The U.S. Geological Survey collected groundwater samples from 48 grid wells and 11 understanding wells screened in aquifers used for domestic supply in San Bernardino County, California, in 2018. The sites were sampled for the Mojave Basin (MOBS) 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 and small system drinking water supplies. The MOBS study unit covered the Upper, Middle, and Lower Mojave River groundwater basins, El Mirage groundwater basin, and part of the Harper Valley groundwater basin [California Department of Water Resources, CA DWR basin designations 6-42, 6-41, 6-40, 6-43, 6-47] in the Desert hydrogeologic province (Johnson and Belitz, 2003). Each study area within the study unit was divided into equal area grid cells and an attempt was made to sample one well per each grid cell.