Analysis Of Contaminant Transport Research Articles

Diffusion and sorption experiments using a DKS permeameter

The analysis of contaminant transport through clay liner is a relevant aspect in the design of industrial, urban and mining waste disposal systems, since these areas must be designed and operated to prevent contaminating substances from reaching underground water systems in unacceptable concentrations. The design requires an estimate of the potential contaminant transport rate. However, before any attempt at quantification can be made, values for transport mechanism control parameters must be established. Clayey materials are frequently used as contaminant barriers. In these materials, which have low hydraulic conductivity, the main contaminant transport mechanism is molecular diffusion. Parameters controlling transport for these conditions are the diffusion coefficient and sorption parameters. These parameters depend on soil constituents and characteristics as well as on the chemical constitution of the contaminant. The great complexity of the factors involved makes it necessary to determine the parameters of each type of soil. This paper discusses an equipment called DKS permeameter (diffusion, convection, sorption), for the study of soil-contaminant transport mechanisms, designed at the Institute for Soil Mechanics of the Ruhr-University Bochum, and some results obtained from its use at COPPE/Federal University of Rio de Janeiro (UFRJ), Brazil. This equipment determines the effective diffusion coefficient and sorption parameter with a better reflection of field conditions. The soil under study is a mix of sodium–bentonite that has low hydraulic conductivity (k=10 −9 cm/ s) with adequate liner characteristics. The result indicated the relevance of determining sorption parameters for structured soils, since the sorption perceived from batch test results using pulverised soil represents maximum soil capacity. Designs based on this parameter would overestimate the attenuation capacity of the liner.

Radial Flow in a Bounded Randomly Heterogeneous Aquifer

Flow to wells in nonuniform geologic formations is of central interest to hydrogeologists and petroleum engineers. There are, however, very few mathematical analyses of such flow. We present analytical expressions for leading statistical moments of vertically averaged hydraulic head and flux under steady-state flow to a well that pumps water from a bounded, randomly heterogeneous aquifer. Like in the widely used Thiem equation, we prescribe a constant pumping rate deterministically at the well and a constant head at a circular outer boundary of radius L. We model the natural logarithm Y = lnT of aquifer transmissivity T as a statistically homogeneous random field with a Gaussian spatial correlation function. Our solution is based on exact nonlocal moment equations for multidimensional steady state flow in bounded, randomly heterogeneous porous media. Perturbation of these nonlocal equations leads to a system of local recursive moment equations that we solve analytically to second order in the standard deviation of Y. In contrast to most stochastic analyses of flow, which require that log transmissivity be multivariate Gaussian, our solution is free of any distributional assumptions. It yields expected values of head and flux, and the variance–covariance of these quantities, as functions of distance from the well. It also yields an apparent transmissivity, T a, defined as the negative ratio between expected flux and head gradient at any radial distance. The solution is supported by numerical Monte Carlo simulations, which demonstrate that it is applicable to strongly heterogeneous aquifers, characterized by large values of log transmissivity variance. The two-dimensional nature of our solution renders it useful for relatively thin aquifers in which vertical heterogeneity tends to be of minor concern relative to that in the horizontal plane. It also applies to thicker aquifers when information about their vertical heterogeneity is lacking, as is commonly the case when measurements of head and flow rate are done in wells that penetrate much of the aquifer thickness. Potential uses include the analysis of pumping tests and tracer test conducted in such wells, the statistical delineation of their respective capture zones, and the analysis of contaminant transport toward fully penetrating wells.

A Fourier-Laplace transform finite element method (FLTFEM) for the analysis of contaminant transport in porous media

In many practical situations, a simplified two-dimensional model is often sufficiently accurate for practical purposes, however on some occasions it is still necessary to consider the full three-dimensional situation. Conventional numerical techniques such as the finite difference and finite element methods are able to deal with contaminant transport under 3D conditions, however the computational effort involved in using them is often very large. Under some circumstances it is possible to perform 3D analyses more efficiently. A particular case is when the geometry is 2D but the problem is actually 3D. In such cases the analysis can be simplified by application of a Fourier transform. In this paper, the dimensions of the contaminant transport problem will be reduced by one and the transient mass transport equation will be transformed into a time-independent mass transport equation through the use of a combination of integral transforms (Laplace and Fourier transforms) and a finite element technique. The mathematical model developed here is called Fourier–Laplace transform finite element method (FLTFEM). By using the FLTFEM a three-dimensional transient problem can be analysed as a two-dimensional time-independent problem and the computational cost will thus greatly be reduced. Copyright © 1999 John Wiley & Sons, Ltd.

A boundary element method for analysis of contaminant transport in porous media—II: non-homogeneous porous media

A boundary element method for analysis of contaminant transport in porous media—II: non-homogeneous porous media

A boundary element method for analysis of contaminant transport in porous media I: homogeneous porous media

A boundary element method is developed for the analysis of contaminant migration in porous media. The technique involves, firstly, taking the Laplace transform with respect to time then followed by a co-ordinate transform and a mathematical transform of the well-known advection-dispersion equation. The series of transforms reduce the equation into the modified Helmholz equation and this greatly facilitates the formulation of the boundary integral equation and a system of approximating algebraic boundary element equations. The algebraic equations are solved simultaneously in the transform space before being inverted numerically to obtain the concentration of the contaminant in real time and space. The application of this technique is demonstrated by some illustrative examples.

A boundary element method for analysis of contaminant transport in porous media I: homogeneous porous media

A boundary element method for analysis of contaminant transport in porous media I: homogeneous porous media

Groundwater pollution by organic compounds: a two-dimensional analysis of contaminant transport in stratified porous media with multiple sources of non-equilibrium partitioning

Transport of pollutants in soil and groundwater often occurs in stratified media under non-equilibrium conditions. Confined aquifers are usually bounded by low-permeability layers of soil which have been shown to exert a significant influence on the fate of contaminants in groundwater. Numerical solutions of transport equations have usually been restricted to single layers and have included single sources of non-equilibrium processes or none at all. The effect of soil stratification itself has sometimes been reduced to a transport-based non-equilibrium process. A boundary element solution of the transport equations in the Laplace domain is extended to include multiple sources of non-equilibrium processes in saturated media under the assumption of rate-limited mass transfer. Green functions accurately model infinite and semi-infinite domains such as soils and Laplace transforms remove the need for time-stepping and the associated numerical complexity. The proposed numerical technique is validated by comparing its results to analytical solutions. Its scope is illustrated through a case study of a sand aquifer bounded by less permeable layers of silt, and infiltrated by pollutants from a neighbouring lake. Copyright © 1999 John Wiley & Sons, Ltd.

Relating lithofacies to hydrofacies: outcrop-based hydrogeological characterisation of Quaternary gravel deposits

Abstract A considerable part of today's drinking water supplies in Europe and North America rely on clean groundwater from gravelly valley aquifers of Quaternary age. The sedimentary architecture, the distribution of lithofacies and of architectural elements in such heterogeneous deposits are of fundamental importance for the analysis of groundwater flow and contaminant transport. As the aquifers are not directly accessible for observation, representative outcrop analogues were used to study the sedimentology on a local scale. Conventional sedimentological classification schemes were adapted for the purpose of hydrogeological evaluations. Measurements of hydraulic properties were then used to define a set of 5 hydrofacies from 23 possible sediment lithofacies. A digital-photographic mapping procedure was developed to allow fast data acquisition in the field. The sedimentologically interpreted outcrops were stored in a GIS style database and thus allow the output for further sedimentological or hydrogeological analysis.

Contaminant transport through an unsaturated soil liner beneath a landfill

Contaminant transport through soil is often modelled mathematically by means of the dispersion-advection equation. When investigating contaminant migration beneath a landfill, a common assumption is that the soil is saturated; however, it is well known that the soil beneath many landfills is only partially saturated. In this paper, steady-state unsaturated moisture distributions are employed in the dispersion-advection equation to find transient contaminant distributions beneath a landfill. A finite layer formulation is used to simplify the contaminant mass transport equation and account for heterogeneous soil profiles. Various assumptions concerning the flow regime beneath the landfill and the functional relation between volumetric water content and the diffusion coefficient in the transport equation are made to highlight differences between contaminant transport through saturated and unsaturated soils. It is found that diffusive contaminant mass transfer through a partially saturated soil liner is comparatively insensitive to variation of the volumetric moisture content at the top of the soil liner alone. However, accounting for the dependence of the diffusion coefficient on the volumetric moisture content of the soil does have a significant effect on diffusive mass transfer through a partially saturated soil liner. Though it is difficult to measure the dependence of the diffusion coefficient on the volumetric moisture content of the soil experimentally, this information appears necessary for a rational analysis of contaminant transport through partially saturated soils.Key words: landfill, contaminant transport, unsaturated soil.

Two‐Dimensional Sensitivity Analysis of Contaminant Transport in the Unsaturated Zone

AbstractA two‐dimensional probabilistic transport model is developed‐by coupling a reliability algorithm to a two‐dimensional unsaturated flow and transport model. The goal is to determine the significance of the uncertainty of each variable to the probability outcome. Model input includes the mean, standard deviation, and probability distribution of each uncertain variable. The uncertain variables are assumed to be uncorrelated to each other. The uncertain variables are saturated water content, residual water content, van Genuchten model parameters alpha (α) and n, saturated hydraulic conductivity, bulk density, coefficient of distribution, molecular diffusion coefficient, and dispersivity. Coefficient of variation is used as the measure of uncertainty for each uncertain variable. Three soil samples are considered for model simulations. One half of the flow surface area is the source of a generic contaminant spill. The reliability analysis determines the probability of exceeding a specific target concentration value and also provides a sensitivity value that measures the relative significance of each uncertain transport variable to the probability outcome. Generally, saturated water content tends to be the most significant uncertain variable for transport in the unsaturated zone. Bulk density and coefficient of distribution also are significant for reactive transport. Molecular diffusion and van Genuchten model parameter n are significant in fine‐grained soils and also at locations far from the contaminant source area. The uncertainty of saturated hydraulic conductivity and dispersivity have no significant influence on the probability outcome and can be considered as deterministic input variables.

A boundary element method for analysis of contaminant transport in fractured and non-fractured porous media

Abstract This paper presents a boundary element method for modelling contaminant transport of a contaminant species in porous media which consists of both fractured and non-fractured material. In the fractured material, blocks of solid matrix comprising the parent soil material are assumed to be separated by discontinuities in the form of fissures, fractures or joints. The method developed in this paper utilises a double porosity model to deal with contaminant transport in the presence of the discontinuities and solid blocks. The underlying assumption of a double porosity model is that the solid matrix blocks, said to have the primary porosity, constitute one continuum while the network of discontinuities, the secondary porosity, form the other continuum. Flux transfer between the two interacting continua is represented by a source/sink term in the governing equation. When there are no discontinuities, the material reduces to a single continuum with one porosity and so the formulation is able to deal with deposits which consist of both fissured and non-fissured material. This approach is used to develop a boundary element technique which can enable a wide range of problems of practical interest to be analysed.

Three-dimensional analysis of airflow and contaminant particle transport in a partitioned enclosure

Determination of the distribution of contaminant particles within zones becomes of great interest, with the increasing concern for indoor air quality. In order to improve the indoor air quality in a realistic building, the air movement and contamination transport in a partitioned enclosure with ventilation have been studied numerically and experimentally. A 3-D analysis of air movement, temperature distribution and contaminant particle transport is made to investigate airflow patterns and deposition of contaminant particle in a partitioned enclosure. An experimental program was also performed to validate the airflow field and temperature distribution. The experiment is carried out in a full-scale environmental control chamber with inlet\\outlet ventilation systems inside a university laboratory. The agreement between the computed and the measured results of temperature and velocity fields are quite well. It is also shown that prediction results of contaminant particle’s trajectory will give valuable information to be used for evaluating the indoor air quality in the design procedure.

Probabilistic sensitivity analysis for one-dimensional contaminant transport in the vadose zone

A reliability algorithm is coupled to a van Genuchten model-based unsaturated flow and transport numerical solution to develop a probabilistic model of one-dimensional non-reactive and reactive contaminant transport in the vadose zone. The reliability approach provides two important quantitative results: an estimate of the probability that a contaminant concentration is exceeded at some time and location, and a probabilistic sensitivity measure which quantifies the relative importance of each uncertain variable to the probabilistic outcome. The method requires that each uncertain variable be assigned at least a mean and standard deviation but marginal distributions and correlation structure may be incorporated. Transport through an idealized soil column with fixed boundary conditions and constant source concentration is analyzed. The uncertain variables include the van Genuchten model shape parameters, the saturated and residual water contents, saturated hydraulic conductivity, dispersivity, dispersion coefficient, first-order decay terms, bulk density and partitioning coefficient. The objective is to examine the relative importance of each uncertain variable and marginal distributions assigned to each variable. The influence of correlation structure is not considered in this study. Results indicate that the probabilistic outcome is generally very sensitive to likely changes in the saturated water content. Uncertainty associated with the diffusion coefficient, residual water content and first-order decay coefficients is often not a significant issue with respect to the probabilistic outcome; thus, these variables may be treated as deterministic constants. Although these results are limited by the assumptions of the numerical code and by the conceptual setup of the idealized soil column, they provide insight into the critical issues to consider in a probabilistic analysis of contaminant transport in the vadose zone. Such information concerning the most important uncertain parameters can be used to guide field and laboratory investigations.

Dimensional Analysis of Colloid-Facilitated Ground-Water Contaminant Transport

Mobile subsurface colloids can carry ground-water contaminants sorbed onto their surfaces to far greater distances than predicted using a conventional advective/dispersive transport equation with normal retardation values. In this study, we present a dimensional analysis of colloid-facilitated ground-water contaminant transport. The significance of the contaminant distribution coefficient with colloids and the colloid distribution coefficient with the solid matrix on the retardation coefficient is demonstrated. Figures plotted in terms of dimensionless parameters show that the smaller the sorption of contaminant with colloids, the smaller the effect of colloid presence on contaminant transport. The effect of retardation coefficient on the normalized contaminant concentration profile is illustrated in terms of dimensionless parameters. It has been demonstrated that a reduction in contaminant retardation coefficient in the presence of colloids is accompanied by a corresponding reduction in the modified hydr...

Numerical stochastic analysis of groundwater contaminant transport and plume containment

First- and second-order reliability methods (FORM and SORM) are applied as alternatives to the Monte Carlo simulation method in the probabilistic analysis of groundwater contaminant transport and remediation. A two-dimensional finite-element model is interfaced with a reliability analysis program to account for uncertainty in aquifer media. Hydraulic conductivity is modeled as a spatial random field with prescribed marginal probability distribution and correlation structure. FORM and SORM provide the probability that a contaminant exceeds a target level at a well, termed the probability of failure. Sensitivity of the probability of failure to basic uncertainty in grid block conductivities is also obtained, at no additional computational effort. Component reliability is used to analyze failure in a single well. Results indicate that, at the most likely failure scenario, grid block conductivities attain their maximum value near the source, the receptor well, and along the stream tubes connecting the two. System reliability is used to analyze the joint probability of failure at several wells in the aquifer. Results indicate that system failure probability is greater than the largest component failure probability. Correlation between component failure events is greater when the individual wells are closer. Sensitivity of the upper bound on system probability with respect to grid block conductivities is highest along the path the contaminant follows to reach the receptor wells. Furthermore, the probability of failure to contain a plume from escaping site boundaries is analyzed, along with the corresponding sensitivity information. Probability of failure to contain the plume decreases as the well pumping rate increases. The presence of regions of lower conductivity dramatically increases the probability of remediation failure. A careful analysis of aquifer material uncertainty and heterogeneity is vital to the success of groundwater remediation systems.

A compositional simulator for modeling surfactant enhanced aquifer remediation, 1 formulation

We describe a three-dimensional, multicomponent, multiphase compositional finite-difference simulator for application to the analysis of contaminant transport and surfactant enhanced aquifer remediation (SEAR) of non aqueous- phase liquid (NAPL) pollutants. Mixtures of surfactant, water and NAPL can form many types of micellar and microemulsion phases with a complex and important dependence on many variables of which the dilute aqueous solution typically assumed in SEAR models is just one example. The phase behavior model is central to our approach and allows for the full range of the commonly observed micellar and microemulsion behavior pertinent to SEAR. The other surfactant related properties such as adsorption, interfacial tension, capillary pressure, capillary number and microemulsion viscosity are all dependent on an accurate phase behavior model. This has proven to be a highly successful approach for surfactant enhanced oil recovery modeling, so it was adapted to SEAR modeling. However, there are many significant differences between petroleum and environmental applications of surfactants, so many new features have been added to model contaminant transport and remediation and these are described and illustrated for the first time here.

A stochastic analysis of contaminant transport through a rough-surfaced fracture

A stochastic model for the calculation of flow and contaminant transport in a single fracture with variable apertures was presented. The spatially varying apertures of the fracture were generated using a geostatistical method, based on a given aperture probability density distribution and a specified spatial correlation length. Fluid flowed between two points in the fracture plane. The fluid potential at each node of the discretization mesh was computed and the steady state flow rates between all the nodes were obtained. Then the contaminant transport was calculated using a particle tracking method. The migration plumes of contaminant between the inlet and the outlet were displayed in contour plots and contaminant elution profiles were also plotted. Calculations showed that fluid flow occured predominantly in a few preferred paths. Hence, the large range of apertures in the fracture gives rise to flow channeling. Simulation results were correlated with the basic input parameters: standard deviation of a lognormal aperture distribution function and the spatial correlation length.

Reliability analysis of contaminant transport in saturated porous media

An approach to probabilistic modeling of contaminant transport based on the first‐ and second‐order reliability methods (FORM and SORM) is presented. FORM and SORM were initially developed for structural reliability applications to estimate the occurrence of low‐probability events. They can be readily used with both analytical and numerical models and do not require restrictive assumptions about the problem geometry or about the properties of the media. Sensitivity information is obtained as an integral part of these analyses and is used to identify the variables or parameters which have a major influence on the estimate of probability. Example reliability analyses of one‐ and two‐dimensional transport are used to illustrate the approach, and the accuracy of the reliability methods is evaluated in comparison with Monte Carlo simulations. The results show that FORM increasingly overestimates the probability of exceedance as the spatial variability of the domain increases. SORM, on the other hand, accounts for the nonlinearity of the limit state surface and gives results consistent with Monte Carlo simulation over a range of coefficient of variation of K from 0.1 to 0.7. In addition, the FORM/SORM analyses are shown to provide a computational advantage over the Monte Carlo simulation for low‐probability events, because the computational effort is independent of the probability and the results also include sensitivity information. Finally, an example application of system reliability using FORM and SORM shows that problems with multiple limit state surfaces can be readily analyzed and the computational effort is proportional to the number and complexity of the limit state functions.

Geochemical Reduction of Hexavalent Chromium in the Trinity Sand Aquifer

AbstractGround water of the Trinity Sand aquifer at the Odessa I site has become contaminated with hexavalent chromium due to release of plating wastes during the 1970s. Measured hexavalent chromium partition coefficients for Trinity Sand aquifer sediments compare favorably with values derived from the literature, and with retardation factors calculated from observed plume migration rates between 1986 and 1991. Dissolved and adsorbed masses of hexavalent chromium were calculated from contoured plume maps using ground‐water data collected between 1986 and 1991 to estimate the total mass of hexavalent chromium in the aquifer. Comparison of the calculations from each year indicated that only 30 percent of the hexavalent chromium present in the aquifer in 1986 remained in 1991.Decreasing hexavalent chromium mass in the Trinity Sand aquifer at the Odessa I site indicates that natural geochemical reactions are occurring to reduce mobile hexavalent chromium to the insoluble trivalent form. Thermodynamic calculations based on platinum electrode redox and pH measurements indicate that solid trivalent chromium hydroxide species are the stable phase in these ground waters. Reduction of hexavalent chromium to the trivalent state results in precipitation from solution of a solid chromium (III) hydroxide, probably in association with iron oxyhydroxides present in the aquifer. Two possible reducing agents in the Trinity Sand aquifer are naturally occurring organic carbon and ferrous iron.Laboratory measurements of the rate of hexavalent chromium reduction indicate that the reaction proceeds slowly at near‐neutral pH values representative of the Odessa I site. Assuming first‐order reaction kinetics, these laboratory reduction rates are consistent with the rate of hexavalent chromium loss at the Odessa I site between 1986 and 1991, and indicate a hexavalent chromium half‐life of approximately 2.5 years. An analytical contaminant transport model was calibrated to simulate historical chromium concentrations for the aquifer, and then used to predict that hexavalent chromium will naturally decrease below the drinking water standard within approximately 10 years.

Boundary element analysis of contaminant transport in fractured porous media : Leo, C J; Booker, J R Int J Num Anal Meth GeomechV17, N7, July 1993, P471–492

Boundary element analysis of contaminant transport in fractured porous media : Leo, C J; Booker, J R Int J Num Anal Meth GeomechV17, N7, July 1993, P471–492