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Area of Concern 57

Soil Gas Survey Results, January - February 2001

Introduction

A soil gas survey was performed at AOC 57 located in the southwestern portion of CSSA. This report contains a summary of the analytical results from the AOC 57 soil gas survey. A detailed description of the methodology is included behind the Soil Gas Surveys tab in Volume 3-1.1. This work was performed in conjunction with soil gas surveys performed at SWMU B-3, SWMU B-4, AOC 55, AOC 63, AOC 65 and the WWTP from January 2, 2001 through February 23, 2001.

The distribution and concentrations for the compounds detected are addressed below. Minor amounts of benzene, toluene, and xylene were detected in soil gas samples throughout the soil gas survey. The detection of these compounds is attributed to artifact contamination from the gas generator used to power the vacuum pump and/or the combustion engine of the geoprobe truck. The frequency of detectable BTEX compounds decreased substantially after the gas generator was moved to the front of the geoprobe rig, and the rig was shut down during sample collection. The BTEX concentrations detected during the soil gas survey are presented in the data tables, but are not discussed in the soil gas findings. The complete laboratory results are presented in Appendix A. Sample locations are presented in the AOC 57 Sample Location Map, Figure AOC 57-1.

Determination of Sampling Locations

Grid locations at AOC 57 were set up based on historical maps that showed locations of temporary buildings in this area. A soil gas survey was performed to confirm that no VOC contamination existed in the vicinity of these buildings, which were reportedly used for gun cleaning and maintenance operations. One hundred twenty-nine (129) grid points were established in 100-foot intervals, which provided adequate coverage of the historical building location areas identified on the aerial photographs. The survey grid would have been tightened to 25-foot grids if any areas indicated contamination.

AOC 57 is long and narrow and extends approximately 4,000 feet from south to north along a 65-acre area. Because the locations of the buildings and topography did lie along a straight-line path, the AOC 57 grid was set up as a 3-part grid (designated North, Central, and South grids) to enable the grid points to provide adequate coverage and consistent spacing within each grid area around the former building locations. The layout of this grid is shown on Figure AOC57-1.

Soil Gas Survey Results

A total of 38 samples and two resamples were collected at AOC 57 North. The sampling locations are shown on Figure AOC57-1. Sampling depths varied from 0.5 to 4.5 feet and depended on refusal. The field team did not observe disturbed soil that would be associated with a landfill environment or concrete footings that would suggest the presence of old buildings. Laboratory-detected constituents are provided in Table 3.1. Plume maps for this AOC are not included because target VOC compounds were not detected.

A total of 67 samples were collected in the AOC 57 Central area. The sampling locations are shown on Figure AOC57-1. Sampling depths ranged from 0.5 to 4.5 feet and depended on refusal. The field team did not observe disturbed soil that would be associated with a landfill environment or concrete footings that would suggest the presence of old buildings. PCE was the only contaminant detected in this portion of the grid, and was reported at a low level in sample location AOC57C-65. This point is located directly east of Building 90 just across the road and is presented on the contour maps associated with the discussion of the AOC 65 results (Volume 3-2, Areas of Concern, AOC 65 site tab). Detected constituents are provided in Table AOC57-1.

Twenty-two samples were collected at AOC 57 South. The sampling locations are shown on Figure AOC57-1. Sampling depths ranged from 0.5 to 5 feet and depended on refusal. The field team did not observe disturbed soil that would be associated with a landfill environment. However, building pads associated with temporary structures were visible in the vicinity west of Building 92. Detected constituents are provided in Table AOC57-1. No figure is included in this section depicting the plume maps because target VOC compounds were not detected at this AOC.

Summary of Findings and Recommendations

From the soil gas survey results, it appears that no significant VOC contamination has been released to, or is present in, the soil and groundwater underlying AOC 57. No additional investigation activities are proposed at AOC 57 with regard to VOC contamination.

Table AOC 57.1 - North Detected Constituents

Table AOC 57.1 - Central Detected Constituents

Table AOC 57.1 - South Detected Constituents