Data Series 258

Data Series 258

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The Monterey Bay and Salinas Valley GAMA study assessed the quality of ground water from 94 public-supply wells and 3 monitoring wells across the approximately 1,000 mi2 study unit. Ground-water samples were analyzed for volatile organic compounds (VOCs), pesticides, pesticide degradates, nutrients, major and minor ions, trace elements, microbial indicators, and noble gases. Naturally occurring isotopes (tritium, carbon-14, oxygen-18, deuterium, and helium-4) also were measured in these samples to provide a data set that will be used to help interpret the source and age of the sampled ground water. In total, 270 constituents and water-quality indicators were investigated for this study.

Results from 91 randomized wells, statistically representative of the study unit, show that only six constituents—alpha radioactivity, N-nitrosodimethylamine, 1,2,3-trichloropropane, nitrate, radon-222, and coliform bacteria—were detected at concentrations higher than those thresholds set for health-based regulatory purposes. Six constituents—total dissolved solids, hexavalent chromium, iron, manganese, molybdenum, and sulfate—were detected at concentrations above levels set for aesthetic concerns.

In this study, one-third of the randomized wells sampled had at least a single detection of a VOC or gasoline additive. Twenty-seven 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 samples include chloroform, a compound resulting from the chlorination of water, and tetrachloroethene, a common solvent.

Pesticides and pesticide degradates 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 samples included the herbicide simazine, and the pesticide degradate deethylatrazine.

Total dissolved solids (TDS), major and minor ions, nutrients, and trace-element samples were collected at 31 public-supply wells and 3 monitoring wells. The median TDS value was 467 mg/L; 16 samples had TDS concentrations above the recommended SMCL of 500 mg/L; and 4 samples had concentrations greater than 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 2 samples had concentrations above the regulatory threshold of 10 mg/L (as nitrogen). The median sulfate concentration in ground-water samples was 138 mg/L, and 5 samples had concentrations above the recommended SMCL of 250 mg/L, while one sample’s concentration was greater than the upper SMCL of 500 mg/L. Iron concentrations above the SMCL of 300 µg/L were measured in 3 samples, and manganese concentrations were above the SMCL of 50 µg/L in 8 samples. A molybdenum concentration above the HAL of 40 µg/L was measured in one sample, and chromium (VI) concentrations above the DLR of 1 µg/L were measured at 86 samples.

Radon-222 was detected in all 31 ground-water samples collected, with activities ranging from 170 to 1,610 pCi/L. Twenty-three radon samples were above the proposed threshold of 300 pCi/L. Alpha radiation was detected above the regulatory 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 mL to 110 colonies per 100 mL.

Future work will combine the data presented in this report with various statistical and qualitative approaches to identify the natural and human factors affecting ground-water quality, and to detect changes in ground-water quality.

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