Introduction

A soil gas survey was performed at SWMU B-3 located in the north-central portion of CSSA. This report contains a summary of the analytical results from the SWMU B-3 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-4, 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-3 Sample Location Map, Figure B3-1. Plume maps for SWMU B-3 are presented in Figure B3-2 and Figure B3-3.

Determination of Sampling Locations

SWMU B-3 is the location of extensive SVE treatability testing. SVE is an existing active treatment system operating at the SWMU. The SVE system was initially installed based on results from the soil gas survey preformed in June of 1995 (Phase I) and November/December of 1995 (Phase II). The 1995 soil gas survey grid covered all of the area reported in the current survey as well as the limestone outcrop to the east. Results from this report are located in the Environmental Encyclopedia (Volume 3-1, SWMUs, Soil Gas Surveys, Technical Memorandum on Soil Gas Surveys, June 1996). High levels of chlorinated VOCs were encountered in the trench at SWMU B-3, and continue to be removed by active SVE treatment. Most of the soil gas samples were collected at this SWMU to assess the relationship of VOC levels measured in the trench during the treatability study and subsequent operations and maintenance of SVE with levels measured using the soil gas survey techniques (maximum depth of 12 feet). They were also collected to investigate soil gas levels present in other portions of the SWMU such as the east trench. The locations of these additional grid points provide current soil gas data for the SWMU B-3 trench located to the east of the main trench, and provide additional data for the soil gas points located along the eastern edge of the main trench in the vicinity of the existing SVE system. Sample B3-4 was actually collected from vapor extraction well 2 (VEW-2) of the SVE system. The SWMU B-3 grid layout is presented in Figure B3-1. This figure includes the existing SVE system layout relative to the sampled soil gas survey grid. A total of 29 additional soil gas samples were collected as part of the soil gas survey at SWMU B-3.

Soil Gas Survey Results for SWMU B-3

A total of 29 soil gas samples were collected at SWMU B-3. PCE, TCE, cis-1,2-DCE, trans-1,2-DCE, and vinyl chloride were detected in soil gas samples collected from SWMU B-3 during this soil gas survey. Soil gas sampling depths varied from 2 feet to 12 feet and sampling depths were generally determined by refusal. A previous soil gas survey was performed at SWMU B-3 in 1995. This survey covered the current survey area and the area to the east. Results from this report are located in the Environmental Encyclopedia (Volume 3-1, SWMUs, Soil Gas Surveys, Technical Memorandum on Soil Gas Surveys, June 1996). SWMU B-3 has an 18-well SVE system that has been in operation since 1997. The purpose of additional soil gas surveying at this site was to determine if VOCs are contaminants of concern that must be addressed within the landfill trench adjacent to, but across the road from, the western (main) trench that currently contains the operating SVE system. No vapor extraction wells were placed in the eastern trench, and little intrusive investigations have been conducted to determine the lateral extent of thickness of this second trench. A secondary objective was to compare the results from this soil gas survey to the results of sampling used for the treatability study and the results from operations and maintenance of the SVE system.

The soil gas samples collected reveal that TCE and PCE contamination persists in the SWMU B-3 main trench despite over four years of mostly continuous operation. Detected constituents for samples collected from SWMU B-3 are presented in Table 3.8. Concentration contour maps for TCE and PCE are presented in Figure B3-2 and Figure B3-3, respectively. Although the levels appear to have decreased from past soil gas and SVE testing results, the lower levels from this soil gas survey could be caused by different sampling intervals, particularly comparing the lengths or surface areas of the screened intervals of the VEWs to those of the soil gas survey probes. Another factor that could be responsible for lower contaminant levels reported in the current soil gas survey is the high soil moisture conditions in the subsurface soils. Higher moisture caused by a wetter than average winter suppresses volatilization from the subsurface soil, especially if the water table within the trench rises above the depth intervals containing the most contamination. The soil gas data does suggest that the contamination within the western (main) trench at SWMU B-3 extends further south than the existing treatment system’s southern-most VEWs.

Data from the eastern trench at SWMU B-3 on the east side of the road also demonstrated elevated levels of TCE and PCE greater than the levels encountered in the western (main) trench. Refusal was encountered at much shallower depths in the eastern trench, generally at depths of two feet bgs, but as deep as 11 feet bgs. The limestone immediately underlying the SWMU B-3 area appears to dip to the west, so that the eastern trench is more readily drained than the down-dip western trench. The higher levels encountered could be associated with better drained soils. The data provide definite indicators that additional VOC contamination exists within the second trench. The results also strongly suggests that PCE is the primary contaminant of concern associated with the eastern trench, and confirmed that TCE is the primary VOC associated with the western (main) trench. PCE concentrations detected during the current soil gas survey indicate significantly higher levels of PCE in the eastern trench than were measured in November 1995.

Summary of Findings and Recommendations

The soil gas survey results at SWMU B-3 re-affirmed the presence of VOCs within the western trench and identified a potential additional source area of PCE contamination within the eastern trench. The SVE system currently operating at SWMU B-3 will be continued while additional investigations or remedial actions are planned. The data indicate that additional investigations and/or remedial actions are necessary to address PCE levels detected in the eastern trench at SWMU B-3. Additional data required to expand the SVE system or to implement different remediation approaches at SWMU B-3 are discussed in the SVE Treatability Study Report and the SVE Operations and Maintenance Assessment Report, located in Volume 4, Treatability Studies.

Table B3-1 - SWMU B-3 Detected Constituents