Because of concerns over ground-water quality, the California State Water Resources Control Board (SWRCB), in collaboration with the U.S. Geological Survey and Lawrence Livermore National Laboratory, has implemented the Ground-Water Ambient Monitoring and Assessment (GAMA) Program. A primary objective of the program is to provide a current assessment of ground-water quality in areas where public supply wells are an important source of drinking water. The San Diego GAMA study unit was the first region of the state where an assessment of ground-water quality was implemented under the GAMA program. The San Diego GAMA study unit covers the entire San Diego Drainages hydrogeologic province, and is broken down into four distinct hydrogeologic study areas: the Temecula Valley study area, the Warner Valley study area, the Alluvial Basins study area, and the Hard Rock study area.

A total of 58 ground-water samples were collected from public supply wells in the San Diego GAMA study unit: 19 wells were sampled in the Temecula Valley study area, 9 in the Warner Valley study area, 17 in the Alluvial Basins study area, and 13 in the Hard Rock study area. Over 350 chemical and microbial constituents and water-quality indicators were analyzed for in this study. However, only select wells were measured for all constituents and water-quality indicators. Results of analyses were calculated as detection frequencies by constituent classification and by individual constituents for the entire San Diego GAMA study unit and for the individual study areas. Additionally, concentrations of constituents that are routinely monitored were compared to maximum contaminant levels (MCL) and secondary maximum contaminant levels (SMCL). Concentrations of constituents classified as "unregulated chemicals for which monitoring is required" (UCMR) were compared to the "detection level for the purposes of reporting" (DLR).

Eighteen of the 88 volatile organic compounds (VOCs) and gasoline oxygenates analyzed for were detected in ground-water samples. Twenty-eight wells sampled in the San Diego GAMA study had at least a single detection of VOCs or gasoline oxygenates. These constituents were most frequently detected in the Alluvial Basin study area (11 of 17 wells), and least frequently detected in the Warner Valley study area (one of nine wells). Trihalomethanes (THMs) were the most frequently detected class of VOCs (18 of 58 wells). The most frequently detected VOCs were chloroform (18 of 58 wells), bromodichloromethane (8 of 58 wells), and methyl tert-butyl ether (MTBE) (7 of 58 wells). Three VOCs were detected at concentrations greater than their MCLs. Tetrachloroethylene (PCE) and trichloroethylene (TCE) were detected in one well in the Hard Rock study area at concentrations of 9.75 and 7.27 micrograms per liter (µg/L), respectively; the MCL for these compounds is 5 µg/L. MTBE was detected in one well in the Alluvial Basins study area at a concentration of 28.3 µg/L; the MCL for MTBE is 13 µg/L.

Twenty-one of the 122 pesticides and pesticide degradates analyzed for were detected in ground-water samples. Pesticide or pesticide degradates were detected in 33 of 58 wells sampled, and were most frequently detected in the Temecula Valley study area wells (9 of 14 wells), and least frequently in the Warner Valley study area wells (3 of 9 wells). Herbicides were the most frequently detected class of pesticides (31 of 58 wells), and simazine was the most frequently detected compound (27 of 58 wells), followed by deethylatrazine (14 of 58 wells), prometon (10 of 58 wells), and atrazine (9 of 58 wells). None of the pesticides detected in ground-water samples had concentrations that exceeded MCLs.

Eight waste-water indicator compounds were detected in ground-water samples. Twenty-one of 47 wells sampled for waste-water indicator compounds had at least a single detection. Waste-water indicator compounds were detected most frequently in the Alluvial Basins study area (9 of 17 wells), and least frequently in the Temecula Valley study area (2 of 6 wells). Phenol was the most frequently detected waste-water indicator compound (14 of 47 wells). Perchlorate was detected in 14 of 50 wells in the San Diego GAMA study; it was most frequently detected in Temecula Valley study area (7 of 11 wells).

Nitrate was detected in 17 of 24 wells at concentrations (0.1 to 9.14 µg/L) less than the MCL of 10 µg/L. Total dissolved solids (TDS) concentrations (148 to 1,800 milligrams per liter, or mg/L) exceeded the recommended secondary maximum contaminant level (SMCL) of 500 mg/L in 12 of the 24 wells in which it was measured. The sample with the highest TDS also had concentrations of chloride (540 mg/L) and sulfate (421 mg/L) that exceeded recommended SMCL levels.

Arsenic and uranium were detected in all 24 wells where they were measured. Arsenic concentrations ranged from less than 0.5 µg/L to 7.8 µg/L, and uranium concentrations ranged from 0.06 µg/L to 17.8 µg/L. The MCLs for these compounds are 50 and 30 µg/L, respectively. The MCL for arsenic will be reduced to 10 µg/L in 2006. Total dissolved chromium (total chromium) was detected in 44 of 50 wells, and hexavalent chromium in 36 of 50 wells. Total chromium concentrations did not exceed 5.7 µg/L, which is well below the 50 µg/L MCL for total chromium. However, hexavalent chromium was detected in 7 of 50 wells at concentrations greater than the detection level for the purposes of reporting (DLR); the DLR for hexavalent chromium is 1 µg/L. Boron was detected at concentrations (6 to 1,054 µg/L) greater than its DLR in 15 of 24 wells, and vanadium was detected at concentrations (1 to 69 µg/L) greater than its DLR in 14 of 24 wells. Concentrations of iron (4 to 2,120 µg/L) and manganese (0.2 to 492 µg/L) exceeded the SMCL in two and four wells, respectively. Mercury was not detected in any samples.

Radon-222 was detected in every ground-water sample in which it was analyzed for, with activities ranging from 180 to 4,820 picoCuries per liter (pCi/L). The proposed MCL and alternative MCL for radon-222 is 300 and 4,000 pCi/L, respectively. Tritium was detected in nearly all samples; the highest activity detected was 23.7 pCi/L, which is well below the tritium MCL of 20,000 pCi/L. Radium-226 and radium-228 were detected together in all but four ground-water samples. Activities detected for radium isotopes in ground water were low; radium-228 activities were below quantification levels in all but one sample. The combined MCL of 5 pCi/L for radium-226 and radium-228 was not exceeded.

Microbial constituents were analyzed in 24 ground-water samples. Total and Escherichia coliform were not detected in any samples. F-specific coliphage was detected in one sample collected in the Alluvial Basins study area. Somatic coliphage was detected in two ground-water samples, one in the Temecula Valley study area and one in the Warner Valley study area.