Formatted Title

Challenges in Soil Vapor Extraction Treatment Design to Deplete Contaminant Mass at the Nation’s Worst Carbon Tetrachloride Landfill Site

Background/Objectives

Approximately 200,000 drums of pesticide manufacturing waste containing carrier solvents of carbon tetrachloride, chloroform, alcohols and ketones were disposed of at the Hardeman County Landfill in open, unlined trenches in three disposal areas. Contamination from the waste drums migrated through sandy vadose zone and into the underlying unconfined drinking water aquifer. Initial remedial actions consisted of construction of low permeability landfill caps coupled with installation and operation of a pump and treat (P&T) system for hydraulic containment and treatment of contaminated groundwater. The P&T system was operated for seven years prior to its shutdown due equipment reaching the end of its operational life and the realization that the Site’s extraction wells provided a pathway for highly impacted unconfined aquifer to cross contaminate an underlying semi-confined aquifer. Initial soil vapor extraction (SVE) pilot testing was performed to assess the performance of this technology to treat vadose zone soils at the smallest of the three disposal areas, the 1-acre South Disposal Area (SDA).  Based on the successful operation of the SVE pilot to treat sols at the SDA, EPA issued an amended Record of Decision for source area depletion using SVE to treat the residual contamination in the vadose zone of the three disposal areas. SVE treatment was successful in treating residual contaminant mass at the 3-acre Middle Disposal Area (MDA). Based on these results, plans were developed to design an SVE system for treatment of the vadose zone contamination at the North Disposal Area (NDA). The challenges in design of SVE treatment system for the NDA included: the size of this disposal area of 20 acres; the presence of free product and residual non-aqueous phase liquids (NAPL) within a vadose zone thickness of 90 feet; the explosive nature of the vapors, the sheer mass residual contamination projected to be removed during the anticipated decade-long operation of the treatment system; the need to incorporate chemical treatment in the caustic scrubber to keep the copious amounts of chlorine gas generated from thermal destruction of the chlorinated volatile organic compounds (CVOCs) in solution; and the considerable volume of acid off-gas scrubber blowdown liquids that would be generated.

Approach/Activities

Based on the size of the NDA of 20 acres and to economize on the size of the treatment system, a phased approach to treat the NDA in thirds was adopted.  In this manner, once an initial third of the vadose zone soils was completed, vapor extraction operations would shift to the second third of the disposal area followed lastly by treatment of the remining third of the disposal area. Due to the length of time projected to be necessary to operate the treatment system to achieve the remedial goal of 97% contaminant mass removal and the overall mass of organics estimated to be removed, off gas treatment by means of a 10,000 cubic feet per minute regenerative thermal oxidizer was incorporated into the design for the NDA. Following destruction of the CVOCs, significant quantities of chlorine gas will be generated requiring the need to provide additional treatment to keep such that these gases would be controlled from being emitted out the discharge stack. Lastly, the addition of a brine reduction system was included to reduce the volume of the liquids generated from the caustic scrubber.

Results/Lessons Learned

The design of the SVE wells and vapor monitoring points for the NDA incorporated the knowledge gained from the successful treatment of vadose zone soils at the SDA and MDA. Additional chemical treatment was added by the chemical addition of sodium thiosulfate in the caustic scrubber to keep the chlorine gas in solution. The phased approach to operation of the SVE system at the NDA significantly reduced the size of the treatment system components for an overall projected remedial time frame of 12 years of SVE system operation. Lastly, operation of the brine reduction system will generate a crystalline solid and eliminate the need for trucking of liquid wastes generated from vapor treatment. Following receipt of a source of funding for implementation in 2022, construction of the SVE system at the NDA is underway with anticipated completion of construction in 2025.