Modeling methane oxidation in landfill cover soils as indicator of functional stability with respect to gas management

ABSTRACT

A performance-based method for evaluating methane (CH₄) oxidation as the best available control technology (BACT) for passive management of landfill gas (LFG) was applied at a municipal solid waste (MSW) landfill in central Washington, USA, to predict when conditions for functional stability with respect to LFG management would be expected. The permitted final cover design at the subject landfill is an all-soil evapotranspirative (ET) cover system. Using a model, a correlation between CH₄ loading flux and oxidation was developed for the specific ET cover design. Under Washington’s regulations, a MSW landfill is functionally stable when it does not present a threat to human health or the environment (HHE) at the relevant point of exposure (POE), which was conservatively established as the cover surface. Approaches for modeling LFG migration and CH₄ oxidation are discussed, along with comparisons between CH₄ oxidation and biodegradation of non-CH₄ organic compounds (NMOCs). The modeled oxidation capacity of the ET cover design is 15 g/m²/day under average climatic conditions at the site, with 100% oxidation expected on an annual average basis for fluxes up to 8 g/m²/day. This translates to a sitewide CH₄ generation rate of about 260 m³/hr, which represents the functional stability target for allowing transition to cover oxidation as the BACT (subject to completion of a confirmation monitoring program). It is recognized that less than 100% oxidation might occur periodically if climate and/or cover conditions do not precisely match the model, but that residual emissions during such events would be de minimis in comparison with published limit values. Accordingly, it is also noted that nonzero net emissions may not represent a threat to HHE at a POE (i.e., a target flux between 8 and 15 g/m²/day might be appropriate for functional stability) depending on the site reuse plan and distance to potential receptors.

Implications: This study provides a scientifically defensible method for estimating when methane oxidation in landfill cover soils may represent the best available control technology for residual landfill gas (LFG) emissions. This should help operators and regulators agree on the process of safely eliminating active LFG controls in favor of passive control measures once LFG generation exhibits asymptotic trend behavior below the oxidation capacity of the soil. It also helps illustrate the potential benefits of evolving landfill designs to include all-soil vegetated evapotranspirative (ET) covers that meet sustainability objectives as well as regulatory performance objectives for infiltration control.

Introduction

In the United States, municipal solid waste (MSW) landfills are regulated under Subtitle D of the Resource Conservation and Recovery Act (RCRA), which requires a landfill owner/operator (hereafter, operator) to monitor and maintain a closed landfill for what is referred to as the post-closure care (PCC) period (Federal Register 1991). Similar regulations exist in other countries and regions, such as Article 13 of the European Union’s (EU) Landfill Directive (European Council 1999). The four main elements of PCC under Subtitle D include leachate management, groundwater monitoring, inspection and maintenance of the final cover, and control and monitoring of offsite methane (CH₄) migration (generally through operation of an active landfill gas control system). Additional federal requirements for control of landfill gas (LFG) emissions exist via the Clean Air Act (CAA). According to the U. S. Environmental Protection Agency (EPA), completion of PCC is demonstrated when potential threats to human health and the environment (HHE) are reduced to acceptable levels at the relevant point of exposure (POE), typically the closest property boundary location at which a receptor could be exposed to contaminants via a defined migration pathway (EPA, 1993). EPA delegates final authority for determining what constitutes completion of PCC to the states, although not all states have developed specific regulations or guidance. State regulations are generally divided into those that specify performance-based demonstrations of functional stability in terms of long-term emission potential (e.g., Washington and Florida) and those that require demonstration of organic stabilization within the waste mass (e.g., Wisconsin).

As an example of performance-based state regulations, Washington issued revised regulations under Chapter 173-351 of the Washington Administrative Code (WAC), “Criteria for Municipal Solid Waste Landfills,” which includes the rule governing PCC under WAC 173-351–500 (Washington Department of Ecology 2012). According to the rule, operators are required to submit PCC plans for both active and closed MSW landfills that include an estimate of when conditions for functional stability are expected for the four main PCC elements and update their financial assurance accordingly. A landfill is defined as being functionally stable when it does not present a threat to HHE at the relevant POE. For the LFG element, conditions for functional stability are that LFG generation and composition must be such that maintenance and operation of the active LFG collection and control system is no longer needed because the landfill does not pose a threat to HHE from emissions of CH₄ or non-CH₄ organic compounds (NMOCs) and criteria for explosive gas control can be met in the absence of active control.

Technical guidance for evaluating functional stability exists in the public domain as the Evaluation of Post-Closure Care (EPCC) methodology (Morris and Barlaz 2011). The target objectives for functional stability under the EPCC methodology are consistent with the definition under WAC 173-351–500 and the no-threat criterion for completion of PCC under Subtitle D. Several case study evaluations of functional stability at sites in North America and Europe have been performed using the EPCC methodology (Morris et al. 2013; O’Donnell et al. 2018). However, Morris et al. (2012) presented an update to the methodology that considers best available control technology (BACT) for LFG management in terms of surface emissions and air quality, for which case studies are lacking. Although few definitive BACT threshold values have been promulgated, procedural guidance issued by EPA suggests selecting the BACT at landfills with low residual LFG levels by identifying all available control technologies, eliminating those technically infeasible on a site-specific basis, and evaluating and ranking remaining controls based on environmental and cost effectiveness (EPA, 2011a). The updated EPCC methodology adopts this strategy, emphasizing reduction or elimination of fugitive CH₄ and NMOCs by installing passive LFG controls such as biocovers (Barlaz et al. 2004) or utilizing the CH₄ oxidation capacity of an existing all-soil final cover system (Abichou et al. 2006a). In this context, functional stability is defined as the site-specific conditions in which the passive control or soil cover may serve as the BACT for residual LFG management.

The requirement to evaluate site-specific conditions for functional stability with respect to CH₄ and NMOC emissions under Washington’s 2012 rule offered an opportunity to apply the EPCC methodology in its updated form. This paper summarizes some LFG-related components of the functional stability assessment completed at an active MSW landfill, which was approved by the Washington Department of Ecology. Note that the rule provides objective targets for LFG management that go beyond Subtitle D (i.e., prevention of subsurface migration of explosive gas and control of odors) and are also independent of emission controls specified under the CAA. However, it should be clarified that compliance with the CAA will remain the driver for LFG management at the subject landfill through the remainder of its active operation.

Technical basis

Dynamics of methane oxidation in landfill cover soils

Anaerobic decomposition of MSW in landfills produces CH₄ and carbon dioxide (CO₂), with lesser quantities of several NMOC species and other volatile organic compounds (VOCs), which collectively constitute LFG (Soltani-Ahmadi 2002). As a result, LFG represents a source of greenhouse gases as well as potential air pollutants, control of which is required at all U.S. landfills that exceed certain size and operational criteria (Federal Register 1996). For landfills with approved all-soil cover systems, several researchers (e.g., Borjesson and Svensson 1997; Czepiel et al. 1996; Rachor et al. 2011) have shown that oxidation of CH₄ in LFG migrating up through the cover can be effective at controlling emissions. There is also increasing evidence that long-term performance criteria for infiltration control can be achieved with final covers in which geosynthetic barriers are eliminated in favor of all-soil evapotranspirative (ET) cover designs to provide increased longevity, stability, and sustainability (EPA, 2011b). ET covers meet performance objectives for infiltration control due to their design, which is to store and release water (Dwyer and Bull 2008). In the process of storing water, the gas permeability of the cover system is minimized, which has a blockage effect on the upward flow of gas and enhances the containment function in a similar fashion to a barrier layer. The degree of blockage is proportional to the moisture content, textural, and other properties of the soil (Abichou et al. 2015a).

Forecasting when passive LFG control can be provided by an all-soil cover requires estimation of when the oxidation capacity of the soil is expected to exceed the CH₄ generation rate. Recent field measurements across a variety of climates have suggested average CH₄ oxidation rates of 30% to 40% (Chanton et al. 2011) with peak rates exceeding 100%, reflecting uptake of atmospheric CH₄ (Bogner, Spokas, and Chanton 2011). Oxidation processes are controlled by interrelated climatic and environmental factors, including soil type, soil texture, gas-filled and total porosity, tortuosity, dynamic water content, moisture-holding capacity, and temperature, as well as CH₄ and oxygen (O₂) supply, nutrient availability, organic matter content, inorganic nitrogen concentration, and formation of exopolymeric substances by soil microbes (Bogner, Meadows, and Czepiel 1997; Maurice and Lagerkvist 2004; Scheutz et al. 2009). Water content has a significant influence on oxidation rates by directly or inversely affecting the optimal environment for microbial activity and the depth of O₂ penetration and extent of gas transport through soil media, respectively (Boeckx, Van Cleemput, and Villaralvo 1996; Einola, Kettunen, and Rintala 2007). The major mechanisms for gas transport through the cover soil are diffusion and advection. Diffusive transport is caused by a concentration gradient through the soil, whereas advective transport results from pressure gradients induced by wind, changing barometric pressure, or internal pressure build-up from LFG generation (Czepiel et al. 2003). Both advection- and diffusion-controlled fluxes need to be accounted for when modeling gas transport through soil covers.

Vegetation is an important component of ET cover design and has a generally positive effect on CH₄ oxidation (Abichou et al. 2014). Soil diffusivity and the availability of O₂, water, and nutrients for methanotrophic microbial activity are limited by physical properties such as soil texture, pore size distribution, and degree of compaction (Majdinasab and Yuan 2017). Vegetated covers exhibit improved agglomeration and mechanical stabilization of soil. Plants also provide thermal insulation (De Visscher et al. 1999). Although uptake of nitrogen by plants may inhibit oxidation to some degree (Bodelier and Laanbroek, 2004), root systems provide channels for O₂ penetration into the rhizosphere and induce a more suitable microbiological environment for methanotrophs (Bohn et al. 2011; Maurice, Ettala, and Lagerkvist 1999). Conversely, however, the potential for roots to extend all the way through cover soils to create preferential pathways for LFG release is of potential concern. Tanthachoon et al. (2008) showed that without vegetation, air influx into cover soil was limited by a decreased share in pores available for gas transport. The O₂ supplied in deeper zones by root systems seems to be an important factor in regulating methanotrophic growth (Reichenauer et al. 2011). This is especially true of fine-grained soils when dampened by watering.

Co-oxidation and degradation of NMOCs

Although much of the literature focuses on the CH₄ oxidation potential of soil, measurement of emissions through final covers consisting of vegetated soil has shown significant potential for co-oxidation of NMOCs in conjunction with methanotrophic activity (Kjeldsen, Dalager, and Broholm 1997; Wang et al. 2015). For example, Scheutz et al. (2008) measured emissions of over 30 NMOC species at two landfills in France, including areas with temporary cover soil, and reported very small net fluxes of 10⁻⁵ to 10⁻⁴ g/m²/day with clear evidence of NMOC co-oxidation with CH₄. Emitted NMOCs generally consisted of chlorinated species recalcitrant to aerobic degradation (e.g., perchloroethylene). Bogner et al. (2010) used an experimental biocover comprising 30- and 60-cm-thick sections of ground garden waste to measure co-oxidation of hydrocarbons with CH₄ at a landfill in Florida, USA. This field study demonstrated substantial reductions in emissions of several hydrocarbons, especially the aromatics, alkanes, and lower-chlorinated groups, with measured emissions in the range of 10⁻⁹ to 10⁻³ g/m²/day. Conservative calculations based on data from the deeper cover section suggested that current EPA methods overpredicted NMOC emissions by more than 2 orders of magnitude. A study in Denmark examined co-oxidation of 18 NMOC species in thicker soil covers and found a high capacity for degradation of NMOCs in parallel with CH₄ oxidation (Scheutz et al. 2003). The study showed that methanotrophic bacteria were active in oxidizing CH₄ and selected trace components down to a depth of 50 cm below the surface, with optimal oxidation activity occurring at 15 to 20 cm depth. Lakhouit, Cabral, and Cabana (2016) evaluated the efficiency of an experimental compost biocover for treating select NMOCs at a landfill in Quebec, Canada. Order-of-magnitude reductions in most NMOC concentrations were observed, with the results suggesting treatment efficiencies ranging from 67% to 100%, with most exceeding 95%.

Evaluation process

Typically, evaluation of functional stability using the EPCC methodology is based on conservative assumptions and driven (to the extent possible) by evaluation of relevant post-closure monitoring data; however, as the subject landfill is currently in active operation, such data are not available. Therefore, the focus of the evaluation is to estimate the capacity of the ET cover design at the subject landfill to oxidize LFG emissions. The evaluation is quantitative for CH₄ but qualitative for NMOCs, since site-specific composition data for NMOC species are lacking. The point of compliance (POC) was conservatively set as the top surface of the cover, which mitigates the need for, and uncertainties associated with, air dispersion modeling at a more distant POE. The CH₄ oxidation capacity of the cover soil was estimated based on the assumed CH₄ flux at the base of the cover system using a numerical soil physics model. From this, a target CH₄ flux into the bottom of the cover was established based on the magnitude of the flux that would meet conditions for functional stability (i.e., de minimis net emissions) consistent with the EPCC methodology. By establishing an emission flux threshold, the allowable sitewide CH₄ generation rate can be back calculated.

Overview of subject landfill

The case study site is a large active MSW landfill located in central Washington, USA. The total permitted liner footprint is 55 ha, which includes some legacy unlined units as well as modern composite-lined units. The maximum thickness of waste at completion of the landfill operation will be approximately 115 m based on the permitted base and final cover grades, with total permitted airspace capacity of about 28 × 10⁶ m³, which roughly equates to 38 × 10⁶ Mg at predicted waste compaction rates. Annual precipitation at the facility is low at approximately 230 mm. Given the favorable climatic conditions and availability of suitable soils, the facility obtained approval for an all-soil ET final cover system. The ET cover design comprises 90 cm of soil with maximum hydraulic conductivity 7 × 10⁻⁴ cm/sec installed over 30 cm of interim soil cover. The existing LFG system, which will be continually expanded as the landfill is developed, currently consists of an active total of about 40 vertical extraction wells and 15 horizontal collection trenches. The system is designed to easily allow for expansion as the landfill continues to receive waste. Each collection structure is fitted with a wellhead control assembly and connected to the LFG conveyance system to deliver LFG to the flare station for thermal destruction.

Modeling the methane oxidation capacity of cover soil

Given the significant interrelation of control factors described previously, modeling CH₄ oxidation in a landfill cover setting is not straightforward. In a landfill cover setting, water content, temperature, and barometric pressure are dynamic and vary depending on climate conditions, soil type, cover thickness, and vegetation. To accurately predict CH₄ oxidation, the changing water content and temperature inside the soil profile must be accounted for (Spokas and Bogner 2011). Researchers at Florida State University (FSU) have developed a predictive finite element model that combines water and heat flow with gas transport and oxidation kinetics (Yuan and Abichou 2010; Yuan et al. 2009). The FSU model is founded on the Richard’s equation–based models (e.g., HYDRUS, UNSAT-H, VADOSE-W) used to predict the hydraulic performance of ET covers. These models are used to predict water balance quantities (surface runoff, soil water storage, evapotranspiration, and deep percolation) as well as daily volumetric water content, potential head, and temperature at specified depths in the cover soil profile (Simunek et al. 2003). In the FSU model, output from a Richard’s equation model is coupled with dynamic parameters associated with water content and temperature. By incorporating dynamic methanotrophic activity, oxidation rates can then be estimated based on known cover thickness, soil layering, vegetative conditions, and daily climatic variability (Abichou et al., 2014, 2015b). Water balance modeling at the subject landfill was conducted with a single 90-cm soil layer to represent the final cover using the model UNSAT-H version 3.0 (Fayer 2000). Input soil properties to the model are shown in Table 1.

Table 1. Soil properties at the subject landfill.

Soil description	USCS classification	Remold density (kN/m^3)	Relative compaction	Specific gravity (assumed)	Hydraulic conductivity (m/sec)	van Genuchten parameters
						Θs	Θr	α	n
SS alluvium “lower end”	CL-ML	15.3	85%	2.65	2.2E-06	0.373	0.002	0.020	1.225

The water balance modeling used daily average climatic conditions from an average year. Water content and soil temperatures at three different depths through the cover profile were modeled for a period of five consecutive average years. Results from the final year were used as input to the FSU model to represent average yearly conditions (Figures 1 and 2).

Figure 1. Average daily water content at different cover depths for an average year.

Figure 2. Average daily soil temperatures at different cover depths for an average year.

The outputs of UNSAT-H modeling (daily water content and daily temperature) were then used by the gas transport module in the FSU model, which considers temperature, moisture, and scaling correction factors as described in Abichou et al. (2010). Michaelis-Menten parameters to predict biological CH₄ oxidation and emissions from the modeled ET cover system were developed using correlations between field measurements and data from laboratory incubation experiments on homogenized and sieved soil samples under fixed environmental conditions according to Abichou et al. (2010).

Because the surface of the cover is open to the atmosphere, the gas composition above the surface node of the model was assumed to be equal to atmospheric gas composition, which in volumetric terms is 21.2% O₂, 1.8 parts per million (ppm) CH₄, 385.0 ppm CO₂, and 78.8% nitrogen gas (N₂).

As described earlier, the soil water content and temperature (Figures 1 and 2) were used as a daily input to the gas transport module in the FSU model. Multiple simulations were run to vary methane loading into the soil profile (5, 10, 20, 50, 100, and 200 g/m²/day). The model outputs were daily concentrations of CH₄, O₂, CO₂, and N₂ at every node of the modeled soil profile. These concentrations were then used to calculate the mass flux of each of these gases into and out of the bottom and top boundaries of the soil profile, respectively. Methane fluxes into/out of the soil profile were then used to determine the daily oxidation rate and percent methane oxidation. The average oxidation rate over the course of the 1-yr modeling period was then determined for each methane loading flux and plotted (Figure 3).

Figure 3. Modeled methane oxidation rates in cover soil at the subject landfill.

The shape of the graph in Figure 3 resembles a Michaelis-Menten enzyme kinetic plot of the response of a biological reaction to increasing substrate concentration (Johnson and Goody 2011). In a landfill ET cover system, soil methane oxidation is a biologically mediated system and behaves as such. When a substrate (CH₄) is supplied, the cover’s rate of substrate uptake is linear to a point, and then the system approaches saturation. This response is due to the O₂ levels to which the bacteria are exposed as well as temporal variability in soil temperature and moisture. The gas transport simulations suggest that the cover design can oxidize as much as 15 g/m²/day under average conditions (Figure 3). Thereafter, the modeled oxidation rate decreases as the loading flux increases, which is consistent with field observations at other sites (e.g., Abichou et al. 2006b; Dever et al. 2005).

Functional stability with respect to methane emissions

Figure 3 shows that the cover soil should be capable of oxidizing 100% of the CH₄ loading flux up to 8 g/m²/day, above which the rate of oxidation as a percentage of the loading flux decreases significantly. Therefore, the target for functional stability with respect to passive control of CH₄ emissions at the case study site was conservatively set at this value, which should result in net-zero CH₄ emissions on an average annual basis. Over the 55-ha surface of the cover soil, a sitewide loading flux of 8 g/m²/day equates to a total CH₄ generation rate of about 260 m³/hr. This threshold rate can then be used with a regulatory-approved CH₄ generation model such as LANDGEM (EPA, 2005) to forecast when CH₄ oxidation could represent the BACT for LFG emissions. However, before making changes to active LFG controls, confirmation monitoring will be critical to demonstrate that decisions based on modeled predictions are appropriate and protective of HHE.

Qualitative assessment of NMOC emissions

From the perspective of NMOC emissions, two NMOC classes of concern are aromatic hydrocarbons and halogenated hydrocarbons. Although not all NMOCs may be oxidized to the same degree, Scheutz et al. (2003) demonstrated that both aromatic hydrocarbons (e.g., benzene) and lower-chlorinated hydrocarbons (e.g., vinyl chloride) showed very low to nondetected emission concentrations under relatively low-flux conditions, indicating co-oxidation with CH₄ in the cover soil. Further, Scheutz et al. (2004) and Scheutz and Kjeldsen (2005) found that high-chlorinated hydrocarbons (e.g., tetrachloromethane) were degradable in the lower, anaerobic zone of cover soils whereas medium-chlorinated hydrocarbons (e.g., trichloroethylene [TCE], trichloromethane [TCM]) were the least likely to degrade. However, Scheutz et al. (2004) also concluded that the capacity of the cover soils to degrade medium-chlorinated hydrocarbons, including TCM and TCE, under optimal conditions far exceeds the maximum measured concentrations in LFG, especially at the tail end of the gas curve. In the absence of site-specific NMOC data, these findings support an assumption that net NMOC emissions will be de minimis under low residual CH₄ flow rates where conditions for passive LFG control by CH₄ oxidation have already been established. For the purposes of demonstrating use of this modeling approach to predict when NMOC emissions would meet conditions for functional stability at the subject landfill, it was thus considered reasonable to assign methane oxidation as a surrogate indicator for NMOC degradation. However, this simplified approach is provided as an example only and may not be applicable at all sites. In all cases, a confirmation monitoring program should be designed to confirm model predictions before elimination of active controls can be made permanent.

Discussion

This study describes a performance-based method for estimating when CH₄ oxidation in cover soils could serve as the BACT for passive management of residual CH₄ and NMOC emissions at a large MSW landfill. The modeling study was performed in response to a regulatory requirement to predict when conditions for functional stability with respect to LFG management might be achieved. As the subject landfill is in active operation, representative post-closure data to calibrate the model are not available. However, the gas transport simulations suggested that the site’s permitted ET cover design could oxidize as much as 15 g/m²/day under average climatic conditions. This corresponds closely to measured values of 16 g/m²/day or greater in the literature (Chanton et al. 2011). The CH₄ oxidation model suggested that bottom fluxes up to 8 g/m²/day would result in complete oxidation (i.e., 100% control of emissions) on an average annual basis, although it is recognized that less-than-complete oxidation might occur periodically if climate and/or cover conditions do not precisely match the model. However, review of global regulations and guidance indicates that such occasional emissions would be de minimis and protective of HHE. For example (cit. in Morris et al. 2012), Austrian regulations establish CH₄ emission limits for temporary soil-capped landfills at 13.7 g/m²/day as a mean value and 27.4 g/m²/day as a “hotspot. maximum,” whereas guidance issued in France suggests CH₄ emission values suitable for uncontrolled passive treatment in cover soils of 8.6 to 17.2 g/m²/day. Occasional periods with less-than-complete or even zero oxidation at the subject landfill would thus not exceed published limit values once an equivalent flux of 8 g/m²/day is routinely achieved. Further, although the upper-bound flux at which 100% oxidation is predicted (Figure 3) was used to establish the functional stability target for transition to cover oxidation as the BACT in this study, these published limit values demonstrate that nonzero net emissions do not necessarily represent a threat to HHE at a POE. Accordingly, a target flux between 8 and 15 g/m²/day might be appropriate for functional stability depending on the site reuse plan, distance to potential receptors, and other site-specific risk factors.

Oxidation rates and patterns for NMOC degradation reported in the literature generally mirror those of CH₄; therefore, in the absence of site-specific NMOC data it is reasonable to assign CH₄ as a surrogate indicator of expected NMOC degradation in soil cover systems. To provide more certainty that the predicted timeframe for functional stability is protective of HHE from the standpoint of both CH₄ and NMOC emissions, a confirmation monitoring (CM) program to investigate surface emissions of CH₄ and NMOCs should be designed to demonstrate cover soil oxidation as the BACT prior to final dismantling of active LFG controls (i.e., to ensure that actual conditions are consistent with modeled predictions). CM requirements will be site specific and driven by the modeling analyses performed but should also reflect the proximity of potential receptors as well as the operator’s experience gained with LFG management at the site. Components of CM could include surface emission scanning and static chamber measurements (Figueroa et al. 2009) or sitewide surveys based on remote sensing techniques (Goldsmith et al. 2012). The CM program should be capable of detecting hotspot emissions of CH₄ and NMOCs. If an emission standard applies as discussed above, measured emissions at hotspots should be below the threshold. Persistent hotspots that exceed the threshold should be fixed by locally enhancing and stabilizing the cover soils. Some hotspots may be associated with redundant appurtenances, which should be removed. CM should continue until hotspot issues are addressed to the satisfaction of the regulator. In most cases, however, by the time CM is formally initiated, the landfill should have decades of iterative emissions monitoring and localized cover repairs/enhancements such that, when combined with declining LFG generation rates, distribution of gas into the cover soil is equalized and hotspots are no longer an issue.

The analysis showed that sustained CH₄ generation below 260 m³/hr should constitute the functional stability target for transition to passive LFG control (i.e., total CH₄ generation below this level should not overwhelm the oxidation capacity of the cover soil). This is a relatively high rate; for example, guidance issued by the Environmental Protection Agency of Ireland (2011) suggests that biofiltration technologies can constitute the BACT at CH₄ flows below 100 m³/hr. This illustrates the value of a well-designed and constructed ET cover in providing very high CH₄ oxidation rates within the specific context of the climate and other conditions at the site. By highlighting a MSW landfill at which permit approval was obtained for an all-soil ET final cover, this study helps illustrate the potential benefits of evolving landfill designs that meet sustainability objectives as well as regulatory performance standards. As alternatives to composite covers featuring geosynthetic barriers, vegetated ET covers meet performance objectives for infiltration control due to their design, which is to store and release water. In the process, cover soils also serve to control gas emissions. As it becomes increasingly difficult to effectively operate an active LFG system during the latter stages of gas generation when the CH₄ content (energy value) is low, a vegetated all-soil cover can thus provide an in-built passive LFG system. Although oxidation as the BACT in this way is only applicable to all-soil covers, it should be noted that all landfills may utilize biofilter technologies such as biofilters or biowindows (Huber-Humer, Gebert, and Hilger 2008; Streese and Stegmann 2003) to similarly oxidize CH₄ and NMOCs.

Additional information

Notes on contributors

Jeremy W.F. Morris

Jeremy W.F. Morris is a principal with Geosyntec Consultants in Washington, DC, USA.

Michael D. Caldwell

Michael D. Caldwell is the Director of Groundwater and Technical Programs at Waste Management in Humble, TX, USA.

James M. Obereiner

James M. Obereiner is the principal and owner at JMO Consulting in Sacramento, CA, USA.

Sean T. O’Donnell

Sean T. O’Donnell is a senior staff engineer with Geosyntec Consultants in Columbia, MD, USA.

Terry R. Johnson

Terry R. Johnson is a senior director at Waste Management in Minneapolis, MN, USA.

Tarek Abichou

Tarek Abichou is a professor in the Department of Civil & Environmental Engineering, FAMU-FSU College of Engineering, at Florida State University in Tallahassee, FL, USA.