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Groundwater Investigation and Associated Source
Characterization Appendix F - Technical Memorandum on Soil Gas Surveys
Section 3 - SWMU B-3
SWMU B-3 is located in the inner cantonment southeast of well 16. This site was a landfill area primarily used for garbage disposal and burning. The landfill was covered in 1990-1991. Miscellaneous debris was found on the ground (plastic, burned wood, fuel filter, metal bands, etc.) during the field investigation. The site is characterized by north-south oriented dirt roads, a short, steep downhill slope to the west, a gradual uphill slope to the east, limestone outcrop and fragments to the east and southeast, and a small depression or "crater" in the north-central area (Figure 3.1). Also present are aboveground steel pipes on the northern perimeter of the site which are believed to continue underground towards well 16 to the northwest. Minor amounts of surface metal are present at isolated locations near the "crater" and at the southwest corner of the site. The perimeter of the site is bounded by trees and other vegetation.
Soil gas samples were collected from this site during both phases of the soil gas investigation. Initially a reconnaissance survey was performed at the site in which several of the target compounds and some non-target compounds were detected in soil gas samples. A more detailed survey was performed during the second phase to better define probable groundwater contamination source areas. The soil gas survey data from the reconnaissance survey are presented in Table 3.1 and from the detailed survey in Table 3.2.
A depth profile was performed at location A,0 near the northeast corner of the landfill to examine the distribution of contaminants with depth. There was not much difference in the concentration of PCE at 6 feet, 0.48 ug/L, versus that at 3 feet, 0.38 ug/L. Soil gas samples could not be obtained from many areas in the northeast corner and along the eastern edge of the site due to shallow soil cover or bedrock outcroppings.
The initial soil gas survey results indicated that both PCE and TCE were detected at concentrations indicative of source areas at SWMU B-3 (Figure 3.1 and Figure 3.2, respectively). A probable source area for TCE and PCE exists near locations C,3 and C,4 as indicated by TCE concentrations greater than 1,000 ug/L and PCE concentrations greater than 100 ug/L. Additionally, another PCE source area may exist near location C,1. cis-1,2-DCE and sometimes trans-1,2-DCE were detected in association with TCE and PCE anomalies (Figure 3.3). Under anaerobic conditions, TCE and PCE can be utilized by mircroorganisms and chemically broken down to cis- and trans-DCE. The occurrence of TCE can also result from the anaerobic decay of PCE. The high concentrations of cis-DCE versus that of trans-DCE at B-3 suggests that the occurrence of these compounds is from the anaerobic decay of PCE and TCE. The soil gas survey indicated anaerobic conditions, low oxygen concentrations in soil gas, exist within the landfill.
A follow-up soil gas survey was performed in November and December 1995 to define the source areas within B-3. This survey was performed on a 20-foot spacing over the previously identified possible source areas. Samples were collected along a 5-foot spacing in areas that had drastic concentration gradients. This sampling defined the source areas previously identified. The additional sampling PCE results were similar to the initial sampling event (Figure 3.4). The second survey identified an additional potential TCE source area within the landfill near location A.4,3.0 (Figure 3.5). DCE results were generally similar to the initial survey, but an area of high cis-1,2-DCE concentrations occurred coincident with the new TCE anomaly (Figure 3.6).