STATUS OF GROUND-WATER RESOURCES AT
U.S. NAVY SUPPORT FACILITY, DIEGO GARCIA:
SUMMARY OF HYDROLOGIC AND CLIMATIC DATA,
JANUARY 1993 THROUGH DECEMBER 1995 *
By Jill D. Torikai
U.S. GEOLOGICAL SURVEY
Open-File Report 96-183
Prepared in cooperation with the
U.S. DEPARTMENT OF THE NAVYNAVY SUPPORT FACILITY, DIEGO GARCIA
Honolulu, Hawaii 1996
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 Earth Science Information Center Open-File Reports Section Box 25286, MS 517 Denver Federal Center Denver, CO 80225
For additional information write to:District ChiefU.S. Geological Survey677 Ala Moana Blvd., Suite 415Honolulu, HI 96813
CONTENTSPage
Executive Summary................................................................................................... 1
IntroductionBackground.................................................................................................... 2Organization of Report.................................................................................. 2
Rainfall....................................................................................................................... 4
Ground-Water Withdrawal........................................................................................ 6
Chloride Concentration of Pumped Ground Water................................................... 8
Chloride Concentration of Ground Water in Monitoring Wells................................ 11
Fuel-Diversion Program at Air Operations................................................................ 14
Hydrologic-Data Section A. Maps of Production and Monitoring Wells atCantonment and Air Operations.................................................................... 17
Hydrologic-Data Section B. Graphs of Monthly Mean Ground-Water Withdrawal,January 1993 through December 1995.......................................................... 22
Hydrologic-Data Section C. Graphs of Chloride Concentration of PumpedWater, January 1993 through December 1995 .............................................. 33
References Cited........................................................................................................ 42
in
FIGURESPage
1. Map showing areas of ground-water production, Diego Garcia............................ 3
2. Graphs of monthly rainfall and monthly departure from mean monthly rainfall atAir Operations, Diego Garcia, January 1993 through December 1995......... 5
3. Graphs of monthly mean ground-water withdrawal island wide and in the ground-water production areas, Diego Garcia, January 1993 through December 1995.............................................................................................. 7
4. Graphs of chloride concentration of pumped water (sampled at weekly intervals) in the ground-water production areas, Diego Garcia, January 1993 through December 1995.......................................................... 9
5. Graphs of chloride concentration of ground water (sampled at monthlyintervals) in monitoring wells at site AW 16 at Cantonment, Diego Garcia, January 1993 through December 1995.......................................................... 12
6. Graphs of chloride concentration of ground water (sampled at monthlyintervals) in monitoring wells at site BW09 at Air Operations, Diego Garcia, January 1993 through December 1995.......................................................... 13
7. Graphs of monthly mean ground-water withdrawal and injection at wells AO-10 through AO-15 at Air Operations, Diego Garcia, January 1993 through December 1995................................................................................. 15
TABLEPage
1. Target and actual withdrawal and injection rates for hydraulic-diversionprogram.......................................................................................................... 14
iv
CONVERSION FACTORS AND ABBREVIATION
Multiply By To obtain
foot (ft)gallon (gal)
gallon per day (gal/d)million gallons per day (Mgal/d)
inch (in.) inch per year (in/yr)
0.30483.7853.7850.04381
25.425.4
meterliterliter per daycubic meter per seconimillimetermillimeter per year
Abbreviation used in water-quality descriptionsmg/L = milligrams per liter
STATUS OF GROUND-WATER RESOURCES AT
U.S. NAVY SUPPORT FACILITY, DIEGO GARCIA:
SUMMARY OF HYDROLOGIC AND CLIMATIC DATA,
JANUARY 1993 THROUGH DECEMBER 1995
EXECUTIVE SUMMARY
This report contains hydrologic and climatic data that describe the status of ground-water resources at U.S. Navy Support Facility, Diego Garcia. Data presented are from January 1993 through December 1995, although the report focuses on hydrologic events from October through December 1995 (fourth quarter of 1995).
1. RAINFALL Cumulative rainfall for October through December 1995 was about 41 inches, which is 32 percent more than the mean cumulative rainfall of about 31 inches for October through December. The period October through December is within the annual wet season. Mean cumulative rainfall is calculated for the fixed base period 1951-90.
2. GROUND-WATER WITHDRAWAL-Ground-water withdrawal during October through December 1995 averaged 931,000 gallons per day. Withdrawal for the same 3 months in 1994 averaged 902,900 gallons per day. Patterns of withdrawal during the fourth quarter of 1995 did not change significantly since 1993 at all five ground-water production areas.
3. CHLORIDE CONCENTRATION OF PUMPED GROUND WATER-At the end of December 1995, the chloride concentration of the composite water supply was 60 milligrams per liter, well below the 250 milligrams per liter secondary drinking-water standard established by the U.S. Environmental Protection Agency. Chloride concentrations of the composite water supply from October through December 1995 ranged between 28 and 67 milligrams per liter.
4. CHLORIDE CONCENTRATION OF GROUND WATER IN MONITORING WELLS- Chloride concentration of ground water in monitoring wells at Cantonment and Air Operatic ns continued to decrease during the fourth quarter of 1995, with water from the deepest monitoring wells decreasing in chloride concentration by as much as 2,000 milligrams per liter. This trend follows increases in chloride concentration during the first half of 1995.
5. FUEL-DIVERSION PROGRAM AT AIR OPERATIONS-A fuel leak at Air Operations caused the shutdown of ten wells in May 1991. Four of the wells resumed pumping for water- supply purposes in April 1992. The remaining six wells are being used to hydraulically divert fuel migration away from water-supply wells by recirculating about 150,000 gallons of water each day.
STATUS OF GROUND-WATER RESOURCES ATU.S. NAVY SUPPORT FACILITY, DIEGO GARCIA:
SUMMARY OF HYDROLOGIC AND CLIMATIC DATA,JANUARY 1993 THROUGH DECEMBER 1995
By Jill D. Torikai
INTRODUCTION
Background
Diego Garcia Atoll is a British Indian Ocean Territory and the site of a U.S. Navy Support Facility. The island's drinking-water supply is derived from ground water by a system of more than 100 shallow wells in five production areas (fig. 1). Water from the Cantonment and Air Operations areas combined account for about 99 percent of total island pumpage. The remainder is pumped for local use at Industrial Site South, Transmitter Site, and GEODSS Site. The system has been in operation since 1978 and has provided about 1 Mgal/d since 1984.
Long-term ground-water management has been facilitated by a cooperative agreement between the Navy Support Facility and the U.S. Geological Survey (USGS) since 1984. The Diego Garcia Long-Term Ground-Water Management Program involves data collection and analysis of daily rainfall, daily pumpage from individual wells, and chloride concentrations of water from all production and monitoring wells. The data are provided to the USGS by the Navy Support Facility, Public Works Department on Diego Garcia. The data are analyzed for hydrologic responses to pumping and climatic variability, and recent trends in the data are identified. This report is part of a series of quarterly data reports regarding Diego Garcia.
Organization of Report
This data summary contains hydrologic and climatic data that describe the status of ground-water resources at Navy Support Facility, Diego Garcia. Data presented are from January 1993 through December 1995. Data of primary relevance to the water supply are:
1. Rainfall2. Volume of ground water withdrawn at production wells3. Chloride concentration of pumped ground water4. Chloride concentration of ground water sampled from monitoring wells5. Volume of ground water injected at Air Operations
The following narrative highlights trends in the data for October through December 1995. Ground-water withdrawal and chloride concentrations of water from individual wells are presented in the "Hydrologic-Data Section." The data section contains the following:
A. Maps of production and monitoring wells at Cantonment and Air OperationsB. Graphs of monthly mean ground-water withdrawal, January 1993 through December
1995 C. Graphs of chloride concentration of pumped water, January 1993 through DecemHr
1995
72° 25' E 72° 30' E
0° ~
40° S .-
DIEGO GARCIA
LOCATION MAP
Figure 1. Areas of ground-water production, Diego Garcia.
RAINFALL
Background.--Rainfall data are available since 1951, and all mean rainfall values in this report are calculated for the fixed base period 1951-90. The mean annual rainfall at Diego Garcia is 105.78 in/yr. Rainfall varies considerably from month to month and from year to yQ,ar. A wet season occurs from about September through February, and a dry season occurs from about March through August.
Recent trends.-Cumulative rainfall for October through December 1995 was 40.77 inches, which is 32 percent more than the mean cumulative rainfall of 30.83 inches for October through December. For the same 3 months in 1993 and 1994, the cumulative rainfall was 25.81 inches and 54.60 inches, respectively. The total rainfall in 1995 was 111.55 inches, which is 5 percent above the mean annual rainfall of 105.78 inches. For comparison, in 1993 the total rainfall of 94.77 inches was 10 percent below the mean annual rainfall, while in 1994 the total rainfall of 131.17 inches was 24 percent above the mean.
Figure 2 shows graphs of recorded rainfall amounts and rainfall departures from mean monthly rainfall values that were averaged for the base period 1951-90. Periods of below average rainfall can be inferred from the graph when the departure from the mean monthly rainfall is less than zero. Rainfall for October through December 1995 was above the respective mean monthly rainfall for October, November, and December.
CO LLJXo
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Departure from mean monthly rainfall
JFMAMJJASOND
1994
-15JFMAMJJASOND
1995
Figure 2. Monthly rainfall and monthly departure from mean monthly rainfall at Ak Operations, Diego Garcia, January 1993 through December 1995.
GROUND-WATER WITHDRAWAL
Background. Withdrawal is measured by flow meters at all production wells and storage tanks in the water system, and is recorded daily. There are 102 production wells in 5 ground-water production areas, of which 80 wells are in the Cantonment area and 18 are in the Air Operations area (figs. Al, A2). The wells in the Cantonment area are further divided into sub-groups, and the measured ground-water withdrawals are reported as such in this summary.
Pumpage from the Cantonment area increased in 1991 when the area began supplying an extra demand due to decreased pumpage at Air Operations (Torikai, 1995). From May 1991 through April 1992, 10 wells at Air Operations were temporarily closed because of an underground fuel pipeline leak near those wells. Pumping resumed at four wells in the water- supply system in May 1992, but six Air Operations wells still do not contribute to the water supply because of their proximity to the fuel leak. Ground-water withdrawals from the other ground-water production areas have been held at fairly steady rates since 1992.
Recent trends. Figure 3 shows time-series graphs of monthly mean withdrawal islandwide and in each ground-water production area from January 1993 through December 1995. Total islandwide withdrawal increased from 902,900 gal/d during October through December 1994 to 931,000 gal/d during October through December 1995. Patterns of withdrawal during the fourth quarter of 1995 have not changed significantly since 1993 in all areas except at the GEODSS site. Withdrawals at the GEODSS site leveled-off since an increase in withdrawal of about 1,000 gal/d in the first half of 1995. Ground-water withdrawals at the Transmitter Site production area remained at less than 1,000 gal/d.
2,000
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Industrial Site South
JFMAMJJASOND
1993
JFMAMJJASOND
1994
JFMAMJJASOND
1995
Figure3 Monthly mean ground-water wiU.dn.wa! Mandwide and in «he ground-waKr production areas.
Diego Garcia. January 1993 through December 1995.
CHLORIDE CONCENTRATION OF PUMPED GROUND WATER
Background. In this report, chloride concentration is used as a quantitative measure of salinity. Chloride concentration in seawater at Diego Garcia is about 19,500 mg/L whereas a concentration of 250 mg/L is the secondary maximum contaminant level (SMCL) under secondary drinking-water standards (U.S. Environmental Protection Agency, 1991). Secondary standards are not enforceable limits, but instead establish goals for constituents that may affect the aesthetic qualities of drinking water, such as taste or color.
Water is sampled at weekly intervals at all wells and storage tanks in the production system. Water in the Air Operations tank is a blend of ground water withdrawn from wells in the Air Operations area only, whereas the Cantonment tank water is a mixture of ground water f-om wells in both the Air Operations and Cantonment areas. This composite tank transmits about 99 percent of total island pumpage. Thus, the chloride concentration of water sampled from the Cantonment tank is considered representative of the overall water supply.
Recent trends. The chloride concentration of the composite water supply at the end of December 1995 was 60 mg/L, well below the 250 mg/L secondary drinking-water standard. Chloride concentrations for the composite water supply ranged from 28 to 67 mg/L from October through December 1995 (fig. 4). Chloride concentrations of the composite water in the Cantonment tank have remained at less than 100 mg/L since 1990, whereas the chloride concentration of water sampled from the Air Operations tank has fluctuated between about 50 to 200 mg/L since 1992 (Torikai, 1995).
During the fourth quarter of 1995, chloride concentrations of water from four of the five ground-water production areas continued to level-off following increases recorded during the first half of 1995. Contrary to this trend was the increase in chloride concentration of about 100 mg/L at the Industrial Site area in December 1995. Chloride concentrations from individual wells during the fourth quarter also remained fairly steady relative to concentrations recorded at the end of the third quarter (figs. Cl through C4). The fourth quarter occurs during the annurl wet season.
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Figure 4. Chloride concentration of pumped water (sampled at weekly intervals) in the ground-water production areas, Diego Garcia, January 1993 through December 1995. Rainfall data are shown for comparison.
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CHLORIDE CONCENTRATION OF GROUND WATER IN MONITORING WELLS
Background. Ground-water chloride concentration is measured monthly at 35 monitoring-well sites (figs. A3, A4) to help estimate the thickness of the freshwater lenses. Each site comprises several wells, with each well having a short screened (open) interval that bottoms at a different depth. The deeper wells typically tap the freshwater-saltwater mixing zones that underlie the freshwater lenses.
Recent trends. Monitoring sites AW16 and BW09 were selected to show trends in ground-water chloride concentration at Cantonment and Air Operations areas, respectively. Figures 5 and 6 show time-series graphs of chloride concentration at three depths at the Cantonment and Air Operations sites, respectively, with rainfall data included in the figures for climatic reference.
Chloride concentrations of the water decreased at both sites during the period October through December 1995. The decreases were recorded at the three wells at each site, with the deepest well at each site showing the largest decrease in concentration. The chloride concentration at the deepest well at sites AW 16 and BW09 decreased about 2,000 and 1,000 mg/ L, respectively. These decreases in chloride concentration occurred during the annual wet season.
11
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JFMAMJJASOND 1995
Figure 5. Chloride concentration of ground water (sampled at monthly intervals) in monitoring wells at site AW 16 at Cantonment, Diego Garcia, January 1993 through December 1995. Rainfall data are shown for comparison.
12
FUEL-DIVERSION PROGRAM AT AIR OPERATIONS
Background. The normal pattern of ground-water withdrawal at Air Operations has been disrupted since May 1991 by a jet-fuel leak at the South ramp parking apron (fig. A2). Wells AO-10 through AO-15 were shut down from May 1991 to April 1992, but have since been used in the closed recirculation loop of withdrawal and injection. In April 1992, a program was initiated to hydraulically divert fuel away from the production wells. The fuel-diversion program consists of pumping about 150,000 gal/d of water from wells AO-14 and AO-15 anl directing this water through the common collection main to the wells nearest the leak (AO-10, 11, 12), where it is injected back into the aquifer. Well AO-13 has been used only intermittently since 1991.
Injection data for wells AO-10, 11, 12 from May 10, 1993 through December 1995 are actual water-meter readings. From April 1992 through early May 1993, meter readings of injection were not available, and daily injection at each of the three wells was estimated to be one-third of the total daily withdrawal from wells AO-13, 14, 15 which provided the injection- supply water. Monthly mean withdrawal and injection at wells AO-10 through AO-15 are shown in figure 7.
Recent /r£/wfe.-Withdrawal and injection rates for wells in the hydraulic-diversion program did not change significantly during the period October through December 1995. Target withdrawal and injection rates are listed in table 1 for wells AO-10 through AO-15. Daily mean withdrawal and injection rates for these wells are also shown.
Table 1. Target and actual withdrawal and injection rates for hydraulic-diversion program. [Injection is denoted by negative values; all values are in gallons per day.]
, ~ . . Daily mean rates, WeU Target rates _ , , iU J ,^ ' irir._October through December 1995
AO-IO -30,000 -25,800
AO-ll -50,000 -49,000
AO-12 -70,000 -64,200
AO-13 0 0
AO-14 70,000 68,400
AO-15 80,000 71,400
14
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HYDROLOGIC-DATA SECTION
TYPES OF DATA INCLUDED
A. Maps of production and monitoring wells at Cantonment and Air OperationsB. Graphs of monthly mean ground-water withdrawal, January 1993 through December
1995 C. Graphs of chloride concentration of pumped water, January 1993 through December
1995
DESCRIPTIONS OF PRINCIPAL PRODUCTION SOURCES AT CANTONMENT AND AIR OPERATIONS AREAS
Cantonment Area
1. Modules A, C through L - each module is a well field of five to eight vertical wells that are pumped to a common collection/transfer tank.
2. Module B is a horizontal well with a collection/transfer tank; has not pump^xl since August 1986.
3. Wells HI through H7 are horizontal wells.4. Quad wells are a well field of four vertical wells; originally, six wells drilled.5. Jungle wells are a well field of 11 vertical wells; have not pumped since
February 1987.
Air Operations Area
1. Wells AO-2 through AO-5 are vertical wells.2. Wells AO-6 through AO-9 are horizontal wells.3. Wells AO-10 through AO-12 are horizontal wells; currently receiving injection
water to divert fuel contaminants from a nearby leak. No samples are currently collected for chloride-concentration analysis.
4. Wells AO-13 through AO-15 are horizontal wells; AO-14 and AO-15 are currently pumping water to injection wells AO-10 through AO-12, while AO-13 is not pumping.
5. AO-16 through AO-19 are horizontal wells.
16
SECTION A
Maps of production and monitoring wells at Cantonment and Air Operations
17
72° 21'30"
7° 15'30"
7° 16'
7° 16'30"
tOQO M2,000i
72° 22'
3;tiOOl=£Ef'
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QUAD WELLS
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MODE
EXPLANATION
VERTICAL WELL AND WELL OR WELL FIELD DESIGNATION-Typical pumping rate 10 to 12 gallons per minute
HORIZONTAL WELL AND DESIGNATION-Typical pumping rate 50 to 75 gallons per minute
WELL MODULE AND DESIGNATION-Vertical wells that pump to a common 1,000-gallon collection and transfer tank
Figure A1. Ground-water production wells and well fields at Cantonment, Diego Garcia.
18
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AW21
EXPLANATION
MONITORING SITE AND DESIGNATION-Consisting of two or more monitoring wells with short (2 to 5 foot) open intervals of different depths
Figure A3. Monitoring wells at Cantonment, Diego Garcia.
20
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SECTION B
Graphs of monthly mean ground-water withdrawal, January 1993 through December 1995
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24
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FigureB2continued,-Monthly mean ground.waterwito^ Diego Garcia, January 1993 through December 1995.
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1995
Figure B6. Monthly mean ground-water withdrawal and injection at wells AO-2 through AO-19 at Air Operations, Diego Garcia, January 1993 through December 1995. Injection is plotted as negative.
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SECTION C
Graphs of chloride concentration of pumped water, January 1993 through December 1995
33
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Figure Cl. Chloride concentration of pumped water (sampled at weekly intervals) at Modules A throrgh L at Cantonment, Diego Garcia, January 1993 through December 1995. Data not available for November 1993 through September 1994.
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Figure C2. Chloride concentration of pumped water (sampled at weekly intervals) at wells HI through H7 at Cantonment, Diego Garcia, January 1993 through December 1995.
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REFERENCES CITED
Torikai, J.D., 1995, Status of ground-water resources at U.S. Navy Support Facility, Diego Garcia: Summary of hydrologic and climatic data through September 1993; U.S. Geological Survey Open-File Report 94-306, 51 p.
U.S. Environmental Protection Agency, 1991, Secondary maximum contaminant levels (section 143.3 of part 143, National secondary drinking water regulations): U.S. Code of Federal Regulations, Title 40, Parts 100 to 149, revised through July 1, 1991.
42
