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Published online 1 March 2008
Published in J Environ Qual 37:459-468 (2008)
DOI: 10.2134/jeq2007.0250
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Transport and Reaction Processes Affecting the Attenuation of Landfill Gas in Cover Soils

S. Molinsa,*, K. U. Mayera, C. Scheutzb and P. Kjeldsenb

a Univ. of British Columbia, Dep. of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, BC, V6T 1Z4, Canada
b Technical Univ. of Denmark, DTU Dep. of Environment & Resources, Building 115, DK-2800 Kgs. Lyngby, Denmark

* Corresponding author (smolins{at}eos.ubc.ca).

Received for publication May 17, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Methods
 Results and Discussion
 Summary
 REFERENCES
 
Methane and trace organic gases produced in landfill waste are partly oxidized in the top 40 cm of landfill cover soils under aerobic conditions. The balance between the oxidation of landfill gases and the ingress of atmospheric oxygen into the soil cover determines the attenuation of emissions of methane, chlorofluorocarbons, and hydrochlorofluorocarbons to the atmosphere. This study was conducted to investigate the effect of oxidation reactions on the overall gas transport regime and to evaluate the contributions of various gas transport processes on methane attenuation in landfill cover soils. For this purpose, a reactive transport model that includes advection and the Dusty Gas Model for simulation of multicomponent gas diffusion was used. The simulations are constrained by data from a series of counter-gradient laboratory experiments. Diffusion typically accounts for over 99% of methane emission to the atmosphere. Oxygen supply into the soil column is driven exclusively by diffusion, whereas advection outward offsets part of the diffusive contribution. In the reaction zone, methane consumption reduces the pressure gradient, further decreasing the significance of advection near the top of the column. Simulations suggest that production of water or accumulation of exopolymeric substances due to microbially mediated methane oxidation can significantly reduce diffusive fluxes. Assuming a constant rate of methane production within a landfill, reduction of the diffusive transport properties, primarily due to exopolymeric substance production, may result in reduced methane attenuation due to limited O2–ingress.

Abbreviations: DGM, Dusty Gas Model • EPS, exopolymeric substances • LFG, landfill gas • CFC, chlorofluorocarbons • HCFC, hydrochlorofluorocarbons • CFC-11, trichlorofluoromethane • CFC-12, dichlorodifluoromethane • HCFC-21, chlorodifluoromethane • HCFC-22, chlorofluoromethane


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Methods
 Results and Discussion
 Summary
 REFERENCES
 
THE anaerobic degradation of organic waste in municipal landfills results in the production of a gas mixture that contains methane (55–60 vol %), carbon dioxide (40–45 vol %), and numerous trace compounds, such as halogenated and aromatic hydrocarbons and sulfur- and oxygen-containing aromatic compounds (Scheutz and Kjeldsen, 2003; Scheutz and Kjeldsen, 2005). Methane is an important greenhouse gas contributing to approximately 22% of the greenhouse effect (Lelieveld et al., 1998), with landfill sources accounting for 9 to 70 Tg yr–1 out of an estimated annual global emission of 600 Tg of methane to the atmosphere (Bogner et al., 1997; Lelieveld et al., 1998).

Column counter-gradient experiments have shown that landfill cover soils can play an important role in attenuating emissions of methane and other trace compounds to the atmosphere. Attenuation is significantly affected by ingress of atmospheric oxygen into the soil, which generates conditions suitable for aerobic methane oxidation. Methane removal efficiencies of up to 81% have been reported in column experiments (De Visscher et al., 1999; Bogner et al., 1997; Stein and Hettiaratchi, 2001; Scheutz and Kjeldsen, 2003). Column and batch experiments demonstrated the capacity of cover soils to attenuate to different extents trace compounds, such as trichlorofluoromethane (CFC-11), dichlorodifluoromethane (CFC-12), chlorodifluoromethane (HCFC-21), chlorofluoromethane (HCFC-22), tetrachloromethane, trichloromethane, dichloromethane, trichloroethylene, vinyl chloride, benzene, and toluene (Scheutz and Kjeldsen, 2003; Scheutz et al., 2004, Scheutz and Kjeldsen, 2005). Furthermore, compost biofilters have been investigated as an alternative to attenuate methane emissions from low-intensity sources where the use of methane as fuel for energy production is not viable (Park et al., 2002; Wilshusen et al., 2004).

Numerical models of column experiments have provided a tool to investigate transport and oxidation processes occurring in landfill cover soils (Hilger et al., 1999, Stein et al., 2001; De Visscher and Van Cleemput, 2003). Typically, gas transport in landfill cover soils involves a multi-component mixture of gases. Thus, multi-component diffusion models such as the Stefan-Maxwell equations provide the most adequate framework to describe the relevant gas transport mechanisms taking place in landfill cover soils (Hilger et al., 1999; De Visscher and Van Cleemput, 2003). In permeable porous media (e.g., silts, sands, and coarser-grained materials), molecule–molecule interactions dominate, and diffusion occurs in the molecular regime. However, in low-permeability porous media, molecule–soil particle interactions become significant, and the Stefan-Maxwell equations need to be cast in the form of the Dusty Gas Model (DGM) to account for diffusion in the Knudsen regime (Thorstenson and Pollock, 1989). In addition, the DGM includes the non-separative flux component that develops as a result of diffusion of non-equimolar gas mixtures: Lighter molecules move faster than heavier molecules (Thorstenson and Pollock, 1989). The DGM has not been included in previous modeling studies (Hilger et al., 1999; Stein et al., 2001; De Visscher and Van Cleemput, 2003). Commonly, Fick's law has been used to simulate diffusive gas transport in cover soils (Stein et al., 2001; Wilshusen et al., 2004), although its applicability is limited to diffusion of dilute species in relatively permeable material or in advection-dominated systems (Thorstenson and Pollock, 1989; Fen and Abriola, 2004). Wind-induced gas advection can be significant under high soil moisture content (Poulsen and Moldrup, 2006). Other models have also included advection, but the contribution of advection to gas transport in landfill cover soils was not investigated (Stein et al., 2001).

Numerical models can be used to identify the sensitivity of cover soils to physical, chemical, and environmental factors in the attenuation of methane and trace gases. Increased moisture content results in low methane oxidation due to a decrease in oxygen supply into the column (Christophersen et al., 2000; De Visscher and Van Cleemput, 2003; Wilshusen et al., 2004). Advection and diffusion can be affected by higher moisture due to the associated decrease in gas-relative permeability and gas tortuosity. Higher moisture contents can be the result of high infiltration and production of water during methane oxidation (Scheutz and Kjeldsen, 2005) and are also affected by the soil type (Kightley et al., 1995). Soil permeability has been found to have little effect on simulations of methane attenuation (Stein et al., 2001). This result may not be generally applicable because permeability is a function of soil type and changes in permeability are associated with changes in moisture content, which profoundly affect the diffusive flux regime. In turn, reaction processes can have a direct effect on gas advection because the pressure decrease caused by the consumption of methane can change the magnitude of advective fluxes (Scheutz and Kjeldsen, 2003; Amos et al., 2005). In addition, the accumulation of exopolymeric substances (EPS) associated with methanotrophic activity can act as a barrier to transport at different scales. At a micro-scale, EPS biofilms limit the diffusion of substrate to methanotrophs (Hilger et al., 1999). At a macro-scale, EPS accumulation can change the effective porosity and hydraulic conductivity of the porous medium (Baveye et al., 1998). For example, Wilshusen et al. (2004) observed a decrease in macro-scale diffusion in compost biofilters. Such macro-scale barriers may result in a decrease in the efficiency of a cover soil to attenuate methane and other trace components of landfill gas due to a limitation of O2 supply.

The aim of this study is to investigate the gas transport and degradation processes occurring in landfill cover soils with the aid of a reactive transport model that includes the aqueous and gaseous phases. The focus is on evaluating the contributions of various gas transport processes and the effect of oxidation reactions on the overall gas transport regime.


   Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
Results and Discussion
 Summary
 REFERENCES
 
The analysis was performed using the results of counter-gradient column experiments conducted by Scheutz and Kjeldsen (2003) to investigate the capacity of landfill cover soils to attenuate methane, CFC (i.e., CFC-11, CFC-12), and hydrochlorofluorocarbons (HCFC) (i.e., HCFC-21, and HCFC-22) emissions. In addition, a sensitivity analysis was conducted to evaluate the effect of local changes in physical and environmental factors on short- and long-term gas transport in landfill cover soils. For this purpose, a reactive transport model that includes flow in the aqueous phase by means of Richard's equations and transport in the aqueous and gas phases was used. Aqueous concentrations are the primary variables of the model, which are assumed to be in equilibrium with gas-phase concentrations by means of Henry's law. The use of the DGM for gas-phase transport allows for the consideration of transport mechanisms that have been neglected in previous landfill cover studies. A detailed description of the model is provided in Mayer et al. (2002) and Molins and Mayer (2007). Here, a brief description is given that focuses on introducing the concepts of relevance to this work, in particular, the formulation for gas transport and relevant reactions.

Model Formulation
Diffusion of gases in the soils can be described by the Stefan-Maxwell equations within the framework of the DGM (Mason and Malinauskas, 1983):

Formula 1[1]
where {chi}ig (–) is the molar fraction of gas species i, cgi is the concentration of gas species i (mol L–1 gas), Mgi (kg mol–1) is the molecular weight of gas species i, and Nd,gi (mol m dm–3 porous medium s–1) is the diffusive molar flux of gas species i. The first term on the left side accounts for molecule–molecule interactions, with Dgij (m2 s–1) being the free-phase binary diffusion coefficient between gas species i and j, {phi} being the porosity (m3 void m–3 porous medium), Sg (m3 gas m–3 void) being the saturation of the gaseous phase, and being the gas tortuosity coefficient (–) (Moldrup et al., 2000):

Formula 2[2]

The second term in Eq. [1] accounts for molecule–sediment interactions, with DK,gi (m2 s–1) being the Knudsen diffusion coefficient for species i. The Knudsen diffusion coefficient (m2 s–1) can be calculated as a function of the Klinkenberg parameter (bi [Pa]), soil permeability (k [m2]), relative gas permeability ( krg[–]), and viscosity of gas species i (µig [Pa s]) (Thorstenson and Pollock, 1989; Massmann and Farrier, 1992; Sleep, 1998; Fen and Abriola, 2004):

Formula 3[3]
where ref indicates the gas species that is used as reference for the calculations. The relative permeability in the gas phase krg(–) is calculated as follows (Parker et al., 1987):

Formula 4[4]
with, m=1-1/n where n and m are soil hydraulic function parameters. The effective saturation of the aqueous phase, Sea (–), is defined as a function of the residual saturation in the aqueous phase, Sra (–):

Formula 5[5]

A nonseparative component may be present as part of the diffusive flux when gases in the mixture have different molecular weights. The non-equimolar flux occurs in systems with walls (soil particles) because diffusing species have different molecular weights, and lighter molecules move faster than heavier molecule (Cunningham and Williams, 1980). Although diffusive in origin, the non-equimolar flux results in a non-separative flux in the same direction as the diffusive flux of the lightest gas species in the mixture. If one adds the DGM equation for each gas species (e.g., Eq. [1]) over all gas species, the first term on the left side cancels out, and the right side can be rearranged as a function of gas-phase pressure (pgi [Pa]) and gas density ({rho}g[kg m–3]):

Formula 6[6]
The magnitude of the non-equimolar flux depends on the Knudsen diffusion coefficient; in particular, it is a function of permeability and the molecular weight of a gas species (Eq. [3]).

Diffusion is not the only mechanism contributing to gas transport in the system. Gas fluxes applied in the experiments range from 0.24 to 4.09 m d–1 and are large enough to generate significant pressure gradients. Darcy's law is used to describe advective fluxes in the gas phase:

Formula 7[7]
where Na,gi (mol m dm–3 porous medium s–1) is the advective flux in the gas phase, and µg (kg m–1 s–1) is the gas phase viscosity. The ideal gas law is used to calculate the partial pressure of each gas species (pgi), which are added to obtain the total pressure in the gas phase (Molins and Mayer, 2007).

Biogeochemical transformations attenuate the emissions of landfill gas (LFG) to the atmosphere. The presence of methanotrophic bacteria results in the oxidation of methane, the consumption of atmospheric oxygen, and the co-oxidation of HCFC-11 and HCFC-21 (Scheutz and Kjeldsen, 2003). These reactions take place in the aqueous phase and are formulated accordingly. During methane oxidation, oxygen is consumed, and carbon dioxide, water, and organic carbon (CH2O) are produced according to the following stoichiometric relationship (De Visscher and Van Cleemput, 2003):

Formula 8[8]
It is assumed that the aerobic oxidation of HCFC-21 and HCFC-22 proceed to completion according to the stoichiometry of these reactions (Table 1 ). In addition, CFC-11, CFC-12, HCFC-21, and HCFC-22 are assumed to undergo reductive dehalogenation under anaerobic conditions (Table 1).


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  		Table 1. Reaction stoichiometry and rate expressions.

 
Methane oxidation rates are calculated using the following Monod-type expression:

Formula 9[9]
where kCH4 (moles L–1 H2O s–1) is the reaction rate constant, and K1 and K2 (mol L–1 H2O) are the half-saturation constants for methane and O2, respectively. Aqueous concentrations of methane and oxygen are denoted with caCH4 and caO2, respectively. Similarly, Monod-type expressions are used for the aerobic oxidation rates of HCFC-21 and HCFC-22 (Table 1). Anaerobic oxidation of the halogenated compounds is described with a Monod-type expression for the concentration of each compound, and an inhibition term is used for the concentration of oxygen to account for the fact that the reaction is hindered under aerobic conditions. Stoichiometry, rate expressions, and half-saturation constants for these reactions are given in Table 1. For simplicity, it is assumed that H2 concentrations are not rate limiting, and H2 is left out of the model.

Diffusive and advective gas fluxes (Nd,gi and Na,gi) and reaction source-sink terms contribute to the mass balance of each component in the aqueous and gas phases. The equations expressing this mass balance are solved simultaneously for all components (Molins and Mayer, 2007).

Model Parameters and Calibration
The simulations are constrained by experimental data from Scheutz and Kjeldsen (2003), who conducted column experiments using soil collected at the Skellingsted Landfill south of Holbæk, Western Sealand, Denmark. The soil consists predominantly of loamy sand, and a value of 10–12 m2 was used for average soil permeability, which is assumed to be representative for this material (Iversen et al., 2001). A detailed description of the soil analyses can be found in Scheutz et al. (2004). The soil was packed in tubes made of rigid PVC, and the resulting average porosity was determined to be 0.52 m3 void m–3 porous medium (Fig. 1 ). The PVC tubes were closed at both ends to form gas-tight columns. The columns were fed from opposite ends with LFG and air through inlets situated at the bottom and top of the columns, respectively. An outlet positioned at the top was used to allow air to pass freely through the top chamber. Artificial LFG consisted of a mixture of 50/50% v/v methane/CO2 and trace concentrations of CFCs and was injected into the bottom inlet of the column at a rate of 0.76 m d–1 for 3 wk (Scheutz and Kjeldsen, 2003). For the simulations, this translates to specified methane and carbon dioxide molar fluxes of 15.6 moles L–1 gas m2, which were used as boundary conditions at the bottom of the column. Specified gas concentrations applied at the top of the column were equal to atmospheric concentrations (i.e., pgN2=0.78, pgO2=0.21, pgCO2=0.01, and pgCH4=0.0001). Simulation parameters are summarized in Table 2 . A detailed description of the experimental setup can be found in Scheutz and Kjeldsen (2003).


Figure 1
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  		Fig. 1. Experimental column set up (adapted from Scheutz and Kjeldsen, 2003).

 

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  		Table 2. Simulation parameters.

 
Simulations were conducted for an active column and a sterilized control column. The maximum zero-order rate constants calculated for the batch experiments conducted by Scheutz and Kjeldsen (2003) were used for initial simulations of the active column. Parameter adjustment was required for the anaerobic degradation of CFC-11 and the aerobic oxidation of HCFCs to provide a better fit to measured concentrations; thus, these values were used for the remaining simulations (Table 1). Experimental results for the control column were simulated by setting all reaction rate constants to zero.

Pore water saturation was used as a fitting parameter for model calibration. Initial simulations were conducted with depth-dependent water saturations, which were determined from experimental porosity and bulk density data. However, consideration of a depth-dependent saturation profile was not warranted because the fit to observed gas concentrations could not be improved relative to simulations with constant water saturations. As a result, pore water saturations were assumed to be constant along the column and over the simulation time. In the active column, an aqueous saturation of 0.22 m3 H2O m–3 void provided the best fit between experimental data and simulated results, which falls in the range of measured water saturations in the soil. In the control column, an aqueous saturation of 0.34 m3 H2O m–3 void provided the best fit to experimental data. The differences in simulated water saturations between active and control columns are consistent with the experimental design. In the control column, higher saturations are the result of the sterilization treatment consisting of the addition of an average of 0.13 m3 of HgCl solution per m3 of soil.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results and Discussion
Summary
 REFERENCES
 
Concentrations of Methane, CFCs, and HCFCs
Model results are in good agreement with the observed concentrations in the columns (Fig. 2 ). Supply of atmospheric oxygen into the column contributes to the aerobic degradation of methane, HCFC-21, and HCFC-22 (Fig. 3a, 3c ), which results in the production of CO2. Organic compounds CFC-11, CFC-12, HCFC-21, and HCFC-22 also degrade in the anaerobic portion of the column (Fig. 3b, 3c). Although results presented are profiles after 3 wk, concentration profiles obtained as early as after 3 d are almost identical to those obtained at steady state, which is an indication that steady-state transport is reached very rapidly. For selected compounds (i.e., CH4, CO2, CFC-12, and HCFC-21), statistical analyses were conducted to further evaluate the model performance. Modeling efficiency was 0.960 for CO2, 0.988 for CH4, 0.935 for CFC-12, and 0.997 for HCFC-21, and the d-index was 0.997 for CO2, 0.999 for CH4, 0.986 for CFC-12, and 0.999 for HCFC-21 (Legates and McCabe, 1999). This analysis confirms the very good agreement between observed and simulated data.


Figure 2
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  		Fig. 2. Gas concentrations. (a) N2, O2, CO2, and CH4. (b) Trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12). (c) Chlorodifluoromethane (HCFC-21) and chlorofluoromethane (HCFC-22). Experimental data in symbols; simulated results in solid lines.

 

Figure 3
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  		Fig. 3. Simulated oxidation rates. (a) CH4. (b) Trichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12). (c) Chlorodifluoromethane (HCFC-21) and chlorofluoromethane (HCFC-22).

 
Relative Contribution of Transport Mechanisms
An analysis of the fluxes shows that advection is the most relevant transport mechanism for CH4 and CO2 near the base of landfill cover soils (Fig. 4a ). However, advection becomes less important with increasing proximity to the ground surface, and diffusion becomes most relevant. As a result, advection does not play a significant role in gas emissions to the atmosphere, contributing only 0.6% of the total methane flux and 3.5% of the carbon dioxide flux. Supply of oxygen into the column is driven by diffusion, whereas an outward advective component offsets 15% of the O2 diffusive influx (Fig. 4c). Nitrogen supplied into the column by diffusion is present at the bottom of the column at a mol fraction of 0.11 (Fig. 2a). Because N2 is nonreactive in this system, at steady state, the downward diffusive flux must be balanced by the upward advective flux, resulting in a zero net flux (Fig. 4d).


Figure 4
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  		Fig. 4. Simulated flux components for (a) CH4, (b) CO2, (c) O2, and (d) N2.

 
During methane oxidation, 2.5 moles of gases are transformed to 0.5 moles of gases (Eq. [8]). This results in a decrease in the gas pressure gradient and the gas flux near the reaction zone (Fig. 5 ). The pressure gradient required to sustain the methane flux in the lower end of the column is 0.16 kPa m–1 (Fig. 5); this gradient is nearly constant with depth except near the main reaction zone, where it decreases significantly to a minimum of 0.07 kPa m–1 near the top. Methane emissions are reduced mostly by methane oxidation but also partly by the decrease in the advective component of the net methane flux (Fig. 4a), which are indirectly caused by the oxidation reaction. Because methane, a relatively light gas, is consumed to produce CO2, a relatively heavy gas, the average molecular weight of the gas mixture increases. This results in the development of a non-separative component of the diffusive fluxes from the bottom of the column (30.9 g mol–1) toward the reaction zone (31.7 g mol–1). However, the contributions of the non-equimolar component of the diffusive fluxes of CH4 and CO2 are small and become insignificant in the upper quarter of the column (Fig. 4). At the top of the column, the non-equimolar component of diffusion is reversed because the atmospheric mixture of gases (28.9 g mol–1) has a smaller molecular weight than the gas mixture in the reaction zone.


Figure 5
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  		Fig. 5. Simulated pressure and advective velocity.

 
Variable Inlet Flow Experiments
In addition to the 3-wk experiment at a constant rate, Scheutz and Kjeldsen (2003) performed column experiments with inlet flows varying between 0.82 and 14.25 mL min–1, which correspond to gas fluxes ranging from 0.24 to 4.09 m d–1. These values encompass the range of gas fluxes observed in landfills covers (Scheutz and Kjeldsen, 2003). Gas fluxes of 0.24 m d–1 are representative for older landfills or sites with gas collection systems, whereas new and active landfills with high gas production rates can have gas fluxes of up to 5 m d–1 (Scheutz and Kjeldsen, 2003). A series of simulations was performed to reproduce the results of these experiments. Simulations for each inlet flow were run for 5 d, consistent with the duration of the experiment. The parameters used for the constant inlet flow simulation (Table 2) were also used in these simulations and provided a good match between measured and simulated data (e.g., Fig. 6a and 6b ).


Figure 6
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  		Fig. 6. Concentrations of N2, O2, CO2, and CH4 (a) at low and (b) high gas influxes. Experimental data in symbols; simulated results in solid lines.

 
Results show that for increasing inflow rates, the efficiency of the soil column to remove methane decreases from 99.8% at 0.24 m d–1 to 8.7% at 4.09 m d–1 (Fig. 7a ). This decrease in efficiency is accompanied by an increase in the relative contribution of advection to methane emission (Fig. 7a). At high LFG flux rates (e.g., 4.09 m d–1), advection dominates in the lower end of the column, as indicated by flat concentration gradients (Fig. 6b), whereas diffusive fluxes dominate in a thin area at the top of the column driven by steep concentration gradients. For increasing gas fluxes, oxygen supply into the column increases at rates below 0.76 m d–1 and decreases slightly above it (Fig. 7b). The net oxygen supply into the column is approximately the same at 0.37 m d–1 as at 4.09 m d–1. As a result, the consumption of methane in absolute value shows a maximum at 0.76 m d–1. The results suggest that above this threshold, methane emissions increase in a linear fashion for the range of fluxes covered in the experiment.


Figure 7
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  		Fig. 7. Simulated fluxes at top of column for variable gas fluxes for (a) CH4 and (b) O2.

 
Sensitivity to Variable LFG Composition
The composition of LFG depends on the makeup of the waste in the landfill and may vary with time. The composition of artificial LFG used in the experiments (50/50% v/v) is within the range of typical compositions of LFG. However, methane concentrations as high as 70% have been measured in landfill cover soils (Kallistova et al., 2005). Furthermore, some column experiments designed to study the ability of filters to attenuate methane emissions were performed using methane as the sole gas (Wilshusen et al., 2004). For a constant gas influx, increasing methane concentrations result in a reduction of the efficiency of methane oxidation (De Visscher and Van Cleemput, 2003).

A series of additional simulations were conducted to analyze the effect of the composition of the LFG on the transport of methane. These simulations were conducted at a LFG influx rate of 0.24 m d–1. Oxygen fluxes into the column become less dominated by diffusion as advection at the top of the column decreases (Fig. 8 ). For a methane fraction of 0.57, the direction of advection is inverted at the top of the column. This reversal is the result of the pressure drop created during methane oxidation. As a consequence, oxygen fluxes into the column occur not only by diffusion but also by advection. However, this was observed only at low LFG fluxes. Pressure gradients in simulation at higher LFG influx rates are not reversed; neither are they in simulations at low LFG influxes with high CO2 concentrations. Reversal of advective fluxes results in a relative increase in N2 concentrations (Fig. 9 ). This peak has been observed in column experiments conducted with methane as the sole LFG (Wilshusen et al., 2004).


Figure 8
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  		Fig. 8. Simulated fluxes at top of column for constant gas influx rate (0.24 m d–1) for O2.

 

Figure 9
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  		Fig. 9. Simulated gas concentrations at influx rate of 0.24 m d–1 for a 70/30% v/v CH4/CO2 mixture.

 
Sensitivity to Moisture Content
Increasing moisture content has been shown to decrease the attenuation of methane within landfill cover soil experiments. Simulation results at a constant LFG flux rate of 0.76 m d–1 with variable moisture content show that for saturations below 0.34 m3 H2O m–3 void, oxygen supply into the soil column is unchanged (Fig. 10 ). As a result, methane attenuation at these saturations in the column is approximately constant. However, for saturations above 0.34 m3 H2O m–3 void, diffusion of oxygen into the column becomes inhibited, whereas O2 advection out of the column remains relatively constant (Fig. 10). Methane and oxygen concentration gradients become steeper, which decreases the width of the zone with active methanotrophic activity (Fig. 11 ). As a result, for constant methane release rates, the simulations suggest that methane consumption in landfill covers decreases with increasing moisture content (Fig. 11).


Figure 10
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  		Fig. 10. Simulated O2 flux at top of column for variable column saturation.

 

Figure 11
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  		Fig. 11. Simulated cumulative consumption of CH4 for different saturations.

 
Sensitivity to Local Changes in Moisture Content, Porosity, and Permeability
A number of processes associated with methane oxidation can affect gas transport in a soil column. As expressed by Eq. [8], in the oxidation of methane, H2O and CH2O are produced, where the organic carbon fraction consists of biomass but mostly of EPS (Thullner et al., 2004). Increased water content and EPS accumulation have been shown to affect attenuation of methane by reducing the supply of oxygen into the column. In addition, both processes can reduce the permeability of the gas phase and therefore change the balance between advection and diffusion. To study the effects of H2O and EPS production on gas transport and methane oxidation in a hypothetical fashion, short-term (3-wk) and longer-term (180-d) simulations were conducted by adding these processes to the model presented herein. The short-term simulations can be used to evaluate if the exclusion of these processes in the assessment of the column experiments is justified, and the long-term simulations provide insight into the potential impact of these processes on the future performance of covers.

H2O Production
Water produced during methane oxidation has not been included in previous models of landfill cover soils. The model can be used to estimate water production during oxidation of methane and calculate its effect on the water budget in a soil column. Water produced contributes to the water balance expressed by Richard's equation (Mayer et al., 2002) through the source-sink term Qa (s–1):

Formula 9A[9]
where Sa is the aqueous saturation (–), Nkin is the number of oxidation reactions, and {nu}H2O,m is the stoichiometric coefficient of H2O in a generic oxidation reaction m (in particular, reactions T1, T2, and T3 in Table 1). Water produced during aerobic oxidation does not accumulate in the reaction zone, but, as described by Richard's equation, water flows from regions of high soil moisture potential to areas of lower soil moisture potential. Loss of water may also occur by evaporation if methane oxidation takes place at shallow depth.

The rate of H2O production integrated over the length of the column due to methane oxidation is 93 mm yr–1, a magnitude comparable to that of infiltration at the Skellingsted site (180 mm yr–1, estimated as 30% of average precipitation; Frich et al. [1997]). This rate of water production from methane oxidation activity increases the water saturation from 0.22 to 0.26 m3 H2O m–3 void at a depth of 9.6 cm in 3 wk of the experiment. This increase has a small effect on methane transport and oxidation, which justifies the exclusion of this process from the calibration simulation. Simulation results after 180 d show a larger increase in aqueous saturation to a maximum of 0.365 m3 H2O m–3 void at 13.8 cm depth (Fig. 12a ). This increase in saturation results in a very small increase in methane release at the cover surface from 5.5 to 5.6 mol m–2 d–1 (Fig. 12b). The drop in methane oxidation efficiency would be more significant if initial saturations in the column were higher so that resulting saturations would be greater than 0.5 where changes in efficiency are greater (see section entitled Sensitivity to Moisture Content). Because of high water saturation, relative permeability in the gas phase decreases (Eq. [4]), but for a specified landfill gas production rate, pressure gradients in the high saturation zone increase, thus resulting in no significant changes in the advective fluxes. This result is partially a consequence of the boundary conditions used, which assume constant concentration at the top and constant influxes at the bottom. Measurement of pressures that develop in the column in response to permeability changes would be necessary to constrain simulation results.


Figure 12
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  		Fig. 12. (a) Simulated aqueous saturation at 180 d and (b) CH4 gas fluxes at 180 d, compared with CH4 net flux at steady-state conditions for the case with no water production.

 
EPS Production
It is assumed that 95% of the biomass produced (CH2O) during methane oxidation is EPS (Thullner et al., 2004) and that this EPS has a density of 0.065 g C m–3, which falls within the range of values reported by Christensen and Characklis (1990). The accumulation of EPS results in a decrease in porosity and therefore in permeability, according to the normalized Carman-Kozeny relationship (MacQuarrie and Mayer, 2005):

Formula 10[10]
where {phi}initial and kinitial are the initial porosity (m3 void m–3 porous medium) and initial permeability (m2), respectively. Simulation results show that the accumulation of EPS results in a small decrease in porosity after 3 wk of experiment. The maximum decrease is observed at a 10-cm depth from 0.52 m3 void m–3 porous medium initially to 0.47 m3 void m–3 porous medium. As a consequence, a decrease of diffusive fluxes is observed near the top of the column, whereas no change is observed in the advective fluxes. This results in a small decrease in methane oxidation from 63% in the base case to 55%, suggesting that EPS production may have an effect on cover performance, even for short time period column experiments.

After 180 d, the decrease in the methane oxidation is more significant, in agreement with the effects observed by Wilshusen et al. (2004) for EPS accumulation in a compost column. As aerobic oxidation reactions occur closer to the top of the column because less oxygen diffuses into the column, EPS accumulation also occurs at shallower depths. As a result, the maximum decrease in porosity (from the initial 0.52 to 0.13 m3 void m–3 porous medium) occurs at a depth of 5.2 cm (Fig. 13a ). The loss of pore space causes a decrease in diffusive properties of the column (Eq. [1], [2], and [3]), and results in a significant decrease in methane oxidation from 63% in the base case to 11%. Despite a significant decrease in permeability from 10–12 m2 to 5 x 10–15 m2, advective fluxes of methane are only slightly different, assuming a constant methane production rate and due to compensation in predicted pressure gradients. Advection dominates the transport of methane and trace gases to the surface, with diffusion being the dominant transport mechanism only in the top 10 cm (Fig. 13b). However, the composition of diffusive fluxes near the reaction zone is significantly different than at earlier times because of the decrease in permeability. The non-equimolar component of diffusion becomes significant at this permeability (Fig. 13b) and makes up 18.1% of the net methane flux at 9.3 cm depth. In the low-permeability area, diffusion occurs in the transition regime between molecular and Knudsen diffusion. Because Knudsen diffusion coefficients in the Knudsen regime are smaller than in the molecular regime, a more significant decrease in permeability can result in further decreases in O2 supply into the column and in the capacity of the cover soil column to attenuate methane emissions.


Figure 13
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  		Fig. 13. Simulated effect of the accumulation of exopolymeric substances on (a) pressure and advective gas flux at 180 d and (b) CH4 gas fluxes.

 
Model results were found to be very sensitive to EPS density, which is in agreement with previous studies (Thullner et al., 2004). Furthermore, the model did not include decay of EPS and subsequent oxidation of this organic material. Further studies are required to constrain model simulations and to investigate the effect of EPS on the oxidation of methane and other trace gases in landfill cover soils.


   Summary
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results and Discussion
 Summary
REFERENCES
 
The role of all transport mechanisms relevant to the attenuation of the emission of methane, CFC-11, CFC-12, HCFC-21, and HCFC-22 in landfill cover soils has been analyzed using a multi-component reactive transport model that includes the Dusty Gas Model for gas diffusion and Darcy's law for gas advection.

Transport in the lower end of the column is dominated by advection controlled by specified methane and CO2 fluxes, whereas diffusion is the mechanism that drives gas fluxes near the top of the soil column. For increasing LFG influxes, the portion of the column dominated by advection is larger, and diffusion is constrained to a shallower section of the column. In consequence, concentration gradients near the top of the column become significantly steeper. The net supply of oxygen into the column does not change significantly, but the efficiency of the column in oxidizing methane decreases because methane influxes are larger. For a LFG influx of 0.76 m d–1, the contribution of advection to methane emissions is small (<1% of net fluxes) in part because methane oxidation results in a decrease in pressure gradients in the reaction zone. The contribution of advection grows slightly with increasing LFG influxes. Model results show that advective fluxes can be reversed near the top of the column, promoting increased O2 ingress and methane oxidation. However, this possibility is limited to cover soils subject to small LFG influxes and mixtures rich in methane.

In addition to the direct effect on gas pressure, methane oxidation can indirectly affect transport of gases in the system by the production of water or accumulation of EPS. Simulations show that both processes could result in a decrease of the diffusive properties of the soil. However, in the 3-wk duration of the experiment, these changes have a relatively small effect on gas concentrations. Long-term simulations showed that EPS accumulation could decrease diffusive O2 ingress significantly and thus decrease oxidation of methane. The results suggest that EPS accumulation affect diffusive transport properties through a decrease in effective porosity that affects diffusion in the molecular regime and partly through a decrease in the Knudsen diffusion coefficients due to reduced permeability that brings diffusion closer to the Knudsen regime. Water production rate is significant (93 mm yr–1) and comparable to recharge at the site. However, water production did not cause a decrease in methane oxidation in the simulation presented. Water accumulation could potentially have a more significant effect for lower permeability soils (e.g., if saturated hydraulic conductivity of the cover soils are not significantly larger than water production rates within the cover). Generally, the production of EPS seems to have a more important effect than water production because EPS accumulation is concentrated in the reaction zone, whereas water produced is distributed along the entire column following Richard's equation. Investigation of the accumulation and decay of EPS is necessary to better constrain models of methane oxidation in landfill cover soils.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 Methods
 Results and Discussion
 Summary
 REFERENCES
 
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   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 Methods
 Results and Discussion
 Summary
 REFERENCES
 





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