The California Groundwater Ambient Monitoring and Assessment Program (GAMA) is a statewide assessment of groundwater quality designed to help better understand and identify risks to groundwater resources. GAMA is implemented by the California State Water Resources Control Board. The U.S. Geological Survey (USGS) is the technical lead for the Priority Basin Project (PBP), one of the components of the GAMA Program. Starting in 2012, GAMA began an assessment of water resources in domestic-supply (shallow) aquifers in California. These aquifers provide water for domestic and small community-supply wells, which are often drilled to shallower depths in the groundwater system than public-supply wells. Domestic-supply aquifers are of interest because shallow groundwater may respond more quickly and be more susceptible to contamination from human activities at the land surface, than the deeper aquifers (Shelton and Tejeda, 2024). To prioritize domestic-supply aquifers, California was divided into 938 groundwater units consisting of California Department of Water Resources (DWR) groundwater basins and highland areas outside of the basins defined by California Groundwater Units (Johnson and Belitz, 2014) or Hydrologic Units (HUC8) from the Watershed Boundary Dataset (USGS and USDA, 2013). The groundwater units were prioritized for sampling based on the number and density of households relying on domestic wells, water-use, and well-location information compiled from well-completion reports submitted to the DWR. The groundwater units were grouped into study units designed to facilitate comparison of groundwater quality between the shallow aquifer systems and the deep aquifer systems assessed by GAMA from 2004 to 2012 (Bennett, 2018). Some study units were further subdivided into study areas. The study units (and study areas when applicable) were divided into equal area polygons (cells) so that all cells within a given study unit (or study area) have an equal area. The study unit boundaries can be found in the GAMA_PBP_SAA_Boundaries shapefile included in this Data Release. The purpose of this publication is to collect and assemble all of the study area grid cells into a single GIS dataset. At the time of this publication, a total of 19 study units comprising of 40 study areas have been sampled. These study areas have been divided into a total of 1137 cells, with the number of cells per study area ranging from 10 to 50 (median 25), and the size of the cells ranging from 14 to 175 km2 (median 60 km2). Two of the Monterey Bay and Salinas Valley Domestic-Supply Aquifer study area cells were further subdivided into subcells to get a greater spatial density of data (Goldrath and others, 2016). Additional information about the design of individual study area grids can be found in the reports listed in the Report_Ref attribute descriptions. Grid cells created for the deep aquifer systems study can be found in Johnson and others, 2018. Version Notes: This Data Release supersedes the data release published in 2020 (version 1) and 2022 (version 2). This data release was updated to include study units sampled since version 2. Also, report references for all study units included in previous versions have been updated to reflect the most recent publications for each study unit. Minor revisions were also made to the metadata text. The version 1 and version 2 data releases may be obtained by contacting the dataset Point of Contact. Revision History First release: 12/28/2020 Revision 2: 11/2022 Revision 3: 7/2025