Groundwater Quality in the South Coast Interior Basins, California

The South Coast Interior Basins (SCI) study unit covers approximately 653 square miles and consists of the Livermore, Gilroy-Hollister, Cuyama Valley, Castac Lake Valley, Cuddy Canyon Valley, Cuddy Ranch Area, Cuddy Valley, and Mil Potrero groundwater basins (California Department of Water Resources, 2003). These basins were grouped into three study areas based on geography: Livermore (LIV), Gilroy (GIL), and Cuyama (CUY) (Mathany and others, 2009).

Average annual precipitation across the study unit varies among the three study areas—from more than 20 inches (in.) along the northwest margin of the LIV study area to less than 8 in. in the northern parts of the CUY study area. The study areas are drained by the Arroyo de la Laguna canal in LIV, the Pájaro River and the San Benito River in GIL, and the Cuyama River in CUY. The primary aquifer system consists of Plio-Pleistocene sedimentary formations, Tertiary to Holocene-age alluvial sedimentary sequences, and unconfined Holocene- age and Pleistoceneage alluvium. The primary aquifer system is defined as those parts of the aquifers corresponding to the perforated intervals of the public-supply wells listed in the California Department of Public Health (CDPH) database. Public-supply wells in the study unit are typically drilled to depths between 200 and 600 feet, consist of solid casing from the land surface to a depth of about 50 to 350 feet, and are perforated below the solid casing. Water quality in the primary aquifer system may differ from that in the shallower and deeper parts of the aquifer system.

Land use within 500-meter buffer areas around the wells in the study unit is approximately 44 percent (%) agricultural, 29% urban, and 27% natural. Agricultural lands are mostly orchards, row crops, vineyards, or pastures. The largest urban areas are the cities of Livermore and Gilroy.

Recharge to groundwater basins occurs as infiltration of spring and streamflow, direct infiltration of precipitation and irrigation, subsurface inflow, and engineered recharge (percolation ponds and river regulation). The primary sources of groundwater discharge are water pumped for irrigation and municipal supply, discharge to rivers and streams, and evaporation.