Use of multi-electrode resistivity profiling to estimate hydraulic properties of preferential flow paths in alluvial floodplains

The Ozark region of eastern Oklahoma is characterized by limestone and sandstone plateaus partitioned by steep-sided stream valleys and occupied by clear, gravel-bed streams. Alluvial floodplain soils are generally cherty with high hydraulic conductivities. Many areas have alluvial floodplains consisting of a mantle of alluvial soils overlying thick subsoil primarily consisting of gravel. Previous work at a site in eastern Oklahoma found preferential flow of a conservative tracer within the gravel subsoil with an estimated hydraulic conductivity of 140 to 230 m/d. Multi-electrode electrical resistivity imaging (ERI) was used as a non-invasive method to determine the nature and extent of the preferential flow path (PFP). The ERI two-dimensional profile found a highly resistive layer in the area of the PFP. The range of resistance for this area was similar to the resistance found in an ERI survey from the surface of a nearby gravel bar, which strongly suggested that the PFP consists of clean, coarse gravel. This study attempted to determine whether PFPs exist at two other alluvial floodplain sites in the Ozarks with differing catchment areas. Since the factors controlling resistivity are similar to the factors controlling hydraulic conductivity, an association between the two was expected. Borehole permeameter tests, using a system specifically designed for gravelly soils, were used to estimate saturated hydraulic conductivity. An equation was developed between the point-measured hydraulic conductivity and larger-scale electrical resistivity mapping. Limitations in current analytical solutions prevented the estimation of hydraulic conductivity for all tests, primarily due to the rates that were induced on the gravel subsoils.