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Ms.Lori Duncan Page 6 of 9 <br /> San Joaquin County Environmental Health Department <br /> August 25,2004 <br /> of impacted soil has probably transferred contamination from the soil to the groundwater, <br /> �1 effectively eliminating soil impacts above the former water table. Since SVE/AS activities have <br /> removed significant hydrocarbon mass from the site, and groundwater elevation has risen at least <br /> 20 feet,impacted soil is no longer considered to be significant to site conditions. <br /> Groundwater is assumed to have a constant thickness of 40 feet. This assumption simplifies <br /> impacted groundwater volume calculations into easily obtained geometric equations.The value of <br /> 40 feet was chosen to conservatively represent the thickness of impacted water,from 25 feet bgs <br /> (approximately the water table and capillary fringe)to 65 feet bgs (the maximum depth explored). <br /> Groundwater samples collected during the August 2004 sampling event are assumed to be <br /> representative of groundwater conditions at the site. <br /> Hydrocarbon mass in soil vapor is assumed to be negligible. This assumption is based on low <br /> influent concentrations that prompted the shutdown of the SVEIAS system(Appendix F). <br /> Isocontour maps presented in Figures 6 through 11 are assumed to represent the actual distribution <br /> of hydrocarbon mass in groundwater. Figures 6, 8, and 10 can be used to calculate impacted water <br /> volumes and estimate average hydrocarbon concentrations in groundwater. Although contaminant <br /> concentrations decrease with depth, average concentrations are assumed to exist throughout the <br /> water column. This assumption provides a conservative estimate of the petroleum hydrocarbon <br /> mass. Appendix I presents the geometry used to calculate impacted water volumes. <br /> Calculations <br /> Impacted groundwater volumes were calculated using the following formula: <br /> Volume(Groundwater) =(Area * Thickness *Porosity)—Circumscribed Volumes <br /> Where: <br /> Area=nr2 r is chosen to fully circumscribe impacted area (feet) <br /> Thickness =40 feet(assumption) <br /> Porosity=0.35 (assumption) <br /> Circumscribed Volumes=Previously calculated volumes for higher concentration intervals. <br /> Calculations must go in order from the highest concentration contours to the lowest. <br /> Hydrocarbon mass was calculated using the following formula: <br /> Mass (Contaminant)= Concentration (In Groundwater)* Volume (Impacted Water) <br /> Where: <br /> Mass=total mass of contaminant in selected interval micrograms <br /> Concentration = highest of all wells in each interval or a reasonable approximated value <br /> where no analytical data exist <br /> Volume=calculated with previous formula <br /> 1 K 1WprocessU5 BP ARCON0054812004 SCMISite Conceptual Model V2.doc <br />