SITE INFORMATION AND CORRESPONDENCE_1988-1991

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Remedial Investigation Report -4- 8 March 1991 DDRW, Sharpe RESPONSE 7. I concur with the finding. However, the manner in which the soil data were collected was biased and may have been based on invalid assumptions. The soils- gas survey was conducted to locate areas of high concentrations of the vapor phase of trichloroethylene (TCE) only. No other volatile organic constituents (VOCs) were considered for this investigation. TCE is known to degrade into other VOCs and TCE is the primary degradation product of tetrachloroethene (PCE) . Therefore, conclusions regarding the different potential source areas may be biased. Additionally, the soil borings samples were collected from the clayey portions rather than the coarser grained portions. This was based on the assumption that dissolved phase of VOCs adsorb to clays. This is incorrect. Non-polar VOCs tend to move through the clays. If it were true that VOCs adsorb to clays, then it would be anticipated that the soil samples would have had consistently higher concentrations throughout the unsaturated zone and that the VOC concentrations in the ground water would be greatly reduced due to the large quantities of clayey sediments in the subsurface. FINDING 8. The sources of VOCs on SHAD are multiple and originate from several former disposal pits/trenches and/or spill areas. Many separate source areas have been identified by the presence of contaminated soil gas, soil , and/or ground water. RESPONSE 8. I concur with this finding. FINDING 9. Seven distinct VOC ground water plumes have been identified at SHAD (see Figs 9.0- 2 through 9.0-5) . They are as follows: Plume 1--South Balloon Area; Plume 2-- wells 441 and 422; Plume 3--well 407; Plume 4 and 5--wells 442, 443, 437, and 504; Plume 6--Firefighter Training Area; Plume 7--North Balloon Area TCE; and Plume 8-- well 413 TCLEE. RESPONSE 9. I do not concur with the finding that there are seven plumes at the site. The contouring of the TCE concentrations on the plan view maps and cross-sections appears to have been done incorrectly in some areas. Therefore, it is possible that there are fewer plumes than currently thought at the site. Based on the concentration contour map (Figure 9.02) and concentration cross- sections, there is strong evidence that Plume I and Plume 3 may be one plume. There are no wells between the leading edge of Plume 1 and the upgradient edge of Plume 3 which have TGE concentrations below 5 micrograms per liter (µg/1 ) . Additionally, as shown on the concentration contour map, the distance between these plumes is only about 300 feet. The Report states that are no reports of waste disposal and that the soil gas survey indicated relatively minor contamination in the area of monitor well cluster 407.