Solute transport and uranium(VI) reactivity in natural heterogeneous sediments.

摘要

The primary goal of this research was to determine the relationship between sedimentary geology and solute transport parameters. The overall hypothesis was that the transport of reactive and nonreactive solutes can be predicted using observable sedimentary depositional characteristics. The objective was to test this hypothesis by performing a variety of quantitative transport experiments in intact layered sediment samples. In this study, I used natural materials to explore (1) how mineralogical composition influenced the fate and transport of uranium(VI), (2) the relationship between heterogeneous sedimentary layering and preference for direction of flow, and (3) the mechanisms of solute transport in a partially-saturated cross-bedded sand. In Chapter 2, uranium(VI) sorption to siliciclastic and carbonate sediments was quantified using batch isotherm, kinetic, and miscible displacement experiments. Uranium(VI) sorption was small (Kd 1), most likely due to the formation of stable anionic and neutral U(VI)-CO3 complexes and competition for sorption sites by dissolved carbonate. Sorption to carbonate-containing sediments was consistently greater than to siliciclastic sediments. Selective extractions and kinetic studies suggested that U(VI) may have co-precipitated with calcite in carbonate sediments, while X-ray absorption spectroscopy suggested that U(VI) reversibly adsorbed to iron oxides and clay minerals in siliciclastic sediments. In Chapter 3, saturated miscible displacement was applied to intact flow bedding parallel (pb) and flow across bedding (xb) samples from six different sedimentary facies. Hydraulic and transport parameters varied over 4 orders of magnitude. Hydraulic and transport parameters (dispersivity, hydraulic conductivity) were slightly anisotropic, i.e., varied with direction of flow with respect to sedimentary bedding. Anisotropy increased predictably with the proportion of fine-grained material, suggesting that flow and/or solute transport across fine-grained beds may have been restricted in comparison to parallel to bedding. In Chapter 4, solute transport in partially-saturated, fine-grained and cross-bedded sediments was influenced by local-scale perching associated with flow across sedimentary layering. The arrivals of the wetting front and nonreactive tracers were concurrent, but the elution of tracers occurred well before the cores became fully saturated. This was suggestive of preferential flow, but changes in the volume of water inside the cores prevented the confirmation of such mechanisms. A reversed pattern of multiple nonreactive tracers was observed, which was characteristic of flow through restrictive media, i.e., media with low hydraulic conductivity. Overall, these results provided quantitative evidence to relate subsurface geology and contaminant transport. My findings will improve the conceptual and quantitative understanding of flow and transport in vadose zones composed of complex layered sediments.