The San Fernando–San Gabriel study unit is approximately 460 square miles and consists of the San Fernando Valley, Raymond, and San Gabriel Valley groundwater basins (California Department of Water Resources, 2003). The study unit has hot, dry summers and cool, moist winters. Average annual rainfall ranges from 17 to 21 inches over the three basins. The study areas are drained by the Los Angeles, San Gabriel, and Rio Hondo rivers and their tributaries (Land and Belitz, 2008).

The San Fernando and San Gabriel Valleys are sedimentologically diverse basins situated within the Transverse Ranges of southern California. These structurally complex basins formed as a result of the dextral slip of the San Andreas Fault system in the late Tertiary-Quaternary (Tinsley, 2001). The valleys have been filled, up to 6,400 feet, with marine and terrestrial sediments of Pleistocene through Holocene age that overlie crystaline basement. The alluvium and underlying marine deposits contain freshwater used for supply. The primary aquifers in the 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. Public-supply wells are typically drilled to depths of 400 to 785 feet, consist of solid casing from the land surface to a depth of about 157 to 300 feet, and are perforated below the solid casing. Water quality in the primary aquifers may differ from that in the shallower and deeper parts of the aquifer system. Groundwater movement is generally from the northern parts of the valleys south towards the Pacific Ocean.

Land use in the study unit is approximately 83 percent (%) urban, 16% natural, and 1% agricultural. The urban landscape consists of residential, commercial, and industrial areas. Natural lands are mostly shrublands and grasslands. The largest urban areas are Los Angeles, San Fernando, Northridge, Burbank, Pasadena, Alhambra, Azusa, and West Covina.

Recharge to the groundwater basins occurs from the infiltration of runoff and imported water at spreading basins, infiltration of precipitation and irrigation, and infiltration of streamflow from the major rivers and their tributaries. Streamflow is a combination of runoff from the surrounding mountains, imported water, industrial discharges, and treated wastewater effluent. The primary sources of discharge are water pumped for municipal supply and for irrigation.