The U.S. Geological Survey collected groundwater samples from 67 domestic wells and 1 spring used for domestic supply in Placer, El Dorado, Amador, and Calaveras Counties, California in 2016-2017. The sites were sampled for the Mokelumne, Cosumnes, and American River Watersheds (MCAW) Shallow Aquifer Study Unit 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 MCAW study unit covered the parts of the Upper Mokelumne, Upper Cosumnes, and South and North Fork American River watersheds [8-digit hydrologic unit codes (HUC8) 18040012, 18040013, 18020129, and 18020128] in the Sierra Nevada hydrogeologic province. The study unit was divided into equal area grid cells and one domestic well or spring was sampled in each cell.