CHARACTERIZING SUBSURFACE FLOW PATHWAYS UNDERLYING AN INTERMITTENT RIVER USING MULTIPLE GEOPHYSICAL METHODS

Many streams in arid and semi-arid landscapes have shifted to non-perennial flow regimes due to pressures including human water use and climatic stresses. Regional pumping in the High Plains aquifer has led to declining groundwater levels and a transition from perennial to intermittent flow in the Middle Arkansas River Basin (KS, USA). The hydro-stratigraphic units at this site are the surficial alluvial aquifer, clay confining unit, and the High Plains aquifer from top to bottom. The confining unit is hypothesized to be leaky. However, the movement of groundwater through this low permeability layer, and therefore the impact of pumping on streamflow, is unclear. This study uses Electrical Resistivity Tomography (ERT) and Ground Electromagnetic (GEM) methods to map the spatial extent and thickness of the confining unit. GEM provides finer lateral resolution, while ERT provides continuous vertical mapping and is thus used to map the horizontal and vertical extent of the clay unit, respectively. The results suggest discontinuities in the clay layer that may act as flow pathways between the alluvial and High Plains aquifers, which could explain the drying of the Middle Arkansas River at this site. However, additional data is needed to support this interpretation. We plan to perform joint inversion with direct push electrical conductivity profiles to better resolve the extent and thickness of the confining clay layer. The improved stratigraphic characterization can assist future hydrologic studies and better explain the river’s response to recharge and pumping in the region.