CANOE-MOUNTED ELECTROMAGNETIC SURVEY IN AN IOWA RIVER TO DELINEATE STREAMBED CONDUCTIVITY

We tested a novel application of electromagnetic (EM) geophysical data collection from a water vessel navigating on a large river. A Geonics, Ltd. EM-31 MK2 EM conductivity meter was mounted to a fiberglass canoe with the goal to collect conductivity data associated with riverbed sediments in the Iowa River in Iowa City, Iowa. The trial was performed as part of an alluvial aquifer investigation for the City of Iowa City. The EM survey was used to provide ancillary data relative to the induced groundwater recharge potential of the alluvial system.

The use of boat-mounted EM surveys has the potential to efficiently characterize riverbed sediments in water bodies with shallow water column depths. Typically, the EM unit’s imaging depth of 6 meters is shown as distance below ground since EM surveys are usually conducted over land. However, in this study, we wanted to determine if the unit can capture: 1) changes in the electrical properties of the surface water present below the device, 2) changes in the electrical properties of the riverbed sediments, or 3) electrical properties related to a quantifiable mix of water column changes and riverbed sediments.

The EM survey was conducted on May 16, 2023. The EM unit was mounted transversely to a canoe, calibrated on land and again on the water. Results from the survey indicated streambed conductivity ranged from 17 to 205 mS/m, with most values falling between 20-50 mS/m. Distinct patterns of higher and lower conductivity were observed in the river sediment coinciding with sands and fines, respectively. Higher conductivity values were observed when traveling near anthropogenic structures, such as bridges, utilities, or other infrastructure.

Results from the EM survey looked promising and have the potential to efficiently characterize bedload sediments. Quantifying groundwater induced from rivers is a valuable component in assessing alluvial aquifers. This rapid, field-based approach to mapping changes in riverbed sediments can be used to inform groundwater assessments by identifying changes in vertical hydraulic conductivity in these systems, which controls the hydraulic connectivity between the river and aquifer. Future work will ground truth the EM results with the collection and analyzation of physical riverbed sediment samples and comparison of river bathymetric data.