Influence of sedimentary bedding on reactive transport parameters under unsaturated conditions

Moisture and contaminant transport in partially saturated, heterogeneous, layered sediments is typically anisotropic. Solute transport parameters, including dispersivity and the adsorption coefficient, and the modeled concentration of reactive minerals may depend on the direction of flow with respect to sedimentary layering. Reaction rates, in contrast, should be independent of flow direction. We determined the influence of flow direction on transport parameters for nonreactive (Br^-) and reactive (cobalt ethylenediaminetetraacetic acid [Co(II)EDTA^2-]) solutes under partially saturated conditions by imposing flow either parallel to or across sedimentary bedding in 11 intact sediment cores of various textures. Higher dispersivity of nonreactive tracers in parallel-bed cores suggested fluid channeling through permeable layers, while low-conductivity layers dampened channeling in cross-bed samples. Rates of transformation of Co(II)EDTA ^2- into Co(III)EDTA^- and of disassociation of Co ²+ and EDTA^4- were modeled assuming that the reaction rates were independent of the flow direction. The concentration of Mn oxides that was responsible for the transformation reaction was dependent on the flow direction, which governed the extent of contact between the solution and the solid phase. Similarly, the adsorption constants of Co(H)EDTA^2- and Co(III)EDTA- were dependent on the flow direction but were also unique for each experiment. The modeled concentration of reactive minerals was the most sensitive parameter describing the reaction and transformation of Co(II)EDTA^2-.