The U.S. Geological Survey collected groundwater samples from 44 wells used for domestic and small system drinking water supplies in the western Central Valley, California in 2023. The wells were sampled for the Western Central Valley domestic-supply aquifer study unit of the State Water Resources Control Board Groundwater Ambient Monitoring and Assessment (GAMA) Program Priority Basin Project’s assessment of the quality of groundwater resources used for domestic and small system drinking water supplies. The study unit consists of two study areas. The western Sacramento Valley study area comprises the Corning, Colusa, Yolo, and Solano subbasins of the Sacramento Valley groundwater basin, and the western San Joaquin Valley study area comprises the East Contra Costa, Tracy, Delta-Mendota, and Westside subbasins of the San Joaquin Valley groundwater basin. The study unit was divided into 49 grid cells and one domestic or small system well was sampled to represent 44 of the grid cells; the remaining 5 grid cells has no accessible wells. Groundwater levels were measured in 28 of 44 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 zipfile is included containing geospatial data (study unit boundaries, grid cell boundaries, site locations) as shapefiles.
