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A Hydrogeophysical Investigation of the Shallow Sandy Aquifers in the Oak Openings Region of Northwest Ohio
  • Adele Metres,
  • Akinwale Ogunkoya,
  • Kennedy Doro
Adele Metres
Bowdoin College

Corresponding Author:[email protected]

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Akinwale Ogunkoya
University of Toledo
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Kennedy Doro
University of Toledo
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The Oak Openings Region of Northwest Ohio has a unique shallow sandy aquifer that is responsible for the wet prairie ecosystem above it. However, groundwater flux and contaminant transport within the 1 – 3 m thick sandy aquifer and a potential flow exchange with the deeper carbonate aquifer in the post glacial regional aquifer system are not well understood. In this study, integrated geophysical methods involving electrical resistivity tomography (ERT) and ground penetrating radar (GPR) are co-located to delineate the sandy aquifer unit at the Stranahan Arboretum and the Sandhill Crane Wetland sites in Toledo and Swanton, Ohio. Parallel ERT profiles were acquired using a SuperSting R8 resistivity meter with a dipole-dipole configuration and unit electrode spacing of 1 m while the GPR profiles were acquired using a PulseEKKO Pro 250 MHz radar system. Additionally, we obtained soil samples extending to a depth of 2.5 m at six locations on three of the profiles at each site. The sand samples were analyzed for their grainsize and to estimate the hydraulic conductivity (K) of the aquifer. Multiple slug tests were also used to estimate the variation in K. We found that the sandy aquifer is somewhat disconnected at the Stranahan Arboretum, with the thickest lenses around 10 - 40 m on the ERT profiles while a continuous and thicker sand sequence is observed at the Sandhill Crane site. The sandy aquifer is underlain by clay-rich silt and glacial till respectively who’s hydraulic leakance controls potential vertical fluxes. The average grain size of the sands was between 0.285-0.33 mm, suggesting fine to medium-grained sands. The average K ranged from 2 × 10-4 to 9 × 10-4 m/s, with generally larger K values found in sands sampled from the thickest lenses. Overall, the correlation of higher K values within thicker sand lenses suggests that in these areas, groundwater would be able to flow more easily, and the aquifer could be more easily contaminated than thinner, less connected sand units. We hope to continue this research and improve K estimates and conceptual models to help devise better plans to protect the groundwater resources and ecosystems of the OOR.