ARCHIVED REPORTS XR0000194 - Laserfiche WebLink

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R10/71-51-1C 6 0 DATA EVALUATION AND DISCUSSION As described above, gasoline-derived contaminants in soil were detected in sampling locations MW13, MW14, and SB17 which are situated around the gravel area where underground tanks were removed Figures 12 through 25 (see Appendix J), present zero concentration maps for benzene, toluene, and xylenes at horizons from 5 feet to 70 feet (the approximate depth where groundwater was encountered) Figures 10 and 11 (see Appendix 1) present geologic cross-sections showing concentrations of benzene, toluene, and xylenes Analysis of their spatial distribution horizontally and vertically suggests that the contaminants migrated generally downward from leaking tank(s) through a sequence of silty clay and silt to a sand unit that generally occurs at approximately between 50 to 60 feet below grade in the affected area Review of the zero concentration maps (for benzene, toluene, and xylenes taken together) suggest that the contaminated area, centered near the former fuel underground tank location, increase from an area of approximately 17,700 square feet from 5-foot below grade, increasing to approximately 24,000 square feet at 25-foot depth, maximizing to approximately 49,000 square feet at 45-foot depth, and then gradually decreasing to approximately 20,000 square feet at the groundwater interface The impacted soil, contaminated with gasoline-derived compounds which include benzene, toluene and xylenes has a volume of approximately 65,200 cubic yards Groundwater contamination was found to be highest in MW10 which is located within the former underground tank area Gasoline-derived contaminants were also found in lower levels in wells MW11, MW12, MW13 and MW14 Contaminant levels in MW10 and MW11 exceeded State action levels for drinking water and EPA maximum contaminant levels (MCLS) for Public Water Supplies (see Table 9, Appendix C) Figures 26, 27, and 28 (See Appendix J) present zero concentration maps of benzene, toluene and xylenes, respectively, in the groundwater The maps indicate that the contamination generally moves in the northeasterly and southeasterly directions This distribution appears to be supported by the groundwater flow direction illustrated in Figure 29 (Appendix K) Based on the water level data collected on October 22, 1993, the groundwater elevation contour map is presented in Figure 29 (see Appendix K) The groundwater flow direction in the vicinity of monitoring well MW8 is to the west, in the vicinity of MW9 is to the east, and in the vicinity of MW1, MW2, MW3, MW4, MW10, MW14, and MW24 is to the north The area of the groundwater impacted by gasoline- derived contaminants which include benzene, toluene, and xylenes has an approximate length of 350 feet (from the north boundary to the south boundary) and a width of approximately 300 feet (from the east boundary to the west boundary) It occupies an 19