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June 1996 - 6- 953-7001002 <br /> Based on water level data and the depths of the samples, it is apparent that these final <br /> confirmation soil samples were taken at depths at or slightly below the depth of shallow <br /> groundwater Because shallow groundwater occurs at a depth at or near the base of the <br /> former tanks it is apparent that groundwater was the mechanism for migration of the <br /> petroleum hydrocarbons away from the former USTs <br /> 3 2 Extent of Petroleum Hydrocarbons in Groundwater <br /> During removal of the underground tanks in 1988, floating product 4 to 6 inches thick was <br />' reported on the surface of the groundwater in the tank excavation Based on the <br /> groundwater sampling results, it appears that this product was removed during tank <br /> removal and/or was excavated <br /> Subsequently,between 1989 and 1992,a total of fourteen shallow and deep monitoring wells <br /> were installed at the site to assess the extent and nature of TPHg and BTEX in groundwater <br /> beneath the site Groundwater sampling activities have been performed since 1989 with <br /> three rounds of sampling conducted in 1995 as a condition for site closure Monitoring well <br /> MW-4S was destroyed and monitoring well MW-4D was damaged during remediation <br /> activities because excavation of soil was performed in these areas In addition, during <br /> remedial excavations at the site groundwater was pumped from the excavation prior to <br /> backfilling Currently there are wells remaining at the site that will be abandoned after case <br /> closure is received <br /> Historical groundwater sampling results and the September 1995 sampling results are <br /> summarized on Table 3 Laboratory reports, field sampling sheets and chain-of-custody <br /> forms for the last two rounds of groundwater sampling in May and September 1995 are in <br /> Appendix B Table 3 also includes the results of the sampling of purge water contained in <br /> drums on site <br /> The results of groundwater sampling and analysis for the site indicate that TPHg and BTEX <br /> have been generally non-detect However,sporadic detections of low concentrations of <br /> BTEX (i e,at or near maximum contaminant levels for drinking water) were reported in <br /> some wells The maximum historical concentrations reported for groundwater samples from <br /> monitoring wells at the site are shown on Table 4 <br /> Monitoring wells MW-4S and MW-4D were constructed immediately east and adjacent to <br /> the former USTs Petroleum hydrocarbons were noted and detected in soil samples near the <br /> shallow groundwater at these locations However, groundwater sampling results have <br /> indicated that TPHg was not detected in thirteen samples collected from well, MW-4s from <br /> March 1990 to October 1992 The well was abandoned and soil in this area was excavated in <br /> March 1993 BTEX was reported at concentrations of 0 003,0 0021, 0 0036 and 0 0074 <br /> milligrams per liter (mg/1) respectively, in only one of the thirteen samples over this time <br /> period BTEX was detected in the initial sampling of the adjacent well MW-4D that is <br /> screened deeper than MW-4S in March 1991 It is likely that the initial detection of BTEX <br /> was a result of smearing of TPH-contaminated soil or water during well construction This <br /> apparent cross contamination is typically suspected when the constituents detected initially <br /> 9537001 R01(7/3M) Golder Associates <br />