USING 3-D MODELS TO VISUALIZE THE HYDROGEOLOGIC SETTING FOR GROUNDWATER RESOURCES PLANNING

As a pilot study of the Central Great Lakes Geologic Mapping Coalition, we are creating 3-D models of the glacial materials above bedrock for 7.5’ quadrangles in Lake and McHenry Counties in northeastern Illinois. The glacial drift is 150 to 350 feet thick and contains several relatively thick, arealy extensive sand and gravel aquifers that are significant sources of water supply for this rapid-growth area. Data used in the modeling are of highly variable quality and primarily are from water-well records (including drillers’ logs), logs of stratigraphic test borings, natural gamma logs, and shallow seismic reflection profiles. Our 3-D mapping has revealed a succession of glacial sediments representing three major events of the last (Wisconsin Episode) glaciation. Tills of these events are generally lithologically distinct and intertongued with proglacial lacustrine and fluvial facies. Saturated sands and gravels of the fluvial facies are significant aquifers in the mapping area. Stacking and burial of debris-rich ice along the western flank of the Lake Michigan lobe during each event also produced abundant reworked supraglacial and ice-marginal facies, which add complexity to the glacial succession. To understand the subsurface distribution of these glacial deposits and visualize them for groundwater applications, we are creating lithologic, stratigraphic, sedimentary facies, and hydrostratigraphic models. We are using a combination of the different types of 3-D models plus histograms, pie charts, and vertical slices to help us provide decision-makers the critical geologic interpretations necessary to understand the hydrogeologic setting of the glacial-drift aquifers and to protect this vital groundwater resource.