Parameter Determination for Reductive Dechlorination of Chlorinated Solvents

The contamination of subsurface due to the chlorinated solvents such as tetrachlorethylene (PCE) and trichlorethylene (TCE) is one of the most difficult environmental problems to treat. Bioremediation has been shown by many researchers to be a remedial alternative for this type of contamination. Chlorinated solvents are not directly mineralized but rather are transformed by microorganisms into one or more intermediate compounds before converting into a final compound. These sequential reactions, termed “reductive dehalogenation”, consist of replacing a chlorine atom by a hydrogen atom. The pathway of degradation of PCE can be expressed by the following scheme PCE → TCE → DCE → VC → ETH, where dichloroethene (DCE), vinyl chloride (VC) and finally ETH is ethylene. Since the biotransformation rate coefficients of each intermediate compound are different, they have to be determined very precisely to establish an effective treatment operation. The sequential decay can be described by Michaelis–Menten’s kinetics, which constitutes a highly nonlinear system of ordinary differential equations (ODEs). This is very sensitive to the changes of biotransformation rate coefficients. In this study we introduce a methodology how to numerically estimate the rate coefficients for Michaelis–Menten’s equations from the knowledge of the concentrations of PCE, TCE, DCE, VC and ETH. The efficiency of the proposed method is demonstrated on some examples. Estimated biotransformation coefficients are employed to predict the concentrations of chlorinated solvents. Computations and measurements show a very good agreement.     

Parameter Determination of Sequential Reductive Dehalogenation Reactions of Chlorinated Hydrocarbons

Chlorinated hydrocarbons such as tetrachloroethylene (PCE) and trichloroethylene (TCE) are not directly mineralized, but rather are transformed into one or more intermediate compounds before converting into a final compound. Since the biotransformation rate coefficients of each intermediate compound are different, the coefficients in each step of reductive dehalogenation must be determined to establish an effective treatment operation. These parameters can be estimated by fitting the experimental data to Michaelis–Menten equation. In this study, we introduce a methodology, using both the curve-fitting and graphical methods, to estimate the rate of maximum biotransformation and half-saturation coefficients of parent and intermediate compounds. First-order rate coefficients are also estimated by simplifying the Michaelis–Menten equation for both curve-fitting and graphical methods. The results show that both methods produce similar parameter values for each rate equation. Estimated first-order kinetic parameters are employed to predict the compound concentrations from the analytical solutions of governing equations for sequential dehalogenation. Comparisons of predicted and experimental values show favorable agreement.     

Transfert du trichloréthylène en milieu poreux à partir d'un panache de vapeursMass transfer of trichloroethylene in a porous medium from a vapor plume

A large scale experiment has been carried out on an experimental facility to study the mass transfer of trichloroethylene (TCE) in a partially saturated porous medium. 5 liters of TCE have been infiltrated in the vadose zone of the site. The mass transfer of TCE from the vapor plume in the unsaturated zone towards the top of groundwater was quantified based on an analytical and a numerical approach. The mass of the pollutant measured at the exit of the model is well represented by the two mathematical approaches. It is found that the transfer of TCE towards the groundwater from the vapor plume is weak, which corresponds to 285 g of TCE, approximately 4% of the initial mass. To cite this article: H. Benremita, G. Schäfer, C. R. Mecanique 331 (2003).     

Transfert du trichloréthylène en milieu poreux à partir d'un panache de vapeurs

A large scale experiment has been carried out on an experimental facility to study the mass transfer of trichloroethylene (TCE) in a partially saturated porous medium. 5 liters of TCE have been infiltrated in the vadose zone of the site. The mass transfer of TCE from the vapor plume in the unsaturated zone towards the top of groundwater was quantified based on an analytical and a numerical approach. The mass of the pollutant measured at the exit of the model is well represented by the two mathematical approaches. It is found that the transfer of TCE towards the groundwater from the vapor plume is weak, which corresponds to 285 g of TCE, approximately 4% of the initial mass. To cite this article: H. Benremita, G. Schäfer, C. R. Mecanique 331 (2003).     

The Role of Biofilm Growth in Bacteria-Facilitated Contaminant Transport in Porous Media

Groundwater contaminants adhered to colloid surfaces may migrate to greater distances than predicted by using the conventional advective-dispersive transport equation. Introduction of exogenous bacteria in a bioremediation operation or mobilization of indigenous bacteria in groundwater aquifers can enhance the transport of contaminants in groundwater by reducing the retardation effects. Because of their colloidal size and favorable surface conditions, bacteria can be efficient contaminant carriers. In cases where contaminants have low mobility because of their high partition with aquifer solids, facilitated contaminant transport by mobile bacteria can create high contaminant fluxes. In this paper, we developed a methodology to describe the bacteria-facilitated contaminant transport in a subsurface environment using the biofilm theory. The model is based on mass balance equations for bacteria and contaminant. The contaminant is utilized as a substrate for bacterial growth. Bacteria are attached to solid surfaces as a biofilm. We investigate the role of the contaminant adsorption on both biofilm and mobile bacteria on groundwater contaminant transport. Also, the effect of bacterial injection on the contaminant transport is evaluated in the presence of indigenous bacteria in porous media. The model was solved numerically and validated by experimental data reported in the literature. Sensitivity analyses were conducted to deduce the effect of critical model parameters. Results show that biofilm grows rapidly near the top of the column where the bacteria and contaminant are injected, and is detached by increasing fluid shear stress and re-attach downstream. The adsorption of contaminant on bacterial surfaces reduces contaminant mobility remarkably in the presence of a biofilm. The contaminant concentration decreases significantly along the biofilm when contaminant partition into bacteria. Bacterial injection and migration in subsurface environments can be important in bioremediation operations regardless of the presence of indigenous bacteria.     

A Three-Dimensional Flow and Transport Modeling of an Aquifer Contaminated by Perchloroethylene Subject to Multi-PRB Remediation

Although subsurface contamination by organic chemicals is a pervasive environmental problem, a permeable reactive barrier (PRB) as a typical in-situ remediation technology is often successful at many sites. Laboratory tests have shown that perchloroethylene (PCE) can be dechlorinated by the combination of zero-valent iron and anaerobic microbial communities (FeMB), and the degradation pathway was: PCE → TCE → 1, 1-DCE → ethylene→  ethane (Ma amd Wu Environ Geol 55(1):47–54, 2008). Based on Ma’s experimental results, we have extended MT3DMS to simulate mother-daughter chain reactions using MODFLOW-2005 V1.7/MT3DMS V5.2. Second, using FeMB as multi-PRB’s reactive media, a 5-component transport model for a three-dimensional aquifer contaminated by PCE subject to multi-PRB remediation was built. Third, the adsorption and degradation parameters of reactive media were estimated by means of genetic algorithm. Finally, the three-dimensional, homogeneous aquifer contaminated by PCE subject to multi-PRB remediation was simulated. Overall, the purpose of this paper was to use FeMB as multi-PRB’s reactive media, and develop a modified MODFLOW/MT3DMS that can simulate a three-dimensional aquifer contaminated by PCE and its daughters. Results demonstrated that FeMB could be a potential reactive media for PCE-contaminated groundwater. Multi-PRB could be a preferred option for secondly pollution caused by application of one-stage PRB. The modified MODFLOW/MT3DMS can effectively simulate multispecies mother–daughter chain kinetic reactions. Sensitivity analysis showed that losses of reactivity and permeability may have a significant effect on remediation’s success.     

Large Scale Experiment on Transport of Trichloroethylene in a Controlled Aquifer

Pollution by dense non-aqueous phase liquids (DNAPLs) represents a major threat to groundwater resources. In a real case of site contamination, the efficiency of remediation techniques is often limited by a lack of knowledge of both the extent of the pollution and the behavior of the different phases of the pollutant in the subsurface. An experiment simulating pollution of an aquifer by a chlorinated solvent (Trichloroethylene: TCE) was conducted on a large controlled experimental site called SCERES. The experiment consisted of an injection of 8.9 liters of TCE under controlled conditions at 35cm below the soil surface with an appropriate set up. The goal was to study the behavior of the three phases of the pollutant (trapped TCE phase forming the impregnation body, vapors in the vadose zone, and dissolved traces in the aquifer) in order to better comprehend the mechanisms which govern the propagation and the transfer of this type of pollution underground. The SCERES experimental data indicate that mass transfer from the saturated zone to the vadose zone is important, affecting the repartition of the vapor plume and causing a significant decrease of dissolved TCE concentrations in the groundwater. Furthermore, vertical leaching of TCE vapors due to rainfall strongly influences the degree of groundwater pollution and its lateral extent. The transient mass balance of the experiment is very satisfactory and shows that the main part of the spilled quantity is lost to the atmosphere.     

Efficient Uncertainty Quantification for Unconfined Flow in Heterogeneous Media with the Sparse Polynomial Chaos Expansion

In this study, we explore an efficient stochastic approach for uncertainty quantification of unconfined groundwater flow in heterogeneous media, where a sparse polynomial chaos expansion (PCE) surrogate model is constructed with the aid of the feature selection method. The feature selection method is introduced to construct a sparse PCE surrogate model with a reduced number of basis functions, which is accomplished by the least absolute shrinkage and selection operator-modified least angle regression and cross-validation. The training samples are enriched sequentially with the quasi-optimal samples until the results are satisfactory. In this study, we test the performance of the sparse PCE method for unconfined flow with the presence of random hydraulic conductivity and recharge, as well as pumping well. Numerical experiments reveal that, even with large spatial variability and high random dimensionality, the sparse PCE approach is able to accurately estimate the flow statistics with greatly reduced computational efforts compared to Monte Carlo simulations.

     

Effect of biotransformation on multispecies plume evolution and natural attenuation

Biological transformation of volatile organic compounds is one of the key factors that influence contaminant-plume evolution and thus natural attenuation. In this study we investigate the effect of biological transformation on the transport of contaminants in the aqueous and gaseous phases. The analysis includes the study of the effect of density-driven advection of contaminants in the gaseous phase on multiphase and multispecies flow, fate and transport modeling in the subsurface. Trichloroethylene (TCE) and its two byproducts, dichloroethylene and vinyl chloride, are analyzed as the target contaminants. Our results indicate that density-driven advection of the gaseous phase, which is initiated by evaporation of TCE as a nonaqueous phase liquid, increases the downward and also the lateral migration of TCE within the unsaturated zone. This process also influences the location of high-concentration zones of the byproducts of TCE in the unsaturated and the saturated zones. Biotransformation of TCE contributes to the reduction of dissolved TCE plume development as expected. The daughter byproducts, which are introduced into the subsurface system, show distinct transport patterns as they are affected by their independent degradation kinetics and density-driven advection. These observations, which are based on our simulation results for biotransformation and transport of TCE and its byproducts, are useful in evaluating the natural attenuation processes, its potential health hazards and also the evaluation of potential plume development at contaminated sites.     

Density-driven Transport of Volatile Organic Compounds and its Impact on Contaminated Groundwater Plume Evolution

Density-driven advection of gas phase due to vaporization of chlorinated volatile organic compounds (VOCs) has a significant effect on fate and transport of contaminants. In this study, we investigated the effects of density-driven advection, infiltration, and permeability on contaminant plume evolution and natural attenuation of VOCs in the subsurface system. To analyze these effects, multiphase flow and contaminant transport processes were simulated using a three-dimensional Galerkin-finite-element-based model. Trichloroethylene (TCE) is selected as a target contaminant. Density-driven advection of gas phase elevated the potential of groundwater pollution in the saturated zone by accelerating downward migration of vaporized contaminant in the unsaturated zone. The advection contributed to increased removal rates of non-aqueous phase liquid (NAPL) TCE source and reduced dissolved TCE plume development in the downstream area. Infiltration reduced the velocity of the density-driven advection and its influence zone, but raised TCE transfer from the unsaturated to the saturated zone. The variation in soil permeability showed greater impact on contaminant migration within water phase in the saturated zone than within gas phase in the unsaturated zone. Temporal variations of TCE mass within two-dimensional (2D) and three-dimensional (3D) domains under several modeling conditions were compared. These results are important in evaluation of natural attenuation processes, and should be considered to effectively design monitored natural attenuation as a remedial option.     

Simulations of Microbial-Enhanced Oil Recovery: Adsorption and Filtration

In the context of microbial-enhanced oil recovery (MEOR) with injection of surfactant-producing bacteria into the reservoir, different types of bacteria attachment and growth scenarios are studied using a 1D simulator. The irreversible bacteria attachment due to filtration similar to the deep bed filtration (DBF) is examined along with the commonly used reversible equilibrium adsorption (REA). The characteristics of the two models are highlighted. The options for bacteria growth are the uniform growth in both phases and growth of attached bacteria only. It is found that uniform growth scenario applied to filtration model provides formation of two oil banks during recovery. This feature is not reproduced by application of REA model or DBF with growth in attached phase. This makes it possible to select a right model based on the qualitative analysis of the experimental data. A criterion is introduced to study the process efficiency: the dimensionless time at which average recovery between pure water injection and maximum surfactant effect is reached. This characteristic recovery period (CRP) was studied as a function of the different MEOR parameters such as bacterial activity, filtration coefficients, and substrate injection concentrations. For both growth scenarios, there is a zone of optimal activity at which the CRP is minimal. Dependence of the CRP on substrate concentration for uniform growth scenario has also an optimal zone. Therefore, growth rate and the substrate concentration should be above a certain threshold value and still not be too high to obtain the minimum CRP. On the other hand, no such zone was found if the bacteria could grow only in the attached phase. Dependencies on both the injected concentration and filtration coefficient are monotonous in this case.     

Multiphase Flow Fields in In-situ Air Sparging and its Effect on Remediation

In-situ air sparging (IAS) is used for the clean-up of soil and groundwater that are contaminated with volatile organic compounds in relatively permeable subsurface environments. In this study, we investigated the secondary groundwater and gas flow fields that develop in the vicinity of single and multiple air sparging wells. The purpose is to evaluate their effects on contaminant plume migration and thus, remediation. Governing equations describing multiphase flow and contaminant transport in a three-dimensional domain were formulated and solved using the Galerkin finite element technique. Trichloroethylene was selected as a target contaminant. The increase in air injection contributed to an increase in the size of the IAS cone of influence and the gas saturation levels within the cone. This reduced the groundwater velocity within the cone and increased the zone of detour of groundwater around the air sparging wells. This outcome was quantified and compared under several IAS operations. Different soil permeability characteristics also affected the groundwater and gas flow patterns, and this impacted the remedial performance of the IAS system. Under high ambient groundwater velocity, an air sparging system that uses a single injection well caused the detour of contaminant plumes around injection wells, regardless of air injection rates, and failed to meet the remedial goal specified here. This system was successful for relatively low ambient groundwater velocity environments used here. An IAS system with multiple injection wells was effective in capturing and remediating the detoured contaminant plume, and showed superior performance when compared to a single injection well IAS system. Using IAS simulation, we also analyzed the impact of injection rates on site remediation using single or multiple wells. Design criteria that are based on the results of this study would be useful in enhancing the performance of the IAS systems.     

Influence of Microbial Growth on Hydraulic Properties of Pore Networks

From laboratory experiments it is known that bacterial biomass is able to influence the hydraulic properties of saturated porous media, an effect called bioclogging. To interpret the observations of these experiments and to predict possible bioclogging effects on the field scale it is necessary to use transport models, which are able to include bioclogging. For these models it is necessary to know the relation between the amount of biomass and the hydraulic conductivity of the porous medium. Usually these relations were determined using bundles of parallel pore channels and do not account for interconnections between the pores in more than one dimension. The present study uses two-dimensional pore network models to study the effects of bioclogging on the pore scale. Numerical simulations were done for two different scenarios of the growth of biomass in the pores. Scenario 1 assumes microbial growth in discrete colonies clogging particular pores completely. Scenario 2 assumes microbial growth as a biofilm growing on the wall of each pore. In both scenarios the hydraulic conductivity was reduced by at least two orders of magnitude, but for the colony scenario much less biomass was needed to get a maximal clogging effect and a better agreement with previously published experimental data could be found. For both scenarios it was shown that heterogeneous pore networks could be clogged with less biomass than more homogeneous ones.     

Efficient Algorithms for Modeling the Transport and Biodegradation of Chlorinated Ethenes in Groundwater

Predicting the fate of chlorinated ethenes in groundwater requires the solution of equations that describe both the transport and the biodegradation of the contaminants. Here, we present a model that accounts for (1) transport of chlorinated ethenes in flowing groundwater, (2) mass transfer of contaminants between mobile groundwater and stationary biofilms, and (3) diffusion and biodegradation within the biofilms. Equations for biodegradation kinetics account for biomass growth within the biofilms, the effect of hydrogen on dechlorination, and competitive inhibition between vinyl chloride and cis–dichloroethene. The overall model consists of coupled, non-linear, partial differential equations; solution of such a model is challenging and requires innovative numerical algorithms. We developed and tested two new numerical algorithms to solve the equations in the model; these are called system splitting with operator splitting (SSOS) and system splitting with Picard iteration (SSPI). We discuss the conditions under which one of these algorithms is superior to the other. The contributions of this paper are as follows: first, we believe that the mathematical model presented here is the first transport model that also accounts for diffusion and non-linear biodegradation of chlorinated ethenes in biofilms; second, the SSOS and SSPI are new computational algorithms developed specifically for problems of transport, mass transfer, and non-linear reaction; third, we have identified which of the two new algorithms is computationally more efficient for the case of chlorinated ethenes; and finally, we applied the model to compare the biodegradation behavior under diffusion-limited, metabolism-limited, and hydrogen-limited (donor-limited) conditions.     

Calculation of flow-induced residual stresses in injection moulded products

Flow-induced residual stresses that arise during the injection moulding of amorphous thermoplastic polymers are calculated in both the filling and post-filling stage. To achieve this a compressible version of the Leonov model is employed. Two techniques are investigated. First a direct approach is used where the pressure problem is formulated using the viscoelastic material model. Secondly, generalized Newtonian material behaviour is assumed, and the resulting flow kinematics is used to calculate normal stresses employing the compressible Leonov model. The latter technique gives comparable results, while reducing the computational cost significantly.     

Modeling Biogeochemical Processes in Leachate-Contaminated Soils: A Review

During subsurface transport, reactive solutes are subject to a variety of hydrological, physical and biochemical processes. The major hydrological and physical processes include advection, diffusion and hydrodynamic dispersion, and key biochemical processes are aqueous complexation, precipitation/dissolution, adsorption/desorption, microbial reactions, and redox transformations. The addition of strongly reduced landfill leachate to an aquifer may lead to the development of different redox environments depending on factors such as the redox capacities and reactivities of the reduced and oxidised compounds in the leachate and the aquifer. The prevailing redox environment is key to understanding the fate of pollutants in the aquifer. The local hydrogeologic conditions such as hydraulic conductivity, ion exchange capacity, and buffering capacity of the soil are also important in assessing the potential for groundwater pollution. Attenuating processes such as bacterial growth and metal precipitation, which alter soil characteristics, must be considered to correctly assess environmental impact. A multicomponent reactive solute transport model coupled to kinetic biodegradation and precipitation/dissolution model, and geochemical equilibrium model can be used to assess the impact of contaminants leaking from landfills on groundwater quality. The fluid flow model can also be coupled to the transport model to simulate the clogging of soils using a relationship between permeability and change in soil porosity. This paper discusses the different biogeochemical processes occurring in leachate-contaminated soils and the modeling of the transport and fate of organic and inorganic contaminants under such conditions.     

Visualization of DNAPL Fingering Processes and Mechanisms in Water-Saturated Porous Media

Groundwater contamination by dense nonaqueous phase liquids (DNAPLs) has received increasing attention in the last decade. The fingering process of DNAPL migration in porous media remains an incompletely understood subject. The main reason is that natural porous media are opaque and hence very few visualizations are available. This paper presents the visual results of two-dimensional experiments in a glass tank in which DNAPL penetrated into water-saturated homogeneous porous media. The results provide a clear reference for conceptual models of DNAPL finger development due to immiscible flow instabilities. The fingering process can be divided into two stages, that is, the finger initiation stage and the finger elongation stage. At the finger initiation stage, many DNAPL protuberances appear at the interface between tetrachloroethylene (PCE) and water along the surface of the porous media. During the finger elongation stage, some relatively larger protuberances develop into primary fingers. Secondary fingers may develop on the existing primary fingers. The fingers grew downwardly in a winding manner with the mechanisms of shielding, tip-growing, splitting, and coalescing. The fingers grew linearly with time and had simi-lar growth rate at the finger elongation stage. The average wavelength (space between fingers) of the primary fingers was 0.051m and the average PCE content in the region with fingers was 2.5% (7.0% saturation).     

疲劳裂纹扩展的随机模型

本文从J.L.Bogdanoff提出的一种新的疲劳裂纹扩展模型出发，假定在裂纹扩展过程中存在不同的等概率均匀扩展区间，采用Z变换和其复数反演理论，推导出疲劳裂纹经任意次载荷循环后的失效概率计算的解析公式，计算结果与实验值符合较好、偏保守。