DEEP VADOSE ZONE REMEDIATION: TECHNICAL AND POLICY CHALLENGES, OPPORTUNITIES, AND PROGRESS IN ACHIEVING CLEANUP ENDPOINTS

Contamination in deep vadose zone environments (sediments below the zone of practicable excavation and removal, but above the water table) is a significant issue in many regions of the world. This definition assumes that such contamination is isolated from exposure and movement of contamination to the groundwater creates the potential for exposure and risk to human health and environmental receptors. Therefore, while deep vadose zone environments are not necessarily considered a resource requiring restoration, limiting contaminant mass flux from vadose zone environments to water resources is necessary. Herein lies the challenge: applying vadose zone remediation to reach an acceptable rate of contaminant mass flux. However, this construct affords the opportunity to view remediation strategies for the vadose zone as targeted to mitigate the source of contamination and reduce transport through the vadose zone to receptors, in contrast to meeting a specific concentration measured at some location within the vadose zone. As such, controlling the flux of contaminants to groundwater to meet groundwater remediation goals can conceptually be viewed as: 1) contaminant mass reduction, 2) stabilization of contaminants so that they move slower, or 3) manipulation to slow the movement of contaminated pore water. These flux-reduction-based approaches recognize that for the deep vadose zone full removal of contamination is likely technically and/or economically impractical. We will present a review of how hydrogeologic and biogeochemical processes (rather than focusing on individual technologies) operate in deep vadose zone environments and are being used to meet remedial objectives, the technical risks and challenges for consideration during evaluation of a proposed remedial action, and the benefits of integrating vadose zone remediation into endpoints for groundwater protection. Specific examples will be presented to highlight significant advancements in defining risk-informed end states that area protective of human health and the environment for technetium, plutonium, americium, uranium, and iodine.