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Groundwater Investigation and Associated Source
Characterization Appendix F - Technical Memorandum on Soil Gas Surveys
Section 10 - Summary
Soil gas surveys were performed at thirteen sites on CSSA to determine possible source area(s) for the chlorinated hydrocarbon contamination present in groundwater at well 16 and other CSSA monitoring wells. During the initial reconnaissance survey, a total of 234 soil gas samples were collected from the sites listed in Table 10.1 and an additional 298 samples were collected in the follow up investigation. A summary of compounds detected and their maximum concentrations is also included in Table 5.3-1 of the report to which this memorandum is appended. Soil gas samples were analyzed for select volatile organic compounds including trichloroethene (TCE), tetrachloroethene (PCE), and cis-1,2-dichloroethene (DCE) because these compounds have been detected in well 16 and other monitoring wells on CSSA. In addition, soil gas samples were also analyzed for benzene, toluene, ethyl benzene, and total xylenes, together referred to as BTEX, to test for fuel contamination during the initial reconnaissance survey. BTEX were not detected in any of the soil gas samples from any site.
The soil gas survey results indicated that both TCE and PCE were detected at concentrations indicative of source areas at B-3 (Figure 5.3-2 of the groundwater report). As indicated by TCE concentrations greater than 1,000 ug/L and PCE concentrations greater than 100 ug/L, a probable source area for TCE and PCE is likely located near the central portion of the site near the existing gravel road with an additional TCE source located along an abandoned road to the west of the main gravel road. DCE was also detected in association with TCE and PCE anomalies. The occurrence of DCE is probably the result of the anaerobic decay of PCE and TCE.
The oxidation pond was determined to be a source of PCE contamination as indicated by PCE concentrations greater than 10,000 ug/L. In relation to PCE concentrations, only minor amounts of TCE were detected at the oxidation pond.
Although PCE was detected at all the other sites and TCE was also detected at several SWMUs, no source areas capable of causing the groundwater contamination observed in well 16 or the other CSSA wells were indicated at the other sites. No soil gas anomalies were present at Gate 6 and only one minor PCE anomaly, less than 1 ug/L, was present at B-2. The occurrence of PCE and TCE at sites adjacent to or near B-3 and the oxidation pond is probably the result of migration from source areas in B-3 and the oxidation pond as suggested by the general decrease in PCE concentrations from those source areas.