssenensnnannngreenesies ET Nor 155 Dredging of Carraizo Reservoir: Restoration of a Critical Resource for San Juan Saale Yer a: Sess ssineyel vdnonseinmses recuse pea ein aerial goi eaaNe “eee 233-2 — Teresita Vega and José J. Terrasa-Soler ...s.sssssssssessseesiveecssseesesstesiecneennnecansecnisenecasscsssecassretessnnesetis setts 161 Improving Water Infiltration and Ameliorating Soil Crusting With Polyacrylamide — Duane Gardiner, Qingguo Sun, Eduardo Méndez, and Todd Carr ....cccccssecsseeececeieeennneesncenenns 167 Restoration of Urban Wetlands for Mitigation Near San Juan, PR. . — WedeL. Nutter and Jeffrey S. TRIE ccs scsvacrrao nn Steecs snot nail oan Ranad aeshive guanine eae LES The Potential Effect of Sea Level Rise on Pterocarpus officinalis — Alejandro Cubifia, Vivian Gerena, Mitchell Aide, Dan Wilkinson, David James, anid José MOntalv0 ..s.s.isecsssseeeeeeessssssssessecseennnnnnnnseceensnonneessecsnnnessessegiens Jascnthsdesa i siaenesaonaeansoiaee ciate re 179 Effects of Conifer Plantations on Water Yields and Regime in Antioquia Colombia — ‘Juliana Gonzdlez-Barney ......sseesscresresreerserserseneeseenenssssesonnees endow Sansa boaevecsusenteecnaomniewweljvin tl uip len 183 CLIMATE CHANGE IMPACT IN THE TROPICS An Assessment of Climate > ac in the Luquillo Mountains of Puerto Rico — FN. Seater fosesssessesseccsecssecusesseeenseeestonsneagacsssnscnsecavenasecanenssenuccanecueensesesancnneayesscsscescanazeenneesesessaegansese 193 - Water Budgets of Forested and Agriculturally-Developed Watersheds in Puerto Rico ~- Mathew C. Larsen and Iris M. CONCEPCION oesssseeesresreeerinseeteesisnsssneeeenessnsnseees seen Maite ee 199 DESALINATION, WATER REUSE, AND OTHER WATER SUPPLY SOURCES Ocean Conversion Seawater Desalination Facility Reverse Osmosis Wellfield Expansion = Wm. Scott MA@nalatt..cc..ccs.ceciescersssssssisistvsvssonsotanssesensbassssssstbsvanenssasnesasbasessenndbossncasonenesceagueneragesestacanstacs 207 _ Mass-Balance Simulation Model for the Guajataca Reservoir, Puerto Rico - Godofredo Canino, Vinio Floris, Roberto Leon, and Ferdinand Quiffones ......ccccccccccecceseeeeeeeeteenes cS Seawater Supply Wells for Reverse Osmosis Desalination Facilities on Grand Cayman Island — Thomas M. Missimer and Harvey Winters...cscceccececseeerereecseesseseseessssscnesenesnenenesenensnenenenenninaransetecty 219 Desalinization Process for Reducing Demand of Raw Water ata Power Plant - — Raril O. McClin and Francisco E. LOpez...sesssecsssessesseseseeseneseseesenesesesesneneneensasssseeesseneseeseneenecneneneensteess 225 Paper Title Index evssessscnssshesssnssntsnsnstneaniessesentneeenrenenenesnensnnesnoseesnsine cient 1231-232 THIRD INTERNATIONAL SYMPOSIUM ON WATER RESOURCES FirtH CARIBBEAN ISLANDS WATER RESOURCES CONGRESS JULY AMERICAN WATER RESOURCES ASSOCIATION 1998 ALTERNATIVE ON-SITE SEWAGE DISPOSAL SYSTEMS IN THE UNITED STATES VIRGIN ISLANDS Janice D. Hodge,’ William F. McComb,* and Henry H. Smith? ABSTRACT: Disposal of residential sanitary waste is a challenge in many areas of the U. S. Virgin Islands not served by the public sewer system. Conventional . septic tank/ seepage pit systems are proving to be inadequate due to the shallow soils, steep slopes and also the increasing numbers of these systems. Proper disposal of sewage is critical in the tourism based Virgin Islands economy ‘not only because of public health concerns but also due to the adverse effects of untreated sewage on the pristine marine waters which are a source of both recreation and food. The Water Resources Research Institute at the University of the Virgin Islands and the Virgin Islands Department of. Planning and Natural Resources have investigated alternative methods of treating sanitary waste for individual residences. Pilot systems installed have proven to be effective and have the potential for widespread application in areas where environmental constraints are similar to those in the U. S. Virgin Islands. KEY TERMS: onsite sewage disposal; nonpoint source; constructed wetlands; wastewater treatment; alternative OSDS. INTRODUCTION Increasing volumes of sanitary waste being produced and discharged from individual residences in the U. S. Virgin Islands, where public waste collection and treatment facilities are confined to limited areas, are a growing concern. Not only is improperly treated sewage unsightly and unsanitary, but much of it makes its way to the shorelines fostering conditions that are unsafe for- swimmers, injurious to marine life and aesthetically unappealing in the tourism based economy. Septic tank/soil absorption systems as used in the USVI consist of a buried tank that discharges into a leach field or seepage pit. The tank is sized to allow for detention of the wastewater for a period of two to three days. Effluent from the septic tank discharges to the seepage area. This area often consists of a network of perforated pipes discharging in to soils but more commonly because of the steep ._ Slopes and limited soil is a -rock-filled hole in the ground. i C2ZM Program Specialist ~- Nonpoint Source Pollution Coordinator, DPNR, Government et the Virgin Islands, St. Thomas, VI, 00802, (Phone: 340/774-3320, Fax: 340/775- 2706, E-mail: jhodge@uvi.edu). ' President, W. F. McComb Engineering, 2Z5A Dronningens Gade, St. Thomas, VI, 00802, {Phone 340/774-8547, Fax: 340/776-1550, E-mail: wfmccomb.eng@worldnet.att.net) . Director, WRRI, University of the Virgin Islands, St. Thomas, VI, 00802-9990," {340/693-1020, Fax:: 340/693-1025, E-mail: hsmith@uvi.edu,). 135 One study has found that septic tank/soil absorption systems are the predominant means by which domestic waste from individual houséhold is discharged in the Usvt and that most of these systems frequently fail. ' Failure ef a septic system refers to the system failing to adequately accept wastewater prior to discharge to the environment. System failure is usually recognized when there is a presence of odors and/or visible discharge of effluent on the ground surface. In the USVI failure generally occurs due to inadequate sizing of the seepage its due to limited space, unsuitable soil characteristics and/or insufficient soil volume. It was found that the widely used septic tank/soil absorption systems are unsuitable for use in the Virgin Islands because. few areas have the two to three feet of pervious soil through which effluent from these systems should be treated prior to discharge to the environment. Because of unsuitable soils. and terrain, these systems very often fail resulting in harmful discharges that very often also make it to the nearshore marine environment. It is jeritiecal that effective and practical procedures are developed for onsite treatment of sanitary wastes in the Virgin Islands. THE STUDY SITES The U. S. Virgin Islands Government's Department of Planning and Natural Resources Management (DPNR) is responsible for controlling, regulating and monitoring water pollution. Under its Nonpoint Pollution Control. Program, DPNR received a grant through Section 308 of the Coastal Zone Management Act to investigate the use of alternative means of onsite disposal of domestic wastewater in the USVI. The Virgin Islands Water Resources Research Institute of the University of- jthe Virgin Islands (WRRI) was contracted by DPNR to conduct the monitoring program. Four systems were monitored as part of this investigation. Two were new systems designed and installed for this study and two were systems that were already in operation and from which it was felt that useful information could he gained. The systems are described below with the two main study sites, Estate Harmony and Cabrita Point, presented first and followed by descriptions of the two systems already in full operation prior to this study, Red Hook and Nazareth. Estate Harmony Site At the Estate Harmony site the residence served by the OSDS consists of a house that includes three bedrooms, two full baths, a fully equipped kitchen and laundry facilities. A. two-bedroom caretaker's house is also located on the property. A water. meter is installed to monitor water withdrawn from the cistern and used as an estimate of water treated by the OSDS. The system's design capacity is 1,200 gallons per day. A two stage secondary tank receives effluent from a traditional septic tank with a capacity of 2,000 gallons. The 600-gallon secondary tank serves to enhance the breakdown of solids which could clog trenches further on in the system. This secondary tank is split in to two sections with the first two-thirds of the volume partitioned off to store solids and provide anaerobic activity additional to that present in the septic tank. The remaining one-third of the tank isolates the solids and floating material from entering the remainder of the treatment system. gett ———— The secondary septic tank discharges its effluent into an 18-foot long primary filter with an 18-inch high central divider running the length of it. This divider serves to slow the passage of the effluent through the trench for the flow distance between the trench's inlet and outlet is almost twice the length of the trench, 32 feet. The effluent's residence time in the primary filter is increased. The primary filter consists of a twelve inch deep trench, for free flowing liquid, that is overlaid with a twelve inch bed of washed gravel. A metal grating covered with a durable permeable fabric serves as the separator. The gravel is covered with twelve inches of soil. The design results in the gravel bed. also being saturated with effluent. The roots of the vegetation growing in the soil then extend into the gravel bed and through to the free flowing liquid at the base of the primary filter. Discharge from the primary filter enters the two secondary filter trenches which also consist of concrete troughs and a central divider. In the troughs are lower layers of gravel 16. inches thick. This gravel is covered with eight inches of soil. The troughs are 12 feet, 6 inches long. The troughs’ outlets are placed so that the gravel remains saturated at all times and allows only free flowing effluent to leave the secondary filter troughs and enter the next troughs,, the hydroponics section. The hydroponics trough consists of a tank 17 feet long and four feet wide. It.is filled with two feet of topsoil in which vegetables. such as lettuce, tomatoes, egg plants and basil and other herbs are grown. Ornamental plants are grown in the primary and secondary filter troughs at this site. Provision has been made for excess water to be discharged from the ‘hydroponics ‘trough in to a gray water cistern. From the gray water cistern the effluent may be used in a drip irrigation system, recirculated to the first stage of the’ septic “tank (primary separator) or to a fish pond where fish such as tilapia may be -cultured. : “Seven monitoring ports were placed in the system and provisions were. made for ‘cleaning the system without disturbing the overlying, soil beds and vegetation by using drains placed in the bottom of each trough. Cabrita Point Site At the Cabrita Point site the residence served by the OSDS consists of two two- bedroom condominiums having a total area of 2,621 square feet. Water meters are -installed at each condominium to obtain estimates of wastewater entering the OSDS. The design capacity of the OSDS is 600 gallons per day. ‘Twenty four 150 gallon stock tanks, each approximately. 3' x 5' x 2' deep, are /connected in series following the existing septic tank. These tanks are piaced on -a downhill slope of 1/8" per foot from the septic tank and filled with a foot and a half of gravel topped with six inches of pea gravel. The final tank is fitted with *@ 30. foot length of 3/4 inch perforated PVC pipe for discharge of excess effluent “Which is expected to occur only under very unusual conditions for the system was “designed for all effluent to undergo evapotranspiration. Any discharge from the final tank is diffused by the perforated pipe into the surrounding earth. “Provision for overflow is important due to the possibility of the tanks being filled by rain or surface runoff during high rainfall events. “Effluent entering each tank fills the voids in each tank until it reaches a level evhere it is high enough to discharge through a port in to the next tank. The *3entle slope downward through the system insures that there is a gravity flow on to ethe-next tank downstream:as each tank is filled. Reeds and ornamental plants such canna lilies with high water uptake are planted in each tank. r E 137 a ee : eee Ee . 5 Sampling ports are placed in the lst, 8th, 16th and 24th tanks for monitoring of the effluent'*s quality. Red Hook Site At the Red Hook site a packaged wastewater treatment plant was monitored. The installed system has a design capacity of 600 gallons and serves a two bedroom residence lived in by three working adults on a year round basis. An aerator in the bottom of the unit's single tank keeps the wastewater completely mixed and aerated at all times. The system is fitted with a pump tc supply pressurized gray water for landscape purposes. Samples in this study were taken from a faucet that was part of this gray water supply system. Nazareth Site The system at the Nazareth site has been in operation for about two years and serves a residence and small architect's office that is in use year round. The system is planted with reeds, ginger lilies, canna lilies and elephant ears. Like those at the Harmony and Cabrita Point sites, this system uses a conventional septic tank for primary. removal of solids. This site is particularly similar to the Cabrita Point site except that two long trenches rather than stock tanks are used for treatment of effluent after it leaves the septic tank. These trenches are each’ about 30 feet long, four feet wide and three feet deep: They are lined with polyethylene (plastic) to keep the effluent contained in the system, filled with pea gravel, and are installed’ parallel to each other on the natural contour of the slope where they are located. The system is gravity fed with excess water from the second trench being pumped by a solar powered pump to a lily pond. Three sampling ports are located in. the system and samples.could also be taken at the lily pond. THE MONITORING PROGRAM Effluent samples, water meter readings and rainfali levels were taken on a monthly | basis and observations made by residents at the site noted. Occasionally, the sites were visited on a weekly basis to assess their progress. Samples were analyzed at the University of Virgin Islands by qualified personnel using approved methods for the examination of wastewater detailed in. Standard Methods for the Examination of Water and Wastewater (APHA,1992). Parameters monitored wete coliform, — dissolved oxygen, total suspended solids, BOD, phosphorous, nitrogen as ammonia and Kjeldahl nitrogen. During the study period, Hurricanes Luis and Marilyn hit St. Thomas in September L995). Though the systems were all installed prior to the hurricanes and some preliminary monitoring done, monitoring was done on a continuous basis for six months. beginning in January 1996. Electrical power interruptions and general disruptions at the study sites had stabilized by January 1996. Occupancy, though, was never at the level for which the systems were desigried. FINDINGS None of the systems experienced any problems that prevented their operation during the study period or required extensive maintenance. . Occasionally, there was insufficient effluent at some sampling ports and samples: could not be taken. This could be reflective of high uptake by the plants but more probably related to low effluent inflow to the OSDS. Water usage rates at the monitored sites were not as high as anticipated. This undoubtedly was due to the unusual circumstances caused by the hurricanes. A summary of the findings for parameters monitored: at the two principal site appears in Table 1. Table 1: Summary of Observed Parameters Parameter Cabrita Harmony Parameter Cabrita Harmony | Coliform " Dissolved 0, 1090 170 (mg/L) 10.3 9/9 col/100m1) 99 88 % change 66 56 % change TSS BOD (mg/L) 60.1 19.5 (mg/L) 6.3 4.0 % change 51 83 % change 23 14 Phosphorus N as ammonia (mg/L) 0.8 4.5 (mg/L) 4.8 44.5 % change 89 40 % change 92 18 Kjeldahl-N (mg/L) 7.8 30.0 % change 89 44 CONCLUSION The biological systems investigated in this study eliminated all the effluent that they were provided with which is particularly significant for then there is no need to be concerned with the quality of the effluent they were discharging to the environment. The mechanical. system operated with no maintenance problems and Produced an effluent that was within, existing standards but nevertheless needed to be disposed. ACKNOWLEDGEMENTS This study was funded partly by a grant from the National Oceanic and Atmospheric Administration under provisions of Section 308 of the Coastal Zone Management Act of. 1972 (Public Law 92-583) and. also financed in part by the. Department -of -the interior, U. S. Géological’ Survey, through the Virgin -Islands Water Resources 139 ae cecenneraeamomeee: 6S eee wey Se eS ee reece NE PC hy me Ot Research Institute. The contents of = pubiiesel on “do: not necessarily reflect the views and policies of the Department of the Interior, nor does mention of trade names. or commercial products constitute their endorsement b Government. y .the United States REFERENCES American Public Health Association, 1992, Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC. 140