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GRANITE CONSTRUCTION—FRENCH CAMP Page 7 <br /> WKA No 2088 09 <br /> October 10, 2005 <br /> 1 The contaminant plume appears to be minor and stable and/or on the decline (not migrating) and <br /> the chemical concentrations of concern in groundwater are below the water quality objectives <br /> Risk-Based Corrective Action Evaluation and Fate and Transport Model <br /> In January 1997 a Risk-Based Corrective Action (RBCA) and a one-dimensional finite difference <br /> vadose zone leaching model (VLEACH) were performed on the site based on existing site <br />' conditions at that time Results of RBCA analysis indicate that the benzene upper confidence <br /> limit(UCL) concentrations in subsurface soil in the vicinity of the former USTs is below the <br /> nsk-based screening levels (RBSLs) of 1 6 mg/kg The remaining UCL concentrations for non- <br /> carcinogenic compounds are below the residual saturation concentrations and are therefore not a <br /> significant health risk The Total Pathway Carcinogenic Rask for the levels of benzene in <br /> subsurface soil is 14 x 10-7, which is well below the target risk of 1 x 10"6 (1 in 1,000,000) The <br /> Hazard Index for non-carcinogenic compounds is 4 4 x 10-5, which is well below the Hazard <br /> Index standard of 1 0 Groundwater UCL concentrations are below the solubility concentrations <br /> for BTEX constituents Results of the VLEACH model predicted that napththalene (diesel) <br /> impact to groundwater at year 10 is 1 597 x 10-6 grams per year <br /> Mass Balance <br /> In addition to the Risk Based Corrective Action Evaluation, the Fate and Transport Modeling and <br /> the Sensitive Receptor Survey, a mass balance calculation was performed in August 1999 to <br /> evaluate the estimated volume of petroleum hydrocarbons remaining in soil and groundwater <br /> beneath the property The calculations were completed using EPA accepted weighted statistical <br /> analyses methods (U S EPA, 1992) The results were presented in the August 1999 Project <br /> Summary and Request for Site Closure Report <br /> The weight/volume of petroleum hydrocarbons present in the soil beneath the site was calculated <br /> to be approximately 160 pounds or 27 gallons (0 09 gallons of benzene) The estimated volume <br /> Mof dissolved petroleum hydrocarbons present in the groundwater beneath the site was calculated <br /> to be approximately 18 gallons (0 18 gallons of benzene) <br /> 1 0 <br /> o <br />'Recycled piper 6 SSOC ATES INC <br />