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AOC 41 Soil Gas Survey Results
December 1999
Introduction
A soil gas survey was performed at Area of Concern (AOC) 41 located in the north-central portion of Camp Stanley Storage Activity (CSSA). This report contains a summary of the analytical results from the AOC 41 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 66, and the entire survey area encompassed approximately 25 acres just north of Well 16 in the outer cantonment area.
Implementation
Sampling locations at AOC 41 are shown on Figure AOC41-1. Forty samples were collected from 6 acres located northeast of Well 16 for BTEX compounds (benzene, toluene, ethylbenzene, m/p/o-xylenes), tetrachloroethene (PCE), trichloroethene (TCE), and cis-1,2-dichloroethene (DCE). A total of 35 samples followed a grid-like pattern over the extent of the site, while the remaining 5 samples focused upon anomalous areas identified in past geophysical and soil gas surveys. All samples were collected and analyzed between December 6-15, 1999 by Parsons ES personnel. 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 40 soil gas samples collected at AOC 41. 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 4.5 feet. PCE was the only target compound detected in soil gas samples from AOC 41.
PCE was detected at the southwestern corner of the site at locations AOC41-1 and AOC41-11. Both samples were reported with a concentration of 0.02 mg/L. In a prior soil gas survey (June-July 1995), PCE had been detected at concentrations of 0.01 to 0.07 mg/L in the vicinity of locations AOC41-20, AOC41-25, AOC41-28, and AOC41-33. However, these results needed to be flagged based on the detection of PCE in blanks. 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 41 is presented in Table AOC41-1 and illustrated in Figure AOC41-2. 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 41 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 AOC41-3 (not available electronically).