Hydrogeology and Ground-Water Resources of Kahlap Island, Mwoakilloa Atoll, State of Pohnpei, Federated States of Micronesia
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HYDROGEOLOGY AND GROUND-WATER RESOURCES OF KAHLAP ISLAND, MWOAKILLOA ATOLL, STATE OF POHNPEI, FEDERATED STATES OF MICRONESIA U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 91-4184 Prepared in cooperation with the STATE OF POHNPEI, DEPARTMENT OF CONSERVATION AND RESOURCE SURVEILLANCE HYDROGEOLOGY AND GROUND-WATER RESOURCES OF KAHLAP ISLAND, MWOAKILLOA ATOLL, STATE OF POHNPEI, FEDERATED STATES OF MICRONESIA U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 91-4184 Prepared in cooperation with the STATE OF POHNPEI, DEPARTMENT OF CONSERVATION AND RESOURCE SURVEILLANCE U.S. DEPARTMENT OF THE INTERIOR BRUCE BABBITT, Secretary U.S. GEOLOGICAL SURVEY Gordon P. Eaton, Director Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government For sale by the U.S. Geological Survey Branch of Information Services Box 25286 Denver, CO 80225-0286 For additional information write to: District Chief U.S. Geological Survey 677 Ala Moana Blvd., Suite 415 Honolulu, HI 96813 CONTENTS Abstract ..................................................................................... 1 Introduction .................................................................................. 1 Purpose and Scope ...................................................................... 1 Background............................................................................ 4 Acknowledgments ...................................................................... 4 Methods of Study.............................................................................. 4 Reconnaissance......................................................................... 5 Installation of Driven-Well Network ........................................................ 5 Surface Geophysical Survey............................................................... 5 Measurement of Rainfall ................................................................. 7 Collection of Water Samples .............................................................. 7 Measurement of Water Levels ............................................................. 7 Slug (Bail) Tests ........................................................................ 7 Hydrogeology................................................................................. 8 General Atoll Features ................................................................... 8 Hydrogeologic Units..................................................................... 8 Conceptual Ground-Water Flow Model..................................................... 11 Hydraulic Characteristics ................................................................ 11 Ground-Water Resources....................................................................... 14 Recharge............................................................................. 14 Occurrence ........................................................................... 17 Storage .............................................................................. 23 Quality .............................................................................. 29 Demand.............................................................................. 30 Sustainable Yield ...................................................................... 30 Development Alternatives ............................................................... 31 Need for additional data ................................................................. 32 Summary ................................................................................... 32 References Cited ............................................................................. 35 FIGURES 1-3. Maps showing: 1. Location of Mwoakilloa Atoll, State of Pohnpei, Federated States of Micronesia ................ 2 2. Mwoakilloa Atoll and location of study area............................................. 3 3. Location of dug wells, driven wells, and lines of section, Kahlap Island. Mwoakilloa Atoll........ 6 4-5. Drawings showing: 4. Schematic cut-away view of an atoll................................................... 9 5. Conceptual hydrogeologic framework of an atoll island................................... 10 6. Drawing showing schematic section of freshwater lens with transition zone, and graph showing relation of salinity and depth ............................................................ 12 7-10. Graphs showing: 7. Relation between A, tidal efficiency and B, tidal lag with distance from the nearest shore for selected driven wells, Kahlap Island, Mwoakilloa Atoll........................... 13 Contents ill 8. Variation in tidal-efficiency with depth for selected driven wells, Kahlap Island, Mwoakilloa Atoll................................................................. 15 9. Variation in hydraulic conductivity with depth for selected driven wells, Kahlap Island, Mwoakilloa Atoll................................................................. 16 10. Mean monthly rainfall, based on intermittent monthly totals between 1962 and 1977, and continuous monthly totals from July 1988 through May 1990, Kahlap Island, Mwoakilloa Atoll................................................................. 18 11-12. Maps showing: 11. Mean annual rainfall in the western Pacific ............................................. 19 12. Chloride concentration of ground water at the water table, November 1989, Kahlap Island, Mwoakilloa Atoll........................................................... 20 13. Graph showing variation in chloride-concentration' with depth at driven-well site B, Kahlap Island, Mwoakilloa Atoll............................................................... 21 14-15. Diagrams showing: 14. Hydrologic section through the Kahlap freshwater lens, November 1989, Kahlap Island, Mwoakilloa Atoll................................................................. 22 15. Hydrologic and geophysical sections through the Kahlap freshwater lens, August 1989, Kahlap Island, Mwoakilloa Atoll..................................................... 24 16-17. Maps showing: 16. Estimated depth to electromagnetic-interpreted interface and thickness of potable fresh water, Kahlap Island, Mwoakilloa Atoll ............................................... 25 17. Estimated depth to base of freshwater nucleus, Kahlap Island, Mwoakilloa Atoll............... 26 18. Graphs showing variation in relative salinity with depth, Kahlap Island, Mwoakilloa Atoll: a. Driven-well site A................................................................ 27 b. Driven-well site B ................................................................ 27 c. Driven-well site E ................................................................ 27 d. Driven-well site F ................................................................ 28 e. Driven-well site H................................................................ 28 f. Driven-well site I................................................................. 28 19. Graph showing monthly rainfall totals for July 1988 through April 1990, Kahlap Island, Mwoakilloa Atoll..................................................................... 29 20-21. Diagrams showing schematic of: 20. Vertical-tube well................................................................. 33 21. Horizontal-infiltration well.......................................................... 34 TABLES 1. Driven-well construction information, Kahlap Island, Mwoakilloa Atoll.............................. 5 2. Temperature, specific conductance, and chloride-concentration data from dug wells, Kahlap Island, Mwoakilloa Atoll.................................................................. 37 3. Temperature, specific conductance, and chloride-concentration data from driven wells, Kahlap Island, Mwoakilloa Atoll.................................................................. 40 4. Water-quality data from selected dug and driven wells collected November 1989, Kahlap Island, Mwoakilloa Atoll........................................................................ 44 5. Tidal response data, Kahlap Island, Mwoakilloa Atoll ........................................... 14 6. Monthly and annual rainfall data, Kahlap Island, Mwoakilloa Atoll................................. 18 7. Nitrate concentrations in water from selected dug wells, Kahlap Island, Mwoakilloa Atoll .............. 30 8. Relation between reduction in recharge and thickness of freshwater lens, Kahlap Island, Mwoakilloa Atoll........................................................................ 31 iv Contents CONVERSION FACTORS AND ABBREVIATIONS Multiply By To obtain acre 4,047 square meter foot (ft) 0.3048 meter foot per day (ft/d) 0.3048 meter per day gallon (gal) 3.785 liter gallon per minute (gal/min) 0.06308 cubic decimeter per minute gallon per day (gal/d) 0.003785 cubic meter per day inch (in.) 25.4 millimeter inch per year (in/yr) 2.54 centimeter per year mile (mi) 1.609 kilometer million gallons (Mgal) 3 735 cubic meter million gallons per day (Mgal/d) 0.04381 cubic meter per second million gallons per day per square mile [(Mgal/d)/mi2] \ 450 cubic meter per day per square kilometer ounce (oz) 28.35 gram square foot (ft) 0.09294 square meter square mile (mi) 2.590 square kilometer______________ Abbreviations used: g/cm3, grams per cubic centimeter; g/d, grams per day; |Limho/m, micromhos per meter; meq/L, milliequivalents per liter; mg/L, milligrams per liter; mS/m, millisiemens per