Geophysical mapping of preferential flow paths across multiple floodplains

In the Ozark ecoregion of Oklahoma, Arkansas and Missouri, the erosion of carbonate bedrock (primarily limestone) by slightly acidic water has left a residuum of chert gravel, producing gravel-bed streams and floodplains generally consisting of coarse chert gravel overlain by a mantle (1 to 300 cm) of gravelly loam or silt loam. Previous research has documented the occurrence of preferential flow paths (PFP) in an alluvial floodplain hypothesized to be a buried gravel bar. Field experiments have shown that the PFP affected alluvial groundwater flow in the floodplain and that water flow in the PFP was transmitted at rates that limited sorption of phosphorus. The implication of these findings depends partly on the frequency and distribution of similar preferential flow features. To this end, four floodplain sites were chosen for comparative mapping. The sites were located in the Ozark region of northeast Oklahoma and had similar underlying geology but differed in watershed area, land cover, and stream order. Subsurface features at the sites were mapped using electrical resistivity imaging (ERI). Vadose zone hydraulic conductivity was measured at three sites using a direct-push borehole permeameter. The ERI profiles at each site showed that the subsurface was heterogeneous and areas of high electrical resistivity formed discrete, possibly continuous features in the vadose zone. Interpolations, based on variograms of resistivity, showed that resistivity within the alluvial aquifers formed patterns that were often linked to geomorphic processes. Hydraulic conductivity within the alluvial aquifers was estimated by applying an empirical linear relationship between electrical resistivity and hydraulic conductivity. Since all of the alluvial floodplain sites were gravel dominated systems, the sites were similar enough that the linear relationship between electrical resistivity and hydraulic conductivity was not site-specific. The positive slope of the relationship suggested that areas of continuous high resistivity could also act as zones of preferential flow within the aquifer under suitable hydrologic conditions. Among the sites, maximum electrical resistivity and hydraulic conductivity generally increased with increasing watershed area.