January - February 2001

Introduction

A soil gas survey was performed at AOC 55 located in the south-central portion of CSSA. This report contains a summary of the analytical results from the AOC 55 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 57, 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 55 Sample Location Map, Figure AOC55-1.

Determination of Sampling Locations

AOC 55 was used as a landfill in the past and is located south of Tenburg Drive, east of Salado Creek, and south of SWMU B-4. AOC 55 was selected for soil gas testing to determine whether VOCs may be associated with this disposal area. A total of 54 grid locations were sampled on a grid layout that extended beyond the estimated lateral extent of the AOC. Grid spacing was approximately 100 feet on the north side of the road and 50 feet on the south side of the road for the 54 grid points. The grid layout for AOC 55 is presented on Figure AOC55-1.

Soil Gas Survey Results for AOC 55

A total of 54 soil gas samples were collected at AOC 55 at locations presented on Figure AOC55-1. PCE was detected in three sample locations (AOC55-17, AOC55-18, and AOC55-27) at concentrations ranging from 0.08-0.11m g/L. The highest detectable level was measured in sample AOC55-27. Upon review of the data, it was discovered that high levels of PCE were detected in the samples collected from SWMU B-3 analyzed immediately prior to testing these three samples, so that PCE detected at AOC 55 may have resulted from cross-contamination within the dessicator, the vacuum pump, rods, or from the laboratory. Based on the results from the remaining sample locations at AOC 55 and the samples preceding these three grid points, the possibility that cross-contamination is the source of the detectable PCE is likely. Because this review occurred after completing the field work, there was not an opportunity to re-sample the locations to verify that PCE does not actually exists in the AOC 55 soil gas.

Detected constituents are provided in Table AOC55-1. No figure is included in this report depicting a contaminant plume in this grid layout because target VOC compounds detected resulted from cross contamination as discussed above. Low levels of benzene in four samples and toluene in one sample are attributed to the geoprobe rig and/or gas generator exhaust. Target VOC compounds detected at this site are not a result of the contaminant plume. Sampling depths varied from 1 foot to 8 feet below grade and were usually determined by refusal. AOC 55 appears to be a landfill. The field team observed numerous horseshoes and assorted metal debris.

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 55. No additional investigation activities are proposed for AOC 55 with regard to VOC contamination.

Table AOC55-1 - AOC 55 Detected Constituents