Integrated geophysical and geological investigation of a heterogeneous fluvial aquifer in Columbus, Mississippi

An integrated field and modeling study is used to test the applicability of geophysical methods for characterizing flow and contaminant transport in a heterogeneous fluvial aquifer at the Macro-Dispersion Experiment (MADE) site in Columbus, Mississippi. An outcrop located 1 km southeast of the MADE site was examined to quantify the relationship between geophysical attributes and lithostratigraphic properties. Seismic reflection, ground penetrating radar (GPR), and DC resistivity profiles were compared with outcrop descriptions, laboratory resistivity measurements, and grain-size analyses conducted on a core collected near the quarry wall. The aquifer is found to be composed of an upper sandy clay meandering fluvial system and a lower gravelly sand braided fluvial system separated by an erosional surface. The meandering fluvial facies is characterized by relatively low resistivity values (500-1250 m) and laterally continuous high-amplitude GPR reflectors. Lateral GPR reflection terminations indicate onlap at channel boundaries, which are filled with silty clay and clayey silt with low resistivity values (50-400 ohm-m). The braided fluvial facies is characterized by higher resistivity values (1600-2600 ohm-m) and low amplitude short bidirectional dipping reflectors. Seismic reflection data images the terrace, but depositional structures within the aquifer are below seismic resolution. Resistivity measurements on the core show a linear correlation to clay fraction, allowing the surface resistivity data to be used as a predictor of clay content in the aquifer. A grid of two dimensional GPR and DC resistivity data collected at the MADE site show similar geophysical facies. A geological model is developed on the basis of these data that agrees with previously collected hydraulic conductivity measurements and migration patterns of a tritium plume produced in a natural gradient tracer test. The tracer experiment is simulated using a hydraulic conductivity field derived from the DC resistivity data, calibrated by an empirical log-log relationship between hydraulic conductivity and DC resistivity derived at one borehole. The agreement between the simulated and observed tritium plumes demonstrates that extremely heterogeneous aquifers can be modeled with minimal hydrological data supplemented with geophysical data at least as well as previous models of the site using extensive well-based hydrologic data.