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CAM B R I A Ms. Lori Duncan November 18,2005 PETROLEUM HYDROCARBON EXTENT IN SOIL AND GROUNDWATER Petroleum Hydrocarbon Extent in Soil Soil sample results indicate the highest hydrocarbon concentrations are located near the former UST complex at depths ranging from 15 to 50 fbg. Soil samples from boring B-3 contained up to 17,000 ppm TPHg and 400 ppm benzene at 15 fbg. Based on the soil analytical data, TPHg is defined laterally in unsaturated soil in all directions by © well borings MW-1 through MW-4 and borings B-2, B-6, B-7, Geo-3, and SB-1. Benzene is defined laterally in unsaturated soil in all directions by well borings MW-1 through MW-5 and borings B-6, Geo-3, and SB-1. Petroleum Hydrocarbon Extent in Groundwater Quarterly groundwater monitoring began at the site in the third quarter 1990. The highest current TPHg and BTEX concentrations detected in on-site wells are from well VEW-2. Petroleum hydrocarbons are defined in groundwater to the north by wells MW-1 and MW-2 and to the west by well MW-5. Benzene is also defined in groundwater east of the site by well MW-12. TPHg is defined below 200 ppb south of the site by well MW-3 and to the east by MW-12. Benzene is defined below 1 ppb in well MW-3. Well MW-7, located cross-gradient of the site, also contains TPHg and benzene. Cambria notes that concentrations in MW-7 are higher than those in wells MW-3 and MW-6, which are located between the former on-site USTs and MW-7. Well MW-3 contained SPH from April 1991 to April 1993. Historically, TPHg and benzene concentrations in wells MW-2,MW-3, MW-4, and MW-6 were significantly higher than they are currently. As discussed above, groundwater depths have risen approximately 25 feet since groundwater monitoring began at the site. Wells MW-1, MW-2, and MW-3 are screened from 45 to 65 fbg, and the well screens are currently submerged. Wells MW-4 through MW-7 are screened from 35 to 55 fbg. These wells are not currently submerged,but have been in the past. Estimated Mass Remaining In Soil The remaining hydrocarbon mass in soil was estimated by correlating an estimated average hydrocarbon concentration with a specific volume of impacted soil. The remaining hydrocarbon mass in soil was calculated using the following equation: Contaminant Mass(lbs)=Soil Volume(cu-ft)x Bulk Density(lbs/cu-ft)x Avg Concentration(mg/kg)x 10 kg/mg Cambria estimated the remaining TPHg and benzene mass in soil as they are the primary constituents of concern addressed by the CAP and this document. Site-specific methods for estimating average benzene concentration and soil volume are described below. 13