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GRANITE CONSTRUCTION — FRENCH CAMP Page 7 WKA No , 2088 .09 October 10, 2005 The contaminant plume appears to be minor and stable and/or on the decline (not migrating) and the chemical concentrations of concern in groundwater are below the water quality objectives . Risk-Based Corrective Action Evaluation and Fate and Transport Model In January 1997 a Risk-Based Corrective Action (RSCA) and a one-dimensional finite difference vadose zone leaching model ( VLEACH) were performed on the site based on existing site conditions at that time. Results of R 3CA analysis indicate that the benzene upper confidence limit (UCL) concentrations in subsurface soil in the vicinity of the former USTs is below the risk-based screening levels (RBSLs) of 1 .6 mg/kg. The remaining UCL concentrations for non- carcinogenic compounds are below the residual saturation concentrations and are therefore not a significant health risk. The Total Pathway Carcinogenic Risk for the levels of benzene in subsurface soil is 1 .4 x 10,', which is well below the target risk of 1 x 10-6 ( 1 in 1 ,000,000). The Hazard Index for non-carcinogenic compounds is 4.4 x 10-5, which is well below the Hazard Index standard of 1 .0. Groundwater UCL concentrations are below the solubility concentrations for BTEX constituents, Results of the VLEACH model predicted that napththalene (diesel) impact to groundwater at year 10 is 1 , 597 x 10-6 grams per year. Mass Balance In addition to the Risk Based Corrective Action Evaluation, the Fate and Transport Modeling and the Sensitive Receptor Survey, a mass balance calculation was performed in August 1999 to evaluate the estimated volume of petroleum hydrocarbons remaining in soil and groundwater beneath the property. The calculations were completed using EPA accepted weighted statistical analyses methods (U.S . EPA, 1992). The results were presented in the August 1999 Project Summary and Request for Site Closure Report. The weight/volume of petroleum hydrocarbons present in the soil beneath the site was calculated to be approximately 160 pounds or 27 gallons (0.09 gallons of benzene). The estimated volume of dissolved petroleum hydrocarbons present in the groundwater beneath the site was calculated to be approximately 18 gallons (0. 18 gallons of benzene) . WALLACE • KUHL _„ FA A550C1Arr5 INC,.