HIGH RESOLUTION GEOPHYSICAL IMAGING AT THE MACRODISPERSION EXPERIMENT (MADE) SITE IN COLUMBUS, MISSISSIPPI

With more than 2000 hydraulic conductivity measurements and three tracer tests, the MADE site serves as an excellent laboratory for examining relationships between geophysical attributes and groundwater flow and contaminant transport properties in extremely heterogeneous aquifers. To better understand these relationships, a grid of two dimensional ground penetrating radar (GPR) and DC resistivity data was collected at the site. The geophysical data, correlated to coring logs and outcrop, delineate four lithostratigraphic units with distinctive geophysical characteristics. From the surface downward, these include 1) a 4-m thick moderately resistive (200-700 ohm-m) horizontally stratified layer interpreted as the modern meandering fluvial system; 2) an 8-m thick highly resistive (1000-4000 ohm-m) complexly stratified unit interpreted as a braided fluvial system; 3) a ca. 3-m thick moderately-low resistivity (100-200 ohm-m) low GPR amplitude horizontally discontinuous unit that pinches out northward interpreted as the sand member of the Eutaw Formation; and 4) a low resistivity (ca. 100 ohm-m) unit at approximately 12 m depth lacking internal GPR reflections interpreted as the clay member of the Eutaw Formation. Water table measurements and tracer test flow models indicate that the braided fluvial system and sand member of the Eutaw Formation is the primary aquifer. The Eutaw Formation clay member serves as a basal aquitard. A paleochannel visible in aerial photographs was mapped in the meandering fluvial system with both resistivity and GPR data. The channel is 2-4 m deep and, based on resistivity values correlated with core data, is predominantly filled with clay and silt. The channel lies above the water table, suggesting that it has little impact on contaminant transport at the site. Hydraulic conductivity variations inferred from GPR stratigraphy and empirical correlations between resistivity and core data are in general agreement with tracer plume concentrations and borehole hydraulic conductivity measurements, indicating that the combination of GPR stratigraphic analysis and resistivity correlations to grain size is an effective tool for predicting hydraulic conductivity in heterogeneous braided fluvial aquifers at scales needed for effective contaminant transport modeling.