摘要:

Introduction of exogenous biocolloids such as genetically engineered bacteria in a bioremediation operation can enhance the transport of contaminants in groundwater by reducing the retardation effects. Because of their colloidal size and favorable surface conditions, bacteria are efficient contaminant carriers. In cases where contaminants have a low mobility in porous media because of their high partition with solid matrix, facilitated contaminant transport by mobile bacteria can create high contaminant fluxes. When metabolically active mobile bacteria are present in a subsurface environment, the system can be treated as consisting of three phases: water phase, bacterial phase, and stationary solid matrix phase. In this work a mathematical model based on mass balance equations is developed to describe the facilitated transport and fate of a contaminant and bacteria in a porous medium. Bacterial partition between the bulk solution and the stationary solid matrix and contaminant partition among three phases are represented by expressions in terms of measurable quantities. Solutions were obtained to provide estimates of contaminant and bacterial concentrations. A dimensional analysis of the transport model was utilized to estimate model parameters from the experimental data and to assess the effect of several parameters on model behavior. The model results matched favorably with experimental data of Jenkins and Lion (1993). The presence of mobile bacteria enhances the contaminant transport. However, bacterial consumption of the contaminant, which serves as a bacterial nutrient, can attenuate the contaminant mobility. The work presented in this paper is the first three‐phase model to include the effects of substrate metabolism on the fate of groundwater contaminant

ISSN:0043-1397    

DOI:10.1029/95WR02183

年代:1995

数据来源: WILEY

摘要:

Bacterial transport through porous media was modeled using detachment functions that incorporate the dependence of detachment rate on bacterial residence time on the collector. Model parameters and the relative merit of alternative forms for the detachment function were evaluated on the basis of comparisons between model simulations and experimentally derived bacterial breakthrough and elution curves. Only detachment functions that provided an initial period in which bacteria were rapidly released, followed by slow bacterial detachment, were able to reproduce the elution portion of the breakthrough curves. In optimal simulations, 90% of the bacteria that were captured by the porous medium detached within 1 min of attachment. Experiments involving saturated flow through columns packed with sand indicated that the time to achieve complete breakthrough was inversely related to the influent bacterial concentration. On this basis and because of the relatively slow approach to breakthrough that was typically observed in transport experiments, it was hypothesized that the experimental medium contained a number of preferred attachment sites that must be essentially filled before breakthrough is achieved. Only when such (irreversible) sorption sites were included in the model formulations was it possible to produce transport simulations that matched both the breakthrough and elution portions of the empirically derived curves. It is concluded that both a time‐dependent detachment function and a degree of sorption site heterogeneity are required to describe bacterial attachment and detachment during transport as observed in our laborator

ISSN:0043-1397    

DOI:10.1029/95WR02311

年代:1995

数据来源: WILEY

摘要:

Predicting contaminant migration within the vadose zone, for performance or risk assessment, requires estimates of unsaturated hydraulic conductivity for field soils. Hydraulic conductivities,K, were experimentally determined as a function of volumetric moisture content,θ, for Hanford sediments. The steady state head control method and an ultracentrifuge method were used to measureK(θ) in the laboratory for 22 soil samples. The van Genuchten model was used to fit mathematical functions to the laboratory‐measured moisture retention data. Unsaturated conductivities estimated by the van Genuchten–Mualem predictive model, using the fitted moisture retention curve and measured saturated hydraulic conductivity,Ks, were compared to those obtained by a scaled‐predictive method that uses a singleK(θ) measurement as a match point near the dry regime. In general, the measuredKvalues and those predicted from van Genuchten–Mualem relationships showed considerable disagreement. This suggests that the use of laboratory‐measuredKsresults in an inadequate characterization ofK(θ) for the desired range of moisture content. Deviations between the measured and predictedKwere particularly severe at relatively low moisture contents; for some samples, there were differences in excess of 2 orders of magnitude at lowθ. However, use of the same moisture retention curve‐fitting parameters and a single steady state head control‐basedK(θ) measurement near the dry regime resulted in considerable improvement. In fact, for the coarse‐textured soils considered in this study, results indicate that aK∥θ) measurement near the dry regime must be used to obtain reliable estimates of unsaturatedKat lowθ. The study provided important insight on application of two different experimental techniques of measuring u

ISSN:0043-1397    

DOI:10.1029/95WR02309

年代:1995

数据来源: WILEY

摘要:

The method of quasi‐reversibility (QR) (Lattes and Lions, 1969) has been used previously to solve the diffusion equation with reversed time. We develop a quasi‐reversible solution to a convection‐dispersion equation by solving the QR diffusion operator in a moving coordinate system. The solution procedure is applied to the problem of recovering the history of a groundwater contaminant plume from observations of its present conditions. This approach to the plume history problem is potentially superior to the Tikhonov regularization approach used by Skaggs and Kabala (1994) because it is easier to implement and readily allows for space‐ and time‐dependent transport parameters. However, our results for a few example problems suggest that the QR procedure is less accurate than the regularization technique. Thus the easy implementation and improved generality of the QR procedure come at the expense of accuracy; this trade‐off will have to be weighed if the QR technique is

ISSN:0043-1397    

DOI:10.1029/95WR02383

年代:1995

数据来源: WILEY

摘要:

A stochastic‐convective reactive (SCR) transport method is developed for one‐dimensional steady transport in physically heterogeneous media with nonlinear degradation. The method is free of perturbation amplitude limitations and circumvents the difficulty of scale dependence of phenomenological parameters by avoiding volume‐averaged specifications of diffusive/dispersive fluxes. The transport system is conceptualized as an ensemble of independent convective‐reactive streamlines, each characterized by a randomized convective velocity (or travel time). Dispersive effects are treated as a component of the randomness in the streamline velocity ensemble, so no explicit expression for hydrodynamic dispersive flux is written in the streamline transport equation. The expected value of the transport over the stream tube ensemble is obtained as an average of solutions to the reactive convection equation according to the stream tube (travel time) probability distribution function. In this way, transport with reaction can be expressed in terms of global‐scale random variables, such as solute travel time and travel distance, which are integrals of the stochastic variables such as velocity. Derivations support the hypothesis that via the SCR the decay process can be factored out of the mechanical transport behavior (as reflected by movement of a passive tracer) and scaled independently. Solution strategies are presented for general linear and nonlinear kinetic reactions. Demonstration simulations show that for Fickian transport with nonlinear reactions the SCR and convection dispersion equation can give different results. Ginn et al. (this issue) extend the SCR solution to coupled nonlinear equations, to accommodate Michaelis‐Menten biodegradation of solute with an accounting of micro

ISSN:0043-1397    

DOI:10.1029/95WR02178

年代:1995

数据来源: WILEY

摘要:

The representation of subsurface flow and reactive transport as an ensemble of one‐dimensional stream tubes is extended to account for nonlinear biodegradation with coupled microbial growth. The Stochastic‐convective reaction (SCR) model is derived for bioreaction of a single solute by a single class of microorganisms coupled with dynamic microbial growth. A new global variable, the integral of the solute degraded per unit length of system traversed, accounts for degradation. Dimensionless scaling and the method of characteristics are used to reduce the model, written for a single convecting reactor (stream tube), to a pair of coupled nonlinear functional equations for solute concentration and microbial biomass. Existence of a solution to the stream tube system is shown, both numerical and approximate analytical approaches to the solution are given, and example computations using both methods are presented. Conditions under which the stream tube solution is “canonical,” or scalable to fit any permissible stream tube travel time function, arise from requirements for invariance (over the stream tube ensemble) of effective one‐dimensional stream tubes used to represent transport along real stream tubes in three‐dimensional space. Averaging of the stream tube solution over travel time and reaction properties representative of physical and chemical heterogeneities is described as a way to separate and upscale the processes of macrodispersion and microbiological reaction. The approach is exercised to simulate Monte Carlo average behavior of bioreactive transport in physically heterogeneous two‐dimensional media. Results show that the method captures the ensemble average large‐scale effects of the nonlinear reactions more accurately than done in the classical reactive convection‐dispersion equation (CDR), even when the appropriate scale dependent dispersion coefficient is af

ISSN:0043-1397    

DOI:10.1029/95WR02179

年代:1995

数据来源: WILEY

摘要:

When water enters a water‐repellent topsoil, fingers develop and a large part of the soil is bypassed. Therefore fingering is expected to accelerate solute leaching to the groundwater. In soils with a groundwater‐affected wettable sublayer, fingering yields a flow pattern consisting of three regions. In the top few centimeters, water flow converges toward the finger top. In the finger, water moves vertically downward. In the wettable subsoil the flow diverges due to matric forces. Analytical steady state solutions are presented for each of these regions. The resulting model provides the breakthrough curve of a pulse of an inert solute and reveals the influence of each region. Calculations showed that the enhancement of leaching caused by fingers could be largely undone by a thick (0.75 m) wettable subsoil. A very thin (0.05 m) wettable subsoil had a much smaller effect. This illustrates the need for accurate field estimates of finger de

ISSN:0043-1397    

DOI:10.1029/95WR02431

年代:1995

数据来源: WILEY

摘要:

A methodology based on fuzzy set theory is developed to incorporate imprecise parameters into steady state groundwater flow models. In this case, fuzzy numbers are used to represent parameter imprecision. As such, they are also used as a measure for the uncertainty associated with the hydraulic heads due to the imprecision in the input parameters. The imprecise input parameters may come from indirect measurements, subjective interpretation, and expert judgment of available information. In the methodology, a finite difference method is combined with level set operations to formulate the fuzzy groundwater flow model. This fuzzy modeling technique can handle imprecise parameters in a direct way without generating a large number of realizations. Two numerical solution methods are used to solve the fuzzy groundwater flow model: the groundwater model operator method proposed in this methodology and the iterative algorithm based on conventional interval arithmetics. The iterative method is simple but may overestimate the uncertainty of hydraulic heads. The groundwater model operator method not only provides the hull of the solution set for the hydraulic heads but also considers the dependence of hydraulic head coefficients which are functions of imprecise parameters. Sensitivity analysis shows that the dependence of hydraulic head coefficients has a critical impact on the model results, and neglecting this dependence may result in significant overestimation of the uncertainty of hydraulic heads. A numerical model based on the methodology is tested by comparing it with the analytical solution for a homogeneous radial flow problem. It is also applied to a simplified two‐dimensional heterogeneous flow case to demonstrate the methodolog

ISSN:0043-1397    

DOI:10.1029/95WR02310

年代:1995

数据来源: WILEY

摘要:

The paper presents a robust Darcy‐based one‐dimensional soil water balance model (SAWAH) for simulating riceland hydrology. Experimental data from an upland rice toposequence showing highly variable groundwater and volumetric soil water contentsθare used to calibrate the model and to validate it by a modified cross‐validation method. Analysis of variance is applied to estimate the mean squared prediction error and to split it into errors in predicting horizontal, vertical, and temporal gradients ofθ. Root mean squared error of prediction (RMSEP) of soil water content at strip‐depth‐time points was about equal to the standard deviation of unreplicated measurements. Measured strip and field means were more accurate than predicted means, but RMSEP remained below 0.03 m3m−3. Ranking of crop and soil parameters for their effects on root zone water content and seasonal flux totals depended strongly on the imposed hydrology regime. Capillary rise contributed 5–55% of seasonal evapotranspiration, depending on season and topog

ISSN:0043-1397    

DOI:10.1029/95WR02330

年代:1995

数据来源: WILEY

摘要:

Using the method of decomposition, new analytical solutions of the nonlinear Boussinesq flow equation and of the exact two‐dimensional groundwater flow equation subject to a nonlinear free‐surface boundary condition are presented and tested with respect to the linearized Boussinesq equation. It is found that for mild regional gradients and for the range of recharge values usually encountered in the field, the extensively used linearized equation with Dupuit assumptions is a reasonable approximation to the exact solution for the hydraulic heads and the regional flow velocities. Discrepancies occur in the presence of high regional hydraulic gradients, unusually high recharge rates, or regions of low hydraulic conductivity. With this result a new nonperturbation solution of the two‐dimensional plan view groundwater flow equation in a heterogeneous (heterogeneity represented as a spatial random field) aquifer is presented, and expressions for the statistical properties of the longitudinal and transverse velocities are derived. The results suggest that scale dependency, or spatial variability, in the flow velocity arises naturally as a result of recharge from rainfall and aquifer heterogeneity and may help explain the scale dependency of aquifer dispersion param

ISSN:0043-1397    

DOI:10.1029/95WR02038

年代:1995

数据来源: WILEY