The San Francisco Bay study unit is approximately 620 square miles and consists of the Marina, Lobos, Downtown, Islais Valley, South San Francisco, Visitacion Valley, Westside, and the Santa Clara Valley groundwater basins (California Department of Water Resources, 2003). These basins were grouped into one study area primarily on the basis of geography (Ray and others, 2009).

The study unit has warm, dry summers and cool, moist winters. Average yearly rainfall across the study unit ranges from 14 inches (in.) in the southern and southwestern parts of the study unit to 28 in. in the north. The study unit is drained primarily by the Guadalupe River, the Coyote Creek, and their principal tributaries. The primary aquifer system consists of alluvial sediments (mixtures of sand, silt, clay, and gravel), fluvial deposits, and marine estuarine deposits (locally referred to as the “Bay Mud”). For this study, the primary aquifer system is defined as the part of the aquifer corresponding to the perforated intervals of public-supply wells listed in the California Department of Public Health database. Public-supply wells are typically drilled to depths between 200 and 700 feet, consist of solid casing from the land surface to a depth of about 175 to 425 feet, and are perforated below the solid casing. Water quality in the shallower and deeper parts of the aquifer system may differ from that in the primary aquifer system.

Land use in the study unit is approximately 77 percent (%) urban, 22% natural, and 1% agricultural. The largest urban areas are the cities of San Francisco, San Jose, Santa Clara, Sunnyvale, Fremont, and Oakland. Natural lands are mostly grassland.

Recharge to groundwater occurs primarily from infiltration of imported water, mountain-front recharge, streamflow, infiltration of water from precipitation and irrigation, and leakage from transmission pipelines. The primary sources of discharge are water pumped for irrigation and municipal supply, evaporation, and discharge to streams and the San Francisco Bay.