The Whittier Narrows Operable Unit (WNOU) addresses contamination in the southern most part of the San Gabriel Valley, where groundwater and surface water flow from the San Gabriel Basin into the Central Basin.

In 1999, the U.S. Environmental Protection Agency (EPA) issued an amendment to the Record of Decision for the WNOU which identified the need for a groundwater extraction barrier approximately ¼ mile north of the Whittier Narrows Dam to halt the flow of contamination traveling toward the Central Basin.

To form an effective containment barrier, five or six extraction sites were required to remove and treat about 12,000 gallons per minute, extracting from both the shallow and intermediate zone aquifers. The EPA built a comprehensive treatment facility operated by the State of California.

Prior to the construction of the EPA project, the San Gabriel Basin Water Quality Authority (WQA) stepped in at the request of EPA to construct a temporary, 1,000-gallon-per-minute early action project to stem the flow of high concentration contaminants in the area. That facility was shut down when EPA completed construction of its project.

Additional information:

Appendix A in WQA’s Groundwater Quality Management and Remediation Plan (406 Plan)