The Western San Joaquin Valley (WSJV) study unit covers approximately 5,600 square miles and includes the Delta–Mendota and Westside subbasins of the San Joaquin Valley groundwater basin (California Department of Water Resources, 2003; Fram, 2017). The freshwater aquifer system in the WSJV study unit primarily is composed of late Tertiary to Quaternary age alluvial and fluvial sediments derived from the Coast Ranges to the west and the Sierra Nevada to the east, and is divided into upper and lower zones by a clay-rich lacustrine unit, the Corcoran Clay (Faunt, 2009).

The groundwater resources used for public drinking water are defined as those parts of the aquifer system tapped by wells listed in the State of California’s database of public supply wells. In the Delta-Mendota subbasin, public supply wells are typically drilled to depths of 150 to 600 feet, consist of solid casing from land surface to depths of 130 to 290 feet, and are screened below the solid casing. In the Westside subbasin, public supply wells are typically drilled to depths of 575 to 1,550 feet, and have solid casing from land surface to depths of 375 to 650 feet. Zones above and below the Corcoran Clay are used for public drinking water in both subbasins. Water quality in groundwater resources used for public drinking water can differ from that in shallower and deeper parts of the aquifer system.

The hydrology of the WSJV study unit has been extensively altered by human activity (Faunt, 2009). Under natural conditions, the primary sources of groundwater recharge were infiltration of sparse streamflow from the Coast Ranges through alluvial fans and from the San Joaquin and Kings Rivers in the basin, primarily during the winter rainy season. Agricultural development began more than a century ago, and about 70 percent of the land now is used for agriculture. Irrigation with groundwater and imported surface water is now the primary source of groundwater recharge, and pumping for agricultural use is now the primary source of discharge from the aquifer system. Drainage from WSJV agricultural lands can have high concentrations of trace elements and salts and, thus, must be managed.