Carpenter, Philip J.
M.S. (Master of Science)
Department of Geology and Environmental Geosciences
Geophysics; Environmental geology; Geology
Wetlands in the Lake Calumet region of Chicago, IL have been filled in with slag for over 100 years. Dissolution of slag at the Big Marsh wetland site near Lake Calumet has led to extremely alkaline ponds and groundwater contamination. The purpose of this research was to delineate contaminated groundwater zones, estimate porosity of the slag, and determine lithology at Big Marsh. A suite of geophysical instrumentation consisting of electromagnetic (EM) conductivity, magnetometry, ground penetrating radar (GPR), vertical electrical soundings (VES), and two-dimensional electrical resistivity imaging (2D-ERI) were used to conduct surface investigations. Average groundwater resistivity (rhow) at the site was measured to be 1.31 O-m in a monitoring well on-site. Terrain conductivity measured in a vertical dipole configuration ranged between -25.80 mS/m to 149.7 mS/m with an average of 68.25 mS/m. Magnetometry surveys were difficult to interpret due to the magnetic properties of the slag-fill. Archie's Law was applied to the EM conductivity data to generate porosity estimates for slag. Porosity is essential to understanding ground water systems for environmental assessments. Porosity estimates ranged from 6%-48% and the average was 28%. True resistivity values gathered from inverse models of vertical electrical soundings (VES) were used to confirm porosity estimates at three locations. Porosities calculated from true bulk rock resistivities were consistent with EM porosity estimates, suggesting sigmaa can be used to estimate porosity at the Big Marsh site. GPR was used to identify geologic interfaces at the water table (2.0 m deep), and the top of the sand (4.5 m deep), lake silts and clays (6.7 m deep), and glacial till (9.1 m deep). VES inversions resulted in three layer models of the subsurface and interpretations included the water table (1.3 m deep), glacial till (11 m deep), and bedrock (21 m deep). A 2D-ERI inversion showed lateral variations in resistivity consistent with fractures and air-filled voids, and detected the water table at approximately 2 m. Interpretations from GPR, VES, and 2D-ERI surveys were consistent with the regional geology of the Lake Calumet Region of Chicago, IL. Electrical and electromagnetic geophysical methods successfully delineated groundwater contamination, estimated slag porosity, and determined lithology.
Divita, Sonny, "Geophysical characterization of the big marsh slag site in Chicago, Illinois" (2017). Graduate Research Theses & Dissertations. 3283.
viii, 211 pages
Northern Illinois University
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