AN IMPROVED CONCEPTUAL HYDROGEOLOGICAL MODEL OF A POST-GLACIAL AQUIFER IN NORTHWESTERN OHIO USING GEOPHYSICAL AND GEOLOGICAL DATASETS

Hydrogeological models offer a great value in conceptualizing aquifer processes for the purpose of understanding the aquifer system and predicting flow and transport within them. Developing a conceptual hydrogeological model based on geophysical data can elucidate aquifer architecture and heterogeneity at meter and smaller scales, which can lead to better predictions of flow and transport pathways. In this research, we combine electrical resistivity, refraction seismic, and geological well logs to develop a conceptual hydrogeological model of a post-glacial aquifer system in northwestern Ohio. We also assess potential connection between the shallow sandy and deeper carbonate aquifers and the implication on solute transport within the aquifers. This study is carried out at the Stranahan Arboretum in Toledo, northwest Ohio. We obtained well logs of boreholes drilled in the area from the ODNR database. Electrical resistivity surveys including vertical electrical sounding and electrical resistivity imaging were conducted using a SuperSting R8 resistivity meter to obtain 1D, 2D and 3D profiles of resistivity distribution at the study site. Refraction seismic profiles were acquired using a 24 channel Geode with geophones spaced every 2 m and a sledgehammer used as the source. Results of the electrical resistivity revealed three major hydrostratigraphic units, which include a 1 – 2m thick shallow sandy unit (170 to 350 Ωm), a glacial-lacustrine sediments unit (16 to 120 Ωm) extending to depth of about 12 m, and a carbonate bedrock aquifer (150 to 300 Ωm) starting at a depth of 8 to 15 m. Velocity models from refraction seismic also isolate the 3 hydrostratigraphic units. In general, the sandy aquifer is very shallow and largely discontinuous within the study area but tend to be well developed towards the south-east region. Preliminary results of slug test estimated the hydraulic conductivity (K) of the sandy aquifer to be about 3.8 x 10^-5 m/s which corresponds to K-values of medium – fine well sorted sands. By combining the geophysical and geological data, we created a 3D conceptual hydrogeological model for the aquifers which will be converted to a starting numerical model for predicting groundwater flow and solute transport in the post-glacial aquifers in the region.