Identification of Aquitard Breaches Using Electrical Methods and Borehole Data in Shelby County, Tennessee

Inter-aquifer exchange of poor-quality water through hydrogeologic windows or breaches (with absent or thin low-permeability layer) in confining units can affect the water quality of confined water-supply aquifers. Identifying breach location and geometry is essential for protecting and managing water resources. Previous studies provide evidence for 16 breaches in the upper Claiborne confining unit (UCCU) overlying the Memphis aquifer, a regionally important water-supply aquifer, in Shelby County, Tennessee. Researchers suggest that the breaches may result from the paleo-channel incision during the Eocene sea-level fall and the Quaternary incision by western Tennessee tributaries to the Mississippi River. This study utilizes electrical resistivity (ER) and airborne electromagnetic (AEM) methods to understand the usability of the methods in subsurface stratigraphic delineation, paleochannel identification, and their relation to the aquitard breaches in Shelby County, USA. ER method was employed to analyze inverted resistivity profiles from two floodplain sites with varying electrode spacing (10m, 7m, 5m, and 2.5m). The results showed that halving the electrode spacing caused an upward shift of about 1.3-1.6 times in stratigraphic depth, while a one-fourth change had a lesser impact. The inverted resistivity profiles of Shelby Farms and Audubon Park revealed the presence of Eocene paleochannels, incised into the Eocene Cook Mountain Formation (CMF). An ER profile along Gray\u27s Creek indicated the CMF\u27s presence but inferred thinning towards the north. Two ER profiles and borehole investigations at Presidents’ Island revealed relatively impermeable strata despite AEM showing a potential breach based on intermediate resistivity (~70 Ωm) responses. The AEM data show with high confidence (resistivity \u3e 100 Ωm) the absence of a protective clay layer to the west of the survey area; however, the area is inaccessible. AEM data were acquired in northern Shelby County along roughly parallel flight lines, 0.5 km apart, and processed with proprietary and GeoScene3D software. The AEM profiles exhibiting high resistivity zones (\u3e70 Ωm) within a low resistive layer (\u3c30 \u3eΩm) are interpreted to indicate the presence of silty or sandy Cockfield Formation (CFF) sediments inset within the CMF. A potential breach identified using AEM in northeastern Shelby County was verified by ER and borehole data, showing the potential of AEM as a tool for identifying breaches and its relation to the Eocene paleo-channel incision. Kilometer-scale AEM profiles were interpreted by integrating geological data and geophysical logs from nearby boreholes, clarifying numerous potential breach locations in northern Shelby County. The results provide a template for future AEM studies and demonstrate the potential for developing more accurate hydrogeologic models. Additionally, the depth and network geometry of paleo-channels inferred from the AEM depth slice at 60 m above sea level offer valuable information regarding the paleogeographic and geologic evolution of the paleo-channel systems and warrant further investigation