SITE INFORMATION AND CORRESPONDENCE

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cm/year(14 incheslyear) based on actual rainfall amounts, with 6 storms per year lasting 2 days each over a 5 month period. 3) Soil Column Data The sail column was modeled with a 25%effective porosity over the entire interval, a conservative estimate based on the silty clay/silty sand lithology at the site.The soil bulk density was assumed to be 1.65 g/cm*3, consistent with other measured bulk densities in the Stockton area (#9-1918).The hydrocarbon source area was assumed to be the area surrounding the former middle UST(determined by soil sampling to be the highest impacted area) and was modeled as measuring 7m by 7m. A) Layer 1 - Practical Scenario Layer 1 was modeled as a 7.62 meter thick(from 0'-25'depth) silty clay layer based on site boring logs.The intrinsic permeability was conservatively estimated to be 1 e-10 cm*2 ( 1e-4 cm/sec) based on lithology.The fraction Organic Carbon was conservatively estimated to be 0.05% (500 mg/Kg)for this layer. Chevron site#9-1916 had a 0.175% (1750 mg/Kg) organic carbon content based on two soil measurements. The mass of BTEX and Naphthalene (surrogate for TPH-gas)were determined for this layer by inputting the highest concentrations of these contaminants found in the onsite soil borings and monitor wells into the following formula: Mass (Kg) = Contamingnt matft*Volume it * *Soil Densily(KUIrn*j 1 e+6 mg/Kg where the volume of impacted soil was defined as 7m by 7m by 7.62 m for Layer 1 or 373 m*3; and the soil density is defined as 1650 Kg/m*3. Benzene was never detected in site soil borings or monitor well soils,therefore,the mass of benzene for this scenario was modeled as 0 Kg. Soil results from MW-415' (To 0.042 mg/Kg, E @ 0.46 mg/Kg, X @ 9.1 mg/Kg and:TPH-g @ 130 mg/Kg) had the highest concentrations of TEX and naphthalene for the site.The toluene mass was calculated to be 0.026 Kg;the ethylbenzene mass was calculated to be 0.28 Kg;the xylene mass was calculated to be 5.6 Kg and the TPH-g mass was calculated to be 80 Kg for the source area. B) Layor 1-Conservative Scenario The modeled parameters for this scenario differed only in the mass of contaminant present in the layer. The analytical results from sample#218 (18 foot depth in UST pit) for benzene,toluene and naphthalene and from MW-4-15'for ethylbenzene and xylene were used to derive BTEX and TPH-g mass for this scenario using the above equation. The results were: Benzene at 7.39 Kg,toluene at 23.4 Kg, ethylbenzene at 0.28 Kg, xylene at 5.6 Kg and TPH-g at 677 Kg. C) Layer 2- Practical and Conservative Scenarios Layer 2 was modeled as a 6.1 m thick(20 feet,from 25'-45') silty sand layer based on lithology logs from the site soil borings.The intrinsic permeability was modeled as 1 e-8 cm*2 (1 a-2 cm/sec) and the fraction Organic Carbon was assumed to be 0.05% (500 mg/Kg). Xylene was detected at a maximum concentration of 0.038 mg/Kg in the MW-4- 35' sample. The mass of xylene in Layer 2 for both Scenarios was 0.019 Kg. Because there was no BTE or TPH-g detected from 25-45 feet in soil boring SBI, located