Determining in‐channel (dead zone) transient storage by comparing solute transport in a bedrock channel–alluvial channel sequence, Oregon

Abstract

Current stream tracer techniques do not allow separation of in‐channel dead zone (e.g., eddies) and out‐of‐channel (hyporheic) transient storage, yet this separation is important to understanding stream biogeochemical processes. We characterize in‐channel transient storage with a rhodamine WT solute tracer experiment in a 304 m cascade‐pool‐type bedrock reach with no hyporheic zone. We compare the solute breakthrough curve (BTC) from this reach to that of an adjacent 367 m alluvial reach with significant hyporheic exchange. In the bedrock reach, transient storage has an exponential residence time distribution with a mean residence time of 3.0 hours and a ratio of transient storage to stream volume of 0.14, demonstrating that at moderate discharge, bedrock in‐channel storage zones provide a small volume of transient storage with substantial residence time. In the alluvial reach, though pools are similar in size to those in the bedrock reach, transient storage has a power law residence time distribution with a mean residence time of >100 hours (estimated at nearly 1200 hours) and a ratio of storage to stream volume of 105. Because the in‐channel hydraulics of bedrock reaches are simpler than alluvial step‐pool reaches, the bedrock results are probably a lower end‐member with respect to volume and residence time, though they demonstrate that in‐channel storage may be appreciable in some reaches. These results suggest that in‐stream dead zone transient storage may be accurately simulated by exponential RTDs but that hyporheic exchange is better simulated with a power law RTD as a consequence of more complicated flow path and exchange dynamics.