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' PROBLEM ASSESSMENT REPORT 6131 Pacific Ave., Stockton, GA Aegis Project No. 90-020 Page 11 4. 0 DISCUSSION OF RESULTS The following subsections discuss the results of the subsurface investigation. Data collected during the preliminary subsurface investigation and tank closure are incorporated in the discussion (Aegis 5/14/90) . 4 .1 Extent of Petroleum_Hydrocarbons in Soil As concluded in the preliminary subsurface investigation /tank closure report the primary source of petroleum hydrocarbons in ry�Q soil appears to have been a hole in the bottom of the regular unleaded tank (Tank #3) . Petroleum hydrocarbons in excess of 1000 ppm PHV in soil samples recovered from VE-2 , drilled directly through the original tank basin, appears to confirm this 1 conclusion. The potentially effected vertical extent of soil containing petroleum hydrocarbons is from the surface to the lowest seasonal occurrence of the ground water table, approximately 57 feet below grade. The horizontal extent of petroleum hydrocarbons in soil, based on the analysis of fifteen soil samples collected from the soil borings is shown on Figure 6a and 6b. Figure 6a shows benzene iso-concentration contours from soil samples collected from between 15 and 25 feet below grade. As shown on the figure, the highest concentrations of benzene in soil are located around the former underground tank location. Figure 6b presents benzene iso-concentration contours from soil samples collected between 40-50 feet below grade. Figure 6b shows a geometric pattern of occurrence similar to that of benzene concentrations at shallower depths. Figures 6a and 6b as well as the laboratory analysis indicate high concentrations of petroleum hydrocarbons centered around the original tank basin. The figures and the laboratory data also indicate a vertical continuity of petroleum hydrocarbons from the surface to the ground water table. The data also indicates that PHV concentrations increase with depth. 4.2 Hydrogeologic Conditions ' As indicated by Figure 5 ground water flow direction is to the southeast. The calculated hydraulic gradient is 0. 008 ft/ft. The values for S ranged from 1. 111 x 10`4 to 1. 111 x 10-2 , T ranged from = 1. 499 x 102 to 1 . 845 x 102 gpd/ft and K ranged from 1. 478 x 102 to 1. 819 x 102 gpd/sq ft. The calculated transmissivity and hydraulic conductivity differ by less than an order of magnitude between the slug in and slug out tests, and can interpreted as reasonable estimates for the hydrogeologic parameters of the materials adjacent to MW-3 .