Hydrogeology and Ground-Water Resources of Ngatik Island, Sapwuahfik Atoll, State of Pohnpei, Federated States of Micronesia
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HYDROGEOLOGY AND GROUND-WATER RESOURCES OF NGATIK ISLAND, SAPWUAHFIK ATOLL, STATE OF POHNPEI, FEDERATED STATES OF MICRONESIA U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 93-4117 Peina Ngatik Island Uataluk Island 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 ........................................................................... 3 Previous Investigations .................................................................. 4 Acknowledgments ...................................................................... 4 Methods of Study ............................................................................. 4 Installation of Driven-Well Network ........................................................ 4 Surface Geophysical Survey .............................................................. 4 Measurement of Rainfall ................................................................. 6 Collection of Water Samples .............................................................. 6 Measurement of Water Levels ............................................................. 8 Slug (Bail) Tests ........................................................................ 8 Hydrogeology ................................................................................ 8 General Atoll Features ................................................................... 8 Hydrogeologic Units ................................................................... 10 Conceptual Ground-Water Flow Model..................................................... 10 Hydraulic Characteristics ................................................................ 12 Ground-Water Resources ...................................................................... 16 Recharge ............................................................................ 16 Occurrence ........................................................................... 17 Storage .............................................................................. 21 Quality .............................................................................. 22 Demand ............................................................................. 27 Sustainable Yield ...................................................................... 28 Development Alternatives ............................................................... 29 Need for Additional Data ................................................................ 29 Summary ................................................................................... 29 References Cited ............................................................................. 32 FIGURES 1-3. Maps showing: 1. Location of Sapwuahfik Atoll, State of Pohnpei, Federated States of Micronesia ................ 2 2. Sapwuahfik Atoll and well location of study area ......................................... 3 3. Locations of dug wells, driven wells, and line of section, Ngatik Island, Sapwuahfik Atoll ........ 5 4 6. Drawings showing: 4. Schematic cut-away view of an atoll ................................................... 9 5. Conceptual hydrogeologic framework of an atoll island ................................... 11 6. Schematic section of freshwater lens with transition zone, and graph showing relation of salinity and depth................................................................. 12 7 9. Graphs showing: 7. Variation in tidal-efficiency with depth for selected driven-well sites, Ngatik Island, Sapwuahfik Atoll................................................................. 14 Contents iii 8. Variation in hydraulic conductivity with depth for selected driven wells, Ngatik Island, SapwuahfikAtoll................................................................. 15 9. Monthly rainfall totals for August 1990 through July 1991, Ngatik Island, Sapwuahfik Atoll...... 17 10-11. Maps showing: 10. Mean annual rainfall in the western Pacific ............................................. 18 11. Chloride concentration of ground water in dug wells, August 1, 1990, Ngatik Island, Sapwuahfik Atoll................................................................. 19 12. Graph showing variation in chloride concentration with depth at driven-well site E, Ngatik Island, Sapwuahfik Atoll...................................................................... 20 13. Diagram showing hydrologic and geophysical section through the Ngatik freshwater lens, November 1990, Ngatik Island, Sapwuahfik Atoll ........................................... 22 14-15. Maps showing: 14. Estimated depth to the electromagnetic-interpreted interface and thickness of potable freshwater, Ngatik Island, Sapwuahfik Atoll............................................ 23 15. Estimated depth to the base of the freshwater nucleus, Ngatik Island, Sapwuahfik Atoll ......... 24 16. Graphs showing variation in relative salinity with depth at: a. Driven-well site A................................................................. 25 b. Driven-well site C................................................................. 25 c. Driven-well site D ................................................................ 25 d. Driven-well site E. ................................................................ 26 e. Driven-well site F................................................................. 26 f. Driven-well site G ................................................................ 26 g. Driven-well site J ................................................................. 27 17-18. Schematic diagrams of: 17. Vertical-tube well................................................................. 30 18. Horizontal-infiltration well.......................................................... 31 TABLES 1. Driven-well construction information, Ngatik Island, Sapwuahfik Atoll.............................. 6 2. Daily and monthly rainfall data, Ngatik Island, Sapwuahfik Atoll................................... 7 3. Temperature, specific conductance, and chloride-concentration data from dug wells, Ngatik Island, Sapwuahfik Atoll.................................................................. 34 4. Temperature, specific conductance, and chloride-concentration data from driven wells, Ngatik Island, Sapwuahfik Atoll............................................................ 36 5. Water-quality data from selected dug and driven wells collected June 1991, Ngatik Island, Sapwuahfik Atoll........................................................................ 44 6. Tidal-response data, Ngatik Island, Sapwuahfik Atoll ........................................... 13 7. Relation between reduction in recharge and thickness of freshwater lens, Ngatik Island, Sapwuahfik Atoll........................................................................ 28 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) 3,785 cubic meter million gallons per day (Mgal/d) 0.04381 cubic meter per second million gallons per day per square mile [(Mgal/d)/mi2] 1,460 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 o Abbreviations used: g/cm , grams per cubic centimeter; g/d, grams per day; |imho/m, micromhos per meter; meq/L, milliequivalents per liter; mg/L, milligrams per liter; mS/m, millisiemens per meter. Specific conductance is given in microsiemens per centimeter (|iS/cm) at 25° Celsius, which is numerically equal to micromhos per centimeter (|imho/cm). Temperature is given in degrees Celsius (°C), which can be converted to degrees Fahrenheit (°F) by using the equation: 1.8x°C + 32 = °F Contents Hydrogeology and Ground-Water Resources of Ngatik Island, Sapwuahfik Atoll, State of Pohnpei, Federated States of Micronesia By Stephen S. Anthony Abstract The water-supply problem on Sapwuahfik was accentuated during a drought in 1983. Although rainfall The lens of fresh ground water on Ngatik was not recorded on Sapwuahfik during 1983, rainfall on the island of Pohnpei, 90 mi to the northeast, was Island contains about 509 million gallons of pota only 13 percent of normal for the period January ble water. Recharge to the freshwater lens is esti through May 1983 (van der Brug, 1986). The subnor mated to be 990,000 gallons per day on the basis of mal rainfall created a severe water shortage because an estimated mean