The Middle Sacramento Valley (MSACV) study unit is located in California's Sacramento Valley. The 3,340-square-mile study unit includes eight groundwater subbasins: East Butte, North Yuba, South Yuba, Sutter, Vina, West Butte, Colusa, and Corning (California Department of Water Resources, 2003). In the MSACV study unit, summers are hot and dry and winters are cool and moist. Average annual rainfall ranges from 17 to 32 inches. Most rivers and streams flowing across the MSACV study unit drain into the Sacramento River.

Aquifers in the study unit consist of discontinuous lenses of gravel, sand, silt, and clay, which primarily are derived from the Sierra Nevada mountain range to the east and the Coast Ranges to the west. The primary aquifers in the MSACV 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 CDPH typically are completed within the primary aquifers to depths of 175– 375 feet below land surface (bls). The wells are constructed with solid casing from land surface to depths of about 100–200 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 the shallow or deep parts of the aquifer system.

Land use in the study unit is about 67 percent (%) agricultural, 30% natural (primarily grassland), and 3% urban. The largest urban areas in the study unit are the cities of Chico and Yuba City.

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 applied surface water (California Department of Water Resources, 2003). The primary sources of groundwater discharge (water leaving the flow system) are from pumping for irrigation and municipal water supply, evaporation from areas with a shallow depth to water, and discharge to streams.