The Santa Barbara study unit covers more than 48 square miles in the Transverse and Selected Peninsular Ranges hydrogeologic province and includes parts of Santa Barbara and Ventura Counties (Davis and Kulongoski, 2016). The study unit is flanked by the Santa Ynez Mountains on the north and the Pacific Ocean on the south. The study unit consists of five coastal plain groundwater basins (California Department of Water Resources, 2003). These five groundwater basins are Goleta, Foothill, Santa Barbara, Carpinteria, and Montecito, from west to east.

The climate in the study unit is classified as Mediterranean, with hot, dry summers and cool, moist winters. Average annual rainfall in the study unit is about 17 inches. The study unit is drained by Mission Creek and other intermittent streams that flow from the Santa Ynez Mountains in the north to the south, where they terminate in the Pacific Ocean.

The main water-bearing units of the primary aquifer system in the Santa Barbara study unit consist of alluvium of Quaternary age (Davis and Kulongoski, 2016). Other water bearing units include the Santa Barbara and Casitas Formations, which underlie the alluvial deposits. The primary aquifer system in the study unit is defined as those parts of the aquifer corresponding to the perforated intervals of wells listed in the California Department of Public Health database. Public-supply wells in the study unit range in depth from 150 to 1,230 feet, consist of solid casing from the land surface to a depth of about 110 to 350 feet, and are perforated below the solid casing. Water quality in the primary aquifer system can differ from that in the shallower and deeper parts of the aquifer system.

Land use in the study unit is approximately 64-percent urban, 24-percent natural land, and 12-percent agricultural. Small areas of natural land use are intermixed with urban and agricultural land uses. Agricultural land use primarily exists in the eastern part of the study unit and along the outskirts of the western part. Topographically, the study area consists of coastal hills sloping toward the ocean.

The groundwater basins are recharged by percolation of agricultural return and precipitation, infiltration of imported water through canals and aqueducts, and seepage losses from the major rivers and their tributaries. The primary sources of discharge are water pumped for municipal supply, subsurface outflow to other groundwater basins, and evaporation.