AOC 65 Soil Gas Survey Results

January - February 2001

Introduction

A soil gas survey was performed at AOC 65 located in the southwestern portion of CSSA. This report contains a summary of the soil gas survey analytical results from AOC 65 and from areas around Building 90. 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 57, AOC 63, and the WWTP.

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 65 Sample Location Maps, Figure AOC65-1 (Fig. 2.2), Figure AOC65-2 (Fig. 2.3), and Figure AOC65-3 (Fig. 2.4). Plume maps for AOC 65 and the surrounding area are presented in Figure AOC65-4 (Fig. 3.1), Figure AOC65-5 (Fig. 3.2), Figure AOC65-6 (Fig. 3.3) and Figure AOC65-7 (Fig. 3.4).

Determination of Sampling Locations

Grid locations at AOC 65 were placed inside and around Building 90. Two below grade vaults were located in Building 90. The first vault was located near the center of the building. This vault was backfilled with base material and capped with concrete prior to 1970. The second vault is located in the south central portion of the building. This vault housed a metal PCE/TCE tank that was used for gun cleaning purposes. Analysis of soil samples collected from beneath this tank in April 2000 found VOC contamination and suggested that a release of PCE/TCE has occurred. The former solvent vat used for weapons cleaning was removed in 1995 and replaced with a new cleaning process that utilizes citrus based material that does not contain VOCs. A 25-foot grid pattern was placed along the western side of the building while the grid attempted 25-foot spacing along the irregularly shaped East side of the building. The grid spacing was increased to 50-feet at locations farther from the building. A second grid was located South of Building 90 in an open field with 50-foot grid spacing to address possible contamination that may have migrated south from Building 90 toward the southern perimeter of the post. Three hundred and nineteen grid points were included in the AOC 65 grid, with 203 points located around Building 90, 40 points inside Building 90, and 76 points located in the Building 90 South grid area. The outer survey grid and the Building 90 South grid locations would have been tightened to 25-foot grids around any of the grid locations if contamination were indicated. The layout of the Building 90 and Building 90 South grids are shown on Figure AOC65-1 (Fig. 2.2) and Figure AOC65-2 (Fig. 2.3), respectively.

Forty grid points were sampled inside Building 90 (AOC 65). Twenty-five-foot grid spacing was attempted to address potential contamination underlying the building’s floor, primarily in the vicinity of the suspected contaminant source areas. The grid was modified to enable soil gas grid points to be located near the drain lines, subsurface metal solvent tanks, and along the centerline of the building. The grid layout inside Building 90 with structural features is presented in Figure AOC65-3 (Fig. 2.4).

Because the primary areas where contamination was encountered were inside Building 90 and within the 25-foot grid spacing outside, no grid tightening was necessary in any portion of AOC 65. CSSA decided to utilize the approximately 150 remaining grid points at other SWMUs or AOCs to assess the potential extent of VOCs in these areas. Results for these additional soil gas surveys are discussed in Volume 3-1.1, behind the Soil Gas Surveys tab.

Soil Gas Survey Results for AOC 65 and Areas around Building 90

A total of 203 samples were collected around the exterior of Building 90. Sampling depths ranged from 0.5 to 4.5 feet with depth determined by refusal. Compounds detected outside Building 90 included trans-1,2-DCE, cis-1,2-DCE, TCE, and PCE. Figure AOC65-4 (Fig. 3.1) presents the concentration contour map for TCE detected, and Figure AOC65-5 (Fig. 3.2) presents the concentration contour map for PCE detected during the soil gas survey. No figure was prepared for cis- and trans-1,2-DCE, which were only detected in samples collected from grid locations BLDG90-15 and BLDG90-16. Trans-1,2-DCE was detected in samples 15 and 16 with concentration of 0.77 and 0.05 mg/L respectively. Cis-1,2-DCE was detected in samples 15 and 16 also, with concentration of 0.88 mg/L and 0.08 mg/L, respectively. Both TCE and PCE were detected in the samples collected from both these grid locations. Detected constituents are provided in Table AOC65-1. The PCE detected in sample AOC57C-65 collected from AOC 57 is also presented in Figure AOC65-5.

TCE was detected in five samples (BLDG90-8, BLDG90-15, BLDG90-16, BLDG90-17, and BLDG90-37) located along the outer western wall of Building 90, as shown on Figure AOC65-4 (Fig. 3.1). TCE concentrations ranged from 0.04 to 8.56 mg/L, with the highest concentrations detected from sample BLDG90-15. PCE was detected in 67 samples at concentrations ranging from 0.08 to 1590.00 mg/L. The highest detected concentration (1590.00 mg/L) was detected at sample location BLDG90-15. The highest readings measured outside Building 90 correlate with the highest PCE levels measured inside the building in the vicinity of the former southern solvent vat (shown on Figure AOC65-5 (Fig. 3.4)). The plume map on Figure AOC65-5 (Fig. 3.4) also demonstrates the extent of the PCE plume migration to the southeast from Building 90. Sample BLDG90-8 was the only other grid location where PCE levels exceeded 5 mg/L. Sample BLDG90-8 is located along the outer western wall in the northern portion of Building 90. The identification of the PCE plume extending to the northeast and southeast of Building 90 plus the detection at sample BLDG90-8, suggests that PCE is present in the air-filled porosity of the fill material underlying the entire building, and that PCE has likely spread from the building into underlying soil, rock, and groundwater.

Soil Gas Survey Results for Building 90 South

A total of 77 samples were collected south of Building 90 at locations presented on Figure AOC65-2 (Fig. 2.3). Sampling depths ranged from 1 to 4 feet, with depth determined by refusal. No target VOC compounds were detected in this portion of the site. Detected constituents are provided in Table AOC65-1. A contaminant plume figure is not included in this section because target compounds were not detected in this portion of the AOC 65 grid.

Soil Gas Survey Results for AOC 65 (Inside Building 90)

A total of 40 samples were collected inside Building 90. Soil gas samples were collected at depths of 1.0 to 2.5 feet with the depth of sampling determined by refusal. TCE and PCE were both detected inside the building. Detected constituents for samples collected inside Building 90 are presented in Table AOC65-1. The sampling grid concentrated around the former metal solvent vault, the metal tank areas, and associated drainage lines. TCE was detected in samples AOC65-6, AOC65-7, AOC65-8, AOC65-9, and AOC65-10 at concentrations ranging from 0.14 to 0.44 mg/L, as shown on Figure AOC65-6 (Fig. 3.3). Of interest was the absence of TCE from samples AOC65-26 and AOC65-27, which are located in the immediate vicinity of the former southern solvent vat. AOC 65-26 has an elevated detection limit for TCE of 200 mg/L due to interference caused by the high levels of PCE encountered in the same sample.

Confirming the conclusion that PCE is present in soil gas underlying the entire building, PCE was detected in every sample at concentrations ranging from 1.77 µg/L to 24,820 mg/L. The maximum results concentrated around sample locations AOC65-8, AOC65-11, and AOC65-27, which were located in the vicinity of the former metal solvent tank located in the southern portion of the building. The PCE concentration contour inside Building 90 is presented on Figure AOC65-7 (Fig. 3.4). Detected constituents are provided in Table AOC65-1. The PCE plume highest levels inside matches the locations of the plume area identified outside Building 90, particularly in the outside grid points (90-15, 90-16, and 90-17) closest to the building near the southern solvent vat location. PCE results measured near the inactive northern solvent vat location suggest the potential for releases from this area were unlikely and that the fill material underlying this portion of the building contains similar levels of PCE as other non-impacted areas of the building.

Summary of Findings and Recommendations

The most significant finding of the soil gas survey was the detection of a PCE contaminant plume underlying Building 90 and extending primarily to the west and southwest from the building. The detection of TCE and cis-1,2-DCE and trans-1,2-DCE at significantly lower levels than PCE suggest that some natural degradation of the PCE has already begun near the southern solvent vat, which has been identified as the most likely source area for the VOC contamination. PCE levels exceeded 24,000 mg/L inside the building near the former solvent vat, decreased to 1,590 mg/L in sample BLDG90-15 approximately 25 feet from the building, and were not detected above 5 mg/L in any of the grid points located more than 50 feet from the building. Based on this result, it appears that the lateral extent of the PCE plume in the soil gas is generally confined to the immediate vicinity of Building 90. Additional investigation of the soil and groundwater around Building 90 are required to determine the extent and primary direction of PCE migration from the source areas and the potential impacted media.

Soil borings have been completed for the AOC 65 and AOC 67 RFIs in and around Building 90 to further investigate the extent and contamination levels of TCE and PCE migrating from beneath the building. The soil boring locations are presented in Figure AOC65-8 (Fig 4.1). Details of the soil borings and monitoring well data around Building 90 are presented in the AOC 65 and AOC 67 RCRA Facility Investigation Reports (Parsons, November 2001) located in Volume 3-2, Areas of Concern behind the AOC 65 and AOC 67 tabs. The completed monitoring wells are being sampled as part of ongoing RFI activities. Source removal of readily accessible contaminated soil or installation of a SVE system will be considered as an interim measure if high levels of contamination are indicated in the soil borings near the building to minimize potential exposure to VOCs accumulating in the building, and possibly migrating vertically to underlying groundwater through the fractured limestone.