A novel application of a geotechnical soil stabilization technology for improved delivery of remedial amendments

Abstract

AbstractAccelerating the remediation of contaminants that have diffused into low‐permeability (low‐k) geologic units such as silts and clays is a key need for the groundwater remediation industry. Injection‐based remedial technologies are often ineffective because the injected amendments are more likely to influence transmissive media (sands and sandy silts) and generally do not penetrate low‐k units. While some emerging technologies are now being tested, they are often expensive or have other technical limitations (e.g., installation depth and rate of amendment emplacement). To address the problem of treating contaminants in low‐k media, a geotechnical technology called the “Grout Bomber” (Keller North America) was repurposed for an environmental application to emplace ~800 vertical “reaction columns” containing zero valent iron (ZVI), sand, and neat vegetable oil for the treatment of chlorinated volatile organic compounds (CVOCs) in lean clays and sandy clays. The reaction columns were closely‐spaced (2–3 ft apart) to greatly accelerate the back diffusion of CVOCs from the low‐k media to the reaction columns where contact with ZVI and vegetable oil promotes abiotic and biotic reductive dechlorination of CVOCs, respectively. Overall, 35 tons of ZVI were emplaced into the 5200 cubic yard (yd3) (~4000 m3) source zone during the project. Key benefits of this technology include rapid installation (average of 107 reaction columns per day), ease of remediation amendment mixing, consistent amendment dosing, and relatively low unit installation costs ($71 per cubic yard of treatment zone). It should be emphasized that this application of the Grout Bomber relies on diffusion of contaminants into the reaction columns from the surrounding low‐k media—a process that occurs slowly over years to decades. A planning‐level model showed that installation of closely spaced vertical reaction columns in a TCE‐laden clay unit reduced the time to achieve maximum contaminant levels from greater than 500 years to approximately 26 years. Groundwater monitoring confirmed the establishment of a concentration gradient between the reaction columns which promotes diffusion of CVOCs toward the reaction columns. The presence of dissolved acetylene and gaseous “higher coupling” products (>C3) provide further evidence of ongoing abiotic reductive dechlorination within the reaction columns.