San Diego Formation

The formation contains the San Diego Formation Basin, a large aquifer under Imperial Beach, Chula Vista, National City, and southern portions of the city of San Diego. The San Diego Formation Basin is a confined shallow aquifer. It has a basin ground surface area of 79,724 acres (32,263 ha) and an estimated groundwater storage capacity of 960,000 AF. The depth to groundwater is about 100 feet (30 m).[3] The groundwater in the San Diego Formation is brackish, and its quality is considered to be fair to poor.[3] Due to its proximity to the ocean, the risk of seawater intrusion is a primary concern regarding water quality. To avoid intrusion, the Sweetwater Authority constantly monitors the basin water levels, which have remained stable since the 1980s. A factor that contributes to limit the groundwater pumping is the importation of Colorado River water. However, to reduce demands for imported water, there have been installed desalination plants. The Richard A. Reynolds Groundwater Desalination Facility, in Chula Vista, was completed in 1999 and holds a production capacity of 4 million gallons of drinking water per day.[3] The Claude “Bud” Lewis Carlsbad Desalination Plant, in San Diego, was completed in 2015 and produces 50 million gallons of water per day.[4] In this way, these plants contribute to reducing vulnerability and secure water supply, especially during drought periods. The use of groundwater contributes to a regional effort to reduce demands for imported water.[5]

Besides those of clams and other mollusks, quite a few bird fossils (which are generally rare) have been found in this geological formation. Among them is a possible ancestor of Cassin's auklet (Ptychoramphus aleuticus) and the loons Gavia concinna and G. howardae. The former was quite likely a close relative or even ancestor of the large black-headed[6] loons; the latter was perhaps a small relative of the large grey-headed[7] loons.[8][9]