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Groundwater Investigation and Associated Source
Characterization Appendix F - Technical Memorandum on Soil Gas Surveys
Section 5 - SWMU O-1 (Oxidation Pond)
SWMU O-1 (the oxidation pond) is located immediately northwest of B-4. The oxidation pond was an evaporation pond constructed in 1975 and lined with vinyl plastic. Waste liquids and sludges from the bluing operation were collected and moved to the pond for disposal. The pond was filled with dirt and closed in the fall of 1985. The plastic liner was deliberately damaged by a bulldozer during the filling operation as evidenced by small scattered pieces of the liner lying on the ground. The oxidation pond is a small site with no vegetation cover and weathered limestone and white caliche at the surface. Limestone bedrock is apparently close to the surface in this area and corresponds with records indicating a depth of the pond of 2 to 5 feet. The site is flat, unobstructed, and surrounded by trees to the north, west, and south (Figure 5.1). There is evidence of scatttered metal debris on the surface, some of which appears to have been subjected to extreme heat.
PCE and TCE were detected in soil gas samples from the oxidation pond. Because the reconnaissance survey indicated that this site was a potential source area, additional sampling was conducted during the second phase of the soil gas investigation to define the extent of the source and to confirm the findings of the first survey. The soil gas survey data for the reconnaissance phase are presented in Table 5.1 and for the follow-up, detailed survey in Table 5.2. Because bedrock occurs at shallow depths at this site, sampling depth was determined by refusal depth. Initially, samples were collected at selected locations along the 25-foot grid spacing laid out over the site. In the second phase, samples were collected over the site at a 12 1/2-foot spacing and a few samples collected at a 6-foot spacing.
The distribution of PCE in the reconnaissance survey indicated a bull-eye pattern typical of solvent disposal or source areas (Figure 5.1). The highest concentration of PCE detected was 60,000 ug/L at location C,5. The highest TCE concentration was 20 ug/L, and it also occurred at location C,5. Location C,5 is near the site where a soil sample had 57 mg/kg of PCE in the soil at a depth of 1 to 2 feet (Parsons ES, 1995d). These high levels in both soil and soil gas suggest that some residual saturation of PCE may exist in the soil and will continue to be a potential source for groundwater contamination. The TCE was probably originally an impurity in the PCE. Significantly, biologically mediated breakdown products, such as the DCE isomers, were not detected. Oxygen readings for the soil gas indicate oxidizing conditions exist in the shallow subsurface. Oxidizing conditions inhibit the growth of the anaerobic bacteria that can metabolize or breakdown PCE and its daughter products.
The distribution of PCE within the oxidation pond was further defined in the detailed survey shown on Figure 5.2. Data from both the reconnaissance and detailed surveys were used to prepare Figure 5.2. the results seem to suggest that the highest concentrations, >10,000 ug/L, are confined to a limited area. the highest concentration detected during the second phase was 14,000 ug/L and occurred at location C.5, 2.5. Figure 5.2a shows data collected only in fall 1995. A comparison of Figure 5.2 and Figure 5.2a shows that the resulting contours are very similar.