Effects of the 1998 Drought on the Freshwater Lens in the Laura Area, Majuro Atoll, Republic of the Marshall Islands
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Prepared in cooperation with the Majuro Water and Sewer Company, Majuro Atoll, Republic of the Marshall Islands Effects of the 1998 Drought on the Freshwater Lens in the Laura Area, Majuro Atoll, Republic of the Marshall Islands Scientific Investigations Report 2005–5098 U.S. Department of the Interior U.S. Geological Survey Cover: Aerial view, looking south, of the Laura area, Majuro Atoll, Republic of the Marshall Islands. (Photograph © by Andre P. Seale. Used with permission.) Effects of the 1998 Drought on the Freshwater Lens in the Laura Area, Majuro Atoll, Republic of the Marshall Islands By Todd K. Presley Prepared in cooperation with the Majuro Water and Sewer Company, Majuro Atoll, Republic of the Marshall Islands Scientific Investigations Report 2005–5098 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey P. Patrick Leahy, Acting Director U.S. Geological Survey, Reston, Virginia: 2005 For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225 For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ 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. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Presley, T.K., 2005, Effects of the 1998 drought on the freshwater lens in the Laura area, Majuro Atoll, Republic of the Marshall Islands: U.S. Geological Survey Scientific Investigations Report 2005-5098, 40 p. iii Contents Abstract ……………………………………………………………………………………… 1 Introduction …………………………………………………………………………………… 1 Purpose and Scope ……………………………………………………………………… 4 Description of Study Area …………………………………………………………………… 4 Land Use ………………………………………………………………………………… 4 Climate and Rainfall ……………………………………………………………………… 4 Geologic and Hydrologic Setting ……………………………………………………………… 10 Geology ………………………………………………………………………………… 10 Hydrology ……………………………………………………………………………… 10 Water Development …………………………………………………………………………… 13 Public and Private Catchment Systems ………………………………………………… 13 Reverse-Osmosis Water-Purification Systems ………………………………………… 13 Ground-Water Resources ……………………………………………………………… 13 Well Design in Laura ……………………………………………………………… 13 Ground-Water Withdrawal ………………………………………………………… 17 Water Supply During the 1998 Drought ………………………………………………… 18 Factors that Affect Thickness of the Freshwater Lens ……………………………………… 19 Determination of Thickness and Area of Freshwater Lens …………………………………… 20 Monitoring-Well Network and Sampling ………………………………………………… 20 Chloride-Concentration Data from Monitoring Wells …………………………………… 20 Interpolation of Freshwater-Lens Thickness …………………………………………… 20 Characteristics of the Freshwater Lens in the Laura Area …………………………………… 21 Lens Thickness During the 1998 Drought ………………………………………………… 21 Chloride Concentration of Water from the Water-Supply Wells ………………………… 30 Saltwater Intrusion Beneath Water-Supply Wells ……………………………………… 33 Comparison of 1998–99 to 1984–85 Data ………………………………………………… 34 Seasonal Variation ……………………………………………………………………… 36 Data Needs and Management Considerations ……………………………………………… 36 Summary and Conclusions …………………………………………………………………… 37 References Cited ……………………………………………………………………………… 39 iv Figures Figure 1. Map showing Western Pacific Ocean and Majuro Atoll, Republic of the Marshall Islands ………………………………………………………………… 2 Figure 2. Map showing monitoring-well and water-supply well sites, and cross-section locations for the Laura area, Majuro Atoll, Republic of the Marshall Islands …… 3 Figure 3. Graphs showing rainfall and rainfall-departures for data from the Majuro Weather Station Office, Dalap, Majuro Atoll, Republic of the Marshall Islands, 1954–2000 … 5 Figure 4. Graphs showing comparison between monthly rainfall for 1981–85 and 1995–99, and the mean monthly rainfall for 1955–99, from the Majuro Weather Station, Dalap, Majuro, Republic of the Marshall Islands ………………………………… 6 Figure 5. Graphs showing mean annual rainfall, departure from 3-month rainfall and number of months with rainfall less than 2 inches during dry seasons of drought years, Majuro Weather Station, Dalap, Majuro Atoll, Republic of the Marshall Islands …………………………………………………………………………… 7 Figure 6. Graphs showing five-month moving mean of departure from mean monthly rainfall for data from the Majuro Weather Station, Dalap, Majuro Atoll, Republic of the Marshall Islands, and the Southern Oscillation Index …………… 9 Figure 7. Cross sections showing atoll island freshwater lens, salinity structure and flow pattern, and freshwater lens at Laura, Majuro Atoll, Republic of the Marsall Islands …………………………………………………………………………… 11 Figure 8. Map showing extent of freshwater lens in 1984, location of hand-dug wells, and monitoring-well sites in the Laura area, Majuro Atoll, Republic of the Marshall Islands …………………………………………………………………………… 12 Figure 9. Schematic diagram showing water-supply well construction …………………… 15 Figure 10. Schematic diagram showing monitoring-well construction ……………………… 15 Figure 11. Graph showing pumpage from water-supply wells in 1992 and 1998, Laura area, Majuro Atoll, Republic of the Marshall Islands …………………………………… 17 Figure 12. Graphs showing monthly-mean pumpage during 1998 for water-supply wells (1-3 and 5-7), Laura area, Majuro Atoll, Republic of the Marshall Islands ………… 18 Figure 13. Graphs showing variation of relative salinity with depth for wells at monitoring-well sites, Laura area, Majuro Atoll, Republic of the Marshall Islands ………………………………………………………………… 22 Figure 14. Cross sections showing estimated thickness of the freshwater lens at the end of the 1998 drought, (on June 8, 1998, August 28, 1998, and January 14, 1999), Laura area, Majuro Atoll, Republic of the Marshall Islands ……………………… 25 Figure 15. Graphs showing chloride concentration of water pumped from water-supply wells, and chloride concentration of water sampled from the adjacent monitoring wells, Laura area, Majuro Atoll, Republic of the Marshall Islands, 1998 …………………………………………………………… 30 v Tables Table 1. Monitoring well depths, chloride concentrations, and relative salinity of samples collected from monitoring-well sites and monitoring wells at pump sites, Laura area, Majuro Atoll, Republic of the Marshall Islands ……………………… 16 Table 2. Monthly pumpage during 1998 for water-supply wells in the Laura area, Majuro Atoll, Republic of the Marshall Islands …………………………………… 17 Table 3. Depths of water with 500 milligrams per liter chloride concentration for monitoring-well sites of this study and for sites in Hamlin and Anthony (1987), and depth change between samplings, Laura area, Majuro Atoll, Republic of the Marshall Islands …………………………………… 35 Conversion Factors and Datums Conversion Factors Multiply By To obtain foot (ft) 0.3048 meter gallon (gal) 3.785 liter gallon per day (gal/d) 0.00004381 cubic decimeter per second inch (in.) 24.4 millimeter inch (in.) 2.54 centimeter inch per year (in/yr) 25.4 millimeter 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 square mile (mi2) 2.590 square kilometer square foot (ft2) 0.09294 square meter Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8. Specific conductance is given in microsiemens per centimeter m( S/cm) at 25° Celsius. Microsiemens per centimeter is numerically equal to micromhos per centimeter. Concentrations of chemical constituents in water are given in milligrams per liter (mg/L). Datums Vertical coordinate information was not available at the time of the study. Horizontal coordinate information: base map used was a navigational chart prepared by the Defense Mapping Agency, surveyed in 1944, 2nd edition printed 1985. Navigational chart was based on a “local datum,” established at the time of survey, and was not updated using WGS 72 for the 2nd edition. vi This page is intentionally blank. Effects of the 1998 Drought on the Freshwater Lens in the Laura Area, Majuro Atoll, Republic of the Marshall Islands By Todd K. Presley Abstract assumed small-scale variability in the thickness of fine and coarse calcareous sediments. This result suggests that a Lower than average rainfall during late 1997 and early monitoring program that uses the same wells through time is 1998 in Majuro Atoll, Republic of the Marshall Islands, needed to adequately describe long-term variability in lens caused a drought and severe drinking-water shortage. Majuro thickness. depends on a public rainfall catchment system, which uses an airport runway and storage reservoirs. The storage reservoirs can supply water for about 30 to 50 days without Introduction replenishment. In February 1998, after a few months with less than one inch of rainfall per month, a drought-related disaster Majuro Atoll, Republic of the Marshall Islands (RMI), is was declared. Reverse-osmosis water-purification systems in the west-central and equatorial Pacific Ocean near latitude were brought to Majuro to help alleviate the water shortage. 7° north and longitude 171° east (fig. 1), with a population of Concurrent with the water-purification program, ground about 27,000. Lower than average rainfall during late 1997 to water from a freshwater lens in the Laura area of the atoll early 1998