A Hydrogeophysical Investigation of the Shallow Sandy Aquifers in the
Oak Openings Region of Northwest Ohio
Abstract
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.