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Ms Vicky Mc Cartney Page 5 of 6 For the groundwater transport modeling it was assumed that the hydraulic conductivity of the aquifer is 8 0"3 cm/sec, the hydraulic gradient is 0 005, the plume width at the source area is 40 feet, the vertical groundwater mixing zone is 10 feet, the distance from the source to the potential receptor is 65 feet (distance to SB2, no actual exposure), effective porosity of the aquifer material is 38%, the organic carbon content of the aquifer material is 0 1%, contaminants disperse with transport and the contaminants decay at the first order rates estimated by Howard To predict contaminant concentrations at our downgradient monitoring point(SB2), we ran the model using two approaches First, we ran the model using the maximum grab groundwater results as source zone concentrations Predicted concentrations under this model are as follows Compound Source Conc Predicted SB2 Conc Actual SB2 Conc Benzene 24 ppb 12 ppb <0 5 ppb Toluene 80 ppb 0 19 ppb <0 5 ppb Ethylbenzene 110 ppb- 16 ppb <0 5 ppb Xylenes 280 ppb 82 ppb <1 0 ppb We also ran the model using the maximum groundwater concentrations from well MW3, which was completed through the UST excavation Predicted concentrations under this model are as follows Compound Source Conc Predicted SB2 Conc Actual SB2 Cone Benzene 3 6 ppb 1 8 ppb <0 5 ppb Toluene 3 1 ppb 0 0073 ppb <0 5 ppb Ethylbenzene 37 ppb 5 3 ppb <0 5 ppb Xylenes 64 ppb 19 ppb <1 0 ppb The model predicts that the dissolved benzene concentration would reach the detection limit of 0 5 ppb at a distance of approxmiately 135 feet (using maximum concentration from well MW3) to 282 feet(using the maximum concentration from"grab" samples These predicted results suggest that the model is overly conservative Actual site conditions indicate that natural attenuation has effectively reduced contaminants to below detection limits approximately 65 feet downgradient of the source Modeling data are included in Attachment 5 5.0 SUMMARY AND CONCLUSIONS 5.1 Summary • Borings SB 1/SB 1 a and SB2 were drilled in the vicinity of the former USTs and downgradient of the USTs, respectively to further investigate the extent of soil and groundwater contamination associated with the former USTs • Groundwater was encountered at a depth of approximately 11 feet bgs The shallow water- bearing zone (well graded sand and well graded gravel) was encountered at a depth of approximately 12 feet bgs and extended to a depth of approximately 24 feet bgs, where an r underlying clay was encountered • Soil samples collected from boring SBI, which was drilled through the former tank pit, . indicated that mild soil contamination is present beneath the former tank excavation to G 1GR0UNDZENSouza\Reports\ADD_SUBSURF_SUM doc