Pre-restoration Shallow Subsurface Characterization of a Wetland in the Oak Openings Region, Ohio Using Drone- and Land-based Geophysical Techniques.

Restoration of wetlands is part of current initiative to restore habitats, manage storm water, and reduce nutrients loading into surface water bodies in areas around the Great Lakes including the Oak Openings Region of Ohio, U.S.A. Most of the wetlands to be restored within the Oak Openings Region are however, old agricultural fields with decades of farming legacies which includes the existence of old drainage tiles for draining the fields, extensively tilled topsoil, and differentially compacted soils due to the use of tractors and heavy farm equipment. Accurately locating the drainage tiles and delineating variations in the soil structures and stratigraphy is pertinent in understanding the hydrological dynamics which are major drivers for achieving the restoration objectives. Our goal in this study is to develop an efficient framework for accurately locating drainage tile networks at a large farm scale within the Oak Openings Region, delineate existing tiles at the proposed 750 m ´ 1,400 m Sandhill Crane Wetland restoration site in Swanton, Ohio, characterize variations in its soil structure and stratigraphy, and assess the implication of these subsurface heterogeneities on water fluxes. Our geophysical survey involved cart-mounted and ATV-towed ground penetrating radar (GPR) measurements using 100 mHz, 200 mHz and 250 mHz antennas along with induced polarization and electrical resistivity measurements using regular electrodes and a capacitive coupling. We also conducted thermal infrared imaging with a UAV mounted FLIR Vue Pro thermal imager flying on a regular grid at 50' or 100' AGL, depending on local airspace restrictions. GPR data using the 250 mHz antenna compared well with the thermal infrared images and reveal the drainage tiles mostly in the East-West direction with an ~ 15 m spacing network. From our study, we recommend drone based thermal infrared imaging for fast characterization of drainage network at large farm scale, while ground-based geophysical methods like GPR should be used to validate the results. Combining the GPR, resistivity, and chargeability models reveal a sandy layer with varying clay content and shallow structures that could favor differential infiltration. Results of this study will guide current efforts in modeling the site hydrology and restoring its habitat.