Combining geophysical and geological datasets to improve a conceptual aquifer model of the Oak Openings Region in northwest Ohio, USA

In this study, geophysical and geologic data were combined to develop a 3D conceptual hydrogeological model of a section of the Oak Openings Region (OOR) in northwest Ohio with a globally rare ecosystem of oak savanna and grassland prairie. The presence of a rhythmically laminated silt and clay unit underlying the study area is now widely realized. Assessing the influence of this unit on the interaction between shallow unconfined, sandy, and confined carbonate bedrock aquifers requires an appropriate groundwater model. A first step to this is to update the current conceptual hydrogeological model of the region, which either neglects the rhythmites or combines it with the till as a single hydrogeological unit. To do this, twenty electrical resistivity soundings and eleven 166 m long 2D transects of electrical resistivity data were combined with refraction seismic data acquired along a 96 m long transect. The geophysics results were validated with lithological data from a public database and two wells drilled during the study. A digital elevation model and stratigraphic layers extracted from the geophysics result were used to create a 3D site model. The model shows four layers consisting of a carbonate bedrock aquifer, confining till layer, rhythmically laminated silt and clay, and a sandy aquifer in a bottom-up sequence. The rhythmites consist of horizontal laminae that are more permeable than the underlying till. Combining geophysics with geologic data distinguished the till from rhythmites and defines the hydrostratigraphic layers used to refine the aquifer conceptual model across sections of the OOR.