USING AIRBORNE ELECTROMAGNETIC GEOPHYSICAL SURVEYS TO UNDERSTAND HYDROGEOLOGIC FRAMEWORKS: A CASE STUDY FROM SHELBY COUNTY, IOWA

Airborne electromagnetic (AEM) surveys have the potential to map and image subsurface features important in understanding groundwater resources. Regional Water: Rural Water Association (RWRWA) in Avoca, Iowa is a rural water supplier that currently produces water from the near-surface West Nishnabotna aquifer. Recently, RWRWA has been interested in expanding its production capacity and improving its resistance to drought.

Southwest Iowa is part of the Southern Iowa Drift Plains, which is characterized by Mississippian-Cretaceous bedrock unconformably overlain by Quaternary glacial and alluvial deposits. Buried channel bodies can be found at the base of the glacial material; however, the depth and scale of these features make them difficult to locate and characterize. The largest of these buried paleochannels is the Pleistocene-age Fremont Channel, a north-south trending incised bedrock channel that transects portions of Fremont, Mills, Pottawattamie, Shelby, and Crawford counties. The Fremont Channel is commonly filled with till and highly variable in thickness, but sand and gravel units are often present, which could create an aquifer. Such an aquifer could provide RWRWA with a second, more drought resistant water source. A lack of subsurface data below the shallow Nishnabotna alluvial aquifer in the study area made identifying and characterizing these deposits challenging. Initial ground-based geophysical surveys did not yield sufficient results.

To solve this issue, RWRWA, in cooperation with AquaGeoFrameworks, flew an AEM survey north of Avoca in July 2018. When coupled with additional datasets, such as local well-log information, AEM surveys can create high-resolution geologic frameworks. Forward modeling conducted before the AEM flight indicated that a buried sand and gravel feature would be identifiable against the underlying carbonate bedrock. The AEM survey identified the channel, but it had a much lower resistivity than predicted, and although the deposits were up to ~50 meters thick in some locations, the sand and gravel beds appeared to be laterally discontinuous. Additional well tests in the area could provide additional details on the exact composition of the Fremont Channel, but currently, RWRWA has begun investigating other potential water sources.