180 research outputs found
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Development of a âsmartâ tracer for the assessment of microbiological activity and sediment-water interaction in natural waters: The resazurin-resorufin system
A âsmartâ tracer is a tracer that provides, directly or through measurement of its concentration or in combination with another compound, at least one âbitâ more of information about the environment through which it travels than a conservative tracer. In this study we propose and present the chemical compound resazurin as a smart tracer to assess the coupling between solute transport and microbiological activity in sediment-water interfaces in freshwaters. Resazurin is a weakly fluorescent redox-sensitive dye that undergoes an irreversible reduction to strongly fluorescent resorufin under mildly reducing conditions, most commonly in the presence of living microorganisms. To investigate the suitability of resazurin as a smart tracer, we characterized the decay, sorption, reaction, and transport behavior of resazurin and resorufin in various waters and sediments using laboratory experiments. Results show that resazurin irreversibly and rapidly reacts to resorufin in colonized sediment with pseudo-first-order behavior and a rate coefficient of 1.41 hâ»Âč. This reaction is 3 orders of magnitude faster than that in stream water alone, indicating the tracer is sensitive to microbiological activity and associated sediment-water interactions. The compounds are affected by significant sorption, with an approximately linear isotherm and a K[subscript d] of 6.63 mL/g for resorufin in sediment with 2.19% organic carbon. The compounds are stable over weeks in natural water, except in the presence of strong light where significant photochemical decay may occur more rapidly
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Quantifying spatial differences in metabolism in headwater streams
Stream functioning includes simultaneous interaction among solute transport, nutrient processing, and metabolism. Metabolism is measured with methods that have limited spatial representativeness and are highly uncertain. These problems restrict development of methods for up-scaling biological processes that mediate nutrient processing. We used the resazurinâresorufin (Raz-Rru) tracer system to estimate metabolism at different spatial scales (habitat, subreach, and reach) in 2 headwater streams of the H. J. Andrews Experimental Forest (Oregon, USA), and present a mathematical framework for its application. We investigated the relationship between metabolism and hydrodynamics, i.e., geomorphic units (e.g., poolâriffle, poolâcascade), bed materials (i.e., alluvium vs bedrock channels) and type of transient storage (i.e., pure hyporheic exchange, pure surface transient storage, and a combination of both). The metabolic hotspots detected by the Raz-Rru system in both watersheds were related to hydrodynamic conditions known to increase biological processing. Higher respiration rate coefficients were found in subreaches with extensive hyporheic flow and flow through large woody-debris complexes, and higher reaeration rate coefficients were found in subreaches with intensive respiration activity and higher flow velocities. Because such hydrodynamic conditions and their effects on stream processing are difficult to quantify in headwater streams without the use of tracer techniques, the Raz-Rru system proved to be a good integrator of solute transport and stream metabolism processes.Keywords: resazurin,
metabolism,
aerobic respiration,
transient storage,
hotspots,
smart tracer,
headwater stream
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Tracer tests in a fractured dolomite: 3. Double-porosity, multiple-rate mass transfer processes in convergent flow tracer tests
Convergent flow tracer tests conducted in the Culebra dolomite (Rustler Formation, New Mexico) are analyzed with both single- and multiple-rate, double-porosity models. Parameter estimation is used to determine the mean and standard deviation of a lognormal distribution of diffusion rate coefficients as well as the advective porosity and longitudinal dispersivity. At two different test sites both multirate and single-rate models are capable of accurately modeling the observed data. The single-well injection-withdrawal test provides more precise estimates of the mass transfer parameters than the convergent flow tracer tests. Estimation of the multirate distribution parameters is consistent across locations for the two types of tests. Limits of resolution are calculated for the multirate distribution, and these limits explain the precision with which the standard deviation of the multirate distribution can be estimated. These limits also explain the necessary increase in the advective porosity for the single-rate model at one location and not the other. Implications of the multirate mass transfer model at time and length scales greater than those of the tracer tests include the instantaneous equilibrium of a significant fraction of the matrix and the possibility of a fraction of the diffusive porosity not reaching an equilibrium solute concentration at long times.Keywords: Hydrology, Groundwater transpor
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Determining in-channel (dead zone) transient storage by comparing solute transport in a bedrock channelâalluvial channel sequence, Oregon
Current stream tracer techniques do not allow separation of in-channel dead zone (e.g., eddies) and out-of-channel (hyporheic) transient storage, yet this separation is important to understanding stream biogeochemical processes. We characterize in-channel transient storage with a rhodamine WT solute tracer experiment in a 304 m cascade-pool-type bedrock reach with no hyporheic zone. We compare the solute breakthrough curve (BTC) from this reach to that of an adjacent 367 m alluvial reach with significant hyporheic exchange. In the bedrock reach, transient storage has an exponential residence time distribution with a mean residence time of 3.0 hours and a ratio of transient storage to stream volume of 0.14, demonstrating that at moderate discharge, bedrock in-channel storage zones provide a small volume of transient storage with substantial residence time. In the alluvial reach, though pools are similar in size to those in the bedrock reach, transient storage has a power law residence time distribution with a mean residence time of >100 hours (estimated at nearly 1200 hours) and a ratio of storage to stream volume of 105. Because the in-channel hydraulics of bedrock reaches are simpler than alluvial step-pool reaches, the bedrock results are probably a lower end-member with respect to volume and residence time, though they demonstrate that in-channel storage may be appreciable in some reaches. These results suggest that in-stream dead zone transient storage may be accurately simulated by exponential RTDs but that hyporheic exchange is better simulated with a power law RTD as a consequence of more complicated flow path and exchange dynamics.Keywords: residence time distribution, transient storage, dead zone, hyporheic exchange, H.J. Andrews Experimental Forest, bedrock channelKeywords: residence time distribution, transient storage, dead zone, hyporheic exchange, H.J. Andrews Experimental Forest, bedrock channe
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Scaling and predicting solute transport processes in streams
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
(10â» Âł to 10Âł mÂł/s), span 5 orders of magnitude in longitudinal scale (10Âč to 10â” m), and
sample different lotic environmentsâforested 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 Œ 1:1860:08,
RÂČ > 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 (RÂČ > 0:86), and the third normalized central moment and the product of the first two
are very strongly correlated (RÂČ > 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)
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A mean residence time relationship for lateral cavities in gravel-bed rivers and streams: Incorporating streambed roughness and cavity shape
Accurate estimates of mass-exchange parameters in transient storage zones are needed to better understand and quantify solute transport and dispersion in riverine systems. Currently, the predictive mean residence time relies on an empirical entrainment coefficient with a range in variance due to the absence of hydraulic and geomorphic quantities driving mass exchange. Two empirically derived relationships are presented for the mean residence time of lateral cavities-a prevalent and widely recognized type of transient storage-in gravel-bed rivers and streams that incorporates hydraulic and geomorphic parameters. The relationships are applicable for gravel-bed rivers and streams with a range of cavity width to length (W/L) aspect ratios (0.2-0.75), shape, and Reynolds numbers (Re, ranging from 1.0 x 10(4) to 1.0 x 10(7)). The relationships equate normalized mean residence time to nondimensional quantities: Froude number, Re, W/L, depth ratio (ratio of cavity to shear layer depth), roughness factor (ratio of shear to channel velocity), and shape factor (representing degree of cavity equidimensionality). One relationship excludes bed roughness (equation (13)) and the other includes bed roughness (equation (14)). The empirically derived relationships have been verified for conservative tracers (R-2 of 0.83) within a range of flow and geometry conditions. Topics warranting future research are testing the empirical relationship that includes the roughness factor using parameters measured in the vicinity of the cavity to reduce the variance in the correlation, and further development of the relationship for nonconservative transport.This is the publisherâs final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/wr/.Keywords: transport, Rectangular cavity, Dead zone, Groyne fields, Flow, Groundwater, Transient storage, Channel, Retention, Exchange processe
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Labile dissolved organic carbon supply limits hyporheic denitrification
We used an in situ steady state Âčâ”N-labeled nitrate (Âčâ”NOââ») and acetate (AcOâ») well-to-wells injection experiment to determine how the availability of labile dissolved organic carbon (DOC) as AcOâ» influences microbial denitrification in the hyporheic zone of an upland (third-order) agricultural stream. The experimental wells receiving conservative (Clâ» and Br) and reactive (Âčâ”NOââ») solute tracers had hyporheic median residence times of 7.0 to 13.1 h, nominal flowpath lengths of 0.7 to 3.7 m, and hypoxic conditions (<1.5 mg Oâ Lâ»Âč). All receiving wells demonstrated Âčâ”Nâ production during ambient conditions, indicating that the hyporheic zone was an environment with active denitrification. The subsequent addition of AcOâ» stimulated more denitrification as evidenced by significant ÎŽÂčâ”Nâ increases by factors of 2.7 to 26.1 in receiving wells and significant decreases of NOââ» and DO in the two wells most hydrologically connected to the injection. The rate of nitrate removal in the hyporheic zone increased from 218 kg haâ»Âč yrâ»Âč to 521 kg haâ»Âč yrâ»Âč under elevated AcOâ» conditions. In all receiving wells, increases of bromide and Âčâ”Nâ occurred without concurrent increases in AcOâ», indicating that 100% of AcOâ» was retained or lost in the hyporheic zone. These results support the hypothesis that denitrification in anaerobic portions of the hyporheic zone is limited by labile DOC supply
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Tracer tests in a fractured dolomite: 2. Analysis of mass transfer in single-well injection-withdrawal tests
We investigated multiple-rate diffusion as a possible explanation for observed behavior in a suite of single-well injection-withdrawal (SWIW) tests conducted in a fractured dolomite. We first investigated the ability of a conventional double-porosity model and a multirate diffusion model to explain the data. This revealed that the multirate diffusion hypothesis/model is consistent with available data and is capable of matching all of the recovery curves. Second, we studied the sensitivity of the SWIW recovery curves to the distribution of diffusion rate coefficients and other parameters. We concluded that the SWIW test is very sensitive to the distribution of rate coefficients but is relatively insensitive to other flow and transport parameters such as advective porosity and dispersivity. Third, we examined the significance of the constant double-log late time slopes (â2.1 to â2.8), which are present in several data sets. The observed late time slopes are significantly different than would be predicted by either conventional double-porosity or single-porosity models and are believed to be a distinctive feature of multirate diffusion. Fourth, we found that the estimated distributions of diffusion rate coefficients are very broad, with the distributions spanning a range of up to 3.6 orders of magnitude. Fifth, when both heterogeneity and solute drift are present, late time behavior similar to multirate mass transfer can occur. Although it is clear that multirate diffusion occurs in the Culebra, the number of orders of magnitude of variability may be overestimated because of the combined effects of drift and heterogeneity.Keywords: Hydrology, Groundwater transportKeywords: Hydrology, Groundwater transpor
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Resazurin as a âsmartâ tracer for quantifying metabolically active transient storage in stream ecosystems
We propose the experimental use of resazurin (Raz) and develop a metabolically active transient storage (MATS) model to include processes that may provide additional information on transient storage from a biogeochemical perspective in stream ecosystems. Raz is a phenoxazine compound that reduces irreversibly to resorufin (Rru) in the presence of aerobic bacteria. Raz was added as a stream tracer to a 128-m reach of the forested second-order Riera de Santa Fe del Montseny (Catalonia, NE Spain), along with a conservative tracer, NaCl. Raz was transformed to Rru at a rate of 0.81 hâ»Âč in the hyporheic zone and only at a rate of 9.9 Ă 10â»âŽ hâ»Âč in the stream surface channel. Raz transformation and decay and Rru production and decay were both correlated with Oâ consumption measured at wells. The ratio of Raz to Rru concentration at the bottom of the reach was moderately correlated with instantaneous rates of net ecosystem production (NEP) measured over the whole reach. Data for Raz, Rru, and chloride were well fitted with the MATS model. The results from this study suggest that Raz transformation to Rru can be used as a ââsmartââ tracer to detect metabolic activity, specifically aerobic respiration, associated with transient storage zones in stream ecosystems. Therefore, the Raz-Rru system can provide an assessment of the amount of transient storage that is metabolically active, an assessment that complements the physical characterization of transient storage obtained from conventional hydrologic tracers. The use of both physical and metabolic parameters of transient storage obtained with these tracers may increase our understanding of the relevance of transient storage on stream biogeochemical processes at whole reach scale, as well as the contribution of the different transient storage compartments to these processes
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Design, modeling, and current interpretations of the H-19 and H-11 tracer tests at the WIPP site
Site-characterization studies at the Waste Isolation Pilot Plant (WIPP) site in southeastern New Mexico, US identified ground-water flow in the Culebra Dolomite Member of the Rustler Formation as the most likely geologic pathway for radionuclide transport to the accessible environment in the event of a breach of the WIPP repository through inadvertent human intrusion. The results of recent tracer tests, as well as hydraulic tests, laboratory measurements, and re-examination of Culebra geology and stratigraphy, have led to a significant refinement of the conceptual model for transport in the Culebra. Tracer test results and geologic observations suggest that flow occurs within fractures, and to some extent within interparticle porosity and vugs connected by microfractures. Diffusion occurs within all connected porosity. Numerical simulations suggest that the data from the tracer tests cannot be simulated with heterogeneous single-porosity models; significant matrix diffusion appears to be required. The low permeability and lack of significant tracer recovery from tracers injected into the upper Culebra suggest that transport primarily occurs in the lower Culebra
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