The U.S. Geological Survey collected groundwater samples from 537 domestic wells for pharmaceutical compounds between 2014 and 2021 from 9 different study units. These wells were sampled as part of the California 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 drinking water supply. The 9 study units are in parts of the San Joaquin Valley, Sacramento Valley, Sierra Nevada Foothills, and the Southern California Desert basins where domestic wells are extensively used for drinking water supply. The well selection for the study units were selected by using a grid-based approach to ensure a representative sampling of the extent of the area where the groundwater resource is being used.