| Summary: | Clay-rich glacial till aquitards are widespread throughout the northern hemisphere. Due to their low hydraulic conductivity, these geologic units are commonly used to contain wastes. Dissolved Organic Carbon (DOC) in natural environments influences the speciation and mobility of contaminants, such as heavy metals and radionuclides, and is present in high concentrations in clay-rich tills (5 to 150 mg l-1). Detailed knowledge of the influence of DOC on the long-term stability, speciation, and mobility of elements is lacking. Studies in this thesis characterize the properties and function of DOC with respect to element speciation and transport at the King research site, an archetypal clay-till aquitard in Saskatchewan, Canada. Characterization of DOC using Asymmetrical Flow-Field Flow Fractionation (AsFlFFF) with on-line UV and DOC detection demonstrated the molecular weight (Mw) of DOC within the aquitard environment is low, ranging from 1160 to 1286 daltons (Da), and the relative amount of aromatic carbon in aquitard DOC is lower than in surface water DOC. These findings imply the complexation ability of DOC in aquitards is lower than surface water DOC. DOC aromaticity decreased with depth in the aquitard, while Mw remained constant. DOC Mw in other aquitards investigated was comparably low (1470-1630 Da). Coupling AsFlFFF with on-line ICP-MS analysis allowed the identification of Fe, U and Zn associated with the DOC, and demonstrated that <4% of total aqueous elements tested at the King site were complexed with DOC. Experimental and numerical modeling results demonstrate the low masses of metals (Cu, Mn, Mo, Ni, Sr, U and Zn) complexed with the DOC can be attributed to competitive complexation with carbonate and sulphate ligands naturally present in the pore water. The in-situ association constant, Kd, for U and Zn as determined by complexation experiments decreased with depth in the aquitard and was attributed to the corresponding decrease in DOC aromaticity. The maximum mass of complexation of U and Zn to DOC takes place at pH 3-6, and decreases above and below this pH range (range tested: pH 1.3-10). These results were supported by geochemical modeling and suggest the complexation of aqueous metals by DOC should be limited in ground water environments of similar chemistries. Laboratory transport studies using double reservoir diffusion cells showed DOC and other similarly sized colloidal material can diffuse through the matrix at the King site aquitard. The effective pore throat diameter in the matrix media was determined to be 2 - 2.2 nm and travel times of colloids increased with increasing colloidal diameter. Colloids >2 nm were prevented from movement in the till by sieving mechanisms, which suggests bacteria and viruses as well as larger colloids should not migrate through till aquitards. Due to preferential sieving of larger Mw DOC, DOC in aquitard environments is typified by a small diameter compared with surface water DOC, and a small range in hydrodynamic diameter and Mw.
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