Characterizing solute transport in coupled stream-hyporheic systems using electrical resistivity imaging

Hyporheic zones contribute a number of ecological services to streams, including buffering of stream temperatures, carbon and nutrient cycling, and providing habitat used by a variety of species. The hyporheic zone has been referred to as the stream's liver because of its ability to process pollutants in a zone of extensive biogeochemical activity. Despite the clear relationship between hyporheic exchange and numerous ecosystem services, the distribution of these services (a function of the hyporheic flowpath network itself) remains unknown. Common techniques to quantify hyporheic exchange rely on solute tracers, where downstream observations are used to infer transport processes occurring upstream. This type of inverse modeling suffers from a lack of physically meaningful parameters, lack of spatial resolution (i.e., providing only reach-average parameter sets), and results that suffer from a window of detection problem (i.e., the inability to quantify flowpaths at a spatial or temporal scale greater than some threshold, which is itself a function of the experimental design). Direct observation in subsurface monitoring wells provides spatially discrete subsurface measurements, but lacks an extensive spatial coverage. Monitoring wells characterize only those flowpaths that directly intersect the well. Three-dimensional modeling of flow and transport is possible, but model validation and calibration are data intensive, and may not capture heterogeneity that exists in natural systems.;This thesis presents work using electrical geophysical methods to overcome the limitations of traditional characterization methods. The first study provides a proof-of-concept in the use of electrical resistivity imaging and electrically conductive tracers to quantify hyporheic exchange. This study demonstrates the ability to quantify physical characteristics of the hyporheic zone through time and discusses the uncertainty in the method. Results demonstrate that not only does the spatial distribution of tracer concentration change in the hyporheic zone through time, but the physical size of the hyporheic zone is temporally variable. The second study demonstrates the use of geophysical imaging to characterize subsurface flowpaths that might be otherwise undetected using traditional geophysical study. The third study includes four replicate solute injections completed during baseflow recession in a headwater stream. This study suggests hyporheic exchange is highly variable as a function of catchment wetness, expressed by baseflow. The fourth study presents a numerical study developed to test the use of temporal moments in characterizing subsurface solute transport processes. Results of this study suggest temporal moments of both solute concentration and electrical resistivity can be used to characterize solute transport processes. Finally, the fifth study applies temporal moment analysis to time-lapse electrical resistivity images collected in the field. This study demonstrates the distributed analysis of physical transport processes in the subsurface. This body of work establishes electrical resistivity imaging as a tool to quantify stream-hyporheic interactions in the field, and represents a substantial advance in our ability to observe coupled surface-subsurface processes in-situ.