Geochemical Transport Model Research Articles

Reactive Transport Process of Earthquake‐induced Hydro‐chemical Changes in Guanding Thermal Spring, Western Sichuan, China

AbstractEarthquake‐related hydrochemical changes in thermal springs have been widely observed; however, quantitative modeling of the reactive transport process is absent. In the present study, we apply reactive transport simulation to capture the hydrochemical responses in a thermal spring following the Wenchuan Ms 8.0 and Lushan Ms 7.0 earthquakes. We first constrain deep reservoir geothermal fluid compositions and temperature by multicomponent geothermometry, and then a reactive geochemical transport model is constructed to reproduce the hydrochemical evolution process. The results show that the recharge from the shallow aquifer increases gradually until it reaches a peak because of the permeability enhancement caused by the Lushan earthquake, which may be the mechanism to explain the earthquake‐related hydrochemical responses. In contrast to the postseismic effect of the Wenchuan earthquake, the chemical evolution can be considered as hydrochemical anomalies related to the Lushan earthquake. This study proves that the efficient simulation of reactive transport processes is useful for investigating earthquake‐related signals in hydrochemical time series.

Long-Term Exposure to Ambient PM2.5, Sunlight, and Obesity: A Nationwide Study in China.

BackgroundAccumulated researches revealed that both fine particulate matter (PM2.5) and sunlight exposure may be a risk factor for obesity, while researches regarding the potential effect modification by sunlight exposure on the relationship between PM2.5 and obesity are limited. We aim to investigate whether the effect of PM2.5 on obesity is affected by sunlight exposure among the general population in China.MethodsA sample of 47,204 adults in China was included. Obesity and abdominal obesity were assessed based on body mass index, waist circumference and waist-to-hip ratio, respectively. The five-year exposure to PM2.5 and sunlight were accessed using the multi-source satellite products and a geochemical transport model. The relationship between PM2.5, sunshine duration, and the obesity or abdominal obesity risk was evaluated using the general additive model.ResultsThe proportion of obesity and abdominal obesity was 12.6% and 26.8%, respectively. Levels of long-term PM2.5 ranged from 13.2 to 72.1 μg/m3 with the mean of 46.6 μg/m3. Each 10 μg/m3 rise in PM2.5 was related to a higher obesity risk [OR 1.12 (95% CI 1.09-1.14)] and abdominal obesity [OR 1.10 (95% CI 1.07-1.13)]. The association between PM2.5 and obesity varied according to sunshine duration, with the highest ORs of 1.56 (95% CI 1.28-1.91) for obesity and 1.66 (95% CI 1.34-2.07) for abdominal obesity in the bottom quartile of sunlight exposure (3.21-5.34 hours/day).ConclusionLong-term PM2.5 effect on obesity risk among the general Chinese population are influenced by sunlight exposure. More attention might be paid to reduce the adverse impacts of exposure to air pollution under short sunshine duration conditions.

The advantages of using a geochemical transport model including thermodynamic equilibrium, kinetic control and surface complexation to simulate the durability of concretes exposed to chlorides and sulphates

In this study, a physically and chemically based model including thermodynamic equilibrium, kinetic control and surface complexation is used. Different cases are proposed and simulated with the same set of parameters. The results are compared with experimental data. They show very good agreement between experimental and numerical results. They also show that the parameters are not very sensible to cement composition, concrete mix-design or environmental exposition. Hence, this numerical model seems to be used as a reliable tool to predict the chloride binding capability and external sulphate attack. It is also convenient because of its limited number of parameters.

Revealing the conditions of Ni mineralization in the laterite profiles of New Caledonia: Insights from reactive geochemical transport modelling

New Caledonia is one of the world's largest nickel laterite deposits that form from intense chemical and mechanical weathering of a peridotite bedrock. As a result of such a weathering process a subsequent downward migration of Si, Mg and Ni takes place, which eventually leads to redistribution of the elements in depth and over time depending on their mobility. Being released from ultramafic parent rock to groundwater, the mobility of nickel is to a great extent controlled by sorption, substitution and dissolution/precipitation processes. Therefore, the final profile of nickel enrichment is the result of the superposition of these possible fates of nickel. The way how Ni is redistributed in between them represents and defines its mineralization in laterite. Knowledge of these processes along with factors, controlling them appears to be a key to detailed understanding of laterite formation. In this study a numerical model, which solves the reaction-transport differential equations, is used to simulate the formation of laterite profile from ultramafic bedrock with particular emphasis on modelled Ni enrichment curve, its comparison with in situ observations, and detailed understanding of trace elements mobility. Since nickel deposits in New Caledonia is characterized by oxide and hydrous Mg silicate ores, three different concurrent fates of Ni deposition in a profile were taken into account in the modelling: i) Ni in a goethite crystal lattice, ii) Ni sorbed on weak and strong goethite sorption sites, and iii) Ni precipitated with silicates (garnierite). Simulations were performed using PHREEQC associated with llnl.dat thermodynamic database that has been edited in order to account garnierite minerals used in the calculations. The work outline is represented by: i) long term (10 Ma) simulation of nickel laterite formation and evolution, ii) analysis of mobility of the elements and understanding its controlling factors, iii) comparison of modelled and in situ Ni enrichment profile and analysis of nickel distribution in between different retention processes, iv) modelling and in depth understanding of these retention processes. The modelling reveals that the vertical progression of the pH front controls thickening of iron-rich zone, explains the vertical mobility of the elements and governs the Ni enrichment. Adsorption itself plays an important role in lateritization process retarding Ni mobility, but i) becomes significant in a narrow range of pH (slightly alkaline) due to competition of Mg and Ni for sorption sites and ii) does not explain such a high nickel content in limonite nowadays, suggesting that Ni is held in goethite mostly by stronger ties i.e. substituted for Fe in the crystal lattice of iron oxide. 1-D modelling appears to be a powerful tool in understanding the general behavior of trace elements upon the formation of laterite and at the same time reveals that locally Ni mineralizations should be explained by more complex processes, such as lateral transfers, convective flows and preferential pathways.

Mobility and transport of copper(II) influenced by the microbial siderophore DFOB: Column experiment and modelling

Acid Cu leaching from the European Kupferschiefer ore deposits is a challenge e.g. due to its high carbonate content. In this study, we investigated the transport behaviour of Cu under conditions related to a biohydrometallurgical leaching approach using neutrophil microorganisms in neutral to slightly alkaline solutions. We studied the effect of the microbial siderophore desferrioxamineB (DFOB) as a model leaching organic ligand on Cu mobility in column experiments with kaolinite. The results revealed that DFOB strongly enhances Cu mobility. The breakthrough of Cu occurs considerably earlier in the presence of DFOB than in the absence of the organic ligand. Furthermore, complete elution of Cu was observed at 5 pore volume exchanges faster compared to elution with deionized water. The established geochemical transport model shows good agreement with the experimental data and suggests a maximum efficiency at a Cu to DFOB molar ratio of 1:1. In addition, results of modelling revealed that in the absence of the ligand, a pH increase from 6.5 to 8.5 significantly retarded Cu breakthrough, whereas in the presence of DFOB, Cu breakthrough curves were nearly insensitive to pH changes and close to the breakthrough curve of a non-reactive tracer.

Using natural CO2 reservoir to constrain geochemical models for CO2 geological sequestration

Numerical modeling of geochemical transport processes is necessary to investigate long-term CO2 storage in deep saline formations, because aluminosilicate mineral alteration is very slow under ambient deep-formation conditions and is not amenable to experimental study. Geochemical transport modeling can solve many problems and answer questions related to CO2 geological sequestration. The numerical modeling provides valuable insights regarding the physical and chemical consequences of CO2 injection in the subsurface environment. However, the reliability and applicability of the models need to be tested and validated if they are applied for CO2 geological sequestration. Issues on model validations are important if CO2 injection technologies are to be implemented safely, efficiently, and predictably. Validation of geochemical transport models could be different from conventional model validation methods for groundwater flow and solute transport. For the short-term behaviors, the models can be validated using laboratory and field experiments. For the long-term mineral alteration and CO2 sequestration, the natural analogue using high-pressure CO2 reservoirs could be a best way to validate the model. In this paper, a natural CO2 reservoir in southern Songliao Basin of China, which is past accumulations of CO2 in geological formation associated with magmatic or volcanic activity, was selected. Although the length of CO2 exposure and hence the rates of reaction for the natural system is not known in detail, we have shown that it is indeed possible to use observation data of mineral alteration in the natural CO2 reservoir to constrain thermodynamic and kinetic data of minerals used in the model, and to confine conditions of temperature, pressure, salinity, and primary mineral composition.

U(VI) Adsorption and Speciation at the Acidic Silica/Water Interface Studied by Resonant and Nonresonant Second Harmonic Generation

Second harmonic generation (SHG) was used to study U(VI) interactions with a buried mineral oxide/water interface using pH 4 uranyl solutions that contained carbonate and environmentally relevant electrolyte concentrations. SHG resonance enhancement at 290 nm was observed at a NaCl concentration of 10 mM, whereas it was necessary to collect the adsorption isotherm using the nonresonant χ(3) technique at 1 mM NaCl concentration. Both methods resulted in the same (within one kT) adsorption free energy of −33 kJ/mol. This value is 10 kJ/mol less favorable than the free energy that was previously reported for the same system at pH 7, indicating that uranyl–surface interactions depend on pH. The charge density due to adsorbed uranyl species was also determined via the χ(3) technique to be +0.0031(5) C/m2, which is half the magnitude previously measured at pH 7. On the basis of the resonant and nonresonant SHG data, it is concluded that the surface-active uranyl species at pH 4 are neutral or univalent, cationic species, which, based on thermodynamic speciation calculations, can consist of the following aquated species present in bulk solution at pH 4: UO2(OH)2, UO2OH+, and UO2CO3. The results of this work are of value as benchmarks for geochemical transport models of uranium pollution resulting from acid mine drainage and nuclear waste storage.

A geochemical transport model for thermo‐hydro‐chemical (THC) coupled processes with saline water

Anhydrous MgSO4 is considered as a potential sealing material for the isolation of high‐level‐waste repositories in salt rock. When an aqueous solution, usually a brine type, penetrates the sealing, different MgSO4 hydrates along with other mineral phases form, removing free water from the solution. The uptake of water leads to an overall increase of solid phase volume. If deformation is constrained, the pore volume decreases and permeability is reduced. In order to simulate such processes, especially for conditions without free water, a coupling between OpenGeoSys and thermodynamic equilibrium calculations were implemented on the basis of the commercially available thermodynamic simulator ChemApp and the object‐oriented programming finite‐element method simulator OpenGeoSys. ChemApp uses the Gibbs energy minimization approach for the geochemical reaction simulation. Based on this method, the thermodynamic equilibrium of geochemical reactions can be calculated by giving the amount of each system component and the molar Gibbs energy of formation for all the possible phases and phase constituents. Activity coefficients in high‐saline solutions were calculated using the Pitzer formalism. This model has the potential to handle 1‐D, 2‐D, and 3‐D saturated and nonsaturated thermo‐hydro‐chemical coupled processes even with highly saline solutions under complex conditions. The model was verified by numerical comparison with other simulators and applied for the modeling of SVV experimental data.

Pseudokinetics arising from the upscaling of geochemical equilibrium

Multicomponent contaminant transport models in groundwater are typically based on assumptions of local geochemical equilibrium on the grid scale. However, in heterogenous systems there may be significant coupling between transport processes and geochemical equilibrium at smaller than grid block scale. Here various pore‐ and field‐scale examples are considered to illustrate the impact of transport processes on assumptions of geochemical equilibrium. In each example the flow length scales required to reach equilibrium are calculated. It is shown that these can be as large as many meters at the pore scale and kilometers at field scales. The influence of heterogeneity in the distribution of the reactive zones is assessed for the pore‐scale example, and it is shown that patchiness of reactive zones within a pore increases equilibration length, with the length and density of reactive zones, pore radius, and diffusion coefficient all playing a role in the equilibration length. When constructing models of field‐scale problems it may not be reasonable to apply geochemical equilibrium, and it may be necessary to explicitly couple pore‐scale and field‐scale models in a multiscale simulation. A field‐scale example is also shown to illustrate that the upscaling of geochemical equilibrium poses a significant practical problem because we usually do not know the spatial location and distribution of geochemically active sites, and this information is essential input to geochemical transport models.

Geochemical transport modelling of drainage from experimental mine tailings cells covered by capillary barriers

The use of covers with capillary barrier effects (CCBEs) for reducing acid mine drainage (AMD) from sulphidic mine tailings is simulated using the MIN3P finite volume model for coupled groundwater flow, O2 diffusion and multi-component reactive transport. The model is applied to simulate five pilot-scale in situ test cells containing reactive tailings from the Manitou mine site, Val d’Or, Que., Canada. Four of the cells were constructed with CCBEs over the tailings, while the fifth tailings cell was left uncovered. Observed and simulated discharge from the base of each cell showed that the capillary barrier covers significantly reduced sulphide oxidation and AMD. Compared to acidic discharge from the uncovered cell, discharge from the four CCBE-covered cells had neutral pH levels and 1–7 orders of magnitude lower concentrations of SO4, Fe, Zn, Cu and Al. The simulations showed that the moisture retaining layer of the CCBEs reduced AMD by inhibiting O2 diffusion into the underlying reactive wastes. Provided the moisture-retention layer of the CCBE remains close to saturation, its thickness had a relatively minor effect. Under such near-saturated conditions, O2 availability is limited by its diffusion rate through the bulk porous medium and not by the diffusion rate through the oxidized grain shells. The model is providing important new insights for comparing design alternatives for reducing or controlling AMD.

Tomographic radiotracer studies of the spatial distribution of heterogeneous geochemical transport processes

Within the scope of the further development of geochemical transport models the consideration of the influence of the heterogeneous structures of the geological layers plays an important role. For the verification and parameter estimation of such models it is necessary to measure the heterogeneous transport and sorption processes inside the samples. Tomographic radiotracer methods (positron emission tomography (PET)) enable nondestructive spatially resolved observations of the transport processes in these layers. A special quantitative evaluation system for geoscientific PET studies was developed. Investigations of the water flow distribution in a drill core of a lignite mining dump and of the migration of Cu ions in a horizontal soil column illustrate the potential of this method. Spatial distribution functions of the flow velocity, the specific mass flow and the longitudinal dispersivity were determined on the basis of PET investigations.

Assessment of the multi-scale leaching behaviour of compacted coal fly ash

Peer experimental–modelling tools were developed and applied in the case of coal fly ashes with the aim to assess the leaching behaviour of ash compacted layers in a use scenario. Laboratory-scale (dissolution kinetics, ANC test, column percolation) and field pilot experimental studies (release monitoring during 18 month, hydrodynamic study, ANC on 44 month leached waste) were performed in order to identify and quantify the main transport phenomena and chemical processes. A quantitative geochemical model was developed taking into account equilibrium chemical reactions as well as kinetic processes for silicate phases like albite, K-feldspar and Ca-olivine. Phases like BaHAsO 4 and a solid solution Ba x Sr 1− x (SO 4) y (CrO 4) 1− y were proposed to explain the complex leaching behaviour of As, Cr, Ba, S; the soluble CaMoO 4 seems to control the Mo concentration. At neutral and acid pH, the model of surface complexation on ferric hydroxides was added for describing the behaviour of As, Cr, and Mo. At each scale the dynamic processes were identified and quantified by modelling. During the first contact with water an equilibration time of about 10 days was identified and then considered in all other laboratory experiments (ANC, column percolation). The hydrodynamic properties of compacted fly ashes were identified: a high water retention capacity (97% of the pores are still filled after draining under normal pressure), a flow regime close to plug type, a low fraction of stagnant zones (<0.03%). The scenario factors like carbonation and rainfall play an important role on the leaching behaviour at field scale. The carbonation diminishes the leachate pH from 11 to 8.5. The alternation of rain periods determines an apparent batch behaviour which slows down the outflow of the initial soluble fraction in pore water, if compared with the laboratory percolation column. The coupled geochemical–transport model was validated by comparison of the simulation results on ANC data obtained on the waste after 44 months of leaching under natural conditions.

Coupled microbial and geochemical reactive transport models in porous media: Formulation and Application to Synthetic and In Situ Experiments

El estudio de la calidad natural y la contaminacion de las aguas subterraneas asi como la evaluacion de la seguridad de las instala¬ciones subterraneas de almacenamiento de residuos radiactivos requiere utilizar modelos acoplados de flujo subterraneo, transporte de calor y solutos, reacciones quimicas y procesos biologicos. Este articulo presenta la formulacion matematica y numerica del modelo acoplado de flujo y transporte reactivo con procesos biologicos para medios porosos que ha sido implementada en el codigo BIO CORE2D© mediante la adicion de los procesos microbiologicos en el codigo de transporte reactivo no isotermo CORE2D©. BIO-CORE2D© incorpora algoritmos numericos eficientes para resolver la version cuasi implicita del metodo de iteracion secuencial y determinar de forma automatica los intervalos de tiempo optimos. Se presentan modelos hidrobiogeoquimicos para una columna sintetica y para el experimento in situ CERBERUS realizado en el laboratorio subterraneo de Mol en Belgica para evaluar el efecto del calor y la radiacion en la arcilla de Boom.

Multicomponent heterovalent chromatography in aquifers. Modelling salinization and freshening phenomena in field conditions

Analysis of hydrochemical data using diagrams and the geochemical transport model PHREEQC confirmed that for the Argos plain aquifer and for other heterogeneous confined alluvial coastal aquifers in Greece, the variation of water qualities along a flow path has basically a chromatographic origin due to seawater intrusion and freshening processes. Additionally, calcite precipitation, dolomite and gypsum dissolution take place. Intense fertilization and overexploitation of the area resulted in degradation of the groundwater quality by high amounts of nitrate ions and increased salinization. The main physical factor affecting groundwater quality in this area is the cation exchange capacity that is calculated to be 4.3 meq(100 gr) −1. Good adaptation of the transport model to the field data showed that the model can be used to predict the freshening process after an artificial recharge. Hence in a first stage for Cl −, there is a continuous decrease to the initial fresh water concentrations corresponding to about 1–1.5 pores volumes that should have been shifted through the aquifer, while for both cations Ca 2+ and Mg 2+ this decrease is even lower and leads to concentrations lower than that of the initial fresh water. Complete restoration of the aquifer requires a great amount of recharge water that should be transported through the aquifer and corresponds to 32 pores volumes.

Positron emission tomography for modelling of geochemical transport processes in clay

Summary The transport of water and salt solutions in unsaturated clay and clay rock was measured using the positron emission tomography (PET). The experimental device is described. KF marked with 18F was used as conservative hydrodynamic tracer to image the flow profiles at different times. Homogeneous and heterogeneous flow regimes were identified. Hydrodynamic model parameters could be determined for the homogeneous regime. The results demonstrate the advantage of PET measurements for investigations of transport processes in geological matrices. They yield quantitative values of the spatial distribution in regard to the flow pattern inside the samples which are important for the validation and parameter estimation of geochemical transport models.

Application of Jacobian-free Newton–Krylov with physics-based preconditioning to biogeochemical transport

Modern multicomponent geochemical transport models require the use of parallel computation for carrying out three-dimensional, field-scale simulations due to extreme memory and processing demands. However, to fully exploit the advanced computational power provided by today’s supercomputers, innovative parallel algorithms are needed. We demonstrate the use of Jacobian-free Newton–Krylov (JFNK) within the Newton–Raphson method to reduce memory and processing requirements on high-performance computers. We also demonstrate the use of physics-based preconditioners, which are often necessary when using JFNK since no explicit Jacobian matrix is ever formed. We apply JFNK to simulate enhanced in situ bioremediation of a NAPL source zone, which entails highly coupled geochemical and biodegradation reactions. The algorithm’s performance is evaluated and compared with conventional solvers and preconditioners. We found that JFNK provided substantial saving in memory (i.e. 30–60%) on problems utilizing up to 512 processors on LANL’s ASCI Q. However, the performance based on wallclock time was less advantageous, coming out on par with conventional techniques. In addition, we illustrate deficiencies in physics-based preconditioner performance for biogeochemical transport problems with components that undergo significant sorption or form a local quasi-stationary state.

Dissolution of a mineral phase in potable aquifers due to CO2 releases from deep formations; effect of dissolution kinetics

Abstract Injection of supercritical CO2 into deep saline aquifers is a technique for sequestration of large amounts of CO2 that is currently being investigated as a means to ameliorate the release of greenhouse gases into the atmosphere. Because complete characterization of these geological formations is not possible, the likelihood that some fraction of the injected CO2 will leak into overlying aquifers needs to be considered. If the leaking CO2 were to reach shallow groundwater aquifers, it could lead to geochemical alterations with detrimental effects on the water quality of these potable aquifers. Identification and assessment of these effects is necessary to analyze risks associated with geologic sequestration of CO2 adequately. In order to assess if there is a potential of solubilizing trace metals, metalloids and/or selected radionuclides by CO2 releases from deep formations into potable aquifers, a series of simulations were conducted. Numerical simulations were conducted for a series of CO2 release scenarios and different aquifer geochemical properties. The effect of CO2 induced pH changes as well as trace metal solubilization was assessed using a geochemical transport model. Results show that elevated CO2 levels in freshwater aquifers can enhance the dissolution of trace metals so that concentrations may reach undesirable levels at the local scale. Transport models demonstrate the importance of assessing the areal extent of this CO2 release, as well as the need to gain thorough understanding of the key kinetic processes related to CO2 solubilization and the dissolution of a trace metal containing mineral phase.

Modeling Np and Pu transport with a surface complexation model and spatially variant sorption capacities: implications for reactive transport modeling and performance assessments of nuclear waste disposal sites

One-dimensional (1D) geochemical transport modeling is used to demonstrate the effects of speciation and sorption reactions on the ground-water transport of Np and Pu, two redox-sensitive elements. Earlier 1D simulations (Reardon, 1981) considered the kinetically limited dissolution of calcite and its effect on ion-exchange reactions (involving 90Sr, Ca, Na, Mg and K), and documented the spatial variation of a 90Sr partition coefficient under both transient and steady-state chemical conditions. In contrast, the simulations presented here assume local equilibrium for all reactions, and consider sorption on constant potential, rather than constant charge, surfaces. Reardon's (1981) seminal findings on the spatial and temporal variability of partitioning (of 90Sr) are reexamined and found partially caused by his assumption of a kinetically limited reaction. In the present work, sorption is assumed the predominant retardation process controlling Pu and Np transport, and is simulated using a diffuse-double-layer-surface-complexation (DDLSC) model. Transport simulations consider the infiltration of Np- and Pu-contaminated waters into an initially uncontaminated environment, followed by the cleanup of the resultant contamination with uncontaminated water. Simulations are conducted using different spatial distributions of sorption capacities (with the same total potential sorption capacity, but with different variances and spatial correlation structures). Results obtained differ markedly from those that would be obtained in transport simulations using constant K d, Langmuir or Freundlich sorption models. When possible, simulation results (breakthrough curves) are fitted to a constant K d advection–dispersion transport model and compared. Functional differences often are great enough that they prevent a meaningful fit of the simulation results with a constant K d (or even a Langmuir or Freundlich) model, even in the case of Np, a weakly sorbed radionuclide under the simulation conditions. Functional behaviors that cannot be fit include concentration trend reversals and radionuclide desorption spikes. Other simulation results are fit successfully but the fitted parameters ( K d and dispersivity) vary significantly depending on simulation conditions (e.g. “infiltration” vs. “cleanup” conditions). Notably, an increase in the variance of the specified sorption capacities results in a marked increase in the dispersion of the radionuclides. The results presented have implications for the simulation of radionuclide migration in performance assessments of nuclear waste-disposal sites, for the future monitoring of those sites, and more generally for modeling contaminant transport in ground-water environments.

Modelling of physical and reactive processes during biodegradation of a hydrocarbon plume under transient groundwater flow conditions

Numerical experiments of non-reactive and reactive transport were carried out to quantify the influence of a seasonally varying, transient flow field on transport and natural attenuation at a hydrocarbon-contaminated field site. Different numerical schemes for solving advective transport were compared to assess their capability to model low transversal dispersivities in transient flow fields. For the field site, it is shown that vertical plume spreading is largely inhibited, particularly if sorption is taken into account. For the reactive simulations, a biodegradation reaction module for the geochemical transport model PHT3D was developed. Results of the reactive transport simulations show that under the site-specific conditions the temporal variations in groundwater flow do, to a modest extent, affect average biodegradation rates and average total (dissolved) contaminant mass in the aquifer. The model simulations demonstrate that the seasonal variability in groundwater flow only results in significantly enhanced biodegradation rates when a differential sorption of electron donor (toluene) and electron acceptor (sulfate) is assumed.

Application of a geochemical transport model to predict heavy metal retention (Pb) by clay liners

PHREEQC, a geochemical transport model, is used to simulate diffusive transport of Pb through a 10-cm-thick clay liner. The results are compared to those of Roehl and Czurda [Applied Clay Science 12 (1998) 387] who studied Pb migration by diffusion in a carefully monitored laboratory experiment. The computer simulation accounts for effects due to adsorption by ion exchange, changes in CEC, variable ion selectivity, and porosity or compacted density. It facilitates evaluation of changes in the diffusion coefficient and solution input parameters. The effective Pb diffusion coefficient determined for the simulation is 3×10 −10 m 2 s −1 and for the 520-day experiment of Roehl and Czurda it is 2.3×10 −10 m 2 s −1. Differences in the retardation factors (23.6 and 503, respectively) indicate that the model does not account for all of the adsorption mechanisms suggested by the experimental investigation. Thus, less Pb is retained and the liner is predicted to fail more rapidly than the actual results indicate. Models have great flexibility, but need to be verified by field data before they can be applied to specific waste site conditions.