The Northern Sacramento Valley (NSACV) study unit is located in California's Sacramento Valley. The approximately 1,200-square-mile study unit includes 2 study areas and 11 groundwater subbasins (subbasins listed clockwise beginning in the north): Enterprise, Millville, South Battle Creek, Bend, Antelope, Dye Creek, Los Molinos, Red Bluff, Bowman, Rosewood, and Anderson (California Department of Water Resources, 2003). In the NSACV study unit, summers are hot and dry and winters are cool and moist. Average annual rainfall ranges from 21 to 33 inches. Most rivers and streams flowing across the study unit drain into the Sacramento River.

Aquifers in the NSACV study unit consist of discontinuous lenses of gravel, sand, silt, and clay, which primarily are derived from the Klamath Mountains and Cascade Range to the north and east, respectively, but also from the Coast Ranges to the west. The primary aquifers in the NSACV 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 200–400 feet below land surface (bls). The wells are constructed with solid casing from land surface to a depth 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 61 percent (%) natural (primarily grassland), 30% agricultural, and 9% urban. The City of Redding 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 from pumping for irrigation and municipal water supply, evaporation from areas with a shallow depth to water, and discharge to streams.