Contaminant Leaching Model for Dredged Material Disposal Facilities

This paper describes the hydrologic evaluation of leachate production and quality model, a screening-level tool to simulate contaminant leaching from a confined disposal facility (CDF) for dredged material. The model combines hydraulics, hydrology, and equilibrium partitioning, using site-specific design specifications, weather data, and equilibrium partitioning coefficients from the literature or from sequential batch or column leach tests of dredged material. The hydraulics and hydrology are modeled using Version 3 of the hydrologic evaluation of landfill performance model. The equilibrium partitioning model includes provisions for estuarine sediments that have variable distribution coefficients resulting from saltwater washout. Model output includes contaminant concentrations in the CDF profile, contaminant concentration and mass releases through the bottom of the CDF, and contaminant concentrations and masses captured by leachate collection systems. The purpose of the model is to provide sound information for evaluating the potential leachate impacts on ground water at dredged material CDFs and the effectiveness of leachate control measures.     

Numerical Evaluation of Landfill Stabilization by Leachate Circulation

This study compares various leachate management scenarios using a biologically reactive transport model, which is proposed in this study. The proposed model can be used to predict the contribution of biodegradation to contaminant attenuation and contaminant concentration in leachate over time. It can also be used to assess the extent of landfill stabilization in terms of local mass per bulk volume of remaining refuse available for transfer. A sensitivity analysis shows that landfill stabilization has significant sensitivity to most biokinetic parameters, the fluid-phase saturation constant, and the dissolution rate, in addition to the half-saturation constant and the retardation factor. The proposed model is applied to assess landfill stabilization under two control scenarios: leachate recycling versus continued input of clean water with no recirculation. The simulation results indicate that leachate recirculation provides more favorable conditions for development of an active anaerobic bacterial population and, hence, accelerates landfill stabilization.     

Numerical Simulation of Transport of Four Heavy Metals in Kaolinite Clay

A multicomponent reactive solute-transport model was used to study the migration of dissolved heavy metals (Cd2+, Pb2+, Cu2+, and Zn2+) in a clay barrier subject to two leachates having different pH values. This solute-transport model is capable of simulating simultaneous processes of water flow, advective-dispersive-solute transport, and chemical reactions. The migration of these metals was simulated in a kaolinite landfill liner, which was assigned realistic physical and chemical properties and boundary conditions to model one-dimensional contaminant transport. The leachate input properties to the model were those of an actual leachate containing the four heavy metals. The numerical simulations were focused on the concentration profiles of these metals in the simulated clay barrier and leachate pH affects their mobilities. The numerical results indicate that with a nearly neutral leachate, the heavy metals mobility follows: Cd2+ < Pb2+ < Cu2+ < Zn2+. With an acidic leachate, the order changes to Pb2+ < Cu2+ < Zn2+ < Cd2+. Leachate pH has a significant effect on Cd2+ and Pb2+ mobility and a small effect on Cu2+ and Zn2+.     

Model of Leaching Behavior from Fly Ash Landfills with Different Age Refuses

A mathematical model that accounts for simultaneous moisture movement and contaminant transport is developed to predict the leaching behavior from fly ash landfills with different age refuses. The approach is based on dividing the refuse layer into separate regions for each age refuse and solving model equations for each region. With the exception of boron in leachate of coal fly ash, good agreement between model simulation and experimental data of laboratory column-leaching tests was obtained. However, the simulated boron concentration profile can be improved by the addition of a retardation factor in the model equation. The important parameters were evaluated in a sensitivity analysis to see their respective effects. Four parameters were identified to influence strongly the model performance: dissolution rate constant (kn), exponential constant (n), ultimate mass of leachable contaminant (M0), and maximum contaminant concentration (Cmax).     

Analytical Solutions for Contaminant Diffusion in Double-Layered Porous Media

Analytical solutions for conservative solute diffusion in one-dimensional double-layered porous media are presented in this paper. These solutions are applicable to various combinations of fixed solute concentration and zero-flux boundary conditions (BC) applied at each end of a finite one-dimensional domain and can consider arbitrary initial solute concentration distributions throughout the media. Several analytical solutions based on several initial and BCs are presented based on typical contaminant transport problems found in geoenvironmental engineering including (1) leachate diffusion in a compacted clay liner (CCL) and an underlying stratum; (2) contaminant removal from soil layers; and (3) contaminant diffusion in a capping layer and underlying contaminated sediments. The analytical solutions are verified against numerical solutions from a finite-element method based model. Problems related to leachate transport in a CCL and an underlying stratum of a landfill and contaminant transport through a capping layer over contaminated sediments are then investigated, and the suitable definition of the average degree of diffusion is considered.     

Numerical Modeling of Contaminant Transport through Soils: Case Study

This paper presents the development and validation of a numerical model for simulation of the flow of water and air and contaminant transport through unsaturated soils. The governing differential equations include two mass balance equations for the water phase and air phase together with a balance equation for contaminant transport through the two phases. In the model the nonlinear system of the governing differential equations was solved using a finite-element method in the space domain and a finite difference scheme in the time domain. The governing equation of the miscible contaminant transport including advection, dispersion-diffusion and adsorption effects are presented. The mathematical framework and the numerical implementation of the model are described in detail. The model is validated by application to standard experiments on contaminant transport in unsaturated soils. The application of the model to a case study is then presented and discussed. Finally, the merits and limitations of the model are highlighted.     

Biogeochemical Evaluation of Mechanisms Controlling CaCO3(s) Precipitation in Landfill Leachate-Collection Systems

A common failure mode for landfills is clogging of the leachate-collection system. The reduction in hydraulic conductivity associated with clogging causes a buildup of leachate head on the underlying liner, potentially increasing advective contaminant transport from the landfill and contaminating adjacent groundwater. In this paper, the biogeochemical model CCBATCH is used to link a primary cause of leachate collection system failure—CaCO3(s) precipitation?to anaerobic degradation of volatile fatty acids (VFAs) in column reactors used to study the clogging phenomena. One key to applying CCBATCH correctly was dividing the VFA conversion into two steps: conversion of propionate to acetate, carbonic acid, and methane; and acetate conversion to methane and carbonic acid. The primary driver for CaCO3(s) precipitation in the columns was acetate fermentation to CH4 and H2CO3, which increased the total carbonate concentration in the leachate and shifted the acid/base control to a weaker acid system, which caused an increase in solution pH. A second key to proper modeling was adding CO2(g) gas transfer to CCBATCH. The modeling results indicate that the kinetics of CO2(g) gas transfer was a key control over leachate chemistry once acetate fermentation was nearly complete. These results suggest that the best approach for the long-term control of CaCO3(s) clogging may be to enhance CO2(g) gas transfer from the leachate while buffering the leachate pH to near neutral. Taken together, these actions should decrease the yield of CaCO3(s) precipitated per mass of acetate removed.     

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A mathematical model for the development of methane production from a landfill bioreactor (LFBR) treating the organic fraction of municipal solid wastes was developed from the Gompertz equation. The model incorporates three biokinetic parameters: methane production lag phase time, rate, and potential. The methane converting capacity test experiment was conducted to monitor the specific methane production rate consuming anaerobic fermentative intermediates, including carbohydrates, proteins, and lipids. The model developed in this study can be used to predict methane production based on the chemical nature and the decomposition characteristics of the organic fraction of municipal solid wastes. The simulative results indicate that the leachate recycle for the LFBR resulted in a more rapid methane production from the consumption of the carbohydrate but in less rapid production from that of the protein and lipid. Moreover, the same specific methane production rate of 2.6 mL∕g volatile solid (VS) per day occurred at the LFBR with∕without leachate recycle; however, a sharp drop in methane production lag phase time, from 125 to 25 days, was obtained at the LFBR incubated with leachate recycle.     

Leaching Behavior of Metals Released from Cement-Stabilized/Solidified Refinery Oily Sludge by Means of Sequential Toxicity Characteristic Leaching Procedure

In this study, stabilization/solidification (S/S) of refinery oily sludge was applied using two types of cement (I and II), in cement-to-waste ratio from 0.1 to 0.7. The leaching behavior of heavy metals was investigated, by means of a five-point sequential toxicity characteristic leaching procedure (TCLP) test. Sequential TCLP was used to provide an improved assessment of long-term contaminant potential leachability, because the acidic leachant is renewed, whereas in the single TCLP, contaminant leachability is limited by the pH neutralization of the alkaline binder. Cement-based S/S of real refinery oily sludge resulted in very low leachability of heavy metals. Pb and Cd were not detected in any TCLP leachate. The maximum leachability of Fe, Zn, and Ni occurred in the pH range between 5.5 and 6.5. The leachability of Cu and Cr increased with increasing pH. Maximum cumulative percentage of Fe, Ni, and Zn leaching after five consecutive TCLP extractions (for worst case conditions, i.e., 10% II42.5) were 0.01, 22, and 1.2%, respectively, on the basis of metal content of each solidified sample. Further, the leaching behavior of Zn and Ni was modeled using the chemical equilibrium program Visual MINTEQ. Using several combinations of suspected solid phases of Zn and Ni hydroxides, carbonates and sulfides, and surface complexation onto ferrihydrite the diffuse double layer model did not accurately describe the leaching behavior of Zn and Ni.     

Plant-Enhanced Subsurface Bioremediation of Nonvolatile Hydrocarbons

In recent years, phytoremediation, i.e., the use of plants to clean up soils contaminated with organics, has become a promising new area of research, particularly for in-situ cleanup of large volumes of slightly contaminated soils. A model that can be used as a predictive tool in phytoremediation operations was developed to simulate the transport and fate of a residual hydrocarbon contaminant interacting with plant roots in a partially saturated soil. Time-specific distribution of root quantity through soil, as well as root uptake of soil water and hydrocarbon, was incorporated into the model. In addition, the microbial activity in the soil rhizosphere was modeled with a biofilm theory. A sandy loam, which is dominant in soils of agricultural importance, was selected for simulations. Cotton, which has well-documented plant properties, was used as the model plant. Model parameters involving root growth and root distribution were obtained from the actual field data reported in the literature and ranges of reported literature values were used to obtain a realistic simulation of a phytoremediation operation. Following the verification of the root growth model with published experimental data, it has been demonstrated that plant characteristics such as the root radius are more dominant than contaminant properties in the overall rate of phytoremediation operation. The simulation results showed enhanced biodegradation of a hydrocarbon contaminant mostly because of increased biofilm metabolism of organic contaminants in a growing root system of cotton. Simulations also show that a high mean daily root-water uptake rate increases the contaminant retardation factors because of the resulting low water content. The ability to simulate the fate of a hydrocarbon contaminant is essential in designing technically efficient and cost-effective, plant-aided remedial strategies and in evaluating the effectiveness of a proposed phytoremediation scheme. The model presented can provide an insight into the selection and optimization of a specific strategy.     

Exposure assessment model for odor causing VOCs volatilization from stored pig slurry

A mathematical model taking into account source depletion with time and the actual thickness of manure layer was derived to evaluate indoor inhalation exposure dynamics to three selected odor causing volatile organic compounds (VOC-odors) of p-cresol, toluene, and xylene volatilization from stored pig slurry. The model assumes that pig slurry is undisturbed and the VOC-odors released in a contaminated layer and transported through a clean layer as well as a manure-air interface boundary. The model simulates time-dependent volatilization, the depletion of source contaminant via both volatilization and degradation, and could be used with a contaminated zone of finite thickness. For a given VOC-odor, the predicted total exposure and resulting manure cleanup criteria can be a large variability depending on the model whether a finite or infinite manure layer thickness was considered. Results obtained from model comparisons suggest that the model incorporating depth-varying of manure layer and contaminant source depletion is more suitable to evaluate the VOC-odor exposure dynamics in swine housing bioclimate.     

Integrated Two-Dimensional Surface and Three-Dimensional Subsurface Contaminant Transport Model Considering Soil Erosion and Sorption

To investigate the complex hydrological, morphodynamic, and environmental processes in watersheds, a physically-based integrated two-dimensional (2D) surface and three-dimensional (3D) subsurface model for flow, soil erosion and transport, and contaminant transport in the surface-subsurface system is presented in this paper. The model simulates the rainfall-induced surface flow by solving the depth-averaged 2D diffusion wave equation and the variably-saturated subsurface flow by solving the 3D mixed-form Richards equation. The surface and subsurface flow equations are coupled using the continuity conditions of pressure and exchange flux at the ground surface. The model uses the concept of nonequilibrium in the depth-averaged 2D simulation of nonuniform total-load sediment transport in upland fields, considering detachments by rainsplash and hydraulic erosion driven by surface flow. The integrated 2D surface and 3D subsurface contaminant transport model takes into account the contaminant changes due to sediment sorption and desorption, as well as exchanges between surface and subsurface domains due to infiltration, diffusion, and bed change. The model applies the same set of surface equations of flow, sediment, and contaminant transport for describing both upland areas and streams, so that no special treatments are required at their interface. The established model has been evaluated by comparisons with published experimental, numerical, and analytical data and then applied in an agricultural watershed. The model is suitable for wetland areas and agricultural watersheds in which streams are not very narrow and deep, and meanwhile a relatively fine mesh that can distinguish the streams is preferred.     

Adjoint Sensitivity Analysis of Contaminant Concentrations in Water Distribution Systems

Sensitivity analysis is used to determine how a system state or a model output changes due to a change in the value of a system parameter or a model input. We present the adjoint approach for determining the sensitivity of the concentration of a contaminant in a water distribution system to a change in a system parameter such as the location of the source of contamination, the reaction rate of the contaminant, and others. With the adjoint method, the sensitivity of the model output to any number of parameters can be obtained with one simulation of the adjoint model. If the number of parameters of interest exceeds the number of model outputs for which the sensitivity is desired, the adjoint method is more efficient than traditional direct methods of calculating sensitivities. We develop the adjoint equations for water quality in a water distribution system, verify the adjoint-based sensitivity equation using an analytical example, and demonstrate the numerical calculation of adjoint sensitivities using EPANET.     

Comparison of Solute Transport in Three Composite Liners

Three composite landfill liners were compared in this study based on leakage rate, mass flux, and sorptive capacity. One composite liner consisted of a geomembrane and a geosynthetic clay liner (GCL). The other two had a geomembrane and a thicker soil barrier (61 or 122 cm). The analyses employed one- and three-dimensional numerical models that were developed for analyzing contaminant transport through defects in the geomembrane component of composite liners and diffusion of volatile organic compounds through intact composite liners (i.e., composite liners without holes in the geomembrane). Cadmium was used to represent inorganic leachate constituents and toluene was used to represent organic leachate constituents. The composite liner, having a GCL had the lowest leakage rate of the three composite liners. For cadmium, the mass flow rate and sorptive capacity for the three composite liners varied within an order of magnitude. However, for toluene, the mass flux from the GCL composite liner was two to three orders of magnitude greater than that through composite liners having a thicker soil liner. Additionally, for leachate having similar concentrations of cadmium and toluene, the mass flux of toluene can be as much as seven orders of magnitude greater than that for cadmium. For toluene, the sorptive capacity of thicker liners was an order of magnitude greater than that for the GCL composite liner. Similar behavior is expected for other inorganic and organic solutes.     

Biologically Reactive Multispecies Transport in Sanitary Landfill

This study proposes a numerical model for gas- and water-phase solute transport describing the change in leachate quality and gas production by microbial activities in a sanitary landfill. The proposed model includes gas and water flows, multispecies transport of gas and water phases, interphase mass transfer of solids and water-phase solutes, microbial growth and death, and aerobic and anaerobic biodegradation. In simulations of gas-phase flow and transport, the influence of gas temperature and composition on flow and transport phenomena area incorporated by changing the gas viscosity and diffusivity. Simulation results using the proposed model are very close to the results of the BIOF&T model and analytical solutions on test problems. Finally, the model is applied to simulate the leachate quality and three gas compositions of a waste decomposition experiment using a lysimeter. The results of the simulation explain the dynamic fluctuation of gas composition with time and match the leachate quality and three gas compositions measured in a limited time span.     

Two-Dimensional Simulation Model for Contour Basin Layouts in Southeast Australia. I: Rectangular Basins

Contour basin irrigation layouts are used to irrigate rice and other cereal crops on heavy cracking soils in Southeast Australia. In this study, a physically based two-dimensional simulation model that incorporates all the features of contour basin irrigation systems is developed. The model’s governing equations are based on a zero-inertia approximation to the two-dimensional shallow water equations of motion. The equations of motion are transformed into a single nonlinear advection–diffusion equation in which the friction force is described by Manning’s formula. The empirical Kostiakov equation and the quasi-analytical Parlange equation are used to model the infiltration process. The governing equations are solved by using a split-operator approach. The numerical procedure described here is capable of modeling rectangular basins; a procedure for irregular shaped basins is presented in Paper II. The model was validated against field data collected on commercial lasered contour layouts.     

Analytical Models for the Design of Iron-Based Permeable Reactive Barriers

The preliminary design of iron-based permeable reactive barriers is often accomplished using analytic expressions for one-dimensional groundwater flow and contaminant transport. Typically, one or more of the governing processes is simplified or neglected to facilitate development of a tractable solution. This paper presents a set of improved design equations that include the effects of dispersion, finite domain boundary, sequential decay, and production processes, and increased flow through high conductivity barriers. When applied to realistic example problems, application of the expanded design equations typically results in the specification of a larger permeable reactive barrier thickness than obtained using conventional approaches.     

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Biofilm Growth and Mineral Precipitation in Synthetic Leachate Columns

The results of a numerical model developed to describe the clogging of the leachate collection systems are compared with data from column experiments conducted with synthetic landfill leachate. A biofilm kinetic model is used to calculate the utilization of organic substrates (acetate and propionate) and growth of active clog material, and the substrate removal is then linked to mineral precipitation. Calculated changes in the chemical oxygen demand and Ca2+ of the effluent leachate and the porosity profiles along the length of the column show encouraging agreement with measured values. Means to obtain reasonable parameter values are presented. The favorable outcome represents completion of an essential first step toward being able to predict what controls clogging in landfill leachate collection systems.     

Interaction of Soil Air Permeability and Soil Vapor Extraction

A new theoretical model to analyze the measurements obtained from a typical soil column venting experiment is proposed. The principles of mass transfer, Darcy's law, and air compressibility in the form of pressure-volume relationships were coupled to calculate the contaminant concentration in the gas phase (air), and the rate of contaminant removal. The proposed model relates soil air permeability with the contaminant removal and is capable of calculating the variation of soil air permeability with time during the venting process. The contaminated sand sample was idealized as a system of straight capillary tubes in the direction of flow, lined by the liquid contaminant. A closed-form solution for radial diffusion of the contaminants in a cylinder, coupled with axial advection of air, was used to model contaminant removal. The results from the mass transfer model were then used to trace the change of soil air permeability with time. The model also uses, as an alternative approach, a modified form of Darcy's law for compressible flow.