HYDROGEOLOGIC INVESTIGATION OF SUBSURFACE STRATIGRAPHY USING GROUND PENETRATING RADAR IN SOUTHWESTERN SHELBY COUNTY, TENNESSEE

Inter-aquifer water exchange between the shallow Mississippi River Valley alluvial (MRVA) aquifer and the deeper Memphis aquifer in Shelby County, Tennessee, poses a hazard due to the downward movement of poorer quality groundwater through breaches in the upper Claiborne confining unit (UCCU), the aquitard for the Memphis aquifer. This research investigates the application of ground-penetrating radar (GPR), a non-invasive, near-surface geophysical tool, to interpret subsurface lithology and identify the extent of known and potential breaches in the UCCU at President’s Island and Ensley Bottoms along the Mississippi River in Shelby County. The presence of breaches in the UCCU beneath the MRVA in southwestern Shelby County was identified in previous studies using borehole drilling, but their lateral extent is unknown. A test GPR survey conducted using a MALA GX system with a 160 MHz hyper-stacking antenna in Ensley Bottoms, acquired 41 survey lines with a total survey length of 9.2 kilometers. The GPR data were processed with bandpass and background removal filters using industry standard software to create radargrams. The GPR radargrams for the east side of the field and parallel to the valley margin show a well-defined, concave upward surface at 12.75 m at its shallowest and 18.4 m at its deepest. Geologic log data from a borehole, 400 m to the south of the profile suggest the surface is likely the top of the UCCU. The concavity of the UCCU surface appears to be an inset paleo-channel. The radargrams also show multiple small (≤1 m) discontinuities in the surface as well as one large discontinuity that is over 30 m long. Though the small discontinuities appear to be fractures, possibly due to desiccation or from seismic shaking in the nearby New Madrid Seismic Zone, it is not clear if they are breaches or subtle changes in lithology. However, the ~30m wide discontinuity appears to show where the UCCU is missing and is likely a breach in the aquitard. This result is consistent with geologic log data from other boreholes in the area that show the UCCU is missing. Future work will include more GPR surveys with the 160 MHz system to test its sensitivity and with a 50 MHz GPR system to image deeper variations in the subsurface stratigraphy where the UCCU is thin or absent.