Preferential Flow Paths in Heterogeneous Glacially-Deposited Aquitards

Background/Need: Preferential flow paths allow for faster movement of fluids than the surrounding matrix due to their hydraulic properties and connectivity. They are important to both groundwater flow and contaminant transport, but are difficult to detect and quantify, especially in aquitards. Preferential flow paths may be caused by fractures and lenses of sediment with high hydraulic conductivity (K) such as sand bodies within a clay matrix. Researchers have discovered that even thick aquitards (greater than 150 ft) may have fractures that are capable of transporting contaminants (Cherry et al., 2006; Gerber et al., 2001) and affecting underlying aquifers. However, few researchers have documented preferential flow paths created by connected sand lenses/bodies. Techniques to delineate preferential flow paths in aquitards are key to determining recharge to underlying confined aquifers and for protection of underlying aquifers.
This project focuses on delineating preferential flow paths in a heterogeneous glacially-deposited aquitard. A representative site has been selected in Outagamie County, Wisconsin where a bedrock valley has been filled with a thick sequence of sediment, dominated by lake sediment with some glacial till and sand lenses of uncertain deposition. This sediment appears to form an extensive aquitard comprised of very low conductivity sediment, occasionally surrounding sand lenses of unknown extent and continuity. Results of this project will be useful to both the municipal and private well owners in Outagamie County. The results can be used in a variety of ways including groundwater management such as siting municipal wells, land use planning such as siting landfills, and public information regarding Wisconsin glacial history.
Objectives: The main objective of this study was to delineate preferential flow paths using multiplepoint geostatistics and groundwater flow models for a representative site in Outagamie County, Wisconsin. Additional objectives included demonstrating the use of multiple-point geostatistics, understanding the flow system in Outagamie County, and reviewing and revising the depositional history of glacial Lake Oshkosh.