EPA Superfund Site

MATHER AIR FORCE BASE (AC&W DISPOSAL SITE)

CA | EPA ID CA8570024143

About this site’s exposure pathways: As of July 2024, Site 23C within the former Mather Air Force Base (AC&W Disposal Site) Superfund site is considered “Insufficient Data to make a Human Exposure evaluation” (HEID) because of the potential for indoor air vapor intrusion in occupied buildings at the Site 23C. The site was previously categorized as "Human Exposures Under Control" (HEUC). The Air Force submitted the Site 23C Soil Vapor Extraction (SVE) Completion Report Final document in December 2017, which details that a large mass of tetrachloroethene (PCE) in the soil vapor remains at the site. To date, a draft Vapor Intrusion Sampling Work Plan has been developed through various negotiations and written positions. Additional soil vapor monitoring wells have been installed at Site 23C and two rounds of sampling have taken place in 2023. EPA is reviewing the results to determine additional sampling recommendations.

EPA uses this Human Exposure status when there is a lack of evidence to suggest that actual or reasonably anticipated human exposures are occurring and that those exposures are above acceptable risk-based levels. The planned activities to collect sufficient information to evaluate this new exposure pathway and/or contaminants(s) include evaluation of soil vapor sampling results and subsequent vapor sampling at Site 23C. This is ongoing and is planned to be completed in December 2025. EPA currently anticipates that there will be sufficient data to make a Human Exposure evaluation by December 2025.