Groundwater Contaminant Transport Research Articles

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.

Dual‐porosity groundwater contaminant transport in the presence of colloids

Prior studies have shown that colloids can facilitate contaminant migration in unimodal porous media. To investigate the effect of no‐flow regions on flow and contaminant transport in dual‐porosity soils, we model a porous medium composed of two different homogeneous, superposed, and interacting regions: the mobile region and the immobile region. We assume that the advective‐dispersive processes govern the transport of contaminant and colloids in the mobile region, while the diffusion process dominates in the immobile region. The contaminant and colloid mass transfer mechanisms between these two regions are represented by a first‐order mass transfer. Colloid deposition on the solid matrix is expressed by a kinetic sorption relationship. The contaminant sorption with the solid matrix and colloidal surfaces is also incorporated into the model. Coupled with mass transfer terms, two sets of governing equations representing the fate and transport of contaminant and colloids in both the mobile and immobile regions are developed and applied to experimental data available in the literature. Numerical solutions are obtained by employing a fully implicit, finite difference scheme. The numerical results indicate that the colloidal facilitation is increased in a dual‐porosity porous medium compared to a unimodal medium. The model is validated by comparing the numerical results with the experimental data available in the literature for colloid‐facilitated contaminant transport in a single‐porosity medium and contaminant transport in a colloid‐free, dual‐porosity medium. A sensitivity analysis is conducted to deduce the effect of major model parameters on contaminant transport. The analysis demonstrates that, although both the volumetric fraction of the mobile region and the mass transfer rate coefficients between the two regions have effects on the dual‐porosity transport, the early breakthrough is affected mainly by the volumetric fraction of the mobile region, while the tailing is affected largely by the mass transfer rate coefficients.

Applying Multimedia Modeling to Karst Systems: Comparing MEPAS, MMSOILS, and RESRAD

Abstract The use of multimedia models has become a popular tool to estimate exposures and assess risks. Computer-based multimedia models compute environmental transport of contaminants in air, surface water, groundwater, and soil and bioaccumulation in plants and animals across varying timeframes. This study examines the applicability of three state-of-the-art multimedia models (MEPAS, RESRAD, and MMSOILS) to karst topography using actual site data obtained from a remedial investigation conducted at Oak Ridge National Laboratory in Tennessee in 1995 and default parameters suggested by the individual model developers.

Crank–Nicolson–Galerkin model for transport in groundwater: Refined criteria for accuracy

The Crank–Nicolson–Galerkin model (CNGFEM) is reported to be accurate in predicting advective–dispersive transport when the product of the mesh Peclet number (Pe) and the Courant number (Cr) is less than, or, equal to 2. However, numerical experimentation involving the application of the CNGFEM to simulate contaminant transport in groundwater has shown that the model prediction can be oscillatory for some values of the Peclet number. Furthermore, the accuracy of prediction has also been found to be a function of the source definition. The objective of this paper has been to develop refined criteria for the CNGFEM to provide oscillation-free accurate results for contaminant transport in groundwater from different types of sources. The refined criteria require that Pe⩽50, PeCr⩽5, and Cr⩽1 for continuous sources. And for pulse sources, oscillation-free accurate results are obtained when Pe⩽20, PeCr⩽5, and Cr⩽1.

Simulating advective-dispersive transport in groundwater: an accurate finite difference model

Finite difference models (FDMs) may provide oscillatory results when employed to predict contaminant transport in groundwater. The oscillation is usually avoided by adapting upwind methods. The upwind methods are reported to introduce large artificial dispersion. An improved FDM has been developed to provide oscillation free results with the introduction of minimum artificial dispersion. The improvement has been achieved by developing an expression of the minimum artificial dispersion needed to provide accurate results. The improved FDM provides results in excellent agreement with the analytical solution. Further, the improved FDM can easily be extended to two- or three-dimensional problems.

A comparison between the advection-dispersion and the Kolmogorov-Dmitriev model for groundwater contaminant transport

The growing interest of the scientific and public community in environmental problems has sparked the undertaking of several activities of protection from pollutants, of remediation of contaminated sites and of design of deposits for toxic or radioactive waste. The implementation and management of these activities require the use of mathematical models for predicting the behaviour of the pollutant in the geosphere; of particular interest is the study of the transport of contaminants in the groundwater system, which provides an important pathway of return of the hazard to the biosphere. In this paper a comparison is made between the traditional advection-dispersion model, which is the most widely used model of groundwater contaminant transport, and a stochastic model based on the Kolmogorov-Dmitriev theory of branching processes. The behaviour of the two models is analysed in the case of transport with and without retardation of contaminant in the solid matrix and, in the latter case, both equilibrium and non-equilibrium processes are considered. The results obtained show that for non-reactive contaminant and for reactive contaminant under equilibrium, the Kolmogorov-Dmitriev parameters are related by simple relations with the advection-dispersion parameters. Moreover, the Kolmogorov-Dmitriev model is capable of describing more realistic non-equilibrium phenomena, that the advection-dispersion cannot treat. This capability arises from the fact of treating explicitly the reactions of contaminant interchange between the liquid and solid phases.

Accuracy of the Taylor–Galerkin model for contaminant transport in groundwater

The Taylor–Galerkin finite element model (TGFEM) is second-order accurate in time. However, it provides oscillatory results for advection dominated transport. The objective of this paper is to develop criteria for the TGFEM to provide oscillation-free accurate results. The accuracy of the TGFEM was evaluated by comparing its prediction with the corresponding analytical solution. The model was found to provide oscillation-free accurate results for Peclet number, Pe, as high as 20 and Courant number Cr⩽0.10.

A general method for obtaining analytical expressions for the first‐order velocity covariance in heterogeneous porous media

Velocity covariance of seepage flow is the essential quantity in understanding the uncertainty of groundwater flow and contaminant transport in heterogeneous porous media. Nevertheless, a closed‐form analytical expression is available only for first‐order analysis. These available expressions are limited to some specific correlation functions of natural log hydraulic conductivity. The present paper develops a general method for obtaining the first‐order velocity covariance for any statistically isotropic log‐hydraulic conductivity correlation function. The general analytical expressions are derived for both two and three dimensions. The results are demonstrated for several frequently used correlation functions.

Mineral Associations and Average Oxidation States of Sorbed Pu on Tuff

Subsurface transport of groundwater contaminants is greatly influenced by chemical speciation, precipitation, and sorption processes. The transport of Pu potentially released from spent nuclear fuel disposal and storage sites will be dependent on its interaction with mineral surfaces and speciation in the subsurface. The sorption of dissolved Pu(V) on a natural zeolitic tuff that was equilibrated with synthetic groundwater was examined using synchrotron-based microanalytical techniques. The tuff contained trace quantities of smectites and iron and manganese oxides, which are present as fracture fill and pore space materials. Synchrotron-based micro-X-ray fluorescence (SXRF) showed that Pu is predominantly associated with manganese oxides (rancieite) and smectites but not with iron oxides (hematite). In situ micro-X-ray absorption near-edge structure (XANES) spectroscopy measurements on two highly enriched regions of Pu indicated that the average oxidation state of sorbed Pu was (+V) in one region and (+VI) in the other. The observed heterogeneous speciation of the sorbed Pu demonstrates the complex nature of this process. Thermodynamic equilibrium calculations indicated that the solution was dominated by negatively charged Pu species, suggesting that sorption to the negatively charged manganese oxide surfaces would be energetically prohibited. Subsequent speciation changes upon sorption to manganese oxide surfaces are discussed.

UNCERT: geostatistics, uncertainty analysis and visualization software applied to groundwater flow and contaminant transport modeling

UNCERT is a 2D and 3D geostatistics, uncertainty analysis and visualization software package applied to ground water flow and contaminant transport modeling. It is a collection of modules that provides tools for linear regression, univariate statistics, semivariogram analysis, inverse-distance gridding, trend-surface analysis, simple and ordinary kriging and discrete conditional indicator simulation. Graphical user interfaces for MODFLOW and MT3D, ground water flow and contaminant transport models, are provided for streamlined data input and result analysis. Visualization tools are included for displaying data input and output. These include, but are not limited to, 2D and 3D scatter plots, histograms, box and whisker plots, 2D contour maps, surface renderings of 2D gridded data and 3D views of gridded data. By design, UNCERT's graphical user interface and visualization tools facilitate model design and analysis. There are few built in restrictions on data set sizes and each module (with two exceptions) can be run in either graphical or batch mode. UNCERT is in the public domain and is available from the World Wide Web with complete on-line and printable (PDF) documentation. UNCERT is written in ANSI-C with a small amount of FORTRAN77, for UNIX workstations running X-Windows and Motif (or Lesstif). This article discusses the features of each module and demonstrates how they can be used individually and in combination. The tools are applicable to a wide range of fields and are currently used by researchers in the ground water, mining, mathematics, chemistry and geophysics, to name a few disciplines.

On Galerkin models for transport in groundwater

The Galerkin finite element model (GFEM) may provide oscillatory results when employed to predict contaminant transport in groundwater unless a very fine mesh is used. Adaptation of a very fine mesh may make the application of the GFEM impractical to field problems. The Petrov—Galerkin finite element models (PGFEMs) can provide oscillation free results for relatively coarser mesh. However, the PGFEM violates the Galerkin principle and introduces large “numerical” dispersion. The objective of this paper has been to develop accurate criteria to improve the applicability of the GFEM to obtain oscillation free accurate results for coarser mesh and compare its performance with that of the PGFEM. It has been shown that the GFEM provides oscillation free accurate results for coarser mesh with Peclet number Pe ⩽ 20. Further, the GFEM prediction has always been more accurate than the PGFEM for a variety of source configurations and flow fields.

A program to create permeability fields that honor single-phase flow rate and pressure data

Accurate prediction (or simulation) of reservoir performance or contaminant transport in groundwater requires a realistic geological model representative of the reservoir/aquifer heterogeneity. Geostatistics provides tools for constructing such complex geological models constrained by different types of available (hard and soft) data and providing an assessment of related uncertainty. Permeability and flow data are nonlinearly related through the flow equations. Derivation of permeability models that honor flow response data is typically an inverse problem. This paper presents a FORTRAN program for generating permeability fields conditional to multiple-well single-phase flow rate and pressure data through an iterative inverse technique, called the sequential self-calibration (SSC) method. The SSC method is geostatistically-based, that is, it generates multiple equiprobable realizations that honor the input geostatistics of permeability and match pressure data for the given flow rate, under the given boundary conditions. The unique aspects of SSC are: (1) the master point concept that reduces the amount of computation, (2) a propagation mechanism based on kriging that accounts for spatial correlations of perturbations and (3) a fast method for computing all sensitivity coefficients within a single flow simulation run. Results from running the SSC code using an example data set are presented.

Modeling the Impact of Oxygen Reaeration on Natural Attenuation

Based on studies of leaking petroleum storage tank (LPST) sites in Texas and California, the average plume of benzene, toluene, ethylene, and xylenes (BTEX) is between 61 and 132 m (200 and 400 ft) long. Standard modeling of BTEX plumes produces plumes well in excess of observed plume lengths. The amount of oxygen carried into the plume zone with clean upgradient water often is insufficient to account for the levels of biodegradation observed in these studies. Traditional recharge of oxygen-containing water into an aquifer adds insufficient oxygen to the system and cannot account for the observed plume lengths. Research has shown that anaerobic processes can contribute to biodegradation in certain cases; however, anaerobic pathways are not included in this work. Reaeration of oxygen-depleted aquifers by diffusive transport of oxygen through the vadose zone has generally been neglected as a way to introduce oxygen into surficial aquifers. The observed plume lengths and preliminary laboratory results indicate that this source of oxygen should be accounted for in any natural attenuation model of BTEX contamination. This approach to modeling reaeration has been incorporated into the finite-element groundwater flow and contaminant transport code, FLOTRAN. Adding diffusion-driven reaeration to the modeling process produces BTEX plumes consistent with observed plume lengths.

A Numerical Simulation of Groundwater Flow and Contaminant Transport on the CRAY T3D and C90 Supercomputers

Numerical simulations of groundwater flow and chemical transport through three-dimensional heterogeneous porous media are described. The authors employ two CRAY supercomputers for different parts of the decoupled calculation: the flow field is computed on the T3D massively parallel computer, and the contaminant migration is simulated on the C90 vector supercomputer. The authors compare simulation results for subsurface models based on homogeneous and heterogeneous conceptual models and find that the heterogeneities have a profound impact on the character of contaminant migration.

MAPPING OF SUBSURFACE CONTAMINANT PROFILES BY NEURAL NETWORKS

Determining the extent of contamination is a frequently encountered problem for private industry and governmental agencies. Numerous groundwater contaminant transport models have been developed involving mathematical relationships based on an understanding of the physical, chemical and microbiological processes that are thought to affect transport of contaminants in subsurface environments. This paper presents the application of neural networks for groundwater contaminant transport modeling. Recently, artificial neural networks (NN) have been successfully used in many different applications. The main advantages of NNs for contaminant transport modeling are their ability to develop contaminant profiles with limited data. The data used for the study were obtained from the monitoring studies conducted for the small potable water wells in Dade County, Florida. The water wells used in the study have less than 100,000 gpd capacity and are located within 1/4 mile of underground petroleum storage tank systems. Laboratory analysis by EPA method 524.2 resulted in the identification and quantification of volatile organic compounds in the wells sampled. An area contaminated from a specific underground storage tank was selected for this NN application. The locations of the contaminated wells in this area were mapped and a 2-dimensional grid system was developed for the area. Input to the neural network program for the learning set consisted of the coordinates of the contaminated wells and the contaminant concentration. The subsurface contaminant profiles for different types of petroleum organics were developed.

Spatial and temporal properties of environmental exposure assessments related to ground water contamination

This paper describes a new methodology for modeling contaminant transport in ground water for (1) better quantifying the magnitude of exposure in a contaminated aquifer, (2) characterizing the spatial and temporal variation of exposure in a heterogeneous aquifer, and (3) providing more tools for characterizing population exposure. This methodology is also applied to a semi-hypothetical case study.

Colloidal culprits in contamination

Contaminant transport in ground water is a contentious issue — especially when it comes to possible movement of radionuclides from nuclear test sites or storage facilities. Analyses carried out in Nevada now implicate colloids in the movement of plutonium from a nuclear detonation site. The case, however, has yet to be clinched, and the broader interest in this example lies in the questions it raises about identifying colloid-associated transport in general.

Outstanding student paper awards

The Hydrology Section presented the following outstanding student paper awards at the 1998 AGU Spring Meeting in Boston, Mass., last May.Robert G. Bruant, Jr. presented a poster titled “Adsorption of Hydrophobic Organic Contaminants at the Air‐Water Interface.” Robert received a B.A. in geosciences from Franklin and Marshall College in 1993. He is currently in his final year of the Hydrology Ph.D. Program at the University of Arizona, advised by Dr. Martha H. Conklin. Rob's research involves qualifying both variations in surface tension as a function of aqueous solute concentrations and adsorption thermodynamics at air‐water interfaces for volatile organic compounds. His goal is to further theoretical development of adsorption phenomena, especially in system with nonideal interactions between components. The application is to promote better understanding of the transport and fate of anthropogenic contaminants in groundwater, surface water, and atmospheric systems.

Calculation and mesh generation of a three-dimensional matrix fracture system

In solid rock aquifers low matrix permeability is often accompanied with fractures and faults of different scales. Therefore, numerical modeling of such coupled systems is very complex. Flow of groundwater and mass transport processes in fractured systems have to be modeled along discrete surfaces joined to the common matrix representation. This leads to high demands on the quality of finite element mesh generation. Another difficulty is the evaluation of field parameters needed in the design of realistic models to simulate natural systems. This paper presents a strategy to generate three dimensional finite element meshes, which allows the modeling of groundwater flow and contaminant transport in a coupled matrix fracture aquifer system. Software development and data evaluation for the discussed application example comes from the research project “Development and Application of Analytical Methods on Investigations of the Reliability of Disposals in Open Mines” sponsored by the German Ministry of Education and Research. The scope of this project is to determine the influence of a rising groundwater level in closed coal mines on the transport of contaminants from waste material deposits in cut coal seams.

Modeling contaminant transport in groundwater: an optimized finite element method

An optimized upwind Taylor-Galerkin finite element method (UTGFEM) has been developed to simulate one-dimensional contaminant transport in groundwater. The model was employed to predict the transport of contaminants from continuous, pulse, and exponentially decaying sources for a wide range of dispersion coefficients and degradation rate constants. The dispersion coefficients considered were 0.0, 1.0, and 5.0 m 2/day. The degradation rate constant was varied between 0.007 and 0.028 (day) −1. The model was found to predict the transport dynamics for all these conditions accurately when the dispersion coefficient was greater than or equal to 0.215 m 2/day. The model would introduce some numerical dispersion if the field dispersion were smaller than this limiting value. The need for numerical dispersion can be reduced significantly by making the length of the finite elements smaller.