COOPERATIVE GEOLOGIC MAPPING FOR APPLIED EARTH SCIENCE IN WAUCONDA, ILLINOIS

Geologic mapping was undertaken in western Lake County, Illinois for the National Cooperative Geologic Mapping Program (NCGMP) to support local land-use and groundwater protection. Through ongoing mapping by the ISGS, new geologic information and interpretations were generated and provided to private consultants and companies, other state agencies, county planning and public health departments, and local citizen groups. In return, these groups shared valuable information regarding groundwater flow direction, provided access to project databases, and allowed us to collect core and conduct geophysical logging during drilling of new wells.

At Wauconda, complex geology comprising Quaternary-age sediments was resolved using more than 300 drillers' logs from residential water wells, environmental monitoring wells, and stratigraphic test borings. The drift comprises multiple tills separated by outwash, ice-marginal deposits, and lacustrine sediment that are locally truncated by channels. The uppermost till and proglacial sediments were deposited at the margin of Wadsworth ice during the last (Wisconsin Episode) glaciation. The Wadsworth till is relatively thin and discontinuous, and where absent or eroded, the underlying outwash is exposed. A second till, deposited by Haeger ice, underlies the Wadsworth deposits and is also discontinuous and locally grades into proglacial sediments. The drift is between 150 and 250 feet thick and lies directly on Paleozoic bedrock composed mostly of Silurian dolomite.

A 3-D model of the approximately 9 square-mile area around the Wauconda Landfill Superfund site demonstrates the geologic complexity. We have identified several unconformities on outwash deposits. In some places, multiple sand and/or gravel units that are aquifers overlie each other from ground surface to the top of bedrock. This model, for the first time, allows users to visualize the relationships among these aquifers in three dimensions and in the future will aid in defining potential pathways for contaminants.