The Northern San Joaquin (NSJ) study unit is located in California’s San Joaquin Valley and includes the Cosumnes, Eastern San Joaquin, and Tracy groundwater subbasins (California Department of Water Resources, 2003). The 2,079-square-mile study unit was divided into four study areas: Cosumnes, Eastern San Joaquin, Tracy, and Uplands (Quaternary-Pleistocene semiconsolidated deposits). The NSJ study unit has hot and dry summers and cool, moist, winters. Average annual rainfall ranges from 11 to 15 inches. Most rivers and streams flowing across the study unit drain into the San Joaquin River, which flows northwestward into the Sacramento-San Joaquin Delta and San Francisco Bay estuary.

Aquifers in the study unit consist of interlayered lenses of gravel, sand, silt, and clay deposited by rivers draining the Sierra Nevada to the east but also from the Coast Ranges to the west. The primary aquifers in the NSJ study unit are defined as those parts of the aquifers corresponding to the perforated intervals of the wells listed in the California Department of Public Health (CDPH) database. The public-supply wells monitored by CDPH typically are completed in the primary aquifers to depths of 250–500 feet below land surface (bls). The wells contain solid casing reaching from the land surface to about 100 to 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 the water in shallow and deep parts of the aquifer system.

Land use in the study unit is about 57 percent (%) agricultural, 36% natural (primarily grassland), and 7% urban. The largest urban area in the study unit is the City of Stockton, with a population of 287,578 (U.S. Census Bureau, 2009 estimate).

Recharge to the groundwater flow system primarily is from percolation of irrigation return water, precipitation, seepage from reservoirs and rivers, and urban runoff (Northeastern San Joaquin County Groundwater Banking Authority, 2004). The primary sources of groundwater discharge are pumping for irrigation and municipal water supply, evaporation from areas with a shallow depth to water, and discharge to streams.