Data Series 301
U.S. GEOLOGICAL SURVEY
Data Series 301
Ground-water quality in the approximately 1,800-square-mile Southern Sierra study unit (SOSA) was investigated in June 2006 as part of the Statewide Basin Assessment Project of Groundwater Ambient Monitoring and Assessment (GAMA) Program. The California State Water Resources Control Board (SWRCB), in collaboration with the U.S. Geological Survey (USGS) and the Lawrence Livermore National Laboratory, is implementing the GAMA Program (http://www.waterboards.ca.gov/gama/). The Statewide Basin Assessment Project was designed by the SWRCB and the USGS in response to the Groundwater Quality Monitoring Act of 2001 (Belitz and others, 2003; State Water Resources Control Board, 2003). The project is a comprehensive assessment of statewide ground-water quality designed to identify and characterize risks to ground-water resources, and to increase the availability of information about ground-water quality to the public. SOSA was the eleventh study unit sampled as part of the project.
SOSA is in the southern portion of the Sierra Nevada hydrogeologic province and includes within it six small ground-water basins defined by the California Department of Water Resources (California Department of Water Resources, 2003). The SOSA study included assessment of the ground-water quality from fifty wells in Kern and Tulare Counties. Thirty-five of the wells were selected using a randomized grid approach to achieve statistically unbiased representation of ground water used for public drinking-water supplies. Fifteen of the wells were selected to provide additional sampling density to aid in understanding processes affecting ground-water quality. Ground-water samples were analyzed for VOCs, pesticides and pesticide degradates, pharmaceutical compounds, wastewater-indicator compounds, nutrients, major and minor ions, trace elements, radioactivity, and microbial indicators. Naturally occurring isotopes (stable isotopes of hydrogen, oxygen, and carbon, and activities of tritium and carbon-14) and dissolved noble gases also were measured to provide a data set that will be used to help interpret the source and age of the sampled ground water. This report describes the hydrogeologic setting of the SOSA region, details the sampling, analytical, and quality assurance used in the study, and presents the results of the chemical and microbial analyses made of the ground-water samples collected during June 2006.
Quality-control samples (blanks, replicates, and samples for matrix spikes) were collected at 10 to 18 percent of the wells, and the results for these samples were used to evaluate the quality of the data for the ground-water samples. Assessment of the quality-control information resulted in censoring of less than 0.2 percent of the ground-water quality data.
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 blended with other waters to maintain acceptable water quality. Regulatory thresholds apply to treated water that is served to the consumer, not to raw ground water. However, to provide some context for the results, concentrations of constituents measured in the raw ground water were compared with health-based thresholds established by the U.S. Environmental Protection Agency (USEPA) and California Department of Public Health (CDPH).
All detections of VOCs and pesticides were below health-based thresholds, and most were less than one one-hundredth of the threshold values. All detections of perchlorate, nitrate, and radioactive constituents were below established thresholds. Only one constituent, arsenic, was detected above a maximum contaminant level (MCL-US), although radon-222 was detected above the proposed MCL-US. Boron and 1,2,3-TCP each had one detection above the CDPH notification level (NL-CA). Total dissolved solids, specific conductance, pH, iron, and manganese were detected at concentrations above secondary maximum contaminant levels (SMCL-CAs), non-enforceable thresholds set for aesthetic concerns, in samples from several of the wells. Future work will interpret the data presented in this report using a variety of statistical, qualitative, and quantitative approaches to assess the natural and human factors affecting ground-water quality.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/ds/301
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