ABSTRACT: A three-dimensional characterization of coarse glacial outwash used for modeling contaminant movement.

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Glacial outwash deposits form important aquifers throughout the northern United States and Canada. They provide high groundwater yields because of their generally high hydraulic conductivity. Because of this high conductivity, they can also rapidly transmit contaminants. The potential threat to water supply wells has stirred interest in better understanding the heterogeneity within these aquifers as a way to predict contaminant movement. Heterogeneity within geologic materials is often conceptualized by groundwater professionals as one or more high conductivity lenses or layers within a matrix of lower conductivity material. When a contaminant is released in an aquifer of this type, it flows preferentially within these lenses. Identification of these preferential flow paths and the geologic processes that control their distribution is critical to predicting contaminant movement and mitigating the impact of contaminants on water resources. This thesis examines the heterogeneity of a gravelly outwash deposit located in southern Wisconsin. Detailed mapping was conducted on a scale typical of point-source contaminant release (on the scale of 1 to 10's of meters). Most hydrogeologic field studies at this scale attempt to infer geologic heterogeneity from measurement of aquifer properties and limited borehole sampling. A different approach was used for this study in that individual sedimentary bodies were mapped as they exist in the deposit. A numerical model was constructed based on the three-dimensional distribution of the mapped heterogeneities to simulate groundwater flow through the deposit. The model was then linked to a particle tracking code to demonstrate the effect of the mapped units on contaminant movement. The heterogeneity present does not fit the typical concept of high conductivity zones in a low conductivity matrix described above. Rather, most of the site is a matrix of high conductivity with rare, discontinuous zones of even more conductive, relatively well-sorted gravel, and layers of fine grained, lower conductivity sand. These two sediment types constitute the high and low conductivity end members of the facies at the site. Because they are relatively rare, these units would intuitively have little impact on contaminant movement. However, areas of the site in which preferential contaminant movement is indicated by the model are closely associated with these two facies. The coarser grained facies affects contaminant movement by its high conductivity, and the fine-grained facies acts as a stratigraphic marker that mantles the basal erosional boundaries of channel scours. These scours in turn, are commonly filled with laterally continuous combinations of coarse-grained facies. The relative hydraulic conductivity of the coarse-grained facies and the directional trend of the channel scours (delineated by the fine-grained facies) was largely responsible for the preferential concentrations of contaminants indicated by the model. Awareness of these relationships during site investigation and characterization can aid consultants and regulators attempting to model contaminant movement in similar outwash settings.

Bibliography of Canadian Geomorphology