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| San Gabriel Valley Groundwater Basin | |
|---|---|
| Name | San Gabriel Valley Groundwater Basin |
| Location | Los Angeles County, California, United States |
| Area | ~600 square miles |
| Type | Alluvial groundwater basin |
| Primary users | Municipal water agencies, Metropolitan Water District of Southern California, City of Los Angeles, Pasadena Water and Power |
| Governing bodies | San Gabriel Basin Water Quality Authority, Upper San Gabriel Valley Municipal Water District, San Gabriel Valley Municipal Water District |
San Gabriel Valley Groundwater Basin The San Gabriel Valley Groundwater Basin is a major alluvial aquifer system beneath the eastern Los Angeles metropolitan area in Los Angeles County, California. It supplies potable water to multiple municipalities, industrial users, and agricultural remnants while interacting with imported supplies from the Colorado River Aqueduct and the State Water Project. The basin has been the focus of intensive management, remediation, and legal actions involving local agencies, federal regulators, and private parties.
The basin underlies communities including Pasadena, California, Alhambra, California, Pomona, California, El Monte, California, Monterey Park, California, San Gabriel, California, and Whittier, California. It is bounded by the San Gabriel Mountains, the Puente Hills, and the San Jose Hills and intersects with the Los Angeles River watershed and the Santa Ana River system via managed recharge and cross-valley flow. Key institutions involved include the U.S. Geological Survey, the California Department of Water Resources, the Environmental Protection Agency, and regional agencies such as the Main San Gabriel Basin Watermaster and the San Gabriel Valley Municipal Water District.
The basin comprises layered alluvium, interbedded gravels, sands, silts, and clays deposited by ancestral streams from the San Gabriel Mountains. Hydrostratigraphic units include shallow perched aquifers, an upper aquifer system, and deeper confined aquifers separated by aquitards. Groundwater flow is controlled by topographic gradients toward the Los Angeles Basin and controlled discharge points including pumping wells, natural springs in the Angeles National Forest, and engineered springs near the Rio Hondo. The basin's transmissivity and storage coefficients have been characterized in studies by the U.S. Army Corps of Engineers, the California Water Commission, and academic researchers at University of California, Los Angeles and California Institute of Technology.
Sources of basin replenishment include percolation of local precipitation from the San Gabriel Mountains', managed aquifer recharge projects run by the San Gabriel River Watermaster, spreading grounds along tributaries such as the San Gabriel River (California), direct injection from treated imported water from the Colorado River and Sacramento–San Joaquin River Delta, and recycled wastewater produced by agencies like the Los Angeles Department of Water and Power and the Three Valleys Municipal Water District. Key recharge facilities include spreading grounds and injection wells operated by the Upper San Gabriel Valley Municipal Water District and the Main San Gabriel Basin Watermaster; partnerships have also involved Metropolitan Water District of Southern California grant-funded recharge enhancements.
The basin has a complex contaminant history including industrial solvents such as trichloroethylene (TCE) and perchloroethylene (PCE) originating from aerospace facilities, dry cleaners, and manufacturing sites tied to entities like North American Aviation and others. Plume investigations and remediation have been led by the San Gabriel Basin Water Quality Authority, the California Environmental Protection Agency, and the U.S. Environmental Protection Agency through Superfund and state cleanup programs. Additional water quality concerns include nitrate from legacy septic systems and fertilizers connected to historical agriculture, volatile organic compounds linked to Lockheed Corporation era activities, and emerging contaminants such as per- and polyfluoroalkyl substances identified in monitoring programs run by the Los Angeles County Department of Public Health and academic partners at California State University, Long Beach.
Management is performed via a network of municipal water districts, groundwater adjudication through the Main San Gabriel Basin Watermaster and court orders, regulatory oversight from the California State Water Resources Control Board, and cooperative planning among Upper Los Angeles County agencies. Legal frameworks include adjudicated pumping rights, recharge credits, and cost-recovery mechanisms adjudicated in state superior courts; prominent court cases shaped allocation and remediation responsibilities involving private companies, municipalities, and state agencies. Funding and capital projects have involved grants from the U.S. Bureau of Reclamation and partnerships with the Metropolitan Water District of Southern California and local bond measures approved by voters in communities such as San Gabriel and Pomona.
Indigenous use by groups associated with the Tongva preceded Spanish and Mexican periods when missions such as Mission San Gabriel Arcángel affected regional water use. American period development accelerated with the arrival of the Southern Pacific Railroad, agricultural expansion in the late 19th century, and suburbanization following World War II driven by defense contractors and aerospace firms including Douglas Aircraft Company and Boeing. Groundwater overdraft and contamination emerged alongside industrialization, prompting the mid-20th century formation of municipal water agencies, later investigations by the U.S. Geological Survey, and landmark litigation and cleanup efforts in the late 20th and early 21st centuries.
Groundwater extraction and contamination have altered baseflows to riparian corridors supporting species in the San Gabriel Mountains National Monument and urban streams monitored by the Los Angeles and San Gabriel Rivers Watershed Council. Remediation and managed recharge projects aim to restore aquifer levels, benefit wetlands and habitat in areas like the Whittier Narrows Recreation Area, and reduce saltwater intrusion risk where the basin interfaces with coastal systems near Long Beach, California. Conservation initiatives have engaged environmental organizations such as the Audubon Society chapters, the Natural Resources Defense Council, and local watershed groups to balance municipal supply, ecological flows, and groundwater-dependent ecosystems.
Category:Water resources in Los Angeles County, California Category:Aquifers of California