GEOPHYSICAL CHARACTERIZATION OF A HETEROGENEOUS BRAIDED STREAM AQUIFER, COLUMBUS, MISSISSIPPI

The MADE (Macro Dispersion Experiment) Site in Columbus, Mississippi has served as an experimental testing ground for numerous researchers interested in field assessment of hydraulic conductivity since the early 1980’s. The main purpose of this project is to characterize the heterogeneities of the MADE site at decimeter horizontal and lateral resolutions using high-resolution geophysical methods including seismology, electrical resistivity, and ground-penetrating radar. These results are compared to similar measurements taken at an open sand and gravel quarry pit located approximately 1 km southeast of the MADE site. Core data and geologic descriptions from both sites confirm that the MADE site and quarry have similar lithologies and facies.

The aquifer is composed of Quaternary alluvium consisting primarily of sand and gravel with varying amounts of clay underlain by a clay-rich aquitard at depths ca. 11 m. At the quarry pit, seismic reflection data images the base of the aquifer and the contact between a laterally continuous sandy clay unit and underlying clay-rich sandy gravel within the aquifer, but depositional structures within the aquifer have proved below seismic resolution. However, initial results from the electrical and radar methods have been extremely useful for this purpose. Meter-scale channels and decimeter-scale channel-fill facies mapped with ground penetrating radar correlate extremely well with the outcrop and with a DC resistivity profile. A close relationship exists between resistivity and lithology. Laboratory measurements of resistivity from a core collected along the quarry wall show a remarkably strong linear correlation to clay content in all units exclusive of those dominated by gravel. Future work includes interpreting the data to develop a depositional model and extrapolating our findings from the quarry to the MADE site itself.