Relationship Between Radionuclide Transport, Water Content, and Flowpaths in Anisotropic Layered Unsaturated Sands

Layering in unsaturated, horizontally-bedded sands tends to result in anisotropic flow and contaminant transport due to the combination of capillary barrier formation in relatively coarse-grained sediments and capillary flow in fine-grained sediments. At the field-scale, such pore-scale processes result in transport parameters being dependent on the scale of measurement. Because of the difficulty in examining flow and transport in the field, few studies have correlated pore structure with contaminant transport, which is the objective of this study. Intact, layered, heterogeneous sediment cores were collected such that flow was imposed parallel to or perpendicular to bedding. Nonreactive tracers, uranium(VI), cobalt-EDTA (ethylenediaminetetraacetate), and Brilliant Blue FCF dye were applied under partially-saturated conditions. Subsequently, the cores were dissembled and sectioned to determine the distribution of particle size, dye, and water content with respect to layering. A relationship was found between particle size, water content and flowpaths within wet, fine-grained layers when flow was imposed parallel to beds. This indicated extensive flow bypassing of dry, coarser-grained sediments and explained observed high dispersivity values, multiple U(VI) peaks, and a low modeled effective water content. Cores in which flow crosses beds are currently in preparation. It is predicted that flow will interact to a greater extent with dry, coarse-grained beds than when flow was exclusively parallel to beds, but that the extent of interaction will be dependent upon water content and characteristics of layering. The final results of this study will determine linkages between sedimentary pore structure, water content, and contaminant transport in anisotropic unsaturated media.