AQUIFER GEOMETRY AND TRANSMISSIVITY PREDICTIONS BASED ON 3-D GEOLOGIC FRAMEWORK MODELING

A large, 3-D geologic framework model (GFM) of Quaternary deposits was developed to guide parameterization of a groundwater flow model in McHenry County, Illinois. The extent of the GFM was defined by regional hydrogeologic boundaries, and its resolution was consistent with the intended groundwater flow modeling objectives. Subsequent data acquisition and mapping at higher resolutions within a portion of the GFM boundaries have allowed for a significant increase in the number of units mapped, primarily through an improved understanding of the sedimentary architecture. Integration of additional information, which includes soils data, continuous core samples, and geophysical data, has provided additional insight on facies relationships within aquifer units. We are exploring new methods for communicating the sedimentologic and hydrogeologic insight that was developed through the mapping process. To that end, we are developing transmissivity maps of aquifer units using two methods. One method relies largely on soil data to provide a map of sediment texture near land surface, which helps resolve sedimentologic relationships of the glacial meltwater deposits that make up the surficial aquifer. A second method relies on the range of field data and sedimentologic conceptual models to provide regionalized estimates of sedimentologic variations within subsurface aquifers. Thus, the additional insight on lithologic variability that is a consequence of the detailed 3-D geologic mapping has allowed for improved transmissivity predictions for the mapped glacial aquifers. These predictive maps will be targeted to consultants, water managers and regulators looking for additional insight on the expected hydraulic properties of the shallow aquifers in McHenry County.