Influence of Clastic Dikes on Vertical Migration of Contaminants in the Vadose Zone at Hanford

Abstract

This research project addressed the effect of clastic dikes on contaminant transport in the vadose zone. Clastic dikes are vertically oriented subsurface heterogeneities that are common at the Hanford Site, including in the subsurface sediments below the tank farms in the 200 West Area. Previous studies have suggested that clastic dikes may provide a fast path for transport of leaking fluid from the tanks through the vadose zone because they cut through the horizontally layered sediments at the site. The research tested the hypothesis that clastic dikes at the Hanford Site provide preferential pathways that enhance the vertical movement of moisture and contaminants through the vadose zone. Current flow and transport models of the vadose zone at the 200 Areas are based on relatively simple hydrogeologic models that assume horizontally layered sediments with no preferential vertical flow paths. To address those scientific needs, our research included field and modeling studies of the large-scale spatial distribution of clastic dikes, the hydrologic properties within dikes, and the potential effect of clastic dikes on fluid flow through the vadose zone. The data and models of the clastic dike networks produced for this project should be directly applicable to fate and transport studies conducted at the Hanford tank farms.