The U.S. Geological Survey collected groundwater samples from 33 wells used for domestic and small system drinking water supplies in Santa Clara and San Benito Counties, California in 2022. The wells were sampled for the Gilroy Hollister basin and surrounding areas 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 includes the Llagas and North San Benito subbasins of the Gilroy-Hollister Valley groundwater basin and surrounding areas. The study unit was divided into 40 grid cells and one or two domestic or small system well was sampled to represent 28 of the grid cells; the remaining 12 grid cells has no accessible wells. Groundwater levels were measured in 17 of 33 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.

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