Breaking the Window of Detection: Using Multi‐Scale Solute Tracer Studies to Assess Mass Recovery at the Detection Limit

Abstract

The time that a parcel of water spends in various locations within a river corridor is a master variable that reflects the integrated effects of physical stores and fluxes and ultimately controls the biogeochemical processes and functions that are realized during transport. Empirical evidence of transit times in river corridors most commonly relies upon naturally occurring tracers to assess relatively long timescales (Cirpka et al., 2007;Gooseff et al., 2003;Lamontagne & Cook, 2007) and injected solute tracers for shorter timescales (Stream Solute Workshop, 1990;Ward et al., 2012). Studies using stream solute tracers are widespread in their application, but for more than 20 years have been known to be biased toward measuring the fastest portion of the transit time distribution, a limitation inherent to the method (J. W. Harvey et al., 1996;Wagner & Harvey, 1997). Despite the recognition of this limit, and the fact that the limitation is itself highly variable as a function of study design (e.g., Schmadel et al., 2016), hydrologists continue to conduct and interpret solute tracer studies. Any tracer that is released but not recovered (i.e., "lost") is attributed to flow that bypasses the monitoring location or transport along flowpaths longer than can be detected by monitoring equipment. In the latter case, such flowpaths could range in timescale from incrementally longer than what is detected to infinitely long. The inherent limits of tracer studies in streams define an arbitrary boundary that separates the tracer we are able to sense and interpret from the tracer we lose to the "black box" of longer timescales. While this demarcation is conceptually understood, we do not understand how this methodological threshold changes our understanding of transport in stream reaches (i.e., individual sections of river that are studied in an experiment, typically 10-100s of meters; Frissell et al., 1986). Moreover, when reaches are combined to represent segments (i.e., sections of a river that are comprised of multiple reaches, typically 100-1000s of meters; Frissell et al., 1986) it is unclear what impact-if any-the study limitations have on our understanding of transport in river corridors. Here, we ask how the well-documented and broadly acknowledged limits of solute tracer studies change our understanding at the reach-and segment-scales. In other words, if we manipulate the observational constraints of solute tracer studies to "peer into the black box", characterizing