Tracking contaminant pathways in groundwater using a geologically based computer code for outwash

Glacial outwash deposits form important aquifers throughout the northern United States and Canada. They provide large groundwater yields because of their generally high hydraulic conductivity. However, their high conductivity also allows them to rapidly transmit contaminants. The threat of contamination to water supply wells has stirred interest in better understanding the heterogeneity within these aquifers as a way to predict contaminent movement. Heterogeneity within geologic materials is often conceptualized 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 districution is critical to predicting contaminant movement and mitigating the impact of contaminants on water resources and, thus, protecting public drinking water supplies. The heterogeneity of a gravelly outwash deposit located in southern Wisconsin was examined by detailed mapping and numerical modeling. The mapping was conducted on a scale typical of point-source contaminant release (a 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.