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Atlantic Richfield Company Page I I <br /> . ARCO Station No 4932 Stockton Cahfomia August 29 2002 <br /> 4.0 EXTENT OF PETROLEUM HYDROCARBON IMPACTS <br /> 4.1 Extent of Adsorbed Petroleum Hydrocarbons <br /> During excavation of the gasoline UST's in May and June 1999, approximately 631 tons of <br /> soil were over-excavated from the UST complex and 150 tons of soil were over-excavated <br /> from the product line and dispenser area Concentrations of TPHg, benzene, and MtBE in <br /> soil analyzed from previous Investigations are included in historical tables presented in <br /> Appendix C <br /> Concentrations of TPHg and BTEX have been detected in soil samples collected up to 25 <br /> feet bgs in the area of the existing northernmost dispensers and product lines Relatively <br /> minor concentrations of TPHg and BTEX have been detected up to 50 feet bgs in the areas <br /> downgradient from the station, near the UST complex and the western product lines and <br /> dispensers The highest concentrations of TPHg were found in the area of the northernmost <br /> dispensers and product lines at 5,430 mg/kg (D-4 @ 11') The maximum benzene and MtBE <br /> concentrations detected in soils across the site were 23 mg/kg (D-5 @ 4') and 684 mg/kg <br /> • (MW-4 25 ), respectively Concentrations of petroleum hydrocarbons, benzene, and MtBE <br /> were not detected in any soil samples from borings for wells MW-3,MW-5, MW-7,MW-8, <br /> MW-9, MW-10,MW-11, and SB-1 <br /> Estimates of the mass of hydrocarbons adsorbed to the soil were calculated using the <br /> formula <br /> AxHxDxX = P <br /> Where <br /> A= impacted area <br /> H= height of impacted area <br /> D =Average soil density <br /> X = average concentration <br /> P= mass of contamination <br /> The soil density was determined by averaging clay and sand soil density The volume was <br /> calculated according to the extent of hydrocarbons reported in samples collected from <br /> monitoring wells, soil borings, and site upgrade activities The total hydrocarbon mass <br /> adsorbed to the soil is approximately 14,083 pounds or 2,012 gallons Tables and figures <br /> showing the mass calculations are included in Appendix K <br /> 4.2 Extent of Dissolved Petroleum Hydrocarbons <br /> 10 Groundwater monitoring wells at the site have been monitored since September 2000 <br /> Elevated concentrations of TPHg, BTEX, and MtBE have been and continue to be detected <br /> I 1ARM4932TARTAR doc <br />