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Duncan A. Austin, P.E. - 2 - January 5, 2015 monitoring wells. The highest concentration monitoring wells were located closest to the highest detection Gore Sorber® location. Extent defined: Yes Estimated Residual Mass: A limited area of PCE vapor and groundwater impacts was identified by the investigations. While soil gas concentrations were not quantified, the groundwater concentrations were below the California Maximum Contaminant (MCL) Level for drinking water (5 pg/L). Threat to Groundwater: Only PCE was detected in the groundwater samples, but was at a concentration of less than the MCL of 5 pg/L. Threat to Human Health: • Vapor Intrusion Hazard: Passive soil vapor samples were recorded as total mass of constituents and 'therefore cannot be compared to screening levels for the vapor intrusion to indoor air pathway. However, using the highest PCE groundwater concentration (3.5 pg/L) with the Department of Toxic Substances Control Vapor Intrusion Screening Model for Groundwater (Version 3.0, DTSC modification March 2014) along with default parameters except for loamy sand soil type based on the site boring logs and a depth to groundwater of 8 feet, the cancer risk is calculated to be less than one in a million and the non-cancer hazard quotient is less than one for a residential vapor intrusion scenario. Therefore, these low levels do not constitute a significant threat to human health through the vapor intrusion pathway at the site. • Dermal Contact Hazard: Based on the low residual concentration levels, there does not appear to be a threat to human health through dermal contact at the site. Summary: Based on the above evaluations the Central Valley Water Board staff concurs that the remaining residual concentrations of PCE at the site do not appear to present either a threat to water quality or human health.