Fact Sheets

Groundwater Quality in the Coastal Los Angeles Basin, California

Fram, M.S., and Belitz, K., 2012, U.S. Geological Survey Fact Sheet 2012-3096, 4 p.

Related Study Unit(s): Coastal Los Angeles Basin Groundwater Resources Used for Public Supply

The Coastal Los Angeles Basin study unit is approximately 860 square miles and consists of the Santa Monica, Hollywood, West Coast, Central, and Orange County Coastal Plain groundwater basins (California Department of Water Resources, 2003). The basins are bounded in part by faults, including the Newport-Inglewood fault zone, and are filled with Holocene-, Pleistocene-, and Pliocene-age marine and alluvial sediments. The Central Basin and Orange County Coastal Plain are divided into a forebay zone on the northeast and a pressure zone in the center and southwest. The forebays consist of unconsolidated coarser sediment, and the pressure zones are characterized by lenses of coarser sediment divided into confined to semi-confined aquifers by lenses of finer sediments. The primary aquifer system in the study unit is defined as those parts of the aquifer system corresponding to the perforated intervals of wells listed in the California Department of Public Health (CDPH) database of public-supply wells. The majority of public-supply wells are drilled to depths of 510 to 1,145 feet, consist of solid casing from the land surface to a depth of about 300 to 510 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 systems.

The study area has hot, dry summers and cool, moist winters. Average annual rainfall is about 15 inches. The Los Angeles, San Gabriel, and Santa Ana Rivers and the Rio Hondo, with headwaters in the mountains to the north and east of the study unit, are the primary streams traversing the study unit. Land use in the study unit is about 86 percent (%) urban, 12% natural, and 2% agricultural. Natural lands include coastal ecological preserves. The urban landscape consists of residential, commercial, and industrial areas, and the large urban centers include the cities of Los Angeles, Long Beach, Anaheim, Santa Ana, and Huntington Beach.

Recharge to the Central Basin and Orange County Coastal Plain occurs primarily by engineered recharge of stormwater, imported water, and reclaimed water along the upper reaches of the San Gabriel and Santa Ana Rivers and the Rio Hondo. Recharge to the West Coast Basin occurs primarily by injection of imported water and reclaimed water into wells of the seawater intrusion barrier and by underflow from the Central Basin. Groundwater flow directions are controlled by the engineered recharge and by groundwater pumping from the many hundreds of wells distributed across the region (Shelton and others, 2001; Dawson and others, 2003).