Geophysical response of slackwater and sandy terrace deposits: A case study near Savanna, Northwestern Illinois

Beth A. Johnson (’01), Department of Geography and Geology, Univ. of Wisconsin-Fox Valley, Menasha, WI 54952; [email protected]

Philip J. Carpenter, Dept. of Geology and Environmental Geosciences, Northern Illinois Univ., DeKalb, IL 60115; [email protected]

Aquifer sensitivity is determined in part by the size of the sediments making up the aquifer: the coarser the sediments, the faster groundwater can move through them. In agricultural areas, this means that agrochemicals and other contaminants can penetrate near-surface aquifers more quickly. This study focuses on the surficial materials of the Savanna Terrace of the Mississippi River north of Savanna, IL and differentiating between them using multiple geophysical methods. Clayey slackwater terrace deposits and sandy terrace deposits are examined using resistivity soundings, ground-penetrating radar (GPR) profiles, and direct-push conductivity logs. The clayey terrace deposits are characterized by low resistivity (10-35 ohm-m) and slow GPR wave velocity (0.07 m/ns), whereas non-clayey terrace deposits exhibit much higher resistivity (169-1762 ohm-m) and faster GPR wave velocities (0.15 m/ns). One-dimensional subsurface models created using resistivity soundings provide reasonably accurate estimates of the thicknesses of many of the slackwater materials, but cannot reveal thin layers of embedded sands that GPR could. The most accurate technique to resolve the thickness of clayey slackwater terrace deposit as well as identifying thin embedded sands was direct-push conductivity logs. This method provided this highest resolution, though is invasive and only reveals stratigraphy at one location. Using the three methods together provides a detailed look at the subsurface and aids in the identification of migration pathways for groundwater and potentially contaminants.