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 (USGS, 2018). 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). 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. These grid cells can be found in the GAMA_PBP_SAA_GridCells shapefile included in this Data Release. Version Notes: This Data Release supersedes the data release published in 2020 (version 1). This data release was updated to include study units sampled since the initial publication. Also, report references for all version 1 study units have been updated to reflect the most recent publications for each study unit. In addition, study unit names were updated to be listed as the full name instead of the study unit abbreviations used in version 1. Minor revisions were also made to the metadata text. The version 1 data release may be obtained by contacting the dataset Point of Contact.