DRESSING THE EMPEROR (GROUNDWATER-FLOW MODEL) OF GLACIAL GEOLOGY: A TALE OF THREE TAILORS

A recent cooperative effort by glacial geologists of the U.S. Geological Survey and several midwestern geological surveys (Central Great Lakes Geologic Mapping Coalition) has resulted in a very detailed map of glacial and proglacial deposits and landforms in Berrien County, Michigan (Stone and others, 2001). Stone subsequently developed surface contours for each of the intersecting and overlapping morphosequences within the 1,350 km² area to facilitate a fully three-dimensional geologic model that is bounded on the top and bottom by the ground and bedrock surfaces, respectively. Our task is to take this detailed morphostratigraphy and merge it with an equally detailed groundwater-flow model (Freeze, 1971), to produce a more realistic understanding of the controls that glacial geology and geomorphology exert on shallow groundwater-flow systems. In an insightful editorial statement in Ground Water, entitled “Groundwater modeling – The Emperor has no clothes,” Anderson (1983) referred to groundwater-flow models as the framework (“underwear”), while parameter values (or, in this case, their distribution) along with the necessary boundary conditions (“clothes”) are the details that need to be tailored to the site-specific setting to be modeled. In this presentation we overview critical issues of spatial resolution and how they are addressed in discretizing complexly juxtaposed bodies of heterogeneous porous media for use in a numerical, variably saturated, groundwater-flow model. Approaches to assigning saturated and unsaturated hydraulic parameters to different material types are discussed, and preliminary results of transient simulations during periods of precipitation-driven recharge are presented.