Comparison of In-Channel Mobile-Immobile Zone Exchange During Instantaneous and Constant Rate Stream Tracer Additions: Implications for Design and Interpretation of Non-Conservative Tracer Experiments

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The stream tracer experiment, including field tracer application and subsequent analysis of solute transport and storage, is an important tool in stream hydrology and ecology. However, there have been few comparisons of tracer dynamics between the commonly applied methods of instantaneous (IA) and constant rate (CRA) tracer additions. To determine whether there are fundamental differences between the two addition techniques due to surface storage zone loading and flushing during experiments, we compare longitudinal distributions of tracer dynamics of stream in-channel dead zones during IA and CRA experiments. Back-to-back IA and CRA additions were carried out in two morphologically distinct tundra stream reaches in Alaska. Dead zone tracer time series are determined by an aggregate of upstream transport and individual dead zone residence time distributions (RTDs). The dead zone breakthrough curves for both tracer addition techniques were not consistent, neither were aggregate RTDs observed in each dead zone. Flushing patterns of tracer from dead zones reveal that stream flushing after IA additions was slower than after CRA additions. However, whole-stream RTDs were similar between IA and CRA techniques in each reach. The implications of these findings are important to design and interpretation of IA and CRA stream tracer experiments, particularly those with reactive solutes whose transformations may depend on solute concentration. Thus, IA and CRA experiments may yield differing conclusions about non-conservative transport in streams because of the inherent differences in loading of transient storage zones between these two addition techniques, and potential differences in biogeochemical processing that may occur as a consequence.