The Mojave and Sonoran Deserts include 57 groundwater basins (California Department of Water Resources, 2003). Basins in the Antelope, Mojave River, Coachella, and Colorado River Valleys were discussed by Dawson and Belitz (2012). The remaining 47 basins compose the Borrego Valley, Central Desert, and Low-Use Basins of the Mojave and Sonoran Deserts study unit (CLUB study unit) and were grouped into three study areas. Basins containing few or no public-supply wells are defined as low-use basins. Public-supply wells are sparse in the 12,103-square-mile area of the CLUB study unit, and this study focused on the 963-square-mile area that is within about 2 miles of a public-supply well.

The climate in the Mojave Desert is characterized as arid high desert, with hot, dry summers and cool winters with limited rainfall. The Sonoran Desert has an arid subtropical climate characterized by long, hot summers, mild winters, and summer and winter rainy seasons. Annual precipitation generally ranges from 0 to 10 inches. Several creeks, washes, and ephemeral streams drain the study unit, flowing into lakes and sinks.

This study evaluated water quality in the part of the aquifer system used for public supply, the primary aquifer system. The primary aquifer system is defined as those parts of the aquifer corresponding to the perforated intervals of wells listed in the California Department of Public Health (CDPH) database. Public-supply wells in the study unit typically are drilled to depths between 350 and 600 feet, consist of solid casing from the land surface to a depth of about 200 to 400 feet, and are perforated below the solid casing. Water quality in the shallower and deeper parts of the aquifer system can differ from that in the primary aquifer system. The primary aquifer system consists of unconfined and confined Pleistocene- to Holocene-age Quaternary alluvium, and to a lesser extent, Tertiary alluvium and sedimentary deposits.

Land use in the CLUB study unit is approximately 91 percent (%) natural, 6% urban, and 3% agricultural. Natural lands are mostly shrubland and bare rock or sediment, with a small percentage of grassland and forest.

Sources of groundwater recharge include runoff from mountains surrounding the basins and infiltration of imported surface water and groundwater used for irrigation. The primary sources of groundwater discharge are water pumped for irrigation and for public supply, natural discharge to streams, and evapotranspiration.