The South Coast Range–Coastal (SCRC) study unit is located in California’s Southern Coast Ranges hydrogeologic province and includes parts of Santa Barbara and San Luis Obispo Counties (Mathany and others, 2010). The SCRC study unit includes five groundwater basins—Los Osos Valley, San Luis Obispo Valley, Santa Maria River Valley, San Antonio Creek Valley, and Santa Ynez River Valley (California Department of Water Resources, 2003)—as well as upland areas of Plio-Pleistocene non-marine and Pliocene marine sediments that are adjacent to the groundwater basins. The 766-square-mile study unit was separated into the two study areas (Basins and Uplands) on the basis of alluvial and terrace deposits found in the five groundwater basins.

The climate in the SCRC study unit is influenced in large part by the Pacific Ocean as well as by the topography of the surrounding areas. The coastal lowlands and valleys near the Pacific Ocean have a coastal climate, with warm, dry summers and cool, wet winters. The slopes of the coastal valleys and within the upland areas have a Mediterranean climate with hot, dry summers and cool, wet winters. Average annual rainfall ranges from 13 inches in the coastal lowlands to 20 inches in the uplands. The Santa Maria and Santa Ynez Rivers and San Antonio Creek are the major streams in the SCRC study unit.

The primary aquifer system consists of alluvial sediments (mixture of gravel, clay, silt, and sand), the Paso Robles Formation (unconsolidated to poorly consolidated coarse sand and gravel), and the Careaga Sand (unconsolidated fine- to medium-grained marine sand). The primary aquifer system is defined as that part of the aquifer corresponding to the perforated depth intervals of wells listed in the California Department of Public Health (CDPH) database. CDPH wells are typically drilled to depths between 265 and 630 feet below land surface and are perforated or screened at depths below 115–350 feet (Burton and others, 2013). Water quality in the primary aquifer system may differ from that in the shallower and deeper parts of the aquifers.

Land use in the study unit is approximately 61 percent (%) natural, 29% agricultural, and 10% urban. The primary agricultural uses are for vegetables and flowers, vineyards, and pasture and hay. The largest urban areas are the cities of Lompoc, Santa Maria, Arroyo Grande, and San Luis Obispo (Burton and others, 2013).

The primary source of recharge to the groundwater flow system is from percolation of precipitation, irrigation waters, seepage from streams and rivers, subsurface inflow, and engineered recharge (California Department of Water Resources, 2003). The primary sources of groundwater discharge are water pumped for irrigation and municipal supply.