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

Objectives: The goal of this research was to develop a basic scientific understanding of clastic dikes at the Hanford Site and their possible role as preferential pathways for vadose zone contaminant transport. Clastic dikes are subvertical structures that form within sedimentary sequences after deposition and cut across the original sedimentary layers. They are common throughout the Hanford Site, often occurring in organized polygonal networks, and their affect on fluid transport is similar to other geologic structures that cut across sedimentary layers, e.g., faults or fractures.

Approach: A major focus of the project was on field, laboratory and modeling studies of the hydrogeological properties of the clastic dikes and the effect that they have on transport of water through the vadose zone. These studies were performed at two field locations at the Hanford Site. We performed an extensive series of field and laboratory measurements of a large number of samples from the clastic dikes linked with infrared and visual imagery of the clastic dikes and surrounding matrix. We developed a series of correlations from the sample data that allowed us to estimate the unsaturated hydraulic conductivity of the dike and matrix at an extremely high resolution (approximately 1 mm) using geostatistical methods. The resulting grids, each of which measured several meters on a side and included nearly four million grid nodes, were used to model the distribution of moisture between the clastic dike and surrounding matrix as well as the relative velocities that moisture would have through the clastic dike and matrix for a number of different recharge scenarios.

Results: Results show the development of complex flow networks in the vadose zone that depend on input flux rates and boundary type and that may sometimes mask the underlying heterogeneity. The networks occupy two complementary states: a high-permeability region in the fine-textured media, principally the clastic dikes, at low fluxes and a high-permeability region in the coarse-textured media at high fluxes. Transition between the two states occurred at an input flux of about 100 mm yr-1. At this input flux, preferential channels essentially disappear with the dike and host matrix and conduct at similar rates. The results suggest that clastic dikes might serve as a conduit for more rapid movement of moisture and mobile contaminants to the water table, but only under a restricted set of recharge (or leak) conditions.

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