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  • Springer  (12)
  • Blackwell Publishing Ltd  (4)
  • 1995-1999  (16)
  • 1
    Digitale Medien
    Digitale Medien
    A Method to Infer In Situ Reaction Rates from Push-Pull Experiments (1998)
    Oxford, UK : Blackwell Publishing Ltd
    Ground water 36 (1998), S. 0 
    Details
    ISSN: 1745-6584
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Energietechnik , Geologie und Paläontologie
    Notizen: A method to evaluate first-order and zero-order in situ reaction rates from a push-pull test is presented. A single-well push-pull test starts with the rapid injection of a well-mixed slug containing a known quantity of a conservative tracer and a reactive solute into the saturated zone. The slug is then periodically extracted and sampled from the same well. For zero- or first-order reactions, in the absence of sorption and assuming negligible background concentrations, these measurements can be used to evaluate reaction rate coefficients directly. The method does not involve computer-based solute transport models and requires no knowledge of regional ground water flow or hydraulic parameters. The method performs well when the dominate processes are advection, dispersion, and zero- or first-order irreversible reactions. Regional flow velocities must be sufficiently low such that the slug stays within the area of the well during the sampling phase. In the case of zero-order reactions, results using the method proposed here are compared with those obtained through the traditional method of calibrating a computer-based transport model. The two methods give similar estimates of the reaction rate coefficient. The method is general enough to work with a broad range of push-pull experiment designs and sampling techniques.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1745-6584.1998.tb02839.x
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  • 2
    Digitale Medien
    Digitale Medien
    How Observations and Structure Affect the Geostatistical Solution to the Steady-State Inverse Problem (1998)
    Oxford, UK : Blackwell Publishing Ltd
    Ground water 36 (1998), S. 0 
    Details
    ISSN: 1745-6584
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Energietechnik , Geologie und Paläontologie
    Notizen: The solution to the steady-state inverse problem can be expanded into a series of spline functions with weights adjusted to reproduce the observations within the observation error. The splines depend on the model spatial structure, the ground water flow model, and the location of the observations. This representation of the solution, which is a rigorous and exact expansion, provides insight into the form of the best estimate and explicitly shows how observations and the conceptual model may affect the solution.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1745-6584.1998.tb02192.x
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  • 3
    Digitale Medien
    Digitale Medien
    Mass-Transfer Limitations for Macroscale Bioremediation Modeling and Implications on Aquifer Clogging (1999)
    Oxford, UK : Blackwell Publishing Ltd
    Ground water 37 (1999), S. 0 
    Details
    ISSN: 1745-6584
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Energietechnik , Geologie und Paläontologie
    Notizen: In engineered in situ bioremediation, substrates are injected into the subsurface to stimulate microbial metabolism and growth. Models are useful in the design and optimization of such systems, such as in devising strategies to prevent clogging of soil by large bacterial populations around wells. Such models are macroscale, i.e., they do not resolve pore-scale variability; rather, substrate and biomass concentrations are bulk averages that vary from block to block. These models give unrealistic predictions, in that they predict monotonically increasing biomass growth everywhere except where the limiting substrate concentration is very small. This work examines the possibility of biofilm mass-transfer limitations at the pore scale using both the traditional biofilm model as well as previously published results from an upscaling model. Results from the biofilm model suggest that limitations on biofilm growth due to mass-transfer resistance could be significant in coarse-grained soils with adequate substrate availability. The upscaling approach confirms this result. While these two approaches do not yield identical results, both do agree that coarser grain sizes tend to cause greater mass transfer resistance. These are the conditions most likely to occur near injection well screens of an enhanced bioremediation system, where clogging is most commonly observed. The upscaling approach also indicates that the degree of mass transfer resistance is reduced at higher ground water velocities, which are also most commonly observed near well screens. These results could be useful for improving macroscale bioremediation models to more accurately predict rates of biomass growth and aquifer clogging.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1745-6584.1999.tb01138.x
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  • 4
    Digitale Medien
    Digitale Medien
    Effects of Shear Detachment on Biomass Growth and In Situ Bioremediation (1999)
    Oxford, UK : Blackwell Publishing Ltd
    Ground water 37 (1999), S. 0 
    Details
    ISSN: 1745-6584
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Energietechnik , Geologie und Paläontologie
    Notizen: The dispersed growth model incorporating Monod kinetics has often been applied to simulate enhanced in situ bioremediation of contaminants. The dispersed growth model might not adequately address the impacts of the growth of large bacterial populations that can often have the unwanted effect of clogging the porous media. Two mechanisms that could affect model predictions of biomass are biofouling and biomass detachment due to shear stress. A mathematical model was developed to evaluate the potential impact of shear detachment on biomass distribution and the prediction of contaminant biodegradation by comparing the impact of both shear detachment and biofouling together with that of biofouling alone. The results of this examination can aid in designing a system for in situ bioremediation using computer simulations and in evaluating a system's ability to meet remediation goals. The model simulations indicate that shear detachment of biomass can be an important process to include in model simulations used to predict the effectiveness of a bioremediation system and the time before significant clogging occurs.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1111/j.1745-6584.1999.tb01142.x
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  • 5
    Digitale Medien
    Digitale Medien
    Analytical expressions of conditional mean, covariance, and sample functions in geostatistics (1996)
    Springer
    Stochastic environmental research and risk assessment 10 (1996), S. 279-294 
    Details
    ISSN: 1436-3259
    Schlagwort(e): Linear estimation ; interpolation ; kriging ; splines ; conditional
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Energietechnik , Geographie , Geologie und Paläontologie
    Notizen: Abstract This work presents analytical expressions for the best estimate, conditional covariance function, and conditional realizations of a function from sparse observations. In contrast to the prevalent approach in kriging where the best estimates at every point are determined from the solution of a system of linear equations, the best-estimate function can be represented analytically in terms of basis functions, whose number depends on the observations. This approach is computationally superior when graphing a function estimate and is also valuable in understanding what the solution should look like. For example, one can immediately see that all “singularities” in the best-estimate function are at observation points.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01581870
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  • 6
    Digitale Medien
    Digitale Medien
    Analysis of macrodispersion through volume averaging: comparison with stochastic theory (1999)
    Springer
    Stochastic environmental research and risk assessment 13 (1999), S. 66-84 
    Details
    ISSN: 1436-3259
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Energietechnik , Geographie , Geologie und Paläontologie
    Notizen: Abstract Velocity variability at scales smaller than the size of a solute plume enhances the rate of spreading of the plume around its center of mass. Macroscopically, the rate of spreading can be quantified through macrodispersion coefficients, the determination of which has been the subject of stochastic theories. This work compares the results of a volume-averaging approach with those of the advection dominated large-time small-perturbation theory of Dagan [1982] and Gelhar and Axness [1983]. Consider transport of an ideal tracer in a porous medium with deterministic periodic velocity. Using the Taylor-Aris-Brenner method of moments, it has been previously demonstrated [Kitanidis, 1992] that when the plume spreads over an area much larger than the period, the volume-averaged concentration satisfies the advection-dispersion equation with constant coefficients that can be computed. Here, the volume-averaging analysis is extended to the case of stationary random velocities. Additionally, a perturbation method is applied to obtain explicit solutions for small-fluctuation cases, and the results are compared with those of the stochastic macrodispersion theory. It is shown that the method of moments, which uses spatial averaging, for sufficiently large volumes of averaging yields the same result as the stochastic theory, which is based on ensemble averaging. The result is of theoretical but also practical significance because the volume-averaging approach provides a potentially efficient way to compute macrodispersion coefficients. The method is applied to a simplified representation of the Borden aquifer.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/s004770050032
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  • 7
    Digitale Medien
    Digitale Medien
    Advection-diffusion in spatially random flows: Formulation of concentration covariance (1997)
    Springer
    Stochastic environmental research and risk assessment 11 (1997), S. 397-422 
    Details
    ISSN: 1436-3259
    Schlagwort(e): Random flow field ; diffusion ; concentration ; fluctuations ; covariance ; variance
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Energietechnik , Geographie , Geologie und Paläontologie
    Notizen: Abstract The concentration c(x,t) of a nonreactive solute undergoing advection and diffusion in a spatially random divergence-free flow field is analyzed. A leading order formulation for the spatial covariance of the concentration field, $$\overline {c'\left( {x,t} \right)c'\left( {x,t} \right)} $$ , is made. That formulation includes the velocity variability induced macrodispersive flux of the covariance field, and the smoothing effects of diffusion. Previous formulations of the concentration covariance had dropped at least one of these effects. It is shown that both these effects need to be included to obtain a qualitatively correct description of the concentration fluctuations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02427926
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  • 8
    Digitale Medien
    Digitale Medien
    Optimization of monitoring well installation time and location during aquifer decontamination (1996)
    Springer
    Water resources management 10 (1996), S. 439-462 
    Details
    ISSN: 1573-1650
    Schlagwort(e): groundwater quality monitoring ; remediation ; dual control ; cost-to-go function
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Architektur, Bauingenieurwesen, Vermessung , Geographie
    Notizen: Abstract An important question in the systematic and objective design of general-purpose ground-water quality monitoring networks is how to evaluate quantitatively the information they provide. However, many applications require the design of a groundwater monitoring network in conjunction with remedial action at a subsurface contamination site. In such a case, it is conceptually clear what is a successful network: One that reduces the net cost of meeting the objectives of cleanup. Uncertainty entails a cost because the natural management response to uncertainty is overdesign for the sake of conservatism (‘safety factor’). The additional information that the network provides must lead to cost reductions that outweigh its cost. This paper presents a method to determine the installation time and location of an additional monitoring well while the aquifer is being cleaned up. While rates of pumping and treatment are determined by the dual control method (a method for optimization with incomplete information) candidate well locations are ranked according to a ‘cost-to-go’ index that measures the costs expected until the goals of remediation are met. This index accounts for the cost associated with uncertainty about the system and thus is useful in appraising the value of information from new measurements in the context of the specific cleanup effort. The usefulness of the method is illustrated through application to a hypothetical two-dimensional aquifer with uncertain initial estimates of the system parameters and variables. Monte Carlo simulations demonstrate the cost effectiveness of solution obtained through this method.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00422549
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  • 9
    Digitale Medien
    Digitale Medien
    Concentration fluctuations and dilution in two-dimensionally periodic heterogeneous porous media (1996)
    Springer
    Transport in porous media 22 (1996), S. 91-119 
    Details
    ISSN: 1573-1634
    Schlagwort(e): concentration fluctuations ; peak concentration ; dilution ; heterogeneity ; local dispersion ; microscale ; fluctuation dissipation ; coefficient of variation
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Technik allgemein
    Notizen: Abstract Dilution of solute in two-dimensionally periodic heterogeneous porous media is assessed by numerically simulating advection-dispersion. The concentration fluctuations, created by advective heterogeneity, are destroyed by local dispersion, over a characteristic variance residence time (VRT). For an impulse introduction of solute, initially, plumes become increasingly irregular with time—the coefficient of variation (CV) of concentration grows with time. A priori, the spatial second moment and mean concentrations are insufficient measures of dilution, because concentration fluctuations can be large. At large times (t 〉 VRT) the relative concentration fluctuations weaken—the concentration CV was observed to slowly decrease with time. At the center of mass the general trend of the decreasing CV follows VRT/t (predicted by Kapoor and Gelhar). The VRT is found to be an increasing function of the log hydraulic conductivity microscale. In employing effective dispersion coefficents to model the mean concentration, it needs to be recognized that excursions of concentrations around the mean are singularly determined by local dispersion.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF00974313
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  • 10
    Digitale Medien
    Digitale Medien
    Stokes Flow in a Slowly Varying Two-Dimensional Periodic Pore (1997)
    Springer
    Transport in porous media 26 (1997), S. 89-98 
    Details
    ISSN: 1573-1634
    Schlagwort(e): Stokes flow ; pore-scale hydrodynamics ; analytical solution ; multiple-scales method ; stream function ; biharmonic equation ; periodic flow.
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Geologie und Paläontologie , Technik allgemein
    Notizen: Abstract This article presents a series solution to the velocity in a two-dimensional long sinusoidal channel. The approach is based on a stream function formulation of the Stokes problem and a series expansion in terms of the width to the length ratio, which is considered small. Results show how immobile zones may appear and even flow separation and nonturbulent eddies, even in the absence of prima facie dead-end pores. It is shown that the flow tends to concentrate in strips connecting pore throats.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1023/A:1006575028391
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