Migration of pollutants in groundwater. III. Numerical dispersion reduction with cartesian coordinates

The partial differential equation governing the movement of a decomposing pollutant undergoing 2-dimensional flow in a saturated aquifer is examined. The analytical solution of the equation is usually not possible, and use of mesh numerical integration techniques causes excessive numerical dispersion to arise from the advection term. We apply two asymmetrical upwind formulas to approximate the advection term. These markedly reduce numerical dispersion without requiring the use of coordinate systems obtained by conformal mapping. Flow within a right angle and flow toward a sink in an otherwise uniform field are analyzed to illustrate the methods.     

Migration of pollutants in groundwater. II. adsorbable pollutants and numerical dispersion reduction

We discuss here the partial differential equations governing the migration of a decomposing pollutant adsorbing according to a Langmuir isotherm and undergoing 2-dimensional flow in a saturated aquifer. The equation governing the mass transfer of the pollutant to the surfaces within the aquifer are solved in closed form, permitting the use of larger values of the time increment t in the numerical integration of the dispersion-advection equation governing the behavior of the dissolved pollutant. In this numerical integration transverse numerical dispersion is eliminated by using conformal coordinates (velocity potential and stream function), and longitudinal numerical dispersion is very substantially reduced by use of an asymmetrical 4-point formula to represent the advection term. Some representative results are given as contour maps. The mass transfer rate coefficient is estimated as the least positive eigenvalue of a diffusion problem.     

OSPM - A Parameterised Street Pollution Model

For many practical applications, as e.g. in support of air pollution management, numerical models based on solution of the basic flow and dispersion equations are still too complex. Alternative are models that are basically parameterised semi-empirical models making use of a priori assumptions about the flow and dispersion conditions. However, these models must, be thoroughly tested and their performance and limitations carefully documented. The Danish Operational Street Pollution Model (OSPM) belongs to this category of parameterised models. In the OSPM, concentrations of exhaust gases are calculated using a combination of a plume model for the direct contribution and a box model for the recirculating part of the pollutants in the street. Parameterisation of flow and dispersion conditions in street canyons was deduced from extensive analysis of experimental data and model tests. Results of these tests were used to further improve the model performance, especially with regard to different street configurations and a variety of meteorological conditions.     

Bioremediation With Oxygen-Releasing Solids: A Mathematical Model

The use of oxygen releasing solids in passive wells has become of considerable interest in the bioremediation of groundwater contaminated with hydrocarbons and other biodegradable contaminants. Labor, operating, and maintenance costs of this technology are quite competitive with more conventional techniques in many instances. Because of the small rates of mass transport by transverse dispersion, however, a pin-stripe pattern of alternating contaminant and oxygen distribution is expected downgradient of a line of wells which is unacceptable if it extends beyond the point of compliance downgradient. A mathematical model is presented which permits determination of the maximum well spacing in an array of wells which will yield effective remediation within a specified distance of a plume of contaminated groundwater passing through the array. Transverse dispersion of both oxygen and the contaminant is assumed, and biodegradation is modeled by means of Monod kinetics. Longitudinal numerical dispersion is minimized by the use of a asymmetrical upwind algorithm for modeling advection. Modeling results are presented showing the dependence of model behavior on input parameters.     

Multipoint source method in air pollution modelling of cold start emission

The paper presents a new method of air pollution modelling on a micro scale. For estimation of concentration of car exhaust pollutants, each car is treated as an instantaneous moving emission source. This approach enables us to model time and spatial changes of emission, especially during cold and cool start of an engine. These stages of engine work are a source of significant pollution concentration in urban areas. In this work, two models are proposed: one for the estimation of emission after cold start of the engine and another for the prediction of pollutant concentration. The first model (defined for individual exhaust gas pollutants) enables us to calculate the emission as a function of time after the cold or cool start, ambient temperature and average speed of motion. This model uses the HBEFA database. The second mathematical model is developed in order to calculate the pollutant dispersion and concentrations. The finite volume method is applied to discretise the set of partial differential equations describing wind flow and pollutant dispersion in the domain considered. Models presented in this paper can be called short-term models on a small spatial scale. The results of numerical simulation of pollutant emission and dispersion are also presented.     

Numerical Simulations of Wind Induced Particle Contamination in Gypsum Landfill Surroundings

The contribution presents numerical simulation of gypsum particles, lifting from a gypsum landfill. First, particle characteristics are presented, resulting from different technologies of gypsum depositing. Next, a laboratory experiment parameter validation tests are described, which served as a means of determination of mass flow of particles from the landfill. The background of the numerical simulations, used in the assessment of landfill impact on the environment, is also described. Simulations consist of two parts: simulation of a long term impact of the particles on the surrounding area, performed by implementation of the Gaussian dispersion model based computer code ISC3, and second, a CFD based simulation for assessing the flow and mass concentration fields in the vicinity of the landfill for several pre-selected flow cases. The results of both computational approaches are presented and compared. In the conclusions, a relation of the simulation results with existing environmental pollution levels is made, and recommendations for landfill management are drawn.     

Experimental and Numerical Analysis of CO Concentration Dispersion of Vehicular Exhaust Emissions in Isolated Environment

Numerical and experimental analyses were applied to carbon monoxide (CO) concentration dispersion to monitor air quality in an enclosed residential complex parking area in Tehran. Firstly, the parking area was preliminary assessed through verifying the characteristics of the problem including the geometry and boundary conditions. Then, proportion of vehicular exhaust emissions was estimated and eventually experimental and numerical analyses were performed. In order to perform numerical calculation, a three-dimensional model was created to numerically simulate the enclosed residential complex parking area by FLUENT software that solves flow governing equations with finite volume method. In FLUENT, species model was selected to assess the dispersion of CO in flow domain. In experimental analysis, CO concentration was measured using sampling bags with a volume of 10 l in 4 min at 6 different points. The sample air was drawn into sampling bags by electric pumps. The findings show that the maximum amount of CO concentration is above the permissible standard recommended by the World Health Organization. Pollutant accumulation was significant in confined areas. In the place where openings exist, the level of accumulation was lower than other areas. The findings obtained from numerical simulation are in complete accord with experimental results.     

A Numerical Study of Airflow and Pollutant Dispersion Inside an Urban Street Canyon Containing an Elevated Expressway

The goal of this study is to investigate numerically the wind flow and pollutant dispersion within an urban street canyon containing an elevated expressway and reveal the impacts of elevated expressway on the atmospheric environment in the canyon. A two-dimensional numerical model for simulating airflow and pollutant dispersion inside urban street canyons is first developed based on the Reynolds-averaged Navier–Stokes equations coupled with the standard k???ε turbulence model and the convection–diffusion equation for passive species transport, and then it is validated against a wind tunnel experiment. It was found that the model-predicted results agree well with the experimental data. Having established this, the wind fields and pollutant distributions in the canyon containing an elevated expressway are evaluated. The numerical results show that the expressway height above the street floor and the gap distance between the expressway and the building wall have considerable influence on airflow and pollutant level inside a canyon: (1) the vortical flow structure in the canyon varies with the expressway height for a constant gap distance, under certain expressway heights, only one main clockwise vortex is formed, while under others one main vortex as well as one or two secondary vortices above and below the expressway are created; (2) the pollutant level within the canyon increases when an expressway is placed in the canyon, especially when the expressway height equals the building height the flow velocities in the canyon are drastically reduced and air exchange in and above the canyon is seriously impeded by the expressway, which leads to a much higher pollution level in the canyon; and (3) the wider gap distance is favorable to pollutant removal from the canyon.     

Numerical Model Inter-comparison for Wind Flow and Turbulence Around Single-Block Buildings

Wind flow and turbulence within the urban canopy layer can influence the heating and ventilation of buildings, affecting the health and comfort of pedestrians, commuters and building occupants. In addition, the predictive capability of pollutant dispersion models is heavily dependent on wind flow models. For that reason, well-validated microscale models are needed for the simulation of wind fields within built-up urban microenvironments. To address this need, an inter-comparison study of several such models was carried out within the European research network ATREUS. This work was conducted as part of an evaluation study for microscale numerical models, so they could be further implemented to provide reliable wind fields for building energy simulation and pollutant dispersion codes. Four computational fluid dynamics (CFD) models (CHENSI, MIMO, VADIS and FLUENT) were applied to reduced-scale single-block buildings, for which quality-assured and fully documented experimental data were obtained. Simulated wind and turbulence fields around two surface-mounted cubes of different dimensions and wall roughness were compared against experimental data produced in the wind tunnels of the Meteorological Institute of Hamburg University under different inflow and boundary conditions. The models reproduced reasonably well the general flow patterns around the single-block buildings, although over-predictions of the turbulent kinetic energy were observed near stagnation points in the upwind impingement region. Certain discrepancies between the CFD models were also identified and interpreted. Finally, some general recommendations for CFD model evaluation and use in environmental applications are presented.     

Numerical simulation on pollutant dispersion from vehicle exhaust in street configurations

The impact of the street configurations on pollutants dispersion from vehicles exhausts within urban canyons was numerically investigated using a computational fluid dynamics (CFD) model. Three-dimensional flow and dispersion of gaseous pollutants were modeled using standard kappa - epsilon turbulence model, which was numerically solved based on Reynolds-averaged Navier-Stokes equations by the commercial CFD code FLUENT. The concentration fields in the urban canyons were examined in three cases of street configurations: (1) a regular-shaped intersection, (2) a T-shaped intersection and (3) a Skew-shaped crossing intersection. Vehicle emissions were simulated as double line sources along the street. The numerical model was validated against wind tunnel results in order to optimize the turbulence model. Numerical predictions agreed reasonably well with wind tunnel results. The results obtained indicate that the mean horizontal velocity was very small in the center near the lower region of street canyon. The lowest turbulent kinetic energy was found at the separation and reattachment points associated with the corner of the down part of the upwind and downwind buildings in the street canyon. The pollutant concentration at the upwind side in the regular-shaped street intersection was higher than that in the T-shaped and Skew-shaped street intersections. Moreover, the results reveal that the street intersections are important factors to predict the flow patterns and pollutant dispersion in street canyon.     

Study on gas diffusion emitted from different height of point source

The flow and dispersion of stack-gas emitted from different elevated point source around flow obstacles in an urban environment have been investigated, using computational fluid dynamics models (CFD). The results were compared with the experimental results obtained from the diffusion wind tunnel under different conditions of thermal stability (stable, neutral or unstable). The flow and dispersion fields in the boundary layer in an urban environment were examined with different flow obstacles. Gaseous pollutant was discharged in the simulated boundary layer over the flat area. The CFD models used for the simulation were based on the steady-state Reynolds-Average Navier-Stoke equations (RANS) with kappa-epsilon turbulence models; standard kappa-epsilon and RNG kappa-epsilon models. The flow and dispersion data measured in the wind tunnel experiments were compared with the results of the CFD models in order to evaluate the prediction accuracy of the pollutant dispersion. The results of the CFD models showed good agreement with the results of the wind tunnel experiments. The results indicate that the turbulent velocity is reduced by the obstacles models. The maximum dispersion appears around the wake region of the obstacles.     

Dispersion of Pollutants in Street Canyon under Traffic Induced Flow and Turbulence

A 3-D Eulerian-Lagrangian approach to moving vehicles is presented that takes into account the traffic induced flow rate and turbulence. The method is applied to pollutants dispersion in a street canyon. The approach is based on CFD calculations using Eulerian approach to the continuous phase and Lagrangian approach to the "discrete phase" of moving objects - vehicles. A commercial CFD code StarCD was used into which the Lagrangian model was integrated. As an example a street canyon is taken into consideration. It has the length of 50 m and the aspect ratio of 1.27. The speed of wind was assigned values of 4, 7 and 12 m/s at the altitude of 300 m. The total height of the domain is 115 m. In the study different traffic situations are considered, namely one-way and two-way traffic with different traffic rates per lane. The predictions show that different traffic situations affect pollutants dispersion in the street canyon and that there are also differences in the pollutants dispersion in case of one- and two-way traffic.     

Modelling Tracer Dispersion from Landfills

Several wind tunnel experiments of tracer dispersion from reduced-scale landfill models are presented in this paper. Different experimental set-ups, hot-wire anemometry, particle image velocimetry and tracer concentration measurements were used for the characterisation of flow and dispersion phenomena nearby the models. The main aim of these experiments is to build an extensive experimental data set useful for model validation purposes. To demonstrate the potentiality of the experimental data set, a validation exercise on several mathematical models was performed by means of a statistical technique. The experiments highlighted an increase in pollutant ground level concentrations immediately downwind from the landfill because of induced turbulence and mean flow deflection. This phenomenon turns out to be predominant for the dispersion process. Tests with a different set-up showed an important dependence of the dispersion phenomena from the landfill height and highlighted how complex orographic conditions downwind of the landfill do not affect significantly the dispersion behaviour. Validation exercises were useful for model calibration, improving code reliability, as well as evaluating performances. The Van Ulden model proved to give the most encouraging results.     

Composite use of numerical groundwater flow modeling and geoinformatics techniques for monitoring Indus Basin aquifer, Pakistan

The integration of the Geographic Information System (GIS) with groundwater modeling and satellite remote sensing capabilities has provided an efficient way of analyzing and monitoring groundwater behavior and its associated land conditions. A 3-dimensional finite element model (Feflow) has been used for regional groundwater flow modeling of Upper Chaj Doab in Indus Basin, Pakistan. The approach of using GIS techniques that partially fulfill the data requirements and define the parameters of existing hydrologic models was adopted. The numerical groundwater flow model is developed to configure the groundwater equipotential surface, hydraulic head gradient, and estimation of the groundwater budget of the aquifer. GIS is used for spatial database development, integration with a remote sensing, and numerical groundwater flow modeling capabilities. The thematic layers of soils, land use, hydrology, infrastructure, and climate were developed using GIS. The Arcview GIS software is used as additive tool to develop supportive data for numerical groundwater flow modeling and integration and presentation of image processing and modeling results. The groundwater flow model was calibrated to simulate future changes in piezometric heads from the period 2006 to 2020. Different scenarios were developed to study the impact of extreme climatic conditions (drought/flood) and variable groundwater abstraction on the regional groundwater system. The model results indicated a significant response in watertable due to external influential factors. The developed model provides an effective tool for evaluating better management options for monitoring future groundwater development in the study area.     

Evaluation of Numerical Modeling Methods for the Management of Produced Water Discharges in the Coastal Region with a Canadian Case Study

This study aims at evaluating two numerical methods for 3D simulation of marine pollutant dispersion problems: the random walk particle tracking (RWPT) method and an explicit second-order finite difference method (FDM) for assessing produced water discharges from offshore oil platform. Test cases in a steady flow field were used to evaluate the efficiency and accuracy of simulating pollutant concentration profiles obtained using both the FDM and RWPT method in comparison with an analytical solution. Additionally, a field study was conducted to simulate the lead concentration distribution of produced water discharged from an oil platform off Canada’s east coast, based on the Princeton Ocean Model modeling the ocean flow in the study area with field verifications. Results indicate that, with proper configuration of grid resolution and particle resolution, both FDM and RWPT method can provide accurate results with reasonable computational costs for complex field cases. Particularly, the satisfactory 3D simulations of marine pollutant dispersion in the far field by both FDM and RWPT numerical methods enable effective assessment and management of offshore waste discharges.     

Multi-scale Atmospheric Dispersion Modelling by Use of Adaptive Gridding Techniques

An accurate prediction of the transport-reaction behaviour of atmospheric chemical species is required to fully understand the impact on the environment of pollution emissions. Elevated levels of secondary pollutants such as ozone in the lower atmosphere can be harmful to the health of both plants and animals, and can cause damage to property present in the urban environment. Detailed models of pollution mechanisms must therefore be developed through comparisons with field measurements to aid the selection of effective abatement policies. Such models must satisfy accuracy requirements both in terms of the number of species represented, and the spatial resolution of species profiles. Computational expense often compels current models to sacrifice detail in one of these areas. This paper attempts to address the latter point by presenting an atmospheric transport-reaction modelling strategy based upon a finite volume discretisation of the atmospheric dispersion equation. The source terms within this equation are provided by an appropriate reduced chemical scheme modelling the major species in the boundary layer. Reaction and transport discretisations are solved efficiently via a splitting technique applied at the level of the non-linear equations. The solution grid is generated using time dependant adaptive techniques, which provide a finer grid around regions of high spatial error in order to adequately resolve species concentration profiles. The techniques discussed are applied in two dimensions employing emissions from both point and area sources. Preliminary results show that the application of adaptive gridding techniques to atmospheric dynamics modelling can provide more accurately resolved species concentration profiles, accompanied by a reduced CPU time invested in solution. Such a model will provide the basis for high resolution studies of the multiple scale interactions between spatially inhomogeneous source patterns in urban and regional environments.     

Modelling of spatial contaminant probabilities of occurrence of chlorinated hydrocarbons in an urban aquifer

In this study, a 3D urban groundwater model is presented which serves for calculation of multispecies contaminant transport in the subsurface on the regional scale. The total model consists of two submodels, the groundwater flow and reactive transport model, and is validated against field data. The model equations are solved applying finite element method. A sensitivity analysis is carried out to perform parameter identification of flow, transport and reaction processes. Coming from the latter, stochastic variation of flow, transport, and reaction input parameters and Monte Carlo simulation are used in calculating probabilities of pollutant occurrence in the domain. These probabilities could be part of determining future spots of contamination and their measure of damages. Application and validation is exemplarily shown for a contaminated site in Braunschweig (Germany), where a vast plume of chlorinated ethenes pollutes the groundwater. With respect to field application, the methods used for modelling reveal feasible and helpful tools to assess natural attenuation (MNA) and the risk that might be reduced by remediation actions.     

Measurement and Interpretation of Concentrations of Urban Atmospheric Organic Compounds

The presence of volatile organic compounds (VOCs) from traffic and other sources in urban areas is a cause for concern about public health. Canister, chemical derivatisation, particulate sampling and adsorption sampling techniques were used to measure VOC concentrations of a wide range of compounds (C6-C40) during a four day campaign in south London with subsequent laboratory analysis of the samples. Compounds quantified included alkanes, mono- and poly-nuclear aromatic hydrocarbons. Also the first sequential measurements of carbonyl compounds (C1-C8) in a UK urban area are presented. Results from canister and adsorption sampling methods are compared. A comparison of the results with other urban data is presented and the temporal variations in VOC concentrations were interpreted with reference to the prevalent wind speeds and directions. The CALINE4 line source dispersion model was generally successful in reproducing the daytime 12 hour average concentrations of selected VOCs.     

Colloid-Associated Groundwater Contaminant Transport in Homogeneous Saturated Porous Media: Mathematical and Numerical Modeling

During the past two decades, significant efforts have been made to study contaminant transport in the presence of colloids. Several researchers reported that colloidal particles could enhance the migration of contaminants in groundwater by reducing retardation factor. When the colloidal particles are present in the aquifer, the subsurface system can be considered as a three-phase system with two solid phases and an aqueous phase. The interaction between contaminants, colloids, and solid matrix should be considered in assessing the fate and transport of the contaminant in the groundwater flow system. In this study, a one-dimensional numerical model is developed by employing a fully implicit finite difference method. This model is based on mass balance equations and mass partition mechanisms between the carriers and solid matrix, as well as between the carriers and contaminants in a saturated homogeneous porous medium. This phenomenon is presented by two approaches: equilibrium approach and fully kinetic first-order approach. The formulation of the model can be simplified by employing equilibrium partitioning of particles. However, contaminant transport can be predicted more accurately in realistic situations by kinetic modeling. To test the sensitivity of the model, the effect of the various chemical and physical coefficients on the migration of contaminant was investigated. The results of numerical modeling matched favorably with experimental data reported in the literature.     

Numerical Modeling of Flow Fields and Dispersion of Passive Pollutants in the Vicinity of the Temelín Nuclear Power Plant

This study deals with numerical simulation of flow fields and dispersion of model passive admixtures in the planetary boundary layer in a 10 × 10 km square area, centered on the Czech Republic’s Temelín Nuclear Power Plant. Numerical calculations of three-dimensional flow fields with eight basic wind directions given for the inlet boundary of the computational domain are performed using the FLUENT CFD code with the standard k – ε turbulence model. The resultant modeling of the flow fields provides information as to probable local directions and velocities of flow vectors on a horizontal scale of 100 m, which are consistent with the data given for the boundaries of the calculation area in the framework of a scale of 10 km. The modeled flow fields generate the input data for related Lagrangian simulation of the ground concentration and deposition fields of passive particles assumedly emitted at the site of the Temelín plant. Simulated plumes describe eight cases of potential ground-level distribution of model passive admixtures in the area.