SITE INFORMATION AND CORRESPONDENCE

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FROM 1997,` 14 17:45 #888 F.08/24 February 14. 1997 Paue 7 data includinc, influent concentrations, flow rates, and the operating time is presented in the table included in Attachment A. The calculations for determining TPPH-g mass in soil and groundwater utilized soil boring and analytical data to estimate the aerial extent of the hydrocarbon plume. Aver- age concentrations were applied to distinct cross sectional areas in order to arrive at the total hydrocarbon mass both prior to and after SVE treatment. Figures 3 through 5 depict soil conditions beneath the site prior to operation of the SVE system (November 1990 - November 1992). Figures 6 through 8 depict these conditions after the operation of the SVE system. Well construction details are presented on the boring logs included in Attachment C. Calculations for determining the TPPH-g mass in soil before and after the operation of the SVE system are presented as Attachment A. Details of the methodology used to calculate the volume and estimate average concentration of the remaining soil contamination are also presented in Attachment A. In their letter dated November 26, 1996 the S)CEHD commented that the report failed to include the June 1-2, 1995 sample's T(11', organic carhrnr and dry hulk densip, res7ilts.vhich ranged from 2.35 to 1.17 gi'em-. Actually, when calculating the estimates of soil contaminants Ln7 average soil density,of/20 lhs was used The above mentioned data was inadvertently omitted and presented in this report as Table 3. The arithmetic average dry bulk density presented on Table 3 is 1.6 g/cm' (100 lb/ft)- Concentrations of TPPH-g tend to reside in sandy to clayey silt, or silty sandy clay. The average bulk density calculated from representative values in Table 3 is 112 lbW. This is in agreement with published values for the density of silt and stiff clay which range from 100 to 135 and 105 to 130 lb&respectively (Engineering Geology,Matheson, 1981, p.12). Therefore PACIFIC used a soil density of 120 lb/ft'for calculating a conservative estimate of remaining petroleum hydrocarbons in soil. RESIDUAL PETROLEUM HYDROCARBONS IN SOIL Currently, residual petroleum hydrocarbons in soil occur below and northeast of the former UST cluster(installed in 1984) and adjacent product island_ The vertical and lateral extent of petroleum hydrocarbons remaining in soil are shown on Figures 6 through 8. These hydrocarbon-affected soils occur in what can be described as two localized vertical soil columns. Hydrocarbon-impacted soils in these two vertical soil _columns occur from a depth of approximately 15 to 50 feet bgs. The horizontal extent of hydrocarbon-affected soils within these soil columns occur primarily within a range of 5 to 10 feet,with occasional thin zones that extend horizontally an additional 10 feet from the soil column. These soil columns occur within vadose zone soils, soils within the capillary fringe and zone of groundwater fluctuation, and currently saturated soils. The estimated mass of petroleum hydrocarbons remaining in place is approximately 32013378)Cr.C)SLMM