San Gabriel Valley (Area 1) is a Superfund site covering roughly 11 square miles of groundwater beneath El Monte, Rosemead, South El Monte, Whittier, and nearby communities in Los Angeles County. EPA placed it on the National Priorities List in 1984, and remedial action began in 1988. The site sits within a larger 170-square-mile valley and spans industrial, commercial, residential, and recreational land. Physical construction of the cleanup is not yet finished, and the site has not been deleted from the list.
The groundwater is contaminated with volatile organic compounds and industrial solvents. Specific contaminants include tetrachloroethene, trichloroethene, 1,1-dichloroethene, carbon tetrachloride, 1,2-dichloroethane, chloroform, 1,4-dioxane, chromium(VI), cis-1,2-dichloroethene, trans-1,2-dichloroethene, and N-nitrosodimethylamine. These chemicals were found across the El Monte, Whittier-Narrows, Suburban, and Richwood Water Company areas. EPA determined they pose an unacceptable risk to human health or the environment. Human exposure is currently under control, meaning no unacceptable pathways exist for people to contact the contamination right now. However, groundwater migration is not under control, and the contamination continues to spread beyond the original area.
EPA oversees four treatment plants that extract and treat contaminated groundwater. Treated deep groundwater meets all drinking water standards and is supplied to the region. Treated shallow groundwater is either reinjected into the aquifer or discharged under permit. The site is divided into nine operable units. Several have completed key phases, including the Richwood Water Company unit, which finished remedial action by 1989. South El Monte transitioned to state-performed operation and maintenance in September 2024. The El Monte Eastside and Westside units are in long-term response actions, with operation and maintenance phases estimated between May and November 2026. EPA has negotiated consent decrees with potentially responsible parties totaling over $25 million for the South El Monte unit alone. The Los Angeles Regional Water Quality Control Board and California Department of Toxic Substances Control oversee property-specific work within the operating units. Soil contamination has been found at nearly 400 individual facilities across the site.
EPA is conducting a fifth Five-Year Review in 2026, with the report due by September 30, 2026. The 2021 review found that cleanup protects human health and the environment in the short term, but noted that modifications to the Westside shallow groundwater remedy are needed to prevent further contaminant migration. Community members can attend El Monte Operable Unit Technical Coordination and Stakeholder meetings, held three times yearly at the San Gabriel Water Quality Authority offices in West Covina. Site records are available at the EPA Region 9 Regional Records Center in San Francisco and at local libraries in Rosemead, West Covina, South El Monte, Whittier, and El Monte. Questions can be directed to EPA staff assigned to the site.
