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

[ 1] 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.