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i v a ASSOCIATES INC 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 1 CRWQCB (Appendix B of the Tri -Regional Recommendations), conditions at the subject site were evaluated to determine if NFA as a "low risk, Groundwater Case" 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 approximately 19 to 25 feet One soil sample collected in 1998 contained a detectable concentration of benzene at 0 0051 mg/kg One sample collected from well MW2 in December 2000 contained benzene above the MCL Benzene has not been detected in ' any other samples collected from the groundwater monitoring wells The estimated mass of TPHg and TPHd remaining in soil is approximately 966 pounds TPHg and TPHd were not detected in any of the groundwater samples collected from wells MW 1, MW3 M and TPHg and TPHd have not been detected in wells MW4, and MW5 since September 2000 None of the wells have ever contained detectable concentrations of fuel oxygenates The surface is the site is paved with asphalt, which will hinder the migration ' of contaminants caused by rain or surface washing from the soil to the groundwater Therefore, the likelihood of contaminants in the vadose zone reversing or threatening to 1 reverse the mass reduction rate of groundwater pollutants is expected to be very low Criterion 2: Separate phase product has been removed to the extent practicable. 1 Free product has never been detected at this site I Criterion 3: No existing water supply wells, deeper aquifers, surface waters, or other receptors are threatened by pollutants remaining in the aquifer. ' Well MW5, located approximately 100 feet downgradient from the former USTs, has not contained detectable concentrations of TPHd since March 2000 No other constituents have been detected in this well Well MW4, located approximately 90 feet crossgradient 1 from the former USTs, has not contained detectable concentrations of TPHd since September 2000 No other constituents have been detected in either of these wells The two closest water supply wells are located approximately 1,100 feet crossgradient and upgradient from the site The nearest surface water is located approximately one mile from the site For the investigation, groundwater wells were installed only in the shallowest aquifer Boring MW2 was advanced to a depth of 45 feet bgs The soil sample collected at this depth contained no detectable concentrations or petroleum w V 30991reportslclosure doc 9