Geologic Mapping Solves Wetland Restoration and Environmental Problems
Geologic research and mapping of the West Chicago Quadrangle in northwestern Cook and DuPage Counties has helped researchers, environmental hydrogeologists, and county agencies better understand topics as diverse as the paleoenvironment of an ancient mire that entombed a mastodon about 13,000 years ago and finding the best places to install monitoring wells in an area with groundwater contaminated with low levels of volatile organic chemicals. The basic understanding of the area's geology will be captured on surficial geology and bedrock topography maps of the West Chicago Quadrangle, which are funded by the STATEMAP component of the National Cooperative Geologic Mapping Program. To assess the geological conditions, an ISGS geologist and his summer intern have been examining sediment cores, sample sets, natural gamma-ray logs, water-well and landfill boring records, and geotechnical reports. Their understanding of the geology has been enhanced by geophysical exploration that included the use of shallow electric earth resistivity, borehole geophysics, and seismic shear transects. The geologic mapping is providing an important context for two very different natural resource and natural history issues.
Contaminated groundwater in the West Chicago Quadrangle is a serious local environmental problem. The geology of the area plays an important role in determining the direction, speed, and changes in concentration of the contaminant along its flowpath. The quadrangle is located in an area where there were at least two stillstands of the Lake Michigan Lobe of the glacier that advanced out of the Lake Michigan basin during the Wisconsin Glacial Episode. The fluctuations of the glacier created a complex succession of glacial, lake, and meltwater stream deposits as well as the formation of the West Chicago Moraine, a ridge composed primarily of fine-grained glacial sediment. The areas with the contaminated groundwater lie within a zone of relatively thin, fine-grained sediment and thick, permeable, sorted sediment. The ISGS is working with the Illinois Environmental Protection Agency to better delineate the details of the geology in the areas affected by the groundwater pollution. ISGS geologic maps and cross sections will be important components to understanding the problem and suggesting potential solutions.
Some of the interesting details of the area's paleoenvironment, learned from this study, are reported in The Late Glacial and Early Holocene Geology, Paleoecology, and Paleohydrology of the Brewster Creek Site, A Proposed Wetland Restoration Site, Pratt's Wayne Woods Forest Preserve and James "Pate" Philip State Park, Bartlett, Illinois, which is in the final stages of review. In this study, an ISGS geologist collaborated with scientists from the Illinois State Museum, the University of Minnesota, Northeastern University, and Northern Illinois University to reconstruct conditions of a drained wetland prior to its reflooding and restoration. Since completion of the report, removal of drainage tiles has resulted in reflooding of the area and an immediate rebound by wetland plant species repopulating the area. During tile removal, three mastodon teeth were discovered, along with an in situ rib thought to be from the same animal. Collagen from one of the teeth dated at about 13,300 calibrated years before present. Among the many discoveries in the study, the most interesting geological finding was that windblown silt ceased its rapid deposition at the onset of the last interglacial period about 14,500 calibrated years ago. This change is revealed in the change from deposition of sediment containing wind-blown clay minerals derived from the High Plains to marl containing little windblown sediment.