ASSESSING HYDRAULIC CONNECTIVITY IN A CARBONATE BEDROCK AND GLACIO-LACUSTRINE AQUIFER SYSTEM USING MULTI-PUMPING TESTS AND GEOPHYSICAL IMAGING

Carbonate bedrock aquifers are major groundwater sources in the United States. In glaciated regions, these aquifers can be overlain by a confining till, and a surficial unconfined aquifers composed of clay-silt-sand glacial-lacustrine sediments. Understanding the geometry, hydraulic variability, and potential connection between these aquifers is currently limited. In this study, we combined electrical resistivity imaging (ERI) and multiple pumping tests to investigate potential hydraulic connections within each aquifer unit and between the confined and unconfined aquifers. This study was conducted at the hydrogeophysics well-field located within the University of Toledo’s Stranahan Arboretum, Toledo, Ohio, consisting of three bedrock wells, each ~ 40 m deep, and eight shallow wells 4-10 m deep. Cross-borehole ERI using 21 electrodes spaced every 1 m and placed in each well was used to image the section across two wells in three different planes. Constant rate pumping and step drawdown tests were conducted in each bedrock well for 8 and 24 hours. Analysis of the time-drawdown curves was used to assess the aquifer type and boundary conditions, while curve matching with analytical solutions was used to estimate the transmissivity, specific storage, and hydraulic leakance. The plotted derivative curves indicate potential fracture dewatering within the bedrock aquifer. The diagnostic and specialized plots generated from the 8-hour test were consistent with theoretical models for confined aquifers. Whereas plots from the 24-hour pumping test align with theoretical models for leaky aquifers, indicating a delayed yield to pumping. Transmissivity estimates range from 4.74E-03 m²/s to 6.49E-03 m²/s. ERI results reveal low resistivity zones (<100 Ωm) interpreted as potential fracture or dissolution cavity zones. These results will be used to improve an existing numerical model for simulating groundwater flow and solute transport within these aquifers.