The Mojave Basin Shallow Aquifer study unit (Mojave) covers approximately 4,680 square kilometers (2,908 square miles) in the western part of the Mojave Desert in San Bernardino County. The study unit consists of groundwater basins along the Mojave River, the El Mirage Valley groundwater basin, and part of the Harper Valley groundwater basin. The Mojave study unit was divided into two study areas—the floodplain study area along the Mojave River and the regional study area surrounding and underlying the floodplain study area. Aquifers in the floodplain study area consist of coarse granitic river-channel and floodplain alluvium deposited by the Mojave River. Aquifers in the regional study area consist of alluvium derived from older stream deposits, locally derived alluvial fans, playa lake deposits, and fractured bedrock (Stamos and others, 2001; Groover and Izbicki, 2019).

This study examined the quality of groundwater resources used for domestic drinking-water supply. Previous studies of groundwater resources used for public drinking-water supply in the Mojave Desert have observed elevated concentrations of some constituents, primarily trace elements, in some wells (Dawson and Belitz, 2012; Metzger and others, 2015). Domestic wells in the study unit typically are drilled to depths of 27–186 meters (Groover and others, 2019), which is shallower than the depths of public-supply wells in the same area (typically 90–300 meters deep; Dawson and Belitz, 2012). Water levels in domestic wells in the study unit typically are 9–140 meters below land surface (Groover and others, 2019).

This study was designed to provide a statistically representative assessment of the quality of groundwater resources used for domestic drinking water. A total of 48 domestic wells were sampled between January and May 2018 (Groover and others, 2019). Eleven additional wells were sampled to evaluate processes affecting groundwater quality, but these wells are not included in this assessment.