HYDROSTRATIGRAPHIC & GROUNDWATER FLOW MODELS FOR GLACIAL LAKE OSHKOSH SEDIMENT, OUTAGAMIE COUNTY, WI

Fine-grained glacial sediment frequently forms aquitards. Aquitards not only can help protect underlying aquifers from contamination, but may also be a source of local water supplies. In Outagamie County, WI, an east-west trending bedrock valley is filled with over 300 feet of sediment, mostly fine-grained glacial Lake Oshkosh sediment with some glacial till and sand bodies of uncertain origin. Outside the valley, fine-grained sediment is significantly thinner but appears to drape over the bedrock. The fine-grained lake sediment appears to form an extensive aquitard of very low conductivity overall, but includes sand bodies of unknown extent and continuity. These sands are likely preferential flow paths within the aquitard and the source of groundwater for many rural residents in the county.

Approximately 2,500 well logs with driller described lithologies of unconsolidated sediment were categorized in order to identify hydrofacies ranging from dominantly clay or silty clay to coarse sand or gravel. Electrical resistivity imaging was used at 8 sites to determine the average and range of sizes of the sand bodies, with geoprobe confirmation at 2 sites. The well log analysis and geophysical images were used in the construction of hydrostratigraphic models, created with multiple-point geostatistics, which uses training images instead of the traditional variogram approach. Statistics of connectivity were calculated for all hydrostratigraphic models, using the code CONNEC3D. Results indicate the presence of connected bodies of sand and gravel, which may form preferential flow paths. A representative set of hydrostratigraphic models were selected for import into groundwater flow and transport models. In on-going work, calibration and particle tracking of the groundwater flow and transport models will determine whether the sand bodies likely act as preferential flow paths within the aquitard.