The Southern Sacramento Valley (SSACV) study unit is located in California's Sacramento Valley. The 2,100-square-mile study unit includes five groundwater subbasins: North American, Solano, South American, Suisun-Fairfield, and Yolo (California Department of Water Resources, 2003). An additional study area, South Sacramento Uplands, was created within portions of the North and South American groundwater subbasins. In the SSACV study unit, summers are hot and dry and winters are cool and moist. Average annual rainfall ranges from 17 to 23 inches. Most rivers and streams flowing across the study unit drain into the Sacramento River, which then flows into the Sacramento-San Joaquin Delta and the San Francisco Bay estuary.

Aquifers in the SSACV study unit consist of discontinuous lenses of gravel, sand, silt, and clay, which primarily are derived from the Sierra Nevada mountains ranges to the east and the Coast Ranges to the west. The primary aquifers in the SSACV study unit are defined as those parts of the aquifers corresponding to the perforated intervals of wells listed in the California Department of Public Health (CDPH) database. The public-supply wells monitored by the CDPH typically are completed within the primary aquifers to depths of 250–500 feet below land surface (bls). The wells are constructed with solid casing from the land surface to a depth of about 150–250 feet bls, and are perforated below the solid casing to allow water into the well. Water quality in the primary aquifers may differ from water quality in shallow or deep parts of the aquifer system.

Land use in the study unit is about 53 percent (%) agricultural, 33% natural (primarily grassland), and 14% urban. The City of Sacramento is the largest urban area in the study unit.

Recharge to the groundwater flow system primarily is from rivers and streams draining the Sierra Nevada and the Coast Ranges, and from infiltration of precipitation and of surface water applied for irrigation (California Department of Water Resources, 2003). The primary sources of groundwater discharge (water leaving the flow system) are pumping for irrigation and municipal water supply, evaporation from areas with a shallow depth to water, and discharge to streams.