Concurrent conservative and reactive tracer tests in a stream undergoing hyporheic exchange

Abstract

[1] Knowledge about the strength and travel times of hyporheic exchange is vital to predict reactive transport and biogeochemical cycling in streams. In this study, we outline how to perform and analyze stream tracer tests using pulse injections of fluorescein as conservative and resazurin as reactive tracer, which is selectively transformed to resorufin when exposed to metabolically active zones, presumably located in the hyporheic zone. We present steps of preliminary data analysis and apply a conceptually simple mathematical model of the tracer tests to separate effects of in-stream transport from hyporheic exchange processes. To overcome the dependence of common parameter estimation schemes on the initial guess, we derive posterior parameter probability density functions using an adaptive Markov chain Monte Carlo scheme. By this, we can identify maximum-likelihood parameter values of instream transport, strength of hyporheic exchange, distribution of hyporheic travel times as well as sorption and reactivity coefficients of the hyporheic zone. We demonstrate the approach by a tracer experiment at River Goldersbach in southern Germany (60 L/s discharge). In-stream breakthrough curves were recorded with online fluorometers and jointly fitted to simulations of a one-dimensional reactive transport model assuming an exponential hyporheic travel-time distribution. The findings show that the additional analysis of resazurin not only improved the physical basis of the modeling, but was crucial to differentiate between surface transport and hyporheic transient storage of stream solutes. Parameter uncertainties were usually small and could not explain parameter variability between adjacent monitoring stations. The latter as well as a systematic underestimation of the tailing are due to structural errors of the model, particularly the exponential hyporheic travel-time distribution. Mean hyporheic travel times were in the range of 12 min, suggesting that small streambed structures dominate hyporheic exchange at the study site.

Exploring Tracer Information and Model Framework Trade‐Offs to Improve Estimation of Stream Transient Storage Processes

“…Approaches to obtain parameter estimates include the use of optimization algorithms (e.g., Briggs et al, ; Kerr et al, ; Runkel, ), Markov Chain Monte Carlo approaches (e.g., Knapp & Cirpka, ; Lemke et al, ), and Monte Carlo approaches coupled with behavioral thresholds (e.g., Kelleher et al, ; Wagener et al, ), as well as a broad literature on approaches to parameter sensitivity (see Pianosi et al, ). In this study, we employed approaches based on Monte Carlo methods to enable a tiered assessment that draws on both the very best and worst simulations and corresponding parameter estimates.…”

The Resazurin‐Resorufin System: Insights From a Decade of “Smart” Tracer Development for Hydrologic Applications

“…Depending on the characteristics of the system where resazurin is injected, tracer salts like sodium bromide or sodium chloride can be used (e.g., Argerich et al, ; González‐Pinzón et al, , , ; Haggerty et al, ; Knapp et al, ). In other studies, the fluorescent tracer fluorescein has been co‐injected with resazurin (e.g., Blaen et al, ; Knapp & Cirpka, ; Lemke et al, ). Importantly, the use of rhodamine WT is not suggested due to its close fluorescent interaction with resazurin and resorufin.…”

“…For these stream‐tracer tests, resazurin is injected into the stream channel together with a conservative tracer, and the concentrations of the three tracers are recorded over time at one or more measurement stations downstream of the injection site. Extensions of the TSM accounting for the turnover of resazurin in metabolically active zones are commonly used for the analysis of mass exchange, residence time distributions and reactivity (e.g., Haggerty et al, ; Knapp & Cirpka, ; Lemke, Liao, et al, ).…”

“…To investigate the applicability of the reactive tracer in bigger streams, resazurin was injected and recorded in a low gradient stream in Germany (Lemke, Liao, et al, ) injected and recorded resazurin in a low gradient stream in Germany. In the same study the authors presented the development and application of a field on‐line fluorometer (GGUN‐Fl30; described in further detail in Lemke, Schnegg, et al, ).…”

“…This approach assumes that the conservative and reactive transport follow the same advection, dispersion, and exchange processes, and that transport and reaction parameters are not correlated. However, Lemke, Liao, et al () showed that the estimates of transport parameters based on a joint fitting of conservative and reactive tracer may differ from those obtained by solely fitting conservative data. In a different approach, Liao and Cirpka () and Liao et al () combined the estimation of the transport parameters with compound‐related parameters, as well as a nonparametric travel time distribution, but found the results to be highly dependent on the initial guess.…”