COMPARISON OF AN URBAN BEDROCK STREAM IN DRY AND WET MONTHS USING ELECTRICAL RESISTIVITY

Exploring the hyporheic exchange in fractured bedrock urban streams is challenging, in part due to complex, joint-controlled flow paths. Electrical Resistivity Tomography (ERT) provides insight into springs, flow pathways, and fracture zones below and along stream beds, leading to a better understanding of hyporheic exchange in urban bedrock stream systems. Cooper Creek is one such fractured bedrock stream located in the Cincinnati, OH metro area. The creek has mostly perennial flow with a reach that is generally dry. The purpose of this study is to better understand fluid flow within and surrounding this dry reach of Cooper Creek.

ERT surveys were collected at the mouth of a Cooper Creek tributary in Sycamore Township, OH. The tributary originates from a retention pond constructed in a commercial area. Water within the tributary flows into the hillside and reemerges farther downstream at multiple points along the hillside, indicating the presence of a spring. At the site, two lines of temporary stainless-steel stakes were installed to serve as an electrode array. Lines 1 and 2 are parallel and perpendicular to the tributary, respectively Line 1 sits atop the potential spring system and Line 2 provides a cross-sectional view of the tributary. Both lines underwent Dipole-Dipole and Werner array measurements on 5/11/23 after a prolonged wet period, and then again on 6/29/23 following an extended dry period.

To assess changes in electrical resistivity profiles over time, timelapse inversions were conducted utilizing the open-source Resipy software. The data show alternating vertical bands of low- and high-resistivity. The lower-resistivity regions are interpreted as preferential fluid flow pathways in the fractured limestone bedrock.