On the value of slug tracer injections and the naturally occurring radon for observing hyporheic exchange

The exchange of stream water and groundwater (hyporheic exchange) plays an important role in hydrological and biogeochemical processes in rivers. Hyporheic flow comprises a distribution of subsurface flow paths characterized by distinct transit times and flow path lengths. Much of the previous research relied on the interpretation of slug tracer experiments that only capture a portion of the overall hyporheic exchange, given their relatively short (minutes to hours) duration. Therefore, there is a need to go beyond the characterization of shorter flow paths in hyporheic research to understand flow paths of the entire transit time distribution. We hypothesize that environmental tracers provide complementary information into longer hyporheic flow paths. Here we derive and compare commonly used transport metrics for hyporheic exchange derived from radon and slug tracer injections and aim to identify combinations of model parameters that predict the concentrations of both tracers along experimental stream reaches. For this purpose, we measured the environmental tracer radon (²²²Rn), that increases with time along hyporheic flow paths, at several stream sections along Oak Creek, Oregon (USA). We conducted slug tracer (sodium chloride) injections at the same stream sections. We employed a transient storage model that includes radon specific processes such as radioactive decay to ensure comparability in the information acquired on hyporheic exchange from radon and the typically applied slug tracer experiments. We calibrated final stream discharge and hyporheic exchange metrics through a global identifiability analysis and subsequently calculated relevant transport metrics using the refined parameters. Results with the field data will be obtained soon. Hence, this study will contribute to a more holistic understanding of hyporheic flow paths and related processes, such as the biogeochemical turnover processes in rivers.