January - February 2001

Introduction

A soil gas survey was performed at SWMU B-4 located in the central portion of CSSA. This report contains a summary of the analytical results from the SWMU B-4 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, AOC 55, 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 SWMU B-4 Sample Location Map, Figure B4-1.

Determination of Sampling Locations

SWMU B-4 is located southeast of a former oxidation pond (SWMU O-1). It was selected for soil gas testing because it is located in the vicinity of SWMU B-3 and SWMU O-1. Like both of these units, SWMU B-4 was identified as a classified burn area, so the potential exists for some VOC contamination to be present. A total of 20 grid locations were sampled on a grid layout that extended over the estimated lateral extent of SWMU B-4. Grid spacing was approximately 25 feet east-west, and 50 feet north-south. The grid layout for SWMU B-4 is presented on Figure B4-1.

Soil Gas Survey Results for SWMU B-4

A total of 20 soil gas samples were collected at SWMU B-4 at locations presented on Figure B4-1. Benzene was detected at sampling point B4-1 at a concentration of 0.02 m g/L. Detected constituents are provided in Table B4-1. No figure is included in this report depicting a contaminant plume in this grid layout because the target compound benzene that was detected at a concentration of 0.02 m g/L is attributed to the geoprobe rig and gas generated exhaust. No other target VOC compounds were detected. Soil gas sampling depths varied from 2 feet to 10 feet, and were usually determined by refusal. Within SWMU B-4 there appears to be three landfill cells associated with past waste disposal activities. Historical data suggest that this area was used to dispose of small caliber ordnance.

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 SWMU B-4. No additional investigation activities are proposed at any of the SWMUs with regard to VOC contamination.

Table B4-1 - SWMU B-4 Detected Constituents