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Remediation Action Report Mountain House High School April 17, 2007 Page 3 Risk characterization requires calculation of cumulative cancer risks and non-cancer hazards, that is, additive by all chemicals and all exposure pathways. In general, the cumulative cancer risk estimation of greater than one-in-one-million or a non-cancer hazard index greater than one, are the points of departure to assess if a clean-up action is required. The RAR provides a calculated cancer risk for the MHHS site of less than one-in- one-million. However, the remainder of the proposed MHHS site has not been investigated nor has groundwater contamination beneath the site been delineated. As previously stated, all pathways have to be evaluated in order to develop a cumulative risk. The remaining portion of the proposed school site needs to be evaluated and the groundwater plume delineated, before a cumulative risk can be determined for the MHHS site. Specific Comment: 1. Benzene in soil gas — DTSC calculated the risk estimate for benzene on the DTSC- revised Johnson & Ettinger vapor intrusion model. The highest benzene concentration (150 ug/m3 at 24 ft bgs, Freon-134a not detected) conveys an approximate excess cancer risk of 4.5E-07 via the vapor intrusion pathway. By it self, benzene concentrations in soil vapor do not appear to convey a significant cancer risk at the MHHS site. 2. Soil matrix contamination - The TPH concentrations detected in the confirmation samples (maximum 360 mg/kg TPHJ do not appear to constitute a significant health risk for future students or staff at the MHHS site. 3. Biogenic methane — DTSC is concerned with the potential for biogenic methane generation from fill placed as backfill into the trench. DTSC's "Advisory on Methane Assessment and Common Remedies at School Sites" (June 16, 2005) states that fill material in excess of 10 ft thickness and a Total Organic Carbon (TOC) content > 0.5% should be rejected, or a methane evaluation should be conducted. Unless it can be shown that the fill material has a TOC content < 0.5%, the fill material should be evaluated for biogenic methane production.