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ISite Background Information Prem Dor 4 Page 5 of 7 Significant petroleum hydrocarbon-impact to soil in the vadose and smear zone is present in bonngs I B l and B2,in the area of the former USTs Total petroleum hydrocarbons quantified as gasoline and diesel(TPH-g,TPH-d)and benzene are present at maximum concentrations of 22,000 mg/kg,7,500 mg/kg and 1 1 mg/kg, respectively, all from sample B2-15 Most of the contamination appears to Ireside above the top of the clay layer,at approximately 29 to 36 feet bsg,except at B3,where TPH-g was detected at 35 feet bsg at a concentration of 900 mg/kg Contamination in ground water was present in all four soil boring locations and in the up-gradient well MW-2, the ground water sample from B4, collected between 53 feet and 57 feet bsg, in the second sand unit, indicated elevated amounts of contamination at a depth of 57 feet bsg ' Concentrations have not been detected in down-gradient well MW-1 and cross-gradient well MW-3 only had one very small detection of toluene and xylene (January 2004), both below State of I California Department of Health Services(DHS)Maximum Contaminant Levels(MCL)for drinking water I Lateral hydrocarbon-impact to soil is not defined toward the north, southeast and southwest of the former UST The vertical extent of soil contamination does not appear to exceed 40 feet bsg, with the exception of B3 at 35 feet bsg, and the low concentrations of hydrocarbons detected in samples Icollected at 35 feet from borings B1, B2 and MW-2 The lateral extent of ground water ie contamination is not defined toward the north, southeast and southwest of the former UST, and is not defined vertically, as contamination is present in the location of B4 at a depth of 57 feet bsg I HYDROGEOLOGIC MODEL ' Based on the soil boring and CPT results and comparison with soil descriptions from borings previously advanced on the site,AGE has developed a preliminary subsurface model of the site soil ' stratigraphy comprised of six informal zones-a vadose(unsaturated) zone and five hydrogeologic units Hydrogeologic units may be defined as distinctive vertical intervals of soil with a significant lateral extent and having similar hydrologic characteristics (i a hydraulic conductivity, Itransmissivity, etc) over its vertical and horizontal extents A `flow unit' may be defined as a hydrogeologic wit characterized by significantly greater hydraulic conductivity values than the ' overlying and underlying units, a flow unit is capable of serving as a preferred migration route,and could potentially allow migration of large contaminant masses over great distances I The model presented is interpretational and will be modified as more data is collected The six vertical zones identified are as follows (depths are approximate and can vary by several feet across the site) Vadose Zone - Extending from surface grade to approximately 18 5 feet bsg (average depth to the • ground water table), the vadose zone is generally comprised of silt, clayey silt, and silty clay, with I Advanced GeoEnvironmental,Inc. I