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Berlin Brandenburg

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  • American Geophysical Union (CrossRef)  (34)
  • 1
    In: Water Resources Research, July 2010, Vol.46(7), pp.n/a-n/a
    Description: Biodegradation of continuously emitted compounds that need a dissolved reaction partner, which is not jointly introduced with the contaminant into the subsurface, is mainly controlled by transverse dispersive mixing. Previous analytical approaches of evaluating mixing‐controlled bioreactive transport in steady state have been based on the assumption that the bulk aqueous‐phase concentration of the reactants is directly available to the specific biomass catalyzing the reaction. These models predict a very narrow stripe of active biomass with high specific biomass concentration. Experimental studies have indicated that such behavior may be unrealistic, particularly for anaerobic biodegradation. I extend the previous analysis to include kinetic solute uptake by the biomass, expressed as a first‐order mass‐transfer process coupled to dual Monod kinetics in the bio‐available domain. The approach is based on the evaluation of conservative components undergoing advective‐dispersive transport, the solution of a quadratic speciation problem within the immobile bio domain, and iterative simulation of linear transport of a single reactive constituent in steady state. Convergence is typically achieved within less than ten iterations. The comparison with simulations assuming instantaneous solute uptake by the biomass indicate that mass‐transfer kinetics may explain larger overlap of reactive constituents and a wider spatial distribution of specific biomass observed in experiments. Depending on the rate coefficient of mass transfer, the overall transformation of the contaminant may be significantly reduced or only slightly shifted to a region farther downstream.
    Keywords: Biodegradation ; Kinetic Mass Transfer ; Reactive Transport ; Monod Kinetics
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 2
    In: Water Resources Research, January 2012, Vol.48(1), pp.n/a-n/a
    Description: We perform a salt tracer experiment, monitored by time‐lapse electrical resistivity tomography, in a quasi‐two‐dimensional sandbox with the aim of determining the hydraulic conductivity distribution in the domain. We use sodium chloride as a tracer, together with cochineal red for visual monitoring. The time series of observed resistance for each electrode configuration is characterized by its temporal moments. We invert the mean arrival time of electrical potential perturbations and a few steady state hydraulic head measurements using the fully coupled hydrogeophysical approach recently introduced by Pollock and Cirpka (2010). This is the first application of the approach to experimental data. The results obtained show a reasonable agreement between the estimated hydraulic conductivity field and the pattern of the actual sandbox filling. Using this estimation, a transient simulation is performed to compute the propagation of the salt tracer plume through the sandbox. The latter is compared to pictures taken during the experiment. These results show an even better agreement, indicating that the lenses of different sand types are not entirely homogeneous and some unexpected preferential flow paths are present. We conclude that temporal moments of potential perturbations obtained during salt tracer tests provide a good basis for inferring the hydraulic conductivity distribution by fully coupled hydrogeophysical inversion. Use temporal moments to invert ERT monitoring data of salt‐tracer experiments Application to laboratory experiments has been successful Inverted results may be better than intended zonation of filling pattern
    Keywords: Electrical Resistivity Tomography ; Fully Coupled Inversion ; Salt Tracer Tests ; Temporal Moments
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 3
    In: Water Resources Research, July 2011, Vol.47(7), pp.n/a-n/a
    Description: The hyporheic zone has been identified as important for river ecology, natural biogeochemical turnover, filtration of particles, degradation of dissolved pollutants—and thus for the self‐cleaning capacity of streams, and for groundwater quality. Good estimation of the traveltime distribution in the hyporheic zone is required to achieve a better understanding of transport in the river system. The transient‐storage model has been accepted as an appropriate tool for reach‐scale transport in rivers undergoing hyporheic exchange, but the choice of the best parametric function for the hyporheic traveltime distribution has remained unclear. We present an approach to obtaining hyporheic traveltime distributions from synchronous conservative and “smart” tracer experiments that does not rely on a particular functional form of the hyporheic traveltime distribution, but treats the latter as a continuous function. Nonnegativity of the hyporheic traveltime distribution is enforced by the application of Lagrange multipliers. A smoothness parameter, needed for regularization, and uncertainty bounds are obtained by the expectation‐maximization method relying on conditional realizations. The shape‐free inference provides the opportunity for capturing unconventional shapes, e.g., multiple peaks, in the estimation. We test the approach by applying it to a virtual test case with a bimodal hyporheic traveltime distribution, which is recaptured in the inversion of noisy data. No particular functional shape of hyporheic travel distribution is assumed Reactive tracers help separating different mixing process in streams Uncommon features in hyporheic traveltime distribution can be revealed
    Keywords: Bayesian Analysis ; Hyporheic Exchange ; Nonnegativity ; Transient‐Storage Model
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 4
    In: Water Resources Research, February 2011, Vol.47(2), pp.n/a-n/a
    Description: Macroscopic transport models calibrated by flux‐averaged breakthrough curves of conservative compounds do not necessarily characterize mixing well because such breakthrough curves do not provide information on fluctuations of concentration within the solute flux, which may influence mean reaction rates. We numerically examine the validity of macroscopic transport models, which are capable of describing all details of flux‐averaged breakthrough curves, for predicting a mixing‐controlled bimolecular precipitation reaction in heterogeneous media. We consider a homogeneous, isotropic medium with an elliptical, low‐permeability inclusion and random heterogeneous fields. For the single‐inclusion case, slow advection through the inclusion results in a multimodal breakthrough curve with enhanced tailing. We vary the hydraulic conductivity contrast and Peclet number to investigate the performance of a “perfect” macroscopic transport model for predicting the total precipitated mass within the domain and the peak concentration difference between the conservative and reactive cases at the outflow boundary. The results indicate that such a model may perform well in media with either very small or very high permeability contrast or at low Peclet number. In the high‐contrast case, most flow takes place in preferential flow paths, resulting in a small variance of the flux‐weighted concentration, even though the offset in the breakthrough between the slow and fast travel paths is substantial. Maximum relative errors in terms of total precipitated mass and the peak concentration difference between the conservative and reactive cases occur at intermediate permeability contrasts and large Peclet numbers. Numerical simulations on random heterogeneous fields confirm the finding of the single‐inclusion case. Thus, in cases with intermediate hydraulic conductivity contrast, making macroscopic models fit flux‐averaged concentration breakthrough curves better may not improve the prediction of mixing‐controlled reactive transport, and it becomes necessary to quantify and account for the variability of conservative concentrations in the flux in order to formulate an appropriate macroscopic transport model that predicts mixing‐controlled reactive transport.
    Keywords: Reactive Mixing ; Low‐Permeability Inclusion ; Breakthrough Tailing ; Heterogeneous Field
    ISSN: 0043-1397
    E-ISSN: 1944-7973
    Source: John Wiley & Sons, Inc.
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  • 5
    In: Water Resources Research, July 2010, Vol.46(7), pp.n/a-n/a
    Description: We present a method for the determination of hydraulic conductivity from monitoring of salt tracer tests by electrical resistivity tomography (ERT). To ensure that the underlying principles of flow, transport, and geoelectrics are obeyed in the inversion, we perform a fully coupled hydrogeophysical analysis using temporal moments of electrical potential perturbations. In the predictive mode, we use moment‐generating equations with corresponding adjoint equations for the evaluation of sensitivities. For inversion, we apply the quasi‐linear geostatistical inversion approach. The method is tested in a synthetic case study mimicking a laboratory‐scale quasi two‐dimensional sandbox, in which 48 electrodes and 8 piezometers are used. The hydraulic conductivity field is estimated from the mean arrival times of electrical potential perturbations and hydraulic heads. The estimated hydraulic conductivity field reproduces most features with, however, a loss of variability. Even though only the temporal moments of the electrical signals are used for inversion, the transient behavior is satisfactorily recovered. Also, the spatial patterns of concentration arrival times in the true and estimated cases are in good agreement, so that the propagation of the tracer plume can be followed fairly accurately. We test the effects of large measurement errors and erroneous prior information on the performance of the inversion. While prior statistical parameters are of minor importance in detecting the major pattern of hydraulic conductivity, a large measurement error could have an important impact on the solution. Also, the choice of electrode configurations appears to be important. In particular, strictly surface‐based geoelectrical surveys do not seem to be very suitable for identifying spatial patterns of hydraulic conductivity by ERT monitoring of salt tracer tests within aquifers.
    Keywords: Electrical Resistivity Tomography Ert ; Tracer Test ; Geostatistical Inversion ; Hydrogeophysics
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 6
    In: Journal of Geophysical Research: Biogeosciences, December 2018, Vol.123(12), pp.3538-3555
    Description: The reactive tracer resazurin is frequently used to investigate metabolic processes and hyporheic exchange in streams, because its transformation to resorufin is considered a function of oxygen turnover in the presence of living cells and thus of ecosystem respiration. This has been investigated and confirmed in a number of laboratory and batch studies but so far not unanimously been verified in the field. In this study we conducted 13 tracer tests with resazurin in different small, low‐gradient streams typical of lowland regions. We determined tracer processing rates and ratios based on metrics commonly used to assess reactive‐tracer tests with resazurin and compared them to flow characteristics and respiration rates determined from diel‐oxygen recordings following a two‐station approach. We found no statistical relationship between tracer processing and parameters of transport, indicating that resazurin‐to‐resorufin transformation cannot be explained by hydrological parameters alone. The statistical relationship between resazurin processing and respiration rates based on diel‐oxygen profiles was strongest if resazurin‐to‐resorufin transformation ratios were used, and uptake lengths were also found to be a good proxy. However, the relationships were not linear, and weak for individual streams, contrary to what was expected based on previous laboratory and batch studies. We attribute this finding to a lack of process understanding of resazurin reactions under field conditions, and generally insignificant hyporheic exchange fluxes in lowland streams. Our findings highlight important limitations of the tracer technique under field conditions. Resazurin is a compound that can be used to track the flow of water and metabolic processes in rivers. This has been tested in laboratory experiments before, but until now the method has not been fully verified in the field. In this study we apply resazurin to 13 different small rivers and measure its transformation. We compare our results to other, independent methods that are often used to determine metabolic processes. We find good relationships between the two different methods for all rivers together, but not for individual rivers. Also, the relationships are not linear, which makes predicting metabolic rates from the transformation of resazurin difficult. Resazurin transformation rates of reach‐scale reactive tracer tests are compared to rates of stream metabolism Uptake lengths and processing ratios are the most suitable indicators of ecosystem respiration The relationship between tracer transformation and ecosystem respiration is weak in lowland streams
    Keywords: Resazurin ; Reactive Tracer Test ; Stream Metabolism ; Hyporheic Zone ; Ecosystem Respiration ; Temporal Moment Analysis
    ISSN: 2169-8953
    E-ISSN: 2169-8961
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  • 7
    In: Water Resources Research, February 2011, Vol.47(2), pp.n/a-n/a
    Description: The correct quantification of mixing is of utmost importance for modeling reactive transport in porous media and for assessing the fate and transport of contaminants in the subsurface. An appropriate measure of mixing in heterogeneous porous formations should correctly capture the effects on mixing intensity of various processes at different scales, such as local dispersion and the mixing enhancement due to heterogeneities. In this work, we use the concept of flux‐related dilution index as a measure of transverse mixing. This quantity expresses the dilution of the mass flux of a conservative tracer solution over the total discharge of the system, and is particularly suited to address problems where a compound is continuously injected into the domain. We focus our attention on two‐dimensional systems under steady state flow conditions and investigate both conservative and reactive transport in homogeneous and heterogeneous porous media at different scales. For mixing‐controlled reactive systems, we introduce and illustrate the concept of critical dilution index, which represents the amount of mixing required for complete degradation of a continuously emitted plume undergoing decay upon mixing with ambient water. We perform two‐dimensional numerical experiments at bench and field scales in homogeneous and heterogeneous conductivity fields. These numerical simulations show that the flux‐related dilution index quantifies mixing and that the concept of critical dilution index is a useful measure to relate the mixing of conservative tracers to mixing‐controlled degradation of reactive compounds.
    Keywords: Mixing ; Dilution ; Reactive Transport ; Entropy ; Transverse Dispersion
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 8
    In: Water Resources Research, July 2011, Vol.47(7), pp.n/a-n/a
    Description: Different measures of dilution have been proposed to describe solute mixing in heterogeneous porous media. Most of these approaches lead to the definition of effective dispersion coefficients. In order to quantify mixing, these up‐scaled parameters should account for both local‐scale dispersion and effects of flow variability in heterogeneous formations (e.g., flow focusing in high‐conductivity and defocusing in low‐conductivity inclusions). The correct quantification of mixing is particularly important for transport of compounds undergoing reactions. Recent results of multitracer laboratory experiments showed a dependency of local transverse dispersion on molecular diffusion over a wide range of flow velocities, implying compound‐specific transverse mixing even at intermediate and high Péclet numbers. The goal of this study is to assess the relevance of a compound‐specific local‐scale transverse dispersion on conservative and reactive mixing in heterogeneous domains at the field scale. We restrict our analysis to steady state two‐dimensional flow and transport with continuous injection from a line source. We present numerical simulations in heterogeneous domains with different characteristics of variability in the conductivity field, and apply as measures of solute mixing: (1) the effective transverse dispersion coefficient derived from second central spatial moments, (2) a dispersion coefficient derived from flux‐related second central spatial moments, (3) the scalar dissipation rate, and (4) a dispersion coefficient derived from the flux‐related dilution index. The results indicate compound‐specific transverse mixing behavior also at the field scale which is particularly significant in case of low to moderately heterogeneous porous media. Moreover, we show that measures of dilution calculated in a flux‐related framework result in an improved quantification of mixing processes and allow to define up‐scaled parameters (i.e., effective transverse dispersion coefficients) affected by a low degree of uncertainty. For mixing‐controlled reactive transport we illustrate the importance of compound‐dependent local effects on the length of reactive solute plumes. Relevance of local compound‐specific transverse dispersion at the field scale Effective parameters from flux‐related measures of dilution show low uncertainty Reactive mixing: critical flux‐related dilution index and scalar dissipation rate
    Keywords: Mixing ; Compound‐Specific Transverse Dispersion ; Dilution ; Reactive Transport ; Effective Dispersion Coefficient
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 9
    In: Water Resources Research, November 2011, Vol.47(11), pp.n/a-n/a
    Description: Spatial variability of hydraulic aquifer parameters causes meandering, squeezing, stretching, and enhanced mixing of steady state plumes in concentrated hot‐spots of mixing. Because the exact spatial distribution of hydraulic parameters is uncertain, the spatial distribution of enhanced mixing rates is also uncertain. We discuss how relevant the resulting uncertainty of mixing rates is for predicting concentrations. We develop analytical solutions for the full statistical distribution of steady state concentration in two‐dimensional, statistically uniform domains with log‐hydraulic conductivity following an isotropic exponential model. In particular, we analyze concentration statistics at the fringes of wide plumes, conceptually represented by a solute introduced over half the width of the domain. Our framework explicitly accounts for uncertainty of streamline meandering and uncertainty of effective transverse mixing (defined at the Darcy scale). We make use of existing low‐order closed‐form expressions that lead to analytical expressions for the statistical distribution of local concentration values. Along the expected position of the plume fringe, the concentration distribution strongly clusters at its extreme values. This behavior extends over travel distances of up to tens of integral scales for the parameters tested in our study. In this regime, the uncertainty of effective transverse mixing is substantial enough to have noticeable effects on the concentration probability density function. At significantly larger travel distances, intermediate concentration values become most likely, and uncertainty of effective transverse mixing becomes negligible. A comparison to numerical Monte Carlo simulations of flow and solute transport show excellent agreement with the theoretically derived expressions. Streamline coordinates give a framework for meandering and transverse mixing Analytical expressions for the concentration pdf could be derived The interplay between mean transverse mixing and uncertain meandering dominates
    Keywords: Concentration Pdf ; Heterogeneity ; Streamfunction ; Transverse Mixing
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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  • 10
    In: Water Resources Research, June 2011, Vol.47(6), pp.n/a-n/a
    Description: Transverse mixing of solutes in steady state transport is of utmost importance for assessing mixing‐controlled reactions of compounds that are continuously introduced into the subsurface. Classical spatial moments analysis fails to describe mixing because the tortuous streamlines in heterogeneous formations cause plume meandering, squeezing, and stretching, which affect transverse spatial moments even if there is no mass transfer perpendicular to the direction of flow. For transverse solute mixing, however, the decisive process is the exchange of solute mass between adjacent stream tubes. We therefore reformulate the advection‐dispersion equation in streamline coordinates (i.e., in terms of the potential and the stream function values) and analyze how flux‐related second central moments of plumes increase with dropping hydraulic potential. We compare the ensemble behavior of these second central moments in random two‐dimensional heterogeneous flow fields with the moments in an equivalent homogeneous system, thus defining an equivalent effective transverse dispersion coefficient. Unlike transverse macrodispersion coefficients derived by traditional moment analysis, our mixing‐relevant, flux‐related coefficient does not increase with travel distance. We present closed‐form solutions for the mean enhancement of transverse mixing by heterogeneity in two‐dimensional isotropic media for linear laws of local‐scale transverse dispersion. The mixing enhancement factor increases with the log conductivity variance but remains fairly low. We also evaluate the variance of our cumulative measure of transverse mixing, showing that heterogeneity causes substantial uncertainty of mixing. The analytical expressions are compared to numerical Monte Carlo simulations for various values of log conductivity variance, indicating good agreement with the analytical results at low variability. In the numerical simulations, we also consider nonlinear models of local‐scale transverse dispersion. To express mixing, transverse moments should be flux related The equivalent transverse mixing coefficient does not grow with distance The uncertainty of transverse mixing decreases only slowly with distance
    Keywords: Solute Mixing ; Steady State Transport ; Stochastic Analysis ; Streamline Coordinates ; Transverse Dispersion
    ISSN: 0043-1397
    E-ISSN: 1944-7973
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