Date of Award

12-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Larry McKay

Committee Members

Phil Jardine, Edmund Perfect, Jaehoon Lee, Maria Uhle

Abstract

At the U.S. Department of Energy’s Hanford Site in Richland, WA nuclear processing wastes, such as Sr-90, organic chelating agents such as EDTA, Co-60, and U(VI) have been detected in the vadose zone beneath the underground storage tanks. There is concern that waste released to the vadose zone could reach the groundwater and eventually flow into the Columbia River. The goal of this paper is to provide an improved understanding of coupled hydrologic and geochemical mechanisms that influence contaminant transport in the Handford vadose zone. Disturbed sediment and undisturbed sediment cores were collected from the Environmental Restoration Disposal Facility (ERDF) at the Handford Site. The disturbed sediment was used in batch sorption experiments as well as saturated repacked column experiments. The undisturbed cores were collected vertically (flow across bedding) and horizontally (flow along bedding) to determine the effect of bedding on fate and transport of contaminants over a range of moisture contents. Multiple non-reactive tracers were used to quantify physical non-equilibrium processes such as preferential flow and media bypass as well as diffusion into immobile water. Miscible displacement experiments were conducted with Sr+2, Co(II)EDTA and U(VI) to quantify the importance of hydrologic processes on the geochemical reactions. Unsaturated undisturbed core experiments indicate the prevalence of unsaturated lateral flow while vertical flow is inhibited due to capillary barriers that develop in the coarse-grain sediment beds resulting in minor perching. During unsaturated lateral flow at low water content, multiple flow paths can develop which may limit the surface sites available for geochemical interactions and therefore “accelerate” transport compared to that predicted from commonly used batch or saturated packed column experiments. The results indicated that sedimentary bedding controls the flow paths (i.e., the hydrology), which then affects the geochemical processes. These coupled processes appear to significantly affect transport in the Hanford vadose zone and need to be considered in the development of conceptual models of vadose zone transport in arid environments.

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