INCORPORATING DETAILED QUATERNARY HYDROSTRATIGRAPHY INTO GROUNDWATER FLOW MODELS

The hydrostratigraphy of unconsolidated sediments typically associated with glaciated terrains is often complex. This complexity presents many challenges when attempting to incorporate the stratigraphy into a groundwater flow model, particularly at a regional or sub-regional scale. Recent advances in developing regional-scale 3-D geologic models for Quaternary sediments have allowed for complex stratigraphy to be easily incorporated into a groundwater flow model. The Hydrogeologic-Unit Flow (HUF) Package for MODFLOW provides an advantageous method because it allows for complex hydrostratigraphic information to be incorporated into a groundwater flow model while maintaining a relatively simple model grid. However, the extra computational demand of the HUF Package needed to iteratively calculate the effective hydraulic conductivity for each grid cell can result long model run times and limit the ability of a model to converge to tolerances sufficient for most automated calibration techniques. These limitations often eliminate the HUF approach from a modeler’s toolbox, particularly for calibration of highly parameterized models. For certain geologic conditions, through careful set up of the model grid and use of external preprocessing, the HUF methodology can be implemented to overcome these drawbacks while still incorporating complex hydrostratigraphy. An example of this approach is presented for a sub-regional water supply management model in Twin Cities metropolitan area of Minnesota.