Influence of Clastic Dikes on Vertical Migration of Contaminants at the Hanford Site

Abstract

Clastic dikes are subvertical sedimentary features that cut through horizontally layered sediments, and they are common at the Hanford Site. Because of their cross‐cutting relationship with the surrounding matrix, they have been proposed as potential fast paths from former contaminant discharge sites at the surface to the water table. However, little was known of the detailed hydrogeologic properties of the dikes, and detailed modeling of flow and transport through the dikes had not been performed. We excavated a 2‐m‐wide clastic dike at the Hanford Site and characterized it using an air minipermeameter, infrared imagery, and grain size analyses. Field injection experiments were also used to characterize the system. The resulting data were used to prepare a detailed numerical model of the clastic dike and surrounding matrix for a portion of the excavation. Unsaturated flow through the system was modeled for several recharge rates. The highly heterogeneous nature of the system led to complex behavior, with the relative flux rates in the matrix and clastic dike being highly dependent on the recharge rates that were imposed on the system. The occurrence of saturation‐dependent complementary flow networks suggests that the contaminant release history may be important to the choice of remedial actions. Contaminants released under high flux conditions could be inaccessible under low fluxes, and vice versa. This phenomenon may also help explain the occurrence of complex breakthrough patterns of contaminants at compliance planes.