The Kern County Subbasin (KERN) study unit is located at the southern end of California’s San Joaquin Valley and consists of the Kern County groundwater subbasin (California Department of Water Resources, 2003; Shelton and others, 2008). The study unit covers about 3,000 square miles in Kern County. It is bounded by the Kern, Kings, and Tulare County lines to the north, the granitic bedrock of the Sierra Nevada and Tehachapi Mountains to the east and southeast, and the marine sediments of the San Emigdio Mountains and Coast Ranges to the southwest and west. The KERN study unit has long, hot summer days and cool nights, and mild, damp winters with dense fog. Average annual rainfall is about 6 inches. The Kern River, which originates in the Sierra Nevada, is the primary stream flowing through the study unit.

The primary aquifers consist of alluvial sediments (mixtures of sand, silt, clay, cobbles, and boulders), and marine and continental deposits in the deeper portion of the aquifers. Downward flow of groundwater is impeded by a subsurface clay layer, known as the Corcoran clay, in the central part of the KERN study unit. The primary aquifers are defined as those parts of the aquifers corresponding to the perforated intervals of wells listed in the California Department of Public Health database. Public-supply wells typically are drilled to depths between 600 and 800 feet below land surface, consist of solid casing from the land surface to a depth of about 275–450 feet, and are perforated or screened below the solid casing. Water quality in the primary aquifers may differ from that in the shallower and deeper parts of the aquifer system.

Land use in the study unit is approximately 66 percent (%) agricultural, 3% urban, and 31% natural. The primary agricultural uses are for field crops (such as vegetables and hay) and fruit and nut orchards. The largest urban area is the City of Bakersfield.

The primary sources of recharge are from the Kern River and artificial recharge at groundwater banking facilities that exist throughout most of the study unit (Tom Haslebacher, Kern County Water Agency, written commun., June 15, 2007). Secondary sources of recharge include return flows from agricultural and municipal irrigation and infiltration of flows from intermittent streams along the edge of the subbasin. The primary sources of groundwater discharge are water pumped for irrigation and municipal supply (California Department of Water Resources, 2003).

Groundwater provides more than 40 percent of California's drinking water. To protect this vital resource, the State of California created the Groundwater Ambient Monitoring and Assessment (GAMA) Program. The Priority Basin Project of the GAMA Program provides a comprehensive assessment of the State's groundwater quality and increases public access to groundwater-quality information. The Kern County Subbasin constitutes one of the study units being evaluated.