Status of ground-water resources at U.S. Navy Support Facility, Diego Garcia; summary of hydrologic and climatic data, 1991-93

This report contains hydrologic and climatic data that describe the status of ground-water resources at U.S. Navy Support Facility, Diego Garcia. Data are presented from January 1991 through December 1993. This report concentrates on data from fourth quarter 1993, and references historic data from 1991 and 1992. Total rainfall for 1993 was 95 inches which is 10 percent below the mean annual rainfall of 106 inches. In comparison, total rainfalls in 1992 and 1991 were 93 inches and 130 inches, respectively. Ground-water withdrawal has averaged 954,000 gallons per day during 1993, while with- drawals in 1992 and 1991 averaged 936,000 gallons per day and 927,000 gallons per day, respectively. In each of the five areas of ground-water produc- tion, withdrawals have remained steady since 1991. At the end of December 1993, the chloride concen- tration of the composite water supply was 36 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 during the last quarter (October through December 1993) ranged between 35 and 75 milligrams per liter. Chloride concentrations in monitoring wells at Cantonment and Air Operations decreased during the last quarter (October through December 1993) after having risen progressively during the previous quarter (July through September 1993). There has been a general trend of increasing chloride concentrations in the deeper monitoring wells since the 1992 dry season, which began in March 1992. A fuel spill 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 contain and divert fuel migration.