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v ATC A S S O C I A T E S I N C .� L- concentration the constituent must be reduced, but only by less than half of its value. The Concentration Reduction Factor(CRF) for benzene is one(Tier 2 Worksheet 9.3, Appendix Q. Tier 2 RBCA assumptions, parameters utilized, and related supporting documentation are included in Appendix C. RBCA Model Output Discussion/Conclusions The model output indicated that on-site groundwater ingestion of benzene exceeded health risk levels. The model calculated the concentration of benzene at the project site must be reduced by a factor of one to achieve the applicable SSTL of 0.029 mg/L. The CRF indicates that the benzene barely exceeded the health risk level as a CRF of<1 requires no reduction to meet excess risk limits. The health risk level for groundwater ingestion of benzene was not exceeded at a residential receptor 100 feet from the site. V The RBCA model also revealed the SSTL values for all other COCs were not exceeded when using a Target Risk value of 1.0E-5. V 7.0 EVALUATION OF LOW RISK CRITERIA 7_1 Low Risk Groundwater Case Evaluation Using the criteria listed in CRWQCB's No Further Action (NFA) requirements, as outlined by CRWQCB (Appendix B of the Tri-Regional Recommendations), conditions at the subject site were evaluated to determine if NFA as a"low risk, soils only site" is appropriate for this site. Criterion 1: Contaminants remaining in the vadose zone must not reverse or threaten to reverse the mass reduction rate of groundwater pollutants. Over the course of the investigation, aquifer levels have been measured at depths of 34 to 37 feet, approximately six to eight feet below the contaminants remaining in the vadose zone. The 11HRA study revealed that benzene was the only COC that barely exceeded health risk levels. Only one soil sample collected in 1996 contained a detectable concentration of benzene at 31 mg/kg. The estimated mass of benzene remaining in soil is less than one and a half pounds. A combined mass of approximately 200 pounds of L toluene, ethyl benzene, and xylenes was estimated to remain in the soil. Concentrations of these constituents in groundwater have never been detected above their respective maximum contaminant levels. Methyl tertiary butyl ether (MTBE) has been detected in groundwater at concentrations above proposed MCL of 0.013 mg/L. Only one soil sample collected in 1998 contained a detectable concentration of MTBE at 0.055 mg/kg. The estimated mass of MTBE remaining in soil is less than 0.2 pounds. Therefore, the likelihood of contaminants in the vadose zone reversing or threatening to reverse the mass reduction rate of groundwater pollutants is very low. w:162574.05\reports\closm-FS3.doc 11 it