Impact of contaminant mass reduction in residuum sediments on dissolved concentrations in underlying aquifer

Contamination of aquifers in many cases occurs because of the downward migration of contaminants from the land surface through the vadose zone, including through residuum sediments in case of fractured or karst aquifers. It is therefore critically important to characterize and model fate and transport of contaminants in both the vadose zone/residuum and the underlying saturated zone as part of remedial design for aquifer restoration. Residuum sediments usually contain a significant fraction of clay minerals and fluid flow is often predominantly in vertical direction. Groundwater modeling is useful when evaluating alternatives of contaminant mass removal in the vadose zone/residuum and their impact on the downgradient dissolved concentrations in the underlying aquifer. Two case studies illustrate applicability of such modeling. The first study includes modeling of the expected natural longevity of a residual DNAPL mass accumulation in the saturated residuum, and its comparison to the effects of remedial DNAPL mass removal. The second study includes modeling contaminant transport from a residual DNAPL source within the vadose zone to the saturated zone in the residuum and the underlying bedrock aquifer. The natural longevity of the unsaturated DNAPL source is modeled, focusing on resulting down-gradient concentrations in the residuum and bedrock aquifer. For comparison, two remedial actions are simulated; 90% mass removal and soil flushing.