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AOC 66 Soil Gas Survey Results
December 1999
Introduction
A soil gas survey was performed at Area of Concern (AOC) 66 located in the north-central portion of Camp Stanley Storage Activity (CSSA). This report contains a summary of the analytical results from the AOC 66 survey. A detailed description of the methodology is included behind the SOIL GAS SURVEYS tab in Volume 3-1. This work was performed in conjunction with adjacent soil gas surveys at AOCs 37 and 41, and the entire survey area encompassed approximately 25 acres just north of Well 16 in the outer cantonment area.
Implementation
Sampling locations at AOC 66 are shown on Figure AOC66-1. A total of 100 samples were collected from 19 acres located northwest of Well 16 for BTEX compounds (benzene, toluene, ethylbenzene, m/p/o-xylenes), tetrachloroethene (PCE), trichloroethene (TCE), and cis-1,2-dichloroethene (DCE). All samples were collected and analyzed between December 8-15, 1999 by Parsons ES personnel. An oversight during the development phase of the investigation resulted in a work plan map that inadvertently omitted location AOC66-50. The adaptation by the field crew to remain consistent with the work plan strategy resulted with a final sampling point numbered AOC66-101, even though only 100 samples were collected. The sampling locations are correctly identified in Figure AOC66-1.
The soil gas samples were analyzed on-site at a field laboratory established within the CSSA Environmental Office. Work plan protocols presented Section 1.1.3 behind the Field Sampling Plan tab in Volume 1-4 were used to plan the soil gas survey. Specific procedures for the December 1999 surveys are presented behind the Soil Gas Surveys tab in Volume 3-1.
Soil Gas Survey Results
The distribution, concentrations, and contaminant associations are discussed below for the compounds detected in 100 soil gas samples collected at AOC 66. The sampling depths varied from 1.5 feet to 6 feet, and were usually determined by the refusal of the probe advancement. The average soil thickness was 4.1 feet. PCE and TCE were the only target compounds detected in soil gas samples from AOC 66. TCE was detected in only one sample, location AOC66-81, at a concentration of 0.04 mg/L. The TCE detection was co-located in the same sample as the highest PCE detection for the survey.
PCE was detected at 19 locations and ranged in concentration from 0.01 to 0.16 mg/L. The highest concentration occurred in the duplicate sample analysis from location AOC66-81. PCE was detected at a concentration equal to, or above 0.10 mg/L at three locations. The concentrations of PCE detected in the soil gas were generally spotty, with more than a third of the reported detections occurring along the boundary with AOC 35. At the outer cantonment fenceline, the PCE concentrations in soil gas appear to be consistent with previous results that indicated low concentrations of PCE extending from AOC 35 into the north pasture. Overall, the concentration and frequency of PCE detections in the north pasture were not as pervasive as in the inner cantonment area, and suggests that the suspected groundwater contamination source does not extend into the north pasture. The isolated occurrence of PCE at locations AOC66-61 and AOC66-81 could be related to gas migration focused along fractures or faults to these locations.
The recent and previous data collected at this site suggest that the site does not contain a source of VOC contamination. The soil gas survey data for AOC 66 is presented in Table AOC66-1 and illustrated in Figure AOC66-2. The gas chromatograms from the field analyses are presented in Appendix A.
The results of this soil gas survey combined with results from previous soil gas surveys performed on the land adjacent to AOC 66 suggest that the PCE plume terminates as it approaches the southern boundary of the north pasture. The PCE results from all soil gas surveys conducted by Parsons ES at CSSA were consolidated and the concentration gradients are illustrated in Figure AOC66-3.