A FIELD COMPARISON OF MULTIPLE TECHNIQUES TO QUANTIFY SURFACE WATER- GROUNDWATER INTERACTIONS (Invited Presentation)

Surface water-groundwater interactions in streams are difficult to quantify due to the heterogeneity in hydraulic and reactive processes that operate across a range of spatial and temporal scales. The challenge of quantifying these interactions has led to the development of several techniques, from cm-scale probes to whole-system tracers. We co-applied several of these techniques within a single experimental reach in a third-order stream, including: conservative and “smart” reactive solute-tracer tests, measurement of hydraulic heads, distributed temperature sensing, vertical profiles of solute tracer and temperature in the streambed, and electrical resistivity imaging. Results from the field experiment consistently indicated that surface water-groundwater interactions were not spatially expansive, but were high in flux through a shallow hyporheic zone surrounding the 450-m study reach. The NaCl and resazurin tracers suggested different surface-subsurface exchange patterns between the upper two thirds and lower third of the reach. Subsurface sampling of tracers and vertical thermal profiles quantified relatively high fluxes through a 10–20 cm deep hyporheic zone with chemical reactivity of the resazurin tracer indicated at 3, 6 and 9 cm sampling depths. Monitoring of hydraulic gradients along transects starting ~40 m away from the stream indicated that groundwater discharge prevented the development of a larger hyporheic zone, which was shown from MINIPOINT samples to progressively decrease from the stream thalweg toward the banks. Finally, distributed temperature sensing did not detect extensive inflow of groundwater into the stream and electrical resistivity imaging showed limited large-scale hyporheic exchange. From the experience gained in our experiment, we recommend the following reasoning to decide which technique(s) should be implemented in a particular study: 1) clearly define the nature of the questions to be addressed, i.e., physical, biological or chemical processes, 2) identify the spatial and temporal scales to be covered explicitly and those required to provide an appropriate context for interpretation, and 3) engage in collaborative research efforts that maximize the generation of mechanistic understanding and reduce the costs of implementing multiple techniques.