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S3 PROSLEM ASSESSMENT REPORT Moore Petroleum, Inc. 14800 W. Frontage Road, Manteca, CA HGA Project No. 10-90-479 Page 13 4.0 nrscnsS1017 of rERuLL- 4.1 Extent of Petroleum Constituents in Soil Analyses of soil samples taken from soil borings and b.�ieath USTs and fuel product lines at the site indicate that the areal extent of petroleum constituents in soil is confined primarily to the vicinity of the USTs and Fuel pump islands. A smaller area where site soils contain detectible? concentrations of petroleum constituents appears to extend from the area of the pump islands to monitoring well MW-4. Soil sample analyses indicate that TPH concentrations in site soils are highest in the vicinity of the USTs and fuel pump islands and decrease rapidly away from these areas. Figure 6 presents the interpreted areal extent of petroleum constituents in site soils with concentrations greater than 10 ppm. The interpreted vertical extent of petroleum constituents in site soils is shown on the geologic cross-sections of Figures 4 and 5. These figures (based upon soil analytical results) indicate that appreciable concentrations of petroleum constituents are generally limited to a depth of about 20 feet.. TPH, concentrations in- soil samples taken at depths, greater than 20 'feet below grade do not exceed .3 ppm for soil samples collected at the site to'date. r. 4.2 Extent of Petroleum Constituents in Ground Water Results of recent ground-water sampling at the site indicates that petroleum constituents are present at concentrations exceeding California: mels at monitoring wells. MW-2, M11-3, MW-4. ,`and MW-5 The highest benzene concentration which has been detected in monitoring wells at the site is 1400 ppb which was detected at monitoring well MW-4 during the May, 1989, sampling event. Benzene concentrations in this well have decreased to 93 ppb at the July, 1990, sampling event. Benzene concentrations at MW-3 increased from 88 ppb (May, 1989) to 190 ppb at the July, ;1990 sampling event. Benzene concentrations at monitoring wells MW-7 and M14-8 have decreased From low levels measured during the first sampling event at these wells (January, 1990) to non-detectible levels during the latest round of sampling. There is inadequate data presently available to determine the significance (if any) of fluctuations in benzene concentrations at MW-3 and MW-4. The decrease in benzene concentrations at MW-7 and MW-8 could be due to the possible transport -of _--petroleum-- constituents _petroleumcons i uents- to-the water- table during drilling of these wells. Should this be the case, subsequent sampling events at these wells will probably indicate low or non-detectible concentrations of petroleum constituents. Figure 7 presents the interpreted extent of petroleum constituents in ground water at the site.