Multitracer Field Fluorometry: Accounting for Temperature and Turbidity Variability During Stream Tracer Tests

Abstract

The use of multitracer field fluorometry is increasing in the hydrological sciences. However, obtaining high‐quality fluorescence measurements is challenging given the variability in environmental conditions within stream ecosystems. Here, we conducted a series of stream tracer tests to examine the degree to which multitracer field fluorometry produces reliable estimates of tracer concentrations under realistic field conditions. Using frequently applied examples of conservative (Uranine) and reactive (Resazurin‐Resorufin) fluorescent tracers, we show that in situ measurements of tracer breakthrough curves can deviate markedly from corresponding samples analyzed under laboratory conditions. To investigate the effects of key environmental variables on fluorescence measurements, we characterized the response of field fluorometer measurements to changes in temperature, turbidity, and tracer concentration. Results showed pronounced negative log‐linear effects of temperature on fluorescence measurements for all tracers, with stronger effects observed typically at lower tracer concentrations. We also observed linear effects of turbidity on fluorescence measurements that varied predictably with tracer concentration. Based on our findings, we present methods to correct field fluorometer measurements for variation in these parameters. Our results show how changing environmental conditions can introduce substantial uncertainties in the analysis of fluorescent tracer breakthrough curves, and highlight the importance of accounting for these changes to prevent incorrect inferences being drawn regarding the physical and biogeochemical processes underpinning observed patterns.

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

“…The last decade has seen an explosion of novel techniques for collecting data used to characterize dynamic hydrologic systems. Tools and techniques that fall under this umbrella include the burgeoning field of hydrogeophysics (e.g., St Clair et al, ; Ward et al, ), the use of unoccupied aerial vehicles (e.g., Brenner et al, ; Vivoni et al, ), high space‐time resolution sensing systems (e.g., Blaen et al, ; Khamis et al, ), and the growing use of smart and conservative tracers in the environment (e.g., Blaen et al, ; González‐Pinzón et al, ; Haggerty et al, ; Knapp et al, ; Runkel, ). Observational data obtained from these techniques have been used to reveal new process dynamics and to refine current understanding of hydrological systems.…”

BIFoR FACE: Water–soil–vegetation–atmosphere data from a temperate deciduous forest catchment, including under elevated CO2

“…An ISCO peristaltic pump (Lincoln, NE, USA) passes 1 L of stream water through these sensors every hour. Stream monitoring is supplemented with campaign‐based sampling facilitated by networks of surface water ISCO autosamplers, for instance during tracer tests (Blaen, Brekenfeld, et al, 2017; Blaen, Khamis, et al, 2017), as well as spatially nested multi‐level mini‐piezometers installed in the streambed to investigate streambed biogeochemical processes and groundwater–surface water interactions (Comer‐Warner et al, 2019; Comer‐Warner et al, 2020). A stage‐discharge relationship for the Wood Brook was developed by Blaen, Khamis, et al (2017).…”

“…Wood Brook is a second-order stream with a 3.1 km 2 catchment ranging in elevation from 90 to 150 m amsl (Blaen, Brekenfeld, et al, 2017) and subsequently draining into the River Severn catchment (the most voluminous river in England and Wales). The entire catchment is undergoing substantial land-use change, having been converted to organic farming since 2019.…”

“…The BIFoR FACE facility (52°48′3.6″ N, 2°18′0″ W, 106 m above mean sea level [amsl]) is situated in a mainly agricultural headwater catchment in the UK drained by the Wood Brook, and consists of the main elevated CO 2 (eCO 2 ) facility and a number of other study sites including tree planting of different age and management (Figure 1). Wood Brook is a second‐order stream with a 3.1 km 2 catchment ranging in elevation from 90 to 150 m amsl (Blaen, Brekenfeld, et al, 2017) and subsequently draining into the River Severn catchment (the most voluminous river in England and Wales). The entire catchment is undergoing substantial land‐use change, having been converted to organic farming since 2019.…”

“…An ISCO peristaltic pump (Lincoln, NE, USA) passes 1 L of stream water through these sensors every hour. Stream monitoring is supplemented with campaign-based sampling facilitated by networks of surface water ISCO autosamplers, for instance during tracer tests(Blaen, Brekenfeld, et al, 2017;Blaen, Khamis, et al, 2017), as well as spatially nested multi-level minipiezometers installed in the streambed to investigate streambed biogeochemical processes and groundwater-surface water interactions(Comer-Warner et al, 2019;Comer-Warner et al, 2020). A stagedischarge relationship for the Wood Brook was developed byBlaen, Khamis, et al (2017).Volumetric soil moisture content in the main BIFoR FACE facility is monitored by 12 cm long frequency domain reflectometry sensors(CS655 by Campbell Scientific [Logan, USA], ±3% v/vfor typical soils) installed diagonally from the surface in triangular groups of three spaced 1 m apart, with two groups in the 'control' and 'treatment' patches and one group in the undisturbed patches, and monitoring at 15 to 30 min resolution.…”