Moving beyond pump and treat toward enhanced attenuation and combined remedies: T-Area, Savannah River Site
ABSTRACT: Ground water beneath T-Area, a former laboratory and semiworks operation at DOE's Savannah River facility, is contaminated by chlorinated solvents (cVOCs). Since the contamination was detected in the 1980s, the cVOCs at T-Area have been addressed by a combination of soil vapor extraction and ground-water pump and treat. The site has received approval to discontinue the active treatments and implement a full-scale test of enhanced attenuation, which is an engineering and regulatory strategy developed by DOE and the Interstate Technology and Regulatory Council. Enhanced attenuation uses active engineering solutions to alter the target site in such a way that the contaminant plume will stabilize passively and shrink. The process involves documenting that the action will be effective, timely, and sustainable. Attenuation remedies fundamentally are based on a mass balance; hence, long-term plume dynamics can be altered either by reducing the contaminant loading from the source or by increasing the rate of natural attenuation processes within all or part of the plume volume. The strategy for T-Area called for a combination of technologies: (1) neat (pure) vegetable oil deployment in the deep vadose zone in the former source area, (2) emulsified vegetable oil deployment within the footprint of the ground-water plume, and (3) identification of attenuation mechanisms and rates for the distal portion of the plume. Field deployment of the amendments was initiated in February 2008. As envisioned by the strategy developers, the neat oil spreads laterally following injection, forming a thin layer on the water table to intercept and reduce future cVOC loading (via partitioning) and reduce oxygen inputs (via biostimulation). The vadose source equilibrates with the deployed neat oil over a period of approximately four years. In the second and third parts, emulsified oil forms active bioremediation reactor zones within the plume footprint to degrade existing ground-water contamination (via reductive dechlorination) and stimulates long-term attenuation capacity in the distal plume (via cometabolism). The enhanced attenuation development process has provided a tool for developing a defensible strategy for T-Area cleanup that promises a high degree of performance while minimizing adverse collateral impacts of remediation (e.g., energy use and wetland damage) and minimizing life-cycle costs. Paper at http://www.osti.gov/energycitations/purl.cover.jsp?purl=/927602-TfHMFq/
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