GROUND-WATER RESOURCES OF THE LAURA AREA, MAJURO ATOLL, MARSHALL ISLANDS By Scott N. Hamlin and Stephen S. Anthony U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 87-4047 Prepared in cooperation with the REPUBLIC OF THE MARSHALL ISLANDS Honolulu, Hawaii 1987 UNITED STATES DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director For additional information Copies of this report write to: can be purchased from: District Chief, Hawaii District U.S. Geological Survey U.S. Geological Survey, WRD Books and Open-File Reports Section 300 Ala Moana Blvd., Rm. 6110 Federal Center, Bldg. 41 Honolulu, Hawai 96850 Box 25425 Denver, Colorado 80225 CONTENTS Page Abstract ------------------------------------------------------------ 1 Introduction -------------------------------------------------------- 2 Purpose and scope ---------------------------------------------- 5 Setting -------------------------------------------------------- 5 Previous investigations ---------------------------------------- 6 Acknowledgments ------------------------------------------------ 8 Methods of study ---------------------------------------------------- 8 Surface geophysical survey ------------------------------------- 8 Installation of driven-well network ---------------------------- 8 Test holes and collection of lithologic samples ---------------- 11 Collection of water samples ------------------------------------ 11 Measurements of water levels ----------------------------------- 13 Geohydrologic framework --------------------------------------------- 13 General atoll features ----------------------------------------- 13 Geology of Laura ----------------------------------------------- 14 Hydraulic characteristics of lithologic units ------------------ 16 Water budget --------------------------------------------------- 19 Ground-water resources ---------------------------------------------- 21 Occurrence ----------------------------------------------------- 21 Storage -------------------------------------------------------- 27 Quality -------------------------------------------------------- 27 Sustainable yield ---------------------------------------------- 32 Development alternatives --------------------------------------- 34 Monitoring program --------------------------------------------- 37 Summary ------------------------------------------------------------- 39 References ---------------------------------------------------------- 41 Appendix A. Water-level and chloride data from dug wells ----------- 45 Appendix B. Specific conductance and chloride data from dug wells, April 1984 ----------------------------- 46 Appendix C. Supplement data from test holes and driven wells ------- 48 Appendix D. Chemical analyses of water from wells ------------------ 68 iii ILLUSTRATIONS Figure Page 1. Map showing location of study area ------------------------ 3 2. Map showing Majuro atoll ---------------------------------- 4 3. Map of water-level and chloride contours, April 1973 ------ 7 4. Map showing locations of dug wells, well-point and test hole sites -------------------------------------- 9 5. Diagram of the hydrogeologic section of Laura showing lines of relative salinity ------------------- 12 6. Graphic lithologic logs of test holes and perforated intervals with tidal efficiencies -------------------- 15 7. Graph of tidal efficiency variation with depth in driven wells ----------------------------------------- 18 8. Graphs of rainfall and ground-water level change at Laura (well number 17) from October 1984 to September 1985 - 20 9. Diagram of the schematic section showing freshwater lens with transition zone --------------------------------- 22 10. Graphs of chloride and salinity profiles of the Laura freshwater lens at selected locations: a. Chloride-depth profile at F-test hole, June 1985 - 24 b. Comparison of test hole and driven-well salinity profiles, June 1985, F-site -------------------- 24 c. Comparison of test hole and driven-well salinity profiles, September 1985, E-site --------------- 24 d. Comparison of test hole and driven-well salinity profiles, September 1985, D-site --------------- 24 11. Diagrams of the hydrologic and geophysical sections and lens-thickness for the Laura freshwater lens: a. East-west hydrogeologic sections through lens based on geophysical data (April 1984) and salinity data (September 1984) ----------------- 25 b. North-south hydrogeologic sections through lens based on geophysical data (April 1984) and salinity data (September 1984) ----------------- 25 c. Contour of the depth of geoelectric interface ---- 26 12. Diagram showing hydrologic sections through the Laura lens for (500 milligram per liter isochlor) September 1984 and 1985 ------------------------------ 29 13. Map showing estimated depth to base of freshwater nucleus (500 milligram per liter chloride) ----------- 30 14. Graph showing relation between specific conductance and chloride concentration in ground water from Laura, Majuro atoll ----------------------------------------- 31 15. Schematic drawing of vertical dug well -------------------- 35 16. Schematic drawing showing plane view and vertical section of infiltration-type well -------------------- 36 iv TABLES Table Page 1. Well-construction information ----------------------------- 10 2. Characteristics of tidal fluctuation in ground water ------ 17 3. Lens storage and rainfall data at Laura village ----------- 28 4. Nitrate concentrations in water from dug wells on September 25, 1984 ----------------------------------- 33 5. Parameters and sampling frequency for Laura area monitoring CONVERSION FACTORS AND ABBREVIATIONS The following table may be used to convert the inch-pound units used in this report to metric units. Multiply inch-pound units To obtain metric units Temperature degree Fahrenheit (°F) °C = 5/9 x(°F-32) --- degree Celsius (°C) Length inch (in.) 25.4 ---------- millimeter (mm) foot (ft) 0.3048 ------- meter (m) mile (mi) 1.609 -------- kilometer (km) Area acre 4,047 ------ square meter (m 2) square foot (ft ) 0.09294 square meter (m 2) square mile (mi ) 2.590 -- square kilometer (km2) Volume acre-foot (acre-ft) -- 1,233 ------------ cubic meter (m3) gallon (gal) --------- 3.785 -------- liter (L) million gallons (Mgal) 3,785 ------------ cubic meter (m3) Volume Per Unit Time (includes Flow) gallon per minute ----------- 0.2070 ------- liter per second per foot [(gal/min)/ft] per meter [(L/s)/m] cubic foot per second (ft 3/s) 0.02832 ------ cubic meter per second (m 3/s) gallon per minute (gal/min) - 0.06308 ------ cubic decimeter per second (dm 3/s) million gallons per day 0.04381 ------ cubic meter per second (Mgal/d) (m 3/s) vi Ground-Water Resources of the Laura Area, Majuro Atoll, Marshall Islands By Scott N. Hamlin and Stephen S. Anthony ABSTRACT The present water system that supplies the heavily populated Dalap- Uliga-Darrit (DUD) area of Majuro atoll relies almost entirely upon airstrip catchment of rain water. Droughts cause severe water-supply problems and water rationing is required, even during periods of normal rainfall. The Laura area contains a substantial lens of fresh ground water that could be developed for export to the DUD area 30 miles to the east. Study of the ground-water resource at Laura involved a survey of existing wells, installation of monitoring wells and test holes, compilation of continuous records of rainfall and water-level fluctuations, and collection of water- quality data. Test hole data permitted the definition of three geohydrologic units, which correlate well with similar units in Bikini and Enewetak atolls. The units consist of two layers of unconsolidated reef and lagoon sediments resting, on a dense, highly permeable limestone. The potable water zone, or freshwater nucleus, of the lens is contained mostly within the unconsolidated layers, which are much less permeable than the basal limestone. Recharge to the Laura freshwater lens is estimated to be 1.8 million gallons per day, based on an average annual rainfall of 140 inches. Sustainable yield is estimated to be about 400,000 gallons per day. Shallow skimming wells or infiltration galleries similar to those used on Kwajalein atoll would be appropriate to develop the freshwater lens. The impact of development on the lens can be determined by monitoring the salinity in developed water and in a network of monitor wells. INTRODUCTION The demand for potable water on Majuro atoll in the Marshall Islands (fig. 1) is increasing as a result of a growing population and new commercial development. During extended dry periods water demand commonly exceeds supply. The water-supply problem on Majuro was accentuated during a drought in 1983 (Van der Brug, 1986). Rainfall was only 13 percent of normal for the period January through May 1983. The subnormal rainfall created a severe problem because nearly all of the water for the central water-supply system on Majuro comes from an airfield rainfall catchment system. Total storage of the catchment system is about 18 Mgal. The average daily demand in the area served by the system is about 450,000 gallons per day. The system cannot provide water continuously during extended dry periods. The volume of water in the system dropped to 7 Mgal on January 1 and 800,000 gallons by the end of May 1983. In addition to stricter rationing of the water supply, it was necessary to construct temporary shallow dug wells and install two saltwater conversion units to meet the water needs of the population. The return of