The U.S. Geological Survey collected groundwater samples from 38 wells used for domestic and small system drinking water supplies in the south-central Sierra Nevada region, California in 2023 and 2024. The wells were sampled for the Western Slope of the Sierra Nevada domestic-supply aquifer study unit of the California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project's (GAMA-PBP) assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. The North High study area comprises the Eastern San Joaquin Highlands, Modesto Highlands, Turlock Highlands, and Merced Highlands groundwater units along the western slopes of the Sierra Nevada range, and the Mad King study area comprises the Yosemite Valley subbasin, Yosemite Highlands, Madera Highlands, and Kings Highlands groundwater units of the western slopes of the south-central Sierra Nevada range. The study unit was divided into 40 grid cells and one domestic or small system well was sampled to represent 38 of the grid cells; the remaining 2 grid cells had no accessible wells. Groundwater levels were measured in 13 of 38 wells. Table 1 contains sample and site information. Groundwater samples were analyzed for field water-quality parameters, microbial indicators, volatile organic compounds, pesticides and pesticide degradants, nutrients, major ions and trace elements, hexavalent chromium, perchlorate, per- and polyfluoroalkyl substances (PFAS), stable isotopes, and groundwater age tracers. Results from these analyses are reported in Tables 2 through 8. Tables 9 and 10 report results from quality control samples. This data release contains two supplementary tables that define abbreviations (Table 11) and provide additional context for the information presented in Tables 1 through 10 (Table 12). A zip file is included containing geospatial data (study unit boundaries, grid cell boundaries, site locations) as shapefiles.
