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City of Stockton A68104 02A June 6, 1996 Page 6 ' 3.0 PURPOSE AND SCOPE The purpose of this work is to provide an estimate of the mass of residual soil contamination at the project site and to provide the specifications for the proper destruction of monitoring well MWA 4.0 PROPOSED PROCEDURES 4.1 Residual Soil Contamination Calculation The purpose of a residual soil contamination calculation is to provide an estimate of the mass of contaminants (i e PPCs) in the sub-surface soil at the project site The parameters required to make this calculation include the volume of soil containing PPCs, and the concentrations of PPCs in the soil Analysis of the soil sample collected from the bottom center of the excavation (B19 5) at 19 5 feet BSG on July 7, 1994 detected PPCs However, PPCs were not detected in soil samples collected at 20 and 25 feet BSG in the soil boring for monitoring well MW-1 drilled below the former UST location This data suggests that PPC migration from the release did not extend vertically to 20 feet BSG The analysis of the soil samples collected on July 7, 1994 from the eastern and western edges of the excavation at 12 and 13 feet BSG, respectively did not detect PPCs This data suggest the PPCs did not extend further than approximately two feet from the former UST location to the east or west The analysis of the sot] sample collected from the northern and southern edges of the ' excavation at 12 and 10 5 feet BSG respectively, did detect PPCs The soil sample analytical results from soil borings B-2 and B-3 were non-detect for the selected PPCs This data suggest that the lateral extent of PPC migration from the former UST location did not exceed approximately 10 feet (the approximate distance from the former UST location to the nearest soil sample collection location at 15 feet BSG in soil boring B-3) ' Soil sample analytical results for the sample collected at a depth of 3 feet BSG from soil boring B-1 were non-detect for PPCs This data suggest that there was no release of PPCs at the former dispenser location ' Evaluation of the extent of PPC migration in subsurface soil is based on the assumptions that the maximum concentrations of PPCs in soil are expected to be directly beneath the source (i e the former UST location) and that PPC concentrations attenuate at a uniform rate as distance from the source increases Based on the findings at this site, the area of soil impacted with PPCs at approximately 15 feet BSG is inferred to be oval shaped, approximately 20 feet long and seven feet wide oriented along a north-south axis as shown in Drawing 6 A cross-section view of the inferred volume of PPC impacted soil is presented in Drawing 7