Comparing tracer techniques to explore flow paths and transit times through the river corridor

Solute tracers are a useful tool for understanding advective transport, time scales of hyporheic flow paths through individual channel morphologic features, changes in discharge along a reach, and much more. However, solute tracers provide a stream-centric point of view, limiting the ability to measure flow paths beyond a single reach or to parse out lateral inputs from the adjacent hillslopes. Some flow paths can take days to weeks to experience complete turnover. These are usually longer than the window of detection or reach length used in most stream tracer studies but might be important for processing of nutrients or for other long-time scale biogeochemical reactions. Here, we combine both salt and uranine tracer studies to identify hyporheic exchange at time scales much longer than those typically accessible via salt tracers alone. Our tracer study was conducted within a ~300-m long reach of a steep, 2nd-order mountain stream, bounded by bedrock at both ends, so that subsurface bypass flow cannot confound the results of our tracer experiment. Dilution gaging was conducted to quantify gross gains and losses, providing an independent estimate of hillslope inputs to the stream. As expected, the ability to measure extremely low concentrations of the fluorescent tracer, uranine, greatly expanded the window of detection, allowing us to observe exchange flows with a time scale of at least 2 weeks, much longer than the exchange flows observed with salt tracers, which were under 24 hours. This study is providing a greater understanding of the time scales and fluxes often missed by solute tracers and their relative contribution to total fluxes in the river corridor.