Imaging Hyporheic Zone Solute Transport Using Electrical Resistivity

Traditional characterization of stream-aquifer interaction relies upon observed in-stream and subsurface monitoring well breakthrough curves. Models are then used to estimate hyporheic zone size and infer exchange rates. Solute data integrate upstream behavior and lack spatial coverage, limiting our ability to accurately quantify spatially heterogeneous exchange dynamics. Furthermore, existing methods do not provide information about solute that has entered less mobile domains in the subsurface, yielding overall non-Fickian behavior. Here, we demonstrate the application of near-surface electrical resistivity imaging methods, coupled with stream tracer experiments (using conservative NaCl), to provide in-situ imaging of spatial and temporal dynamics of hyporheic exchange. Tracer-labeled water is more electrically conductive than groundwater and in-stream background levels, reducing electrical resistance of the subsurface actively communicating with the stream (the hyporheic zone). Comparison of background measurements with those recording tracer presence provides distributed characterization of hyporheic area at a given transect (in this application, peak of 0.5 m2), and hyporheic volume in a reach (peak of 2.0 m3 in our 20 m study reach). Results demonstrate the first two- and three- dimensional imaging of stream-aquifer exchange and hyporheic zone extent. Inverted resistivity tomograms map the extent of hyporheic exchange throughout the reach of interest, and provide insight to subsurface heterogeneity and localized processes, which would otherwise be averaged over the reach using traditional characterization methods. Future application will greatly enhance our ability to understand Fickian and non-Fickian solute transport and fate in hyporheic zones.