PNNL

Abstract

Contaminants may persist for long time periods within low permeability portions of the vadose zone where they cannot be effectively treated and are a potential continuing source of contamination to groundwater. Setting appropriate vadose zone remediation goals requires evaluating these persistent sources in terms of their impact on meeting groundwater remediation goals. One-dimensional approaches for estimating transport of volatile contaminants in the vadose zone are considered and compared to a one-dimensional flux-continuity-based assessment of vapor-phase contaminant movement from the vadose zone to the groundwater. The flux-continuity-based assessment demonstrates that the ability of the groundwater to move contaminant away from the water table controls the vapor-phase mass flux from the vadose zone across the water table. Limitations of the one-dimensional approaches are then discussed with respect to the need for further method development and application of two- or three-dimensional numerical modeling. The carbon tetrachloride (CT) plume at the U.S. Department of Energy Hanford Site is used as an example of a site where persistent vadose zone contamination needs to be considered in the context of groundwater remediation.