The resazurin‐resorufin tracer system has been used to quantify surface water‐sediment interactions and microbial metabolic activity in stream ecosystems for one decade. This review describes the ...evolution of the tracer technique and summarizes how it has been used by the hydrologic and stream ecology communities. We highlight major hydrologic applications and milestones in the advancement of the reactive tracer system on scales ranging from cells to river reaches and catchments. We discuss the advantages and limitations of the resazurin‐resorufin system for hydrologic applications and suggest new directions of research, including how to address existing knowledge gaps. Beyond the goal of summarizing information that is specific to the development of the resazurin‐resorufin system, this review seeks to inform on the development of new “smart” tracer techniques as they, very likely, will face the same or similar challenges and opportunities encountered in the development of the resazurin‐resorufin system. The supporting information furthermore contains a detailed manual for the application of the resazurin‐resorufin system as hydrologic tracer and MATLAB codes for the analysis of their reactive transport.
Plain Language Summary
The reactive tracer compound resazurin has been used for 10 years to investigate how surface water interacts with the sediment and to calculate rates of metabolic activity. This review paper describes the development and applications of the tracer compound resazurin in hydrology and ecology. We also discuss advantages and limitations of the tracer and show future directions of research. The development of the resazurin‐resorufin system can also be used as a model for the development of other tracer techniques.
Key Points
Past applications of resazurin in hydrology on a range of spatial scales are summarized
New developments and challenges of the reactive tracer system are outlined
Details of the resazurin‐resorufin “smart” tracer system for the estimation of metabolic activity are presented
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
The fate of biologically available nitrogen (N) and carbon (C) in stream ecosystems is controlled by the coupling of physical transport and biogeochemical reaction kinetics. However, determining the ...relative role of physical and biogeochemical controls at different temporal and spatial scales is difficult. The hyporheic zone (HZ), where groundwater–stream water mix, can be an important location controlling N and C transformations because it creates strong gradients in both the physical and biogeochemical conditions that control redox biogeochemistry. We evaluated the coupling of physical transport and biogeochemical redox reactions by linking an advection, dispersion, and residence time model with a multiple Monod kinetics model simulating the concentrations of oxygen (O2), ammonium (NH4), nitrate (NO3), and dissolved organic carbon (DOC). We used global Monte Carlo sensitivity analyses with a nondimensional form of the model to examine coupled nitrification‐denitrification dynamics across many scales of transport and reaction conditions. Results demonstrated that the residence time of water in the HZ and the uptake rate of O2 from either respiration and/or nitrification determined whether the HZ was a source or a sink of NO3 to the stream. We further show that whether the HZ is a net NO3 source or net NO3 sink is determined by the ratio of the characteristic transport time to the characteristic reaction time of O2 (i.e., the Damköhler number, DaO2), where HZs with DaO2 < 1 will be net nitrification environments and HZs with DaO2 ≪ 1 will be net denitrification environments. Our coupling of the hydrologic and biogeochemical limitations of N transformations across different temporal and spatial scales within the HZ allows us to explain the widely contrasting results of previous investigations of HZ N dynamics which variously identify the HZ as either a net source or sink of NO3. Our model results suggest that only estimates of residence times and O2uptake rates are necessary to predict this nitrification‐denitrification threshold and, ultimately, whether a HZ will be either a net source or sink of NO3.
Key Points
Hyporheic (HZ) N is controlled by coupled transport and reaction kinetics
Ratio of HZ residence time to O2 reaction time controls N source‐sink dynamics
We present a process‐based scaling relationship for hyporheic N transformations
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BFBNIB, CEKLJ, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
•We quantified mixing lengths downstream of a WWTP over multiple flow conditions.•Empirical equations consistently failed at estimating experimental mixing lengths.•Erroneous predictions of mixing ...lengths can increase exposure to emerging contaminants.
Despite advances in wastewater treatment plant (WWTP) efficiencies, multiple contaminants of concern, such as microplastics, pharmaceuticals, and per- and poly-fluoroalkyl substances (PFAS) remain largely untreated near discharge points and can be highly concentrated before they are fully mixed within the receiving river. Environmental agencies enforce mixing zone permits for the temporary exceedance of water quality parameters beyond targeted control levels under the assumption that contaminants are well-mixed and diluted downstream of mixing lengths, which are typically quantified using empirical equations derived from one-dimensional transport models. Most of these equations were developed in the 1970s and have been assumed to be standard practice since then. However, their development and validation lacked the technological advances required to test them in the field and under changing flow conditions. While new monitoring techniques such as remote sensing and infrared imaging have been employed to visualize mixing lengths and test the validity of empirical equations, those methods cannot be easily repeated due to high costs or flight restrictions. We investigated the application of Lagrangian and Eulerian monitoring approaches to experimentally quantify mixing lengths downstream of a WWTP discharging into the Rio Grande near Albuquerque, New Mexico (USA). Our data spans river to WWTP discharges ranging between 2-22x, thus providing a unique dataset to test long-standing empirical equations in the field. Our results consistently show empirical equations could not describe our experimental mixing lengths. Specifically, while our experimental data revealed “bell-shaped” mixing lengths as a function of increasing river discharges, all empirical equations predicted monotonically increasing mixing lengths. Those mismatches between experimental and empirical mixing lengths are likely due to the existence of threshold processes defining mixing at different flow regimes, i.e., jet diffusion at low flows, the Coanda effect at intermediate flows, and turbulent mixing at higher flows, which are unaccounted for by the one-dimensional empirical formulas. Our results call for a review of the use of empirical mixing lengths in streams and rivers to avoid widespread exposures to emerging contaminants.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The Navigator is an autonomous system for Lagrangian monitoring of surface waters.•It generates and shares water quality and GPS data, site photos, and depth surveys.•It can support informed ...decisions to improve environmental and health outcomes.
Most freshwater aquatic studies rely on Eulerian monitoring, i.e., water quality and quantity are monitored using grab samples or semi-continuous sensors deployed at fixed cross-sections. While Eulerian monitoring is practical, it provides a limited understanding of spatial and temporal heterogeneity. We designed and built The Navigator, a Lagrangian (i.e., along a flow path) monitoring system that offers cost-effective solutions for in-situ, real-time data collection in surface freshwater ecosystems. The Navigator features a suite of technologies, including an autonomous surface vehicle with GPS and LTE connectivity, water quality sensors, a depth sonar, a camera, and a webpage dashboard to visualize real-time data. With these technologies, The Navigator provides insight into where, how, and why water quality and quantity change over time and space as it moves with the current or follows user-specified pathways. We tested The Navigator monitoring water quality parameters at high spatial-temporal resolution in multiple surface water bodies in New Mexico (USA) to: (1) identify water quality changes associated with land use changes along a 7th-order reach in the Rio Grande, (2) identify the fate of wildfire disturbances ∼175 km downstream of a burned watershed affected by the largest wildfire ever recorded in the state, (3) monitor the water quality of a recreational fishing pond in the City of Albuquerque. Our three successful tests confirm that The Navigator is an affordable (USD 5,101 in 2023) monitoring system that can be used to address questions involving mass and energy balances in surface waters.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The use of smart tracers to study hydrologic systems is becoming more widespread. Smart tracers are compounds that irreversibly react in the presence of a process or condition under investigation. ...Resazurin (Raz) is a smart tracer that undergoes an irreversible reduction to resorufin (Rru) in the presence of cellular metabolic activity. We quantified the relationship between the transformation of Raz and aerobic bacterial respiration in pure culture experiments using two obligate aerobes and two facultative anaerobes, and in colonized surface and shallow (<10 cm) hyporheic sediments using reach‐scale experiments. We found that the transformation of Raz to Rru was nearly perfectly (minr2 = 0.986), positively correlated with aerobic microbial respiration in all experiments. These results suggest that Raz can be used as a surrogate to measure respiration in situ and in vivoat different spatial scales, thus providing an alternative to investigate mechanistic controls of solute transport and stream metabolism on nutrient processing. Lastly, a comparison of respiration and mass‐transfer rates in streams suggests that field‐scale respiration is controlled by the slower of respiration and mass transfer, highlighting the need to understand both biogeochemistry and physics in stream ecosystems.
Key Points
The smart tracer resazurin (Raz) can be used to measure aerobic respiration
Raz was correlated with aerobic respiration in pure culture and field tests
Raz can be used to measure respiration in situ and in vivo in stream ecosystems
We investigated scaling of conservative solute transport using temporal moment analysis of 98 tracer experiments (384 breakthrough curves) conducted in 44 streams located on five continents. The ...experiments span 7 orders of magnitude in discharge (10−3 to 103 m3/s), span 5 orders of magnitude in longitudinal scale (101 to 105 m), and sample different lotic environments—forested headwater streams, hyporheic zones, desert streams, major rivers, and an urban manmade channel. Our meta‐analysis of these data reveals that the coefficient of skewness is constant over time (
CSK =1.18±0.08,
R2>0.98). In contrast, the
CSK of all commonly used solute transport models decreases over time. This shows that current theory is inconsistent with experimental data and suggests that a revised theory of solute transport is needed. Our meta‐analysis also shows that the variance (second normalized central moment) is correlated with the mean travel time (
R2>0.86), and the third normalized central moment and the product of the first two are very strongly correlated (
R2>0.96). These correlations were applied in four different streams to predict transport based on the transient storage and the aggregated dead zone models, and two probability distributions (Gumbel and log normal).
Key Points
There are scale‐invariant patterns in stream solute transport
There is persistence of skewness in stream solute transport
A revised transport theory is needed to correctly represent experimental results
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We evaluated groundwater quality, pollution, and its effects on human health in the eastern part of the Lake Urmia basin, the largest lake in the Middle East. Although groundwater quality is suitable ...for drinking and irrigation purposes, an index-based approach quantifying heavy metal pollution revealed that most sampling sites exhibited moderate to high pollution levels in the northern and southern regions. The positive matrix factorization (PMF) and principal component analysis-multi linear regression (PCA-MLR) receptor models suggest that the main contributors to the observed groundwater pollution, expressed as percentages by model, were: lake water infiltration and dissolution of minerals and fertilizers (46% and 63%), infiltration of leachates from solid wastes (29% and 15%), mixing with industrial-municipal wastewaters (18% and 13%), and vehicular emissions (7% and 9%). The PMF model indicated better correlations between observed and predicted concentrations (R2 = 0.96) than the PCA-MLR (R2 = 0.89). Our results from the human health risk assessments (HHRA) highlight non-carcinogenic and carcinogenic risks for Pb and Cr, respectively. Also, the PMF-based assessment of human health risk indicated that wastewaters and solid waste leachates are responsible for the cancer risk from Cr for children.
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•Groundwater pollution by heavy metals indicated a moderate to high level.•The chemical composition of Lake Urmia groundwaters reflects a combination of natural and anthropogenic sources.•The PMF receptor model is preferred for estimating the source apportionment.•Pollution by wastewaters and solid waste leachates are responsible for the cancer risk from Cr for children.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Wildfires are increasing globally in frequency, severity, and extent, but their impact on fluvial networks, and the resources they provide, remains unclear. We combine remote sensing of burn ...perimeter and severity, in-situ water quality monitoring, and longitudinal modeling to create the first large-scale, long-term estimates of stream+river length impacted by wildfire for the western US. We find that wildfires directly impact ~6% of the total stream+river length between 1984 and 2014, increasing at a rate of 342 km/year. When longitudinal propagation of water quality impacts is included, we estimate that wildfires affect ~11% of the total stream+river length. Our results indicate that wildfire activity is one of the largest drivers of aquatic impairment, though it is not routinely reported by regulatory agencies, as wildfire impacts on fluvial networks remain unconstrained. We identify key actions to address this knowledge gap and better understand the growing threat to fluvial networks, water security, and public health risks.
Fluvial networks integrate, transform, and transport constituents from terrestrial and aquatic ecosystems. To date, most research on water quality dynamics has focused on process understanding at ...individual streams, and, as a result, there is a lack of studies analyzing how physical and biogeochemical drivers scale across fluvial networks. We performed tracer tests in five stream orders of the Jemez River continuum in New Mexico, USA, to quantify reach‐scale hyporheic exchange during two different seasonal periods to address the following: How do hyporheic zone contributions to overall riverine processing change with space and time? And does the spatiotemporal variability of hyporheic exchange scale across fluvial networks? Combining conservative (i.e., bromide) and reactive (i.e., resazurin) tracer analyses with solute transport modeling, we found a dominance of reaction‐limited transport conditions and a decrease of the contributions of hyporheic processing across stream orders and flow regimes. Our field‐based findings suggest that achieving knowledge transferability of hyporheic processing within fluvial networks may be possible, especially when process variability is sampled across multiple stream orders and flow regimes. Therefore, we propose a shift in our traditional approach to investigating scaling patterns in transport processes, which currently relies on the interpretation of studies conducted in multiple sites (mainly in headwater streams) that are located in different fluvial networks, to a more cohesive, network‐centered investigation of processes using the same or readily comparable methods.
Key Points
Reactive tracer tests revealed reaction‐limited conditions along a first‐ to fifth‐order fluvial network during contrasting flow regimes
Processing rate coefficients and Damköhler numbers decreased along the continuum
Results support modeling expectations of decreasing hyporheic contributions along fluvial networks
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Hyporheic exchange influences hydrologic transport and water quality through transient storage, which extends solute transit time, and leads to mixing of surface water and groundwater. Despite its ...importance, estimating the extent and spatiotemporal variability of the hyporheic zone remains challenging due to limitations in assessing the subsurface with discrete point‐scale sampling. Analysis of time‐lapse electrical resistivity (ER) data from tracer studies has shown potential to ameliorate such limitations. However, its utility in objectively delimiting hyporheic extent and quantifying changes in surface‐groundwater exchange has been impeded by reliance on qualitative analysis of hyporheic extent or the use of a priori assumptions about data quality and signal strength. This study applies a novel unsupervised clustering method to time‐lapse ER models derived from a benchmark dataset collected throughout baseflow recession in a mountain stream. We demonstrate that unsupervised clustering of inverted ER model time series can delimit hyporheic extent by distinguishing solute transport signals from noisy background inversions and identify functional zones defined by unique transport characteristics. We found that the structure of these zones was stable even as discharge changed by an order of magnitude, likely due to morphological constraints in this steep, narrow valley. Compared to traditional methods utilizing a priori thresholds to delimit hyporheic extent, clustering is robust to unintentional variations in tracer breakthrough curves that are typical of field‐based studies. Therefore, clustering of inverted ER models represents a more robust and data‐driven functional zonation representation of hyporheic exchange than has been possible with point‐scale sampling or transport modelling, which usually assumes a single well‐mixed hyporheic zone.
A novel hierarchical clustering approach is used to approximate hyporheic extent from stream tracer and geophysical data without a priori assumptions. Evidence is presented of stable functional zonation characterized by differences in transport behaviour within an Oregon streambed, even as streamflow changes by an order of magnitude.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
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