A new analytical model for transport of multiple contaminants considering remediation of both NAPL source and downgradient contaminant plume in groundwater

The contamination of groundwater aquifers by chlorinated solvents is a water-resource issue of great importance worldwide. Such contaminants are difficult to treat because they are often released as dense non-aqueous phase liquids (DNAPLs). Research shows that the application of remediation technologies to both the NAPL source and dissolved plume can lead to more efficient remediation, rather than to either alone. In these remediation efforts, analytical models that evaluate behavior and fate of contaminants do provide a better understanding of the performance of these remedial technologies. To the best of our knowledge, there exist no analytical model of simulating the plume migration of multiple contaminants with capabilities of accounting for both NAPL source and plume remediation simultaneously and different retardation for original chlorinated solvent contaminant and its degradation byproducts. In this study, we present a new analytical model for remediating both NAPL source and downgradient contaminant plume in groundwater at sites contaminated with chlorinated solvents and their degradation products with different retardation factors as well as considering both NAPL source and plume remediation simultaneously. A source model that accounts for the depletion of mass by the processes of dissolution or first-order decay reactions, corresponding with the removal or destruction of the source mass, is coupled to a plume reactive transport model. The source model is accounted for by relating source mass to the flux-averaged source discharge concentration through a power function. The developed analytical model considers 1-D advection, 3-D dispersion, first-order decay reactions and ingrowth as well as linear isothermal equilibrium sorption. The proposed analytical solution was derived through successive application of the Laplace transform in time and the double finite Fourier cosine transform regarding y and z. The correctness of the analytical model and its auxiliary FORTRAN computer program code are proved by showing excellent agreements between the simulated plume concentrations of all contaminants obtained from the derived analytical model and from a semi-analytical model available in the literature. Application of the proposed analytical solutions illustrates that the use of identical retardation factors for all contaminants may lead to underestimation or overestimation of the mobility of the contaminants, in cases when the retardation factors of the individual contaminants are greatly different from the identical retardation factor value adopted in all contaminants. From the experiments on six scenarios corresponding six remedial treatments, we found out that both the enhanced source decay and partial removal of source mass are main controlling factors at reducing the concentrations of all the contaminants, whereas plume decay leads to effective reduction in the concentrations of PCE, however, rather it causes unfavorable increases of the concentrations of the degradation byproducts. Ultimately, the developed model is used to better understand the impacts of various possible combinations of remedial efforts and management decisions on remediation of the subsurface contamination and quantify the benefit of a certain remediation decision.補正完畢GB

Multiphase Flow Modeling With General Boundary Conditions And Automatic Phase-Configuration Changes Using A Fractional-Flow Approach

The multiphase flow simulator moving particle semi-implicit (MPS) method is developed based on the fractional-flow approach, originated in the petroleum engineering literature, considering the fully three-phase flow with general boundary conditions. The fractional flow approach employs water saturation, total liquid saturation, and total pressure as primary variables. Most existing models based upon fractional flow are limited to two-phase flow and specific boundary conditions. Although there appear a number of three-phase flow models, they were mostly developed using pressure-based approaches, which require variable-switch techniques to deal with phase appearance and disappearance. The use of fractional flow-based approaches in MPS makes it unnecessary to use variable-switching to handle the change of phase configurations because the water saturation, total liquid saturation, and total pressure exist throughout the solution domain regardless of whether certain phases are present or not. Furthermore, most existing fractional flow-based models consider only specific boundary conditions, usually Dirichlet-type pressure for water phase and flux-type boundary for nonaqueous phase liquid or particular combinations for individual phase. However, the present model considers general boundary conditions of ten most possible and plausible cases. The first eight cases are the combinations of the phase pressure or the phase flux of each of the three individual phases. The other two cases are the variable boundary conditions: one for water-medium interface and the other for the air-medium interface when the directions of fluxes are not known a priori. Thus, the model\u27s capabilities of handling general boundary conditions extend the simulators\u27 usefulness in the field system. © Springer Science+Business Media B.V. 2008

3D, Three-Phase Flow Simulations Using The Lagrangian-Eulerian Approach With Adaptively Zooming And Peak/Valley Capturing Scheme

A fully three-dimensional (3D) multiphase flow model (3DMPS) is developed to simulate the migration of three phases (water, non-aqueous phase liquid and gas) using a fractional flow formulation for the governing equations. This model can incorporate general boundary and initial conditions and automatic phase appearance and disappearance. Numerically, the Lagrangian-Eulerian decoupling method with an adaptive zooming and peak/valley capturing scheme (LEZOOMPC) algorithm is employed to solve multiphase flow problems. A total of seven examples are given in this paper. First, verification is performed against an analytical solution in one case and against other numerical models in another. Second, two examples were used to demonstrate the ability of the model to treat general boundary conditions. Third, the comparison of CPU time in one example illustrated that the efficiency of the LEZOOMPC algorithm is superior when compared to traditional upstream finite-element methods. Finally, two examples are presented to show the applicability of 3DMPS to real 3D problems. © 2007 ASCE

Multidimensional Finite-Element Particle Tracking Method For Solving Complex Transient Flow Problems

This paper proposes a particle tracking technique for applying the Eulerian-Lagrangian localized adjoint method or the Eulerian-Lagrangian method to solve transport equations in a rapidly changing unsteady flow. The proposed particle tracking technique uses a linear temporal interpolation of velocity to avoid the error introduced when using a stepwise temporal approximation of the velocity, which has been adopted in many existing particle tracking techniques. Numerical experiments were carried out on several example problems to demonstrate the superior accuracy and efficiency of the proposed particle tracking technique over Cheng\u27s and Pollock\u27s methods. The numerical results show that the use of a linear temporal interpolation of velocity can allow an increase in time step size without causing a significant decrease in accuracy, particularly under a complicated transient flow or a rapidly changing velocity field with time. © 2009 ASCE

Multiphase Flow Simulation In Fractional Flow Approach With General Boundary Condition Considering Phase Configuration Change In The System

The multiphase flow simulator, MPS, is developed based on the fractional flow approach originated in the petroleum engineering literature considering the fully three phase flow with general boundary condition. The fractional flow approach employs water saturation, total liquid saturation, and total pressure as primary variables. Most existing fractional flow-based models are limited to two-phase flow and specific boundary conditions. Although there appears a number of three-phase flow models, they were mostly developed using pressures based approaches. As a result, these models require cumbersome variable-switch techniques to deal with phase appearance and disappearance. use use of fractional flow-based approaches in MPS makes it unnecessary to use variable switches to handle the change of phase configurations, because the water saturation, total liquid saturation, and total pressure exist throughout the solution domain regardless of whether certain phases are present or not. Also most existing fractional flow-based models consider only specific boundary conditions, which are usually Dirichlet type pressure for water phase, and flux type boundary for NAPL phase or particular combinations for individual phase. The present model considers general boundary conditions of most possible and plausible cases that consist of eight cases. These are the combinations of the phase pressure or phase flux of each of the three individual phases. Thus, the model\u27s capabilities of handling general boundary conditions extend the simulators\u27 usefulness in the field system. © 2002 Elsevier B.V. All rights reserved

Application of a Developed Numerical Model for Surfactant Flushing Combined with Intermittent Air Injection at Field Scale

Surfactant flushing with intermittent air injection, referred to as enhanced flushing, has been proposed at a site in Korea contaminated by military activity to overcome the difficulty of treatment caused by a layered geological structure. In this study, we developed a simple numerical model for exploring the effects of various physical and chemical processes associated with enhanced flushing on pollutant removal efficiency and applied it in a field-scale test. This simple numerical model considers only enhanced hydraulic conductivity rather than all of the interacting parameters associated with the complex chemical and physical processes related to air and surfactant behavior during enhanced flushing treatment. In the numerical experiment, the removal efficiency of residual non-aqueous phase liquid (NAPL) was approximately 12% greater with enhanced, rather than conventional, flushing because the hydraulic conductivity of the low-permeability layer was enhanced 5-fold, thus accelerating surfactant transport in the low-permeability layer and facilitating enhanced dissolution of residual NAPL. To test whether the enhanced flushing method is superior to conventional flushing, as observed in the field-scale test, successive soil flushing operations were simulated using the newly developed model, and the results were compared to field data. Overall, the simulation results aligned well with the field data

Generalized Analytical Solutions of The Advection-Dispersion Equation with Variable Flow and Transport Coefficients

Demand has increased for analytical solutions to determine the velocities and dispersion coefficients that describe solute transport with spatial, temporal, or spatiotemporal variations encountered in the field. However, few analytical solutions have considered spatially, temporally, or spatiotemporally dependent dispersion coefficients and velocities. The proposed solutions consider eight cases of dispersion coefficients and velocities: both spatially dependent, both spatiotemporally dependent, both temporally dependent, spatiotemporally dependent dispersion coefficient with spatially dependent velocity, temporally dependent dispersion coefficient with constant velocity, both constant, spatially dependent dispersion coefficient with spatiotemporally dependent velocity, and constant dispersion coefficient with temporally dependent velocity. The spatial dependence is linear, while the temporal dependence may be exponential, asymptotical, or sinusoidal. An advection–dispersion equation with these variable coefficients was reduced to a non-homogeneous diffusion equation using the pertinent coordinate transform method. Then, solutions were obtained in an infinite medium using Green’s function. The proposed analytical solutions were validated against existing analytical solutions or against numerical solutions when analytical solutions were unavailable. In this study, we showed that the proposed analytical solutions could be applied for various spatiotemporal patterns of both velocity and the dispersion coefficient, shedding light on feasibility of the proposed solution under highly transient flow in heterogeneous porous medium

A Machine Learning Approach for Spatial Mapping of the Health Risk Associated with Arsenic-Contaminated Groundwater in Taiwan’s Lanyang Plain

Groundwater resources are abundant and widely used in Taiwan’s Lanyang Plain. However, in some places the groundwater arsenic (As) concentrations far exceed the World Health Organization’s standards for drinking water quality. Measurements of the As concentrations in groundwater show considerable spatial variability, which means that the associated risk to human health would also vary from region to region. This study aims to adapt a back-propagation neural network (BPNN) method to carry out more reliable spatial mapping of the As concentrations in the groundwater for comparison with the geostatistical ordinary kriging (OK) method results. Cross validation is performed to evaluate the prediction performance by dividing the As monitoring data into three sets. The cross-validation results show that the average determination coefficients (R2) for the As concentrations obtained with BPNN and OK are 0.55 and 0.49, whereas the average root mean square errors (RMSE) are 0.49 and 0.54, respectively. Given the better prediction performance of the BPNN, it is recommended as a more reliable tool for the spatial mapping of the groundwater As concentration. Subsequently, the As concentrations estimated obtained using the BPNN are applied to develop a spatial map illustrating the risk to human health associated with the ingestion of As-containing groundwater based on the noncarcinogenic hazard quotient (HQ) and carcinogenic target risk (TR) standards established by the U.S. Environmental Protection Agency. Such maps can be used to demarcate the areas where residents are at higher risk due to the ingestion of As-containing groundwater, and prioritize the areas where more intensive monitoring of groundwater quality is required. The spatial mapping of As concentrations from the BPNN was also used to demarcate the regions where the groundwater is suitable for farmland and fishponds based on the water quality standards for As for irrigation and aquaculture