The U.S. Geological Survey collected groundwater samples from 49 wells used for domestic and small system drinking water supplies and 10 monitoring wells in San Bernardino County, California in 2018. The wells 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. Domestic and small-system wells commonly are screened at shallower depths than public-supply wells. The MOBS study unit covered the Upper, Middle, and Lower Mojave River groundwater basins, the El Mirage groundwater basin, and part of the Harper 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 (California Department of Water Resources, 2003; Johnson and Belitz, 2003). Wells in the MOBS study unit generally tap unconsolidated alluvial aquifers composed of sediment eroded from granitic and metamorphic rocks, with portions of aquifer material in the northeast part of the study unit eroded from volcanic rock. The MOBS study unit was divided into two study areas, the floodplain study area along the course of the Mojave River, and the regional study area covering the rest of the study unit. The study areas were further divided into equal area grid cells, with 35 grid cells of 85 square-kilometers each in the regional study area, and 15 grid cells of 29 square-kilometers in the floodplain study area. Domestic or small-system wells were sampled in 48 of the 50 grid cells. The 11 understanding wells (10 monitoring wells and 1 domestic well) were selected to provide additional data for evaluation of the effect of septic systems on groundwater resources used for domestic drinking water supply. Groundwater samples were analyzed for field water-quality parameters, volatile organic compounds, pesticides and pesticide degradates, nutrients, major ions and trace elements, hexavalent chromium, perchlorate, gross alpha and gross beta particle activities, noble gases, tritium, carbon-14 in dissolved inorganic carbon, stable isotopic ratios of water and dissolved nitrate, and microbial indicators. Groundwater levels were measured in nearly all of the sampled wells. All results are presented in this data release, except results for pesticide and pesticide degradates, sulfur hexafluoride, and noble gases.

These data support the following publication:
Groover, K.D., and Goldrath, D.A., 2019, Groundwater quality in shallow aquifers of the western Mojave Desert, California: U.S. Geological Survey Open-File Report, 4 p. (In Press)

References cited:
California Department of Water Resources, 2003, California’s Groundwater: California Department of Water Resources Bulletin 118, 246 p., http://www.water.ca.gov/groundwater/bulletin118/update_2003.cfm (accessed August 2018)
Johnson, T.D., and Belitz, K., 2003, Hydrogeologic Provinces for California based upon established groundwater basins and watershed polygons: U.S. Geological Survey Open-File Report 03-470, digital data, https://doi.org/10.3133/ofr03470