HYDROSTRATIGRAPHIC AND GROUNDWATER FLOW MODELS: TROY VALLEY GLACIAL AQUIFER, SOUTHEASTERN WISCONSIN

Glacial deposits in the Troy Valley, a buried bedrock valley, could be a source of groundwater for municipalities in southeastern Wisconsin. However, municipal pumping may divert significant amounts of water from lakes, streams, and wetlands. Three-dimensional hydrostratigraphic and groundwater flow models were constructed to determine the effects of pumping four recently installed municipal wells near Lake Beulah and Vernon Marsh. The 3D hydrostratigraphic model was produced using Rockworks^TM v. 2006 software and imported into a regional 3D groundwater flow model using the computer code MODFLOW.

Well logs for the unconsolidated sediment were used to identify four distinct hydrofacies ranging from dominantly silty clay to gravel, each of which was assigned a hydraulic conductivity value based on literature values. Hydrostratigraphic units were constructed by interpolation between well logs using inverse-distance with weighting; this method allows for the closest data points to have the greatest effect on an interpolated value. Additionally, weight can be set preferentially in the horizontal or vertical direction. Eleven models with different horizontal verses vertical weighting were constructed. An initial model with equal weights in the horizontal and vertical directions was selected to test the effects of not using directional weighting. This model showed vertical tubes of sand and gravel, which does not make geologic sense. Therefore, all subsequent models were selected with more weighting in the horizontal direction to reduce this effect. The selection of the final hydrostratigraphic model was guided by geologic reasoning and the fit to six hydrostratigraphic cross sections constructed independently from field information.

Results from the regional and local scale groundwater flow models showed that pumping in the Troy Valley near Vernon Marsh and Lake Beulah will cause an average 18% reduction in groundwater inflow in the northern section of Vernon Marsh and an average of 20% reduction in groundwater flow to Lake Beulah, where groundwater supplies 30% and 20%, respectively, of the total water inflow to these surface water features. Uncertainty analysis indicated that the hydraulic conductivity of the glacial deposits can significantly affect the predicted heads under pumping conditions.