Data Series 258
U.S. GEOLOGICAL SURVEY
Data Series 258
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Ground-water quality in the approximately 1,000-square-mile Monterey Bay and Salinas Valley study unit was investigated from July through October 2005 as part of the California Ground-Water Ambient Monitoring and Assessment (GAMA) program. The study was designed to provide a spatially unbiased assessment of raw ground-water quality, as well as a statistically consistent basis for comparing water quality throughout California. Samples were collected from 94 public-supply wells and 3 monitoring wells in Monterey, Santa Cruz, and San Luis Obispo Counties. Ninety-one of the public-supply wells sampled were selected to provide a spatially distributed, randomized monitoring network for statistical representation of the study area. Six wells were sampled to evaluate changes in water chemistry: three wells along a ground-water flow path were sampled to evaluate lateral changes, and three wells at discrete depths from land surface were sampled to evaluate changes in water chemistry with depth from land surface.
The ground-water samples were analyzed for volatile organic compounds (VOCs), pesticides, pesticide degradates, nutrients, major and minor ions, trace elements, radioactivity, microbial indicators, and dissolved noble gases (the last in collaboration with Lawrence Livermore National Laboratory). Naturally occurring isotopes (tritium, carbon-14, helium-4, and the isotopic composition of oxygen and hydrogen) also were measured to help identify the source and age of the sampled ground water. In total, 270 constituents and water-quality indicators were investigated for this study. This study did not attempt to evaluate the quality of water delivered to consumers; after withdrawal from the ground, water typically is treated, disinfected, and (or) blended with other waters to maintain water quality. In addition, regulatory thresholds apply to treated water that is served to the consumer, not to raw ground water.
In this study, only six constituents, alpha radioactivity, N-nitrosodimethylamine, 1,2,3-trichloropropane, nitrate, radon-222, and coliform bacteria were detected at concentrations higher than health-based regulatory thresholds. Six constituents, including total dissolved solids, hexavalent chromium, iron, manganese, molybdenum, and sulfate were detected at concentrations above levels set for aesthetic concerns.
One-third of the randomized wells sampled for the Monterey Bay and Salinas Valley GAMA study had at least a single detection of a VOC or gasoline additive. Twenty-eight of the 88 VOCs and gasoline additives investigated were found in ground-water samples; however, detected concentrations were one-third to one-sixty-thousandth of their respective regulatory thresholds. Compounds detected in 10 percent or more of the wells sampled include chloroform, a compound resulting from the chlorination of water, and tetrachloroethylene (PCE), a common solvent.
Pesticides and pesticide degradates also were detected in one-third of the ground-water samples collected; however, detected concentrations were one-thirtieth to one-fourteen-thousandth of their respective regulatory thresholds. Ten of the 122 pesticides and pesticide degradates investigated were found in ground-water samples. Compounds detected in 10 percent or more of the wells sampled include the herbicide simazine, and the pesticide degradate deethylatrazine.
Ground-water samples had a median total dissolved solids (TDS) concentration of 467 milligrams per liter (mg/L), and 16 of the 34 samples had TDS concentrations above the recommended secondary maximum contaminant level (SMCL—a threshold established for aesthetic qualities: taste, odor, and color) of 500 mg/L, while four samples had concentrations above the upper SMCL of 1,000 mg/L. Concentrations of nitrate plus nitrite ranged from 0.04 to 37.8 mg/L (as nitrogen), and two samples had concentrations above the health-based threshold for nitrate of 10 mg/L (as nitrogen). The median sulfate concentration in ground-water samples was 138 mg/L, and five samples had concentrations above the recommended SMCL of 250 mg/L, while only one sample had a concentration above the upper SMCL of 500 mg/L. Iron concentrations above the SMCL of 300 micrograms per liter (µg/L) were measured in three samples, and manganese concentrations were above the SMCL of 50 µg/L in eight samples. A molybdenum concentration above the Lifetime Health Advisory of 40 µg/L was measured in one sample, and hexavalent chromium (VI) concentrations above the detection level for the purpose of reporting (DLR) of 1 µg/L were measured in 86 samples.
Radon-222 was detected in all 31 ground-water samples collected, with activities ranging from 170 to 1,610 picocuries per liter (pCi/L). Twenty-three radon samples were above 300 pCi/L, a proposed health-based threshold. Alpha radiation was detected above the health-based threshold of 15 pCi/L in one sample.
Microbial constituents were analyzed in 30 ground-water samples. Coliform bacteria was detected in four samples. Counts ranged from an estimated 1 colony per 100 milliliter (mL) to 110 colonies per 100 mL. Thresholds for microbial constituents are based on recurring detection, and these constituents will be monitored during future sampling.
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