TRANSPORT FACTORS IMPORTANT FOR ESTIMATING MASS FLUX OF VOLATILE CONTAMINANTS TO GROUNDWATER

Soil Vapor Extraction (SVE) is a baseline approach to remediation of volatile contaminants in the vadose zone. During SVE, contaminated soil gas is removed from the subsurface. This process removes contamination from portions of the vadose zone readily accessible to induced soil gas movement. Some portions of the vadose zone are more slowly remediated due to low induced soil gas flow, large vapor diffusion distances, and/or presence of NAPL. In particular, contamination within low permeability portions of the vadose zone represent potential zones of persistent contamination that need to be considered in SVE closure and transition decisions.

There are several methods to estimate the impact of persistent vadose zone volatile contamination on the groundwater. The suitability of each technique depends on the site setting and associated primary transport mechanisms from the vadose zone to the groundwater. Each method requires some assumptions and some type of measurement in the vadose zone to quantify the strength of the source. Where aqueous transport dominates, contaminant transport in the vadose zone is primarily vertical (one-dimensional), the contact area of vadose zone contamination on the water table can be readily estimated, and transport across the water table can be represented simply. As vapor-phase transport becomes more important (e.g., for arid sites with low aqueous recharge), three-dimensional contaminant movement in the vadose zone may be more important, the contact area of vadose zone contamination on the water table is more difficult to estimate, and transport of contaminants across the water table includes a mass transfer resistance. Thus, when vapor-phase transport is significant, these issues should be considered in terms of computing contaminant flux to the groundwater and the resultant groundwater contaminant concentrations. Numerical modeling was applied to evaluate the factors that impact estimates of mass flux to the groundwater and the resultant groundwater concentration for persistent sources of volatile contaminants under conditions where vadose zone transport is predominantly in the vapor-phase. These results have implications in selecting appropriate transition or closure goals for SVE that are protective of groundwater.