Application of cavity ring-down spectroscopy and a novel near surface Gaussian plume estimation approach to inverse model landfill methane emissions

Fugitive methane emissions from municipal solid waste landfills impact global climate change and reliable emissions quantification is of increasing importance. Ground-based cavity ring-down spectrometer (CRDS) measurements were used to determine methane concentrations and isotopic compositions of carbon in CH₄. Then, CH₄ oxidation through various cover materials was assessed using the Keeling plot method. A novel inverse modeling approach using Gaussian dispersion analysis, termed near-surface Gaussian plume estimation (NSGPE), was developed to predict whole-site landfill methane emissions. The concentration data obtained around the landfill perimeter with the mobile ground-based CRDS were used. Methane concentration data were integrated to parameterize discretized point source emissions from a Gaussian dispersion model. Post-processing algorithms were applied to refine modeling predictions to account for the influence of topographical and meteorological conditions on methane transport. Results indicate spatially resolved and consistent emissions estimates among multiple optimization simulations, with refinements increasing the resolution and spatial trends of emissions. Post-processing algorithms resolve consistent overestimation of emissions commonly observed using conventional Gaussian dispersion models.•Ground-based CRDS used to obtain methane concentration and oxidation data.•Novel inverse Gaussian dispersion modeling approach developed to predict methane emissions from landfills accounting for site-specific topography and meteorology.•Post-processing algorithms refine emissions estimates.

Assessment of methane emissions from a California landfill using concurrent experimental, inventory, and modeling approaches

Methane flux and emissions were obtained at a California landfill concurrently using field measurements, inventory analyses, and modeling. Measured fluxes ranged from -3.7 to 828 g/m2-day and generally decreased from daily to intermediate to final covers. Soil covers with high-plasticity clay had the lowest fluxes. Whole-site emissions ranged from 406 to 47,414 tonnes/year (11,368 to 1,327,592 tonnes CO2-eq./year), and were dominated by intermediate covers with high relative surface area. Emissions estimates from flux chamber tests and California Landfill Methane Inventory Model (CALMIM) with oxidation were similar and low, whereas emissions from aerial measurements and CALMIM without oxidation were similar and high. The inventory analyses provided intermediate emissions and a new Gaussian plume model based on ground cavity ring-down spectrometer measurements provided the highest emissions. The assumptions used and the inherent strengths and limitations of the different approaches resulted in the flux and emissions differences. With varied attributes (experimental/modeling; flux/emissions; whole-site/cover-specific, top-down/bottom-up), the approaches provide envelopes of methane emissions and can be used selectively for the two main purposes of landfill methane emissions analysis: to mechanistically determine the factors that control/limit surface emissions and to provide data for atmospheric methane analysis. To reduce emissions, progression from temporary to permanent cover areas can be accelerated and covers with coarser materials can be amended with plastic fines.

Anthropogenic land use and urbanization alter the dynamics and increase the export of dissolved carbon in an urbanized river system

Greenhouse gas emissions from urban rivers play a crucial role in global carbon (C) cycling, this is tightly linked to dissolved C in rivers but research gaps remain. The effects of urbanization and anthropogenic land-use change on riverine dissolved carbon dynamics were investigated in a temperate river, the River Kelvin in UK. The river was constantly a source of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere (excess concentration of CH₄ ranged from 13 to 4441 nM, and excess concentration of CO₂ ranged from 2.6 to 230.6 μM), and dissolved C concentrations show significant spatiotemporal variations (p < 0.05), reflecting a variety of proximal sources and controls. For example, the concentration variation of dissolved CH₄ and dissolved CO₂ were heavily controlled by the proximity of coal mine infrastructure in the tributary near the river head (~ 2 km) but were more likely controlled by adjacent landfills in the midstream section of the rivers main channel. Concentration and isotopic evidence revealed an important anthropogenic control on the riverine export of CO₂ and dissolved organic carbon (DOC). However, dissolved inorganic carbon (DIC) input via groundwater at the catchment scale primarily controlled the dynamics of riverine DIC. Furthermore, the positive relationship between the isotopic composition of DIC and CO₂ (r = 0.79, p < 0.01) indicates the DIC pool was at times also significantly influenced by soil respiratory CO₂. Both DIC and DOC showed a weak but significant correlation with the proportion of urban/suburban land use, suggesting increased dissolved C export resulting from urbanization. This research elucidates a series of potentially key effects anthropogenic activities and land-use practices can have on riverine C dynamics and highlights the need for future consideration of the direct effects urbanization has on riverine C dynamics.

Carbon isotopic characterisation and oxidation of UK landfill methane emissions by atmospheric measurements

Biological oxidation of methane in landfill cover material can be calculated from the carbon isotopic signature (δ¹³C_CH4) of emitted CH_4. Enhanced microbial consumption of methane in the aerobic portion of the landfill cover is indicated by a shift to heavier (less depleted) isotopic values in the residual methane emitted to air. This study was conducted at four landfill sites in southwest England. Measurement of CH₄ using a mobile vehicle mounted instrument at the four sites was coupled with Flexfoil bag sampling of ambient air for high-precision isotope analysis. Gas well collection systems were sampled to estimate landfill oxidised proportion. Closed or active status, seasonal variation, cap stripping and site closure impact on landfill isotopic signature were also assessed. The δ¹³C_CH4 values ranged from -60 to -54‰, with an average value of -57 ± 2‰. Methane emissions from active cells are more depleted in ¹³C than closed sites. Methane oxidation, estimated from the isotope fractionation, ranged from 2.6 to 38.2%, with mean values of 9.5% for active and 16.2% for closed landfills, indicating that oxidised proportion is highly site specific.

Isotopic evidence for axial tree stem methane oxidation within subtropical lowland forests

Knowledge regarding mechanisms moderating methane (CH₄ ) sink/source behaviour along the soil-tree stem-atmosphere continuum remains incomplete. Here, we applied stable isotope analysis (δ¹³ C-CH₄ ) to gain insights into axial CH₄ transport and oxidation in two globally distributed subtropical lowland species (Melaleuca quinquenervia and Casuarina glauca). We found consistent trends in CH₄ flux (decreasing with height) and δ¹³ C-CH₄ enrichment (increasing with height) in relation to stem height from ground. The average lower tree stem δ¹³ C-CH₄ (0-40 cm) of Melaleuca and Casuarina (-53.96‰ and -65.89‰) were similar to adjacent flooded soil CH₄ ebullition (-52.87‰ and -62.98‰), suggesting that stem CH₄ is derived mainly by soil sources. Upper stems (81-200 cm) displayed distinct δ¹³ C-CH₄ enrichment (Melaleuca -44.6‰ and Casuarina -46.5‰, respectively). Coupled 3D-photogrammetry with novel 3D-stem measurements revealed distinct hotspots of CH₄ flux and isotopic fractionation on Melaleuca, which were likely due to bark anomalies in which preferential pathways of gas efflux were enhanced. Diel experiments revealed greater δ¹³ C-CH₄ enrichment and higher oxidation rates in the afternoon, compared with the morning. Overall, we estimated that c. 33% of the methane was oxidised between lower and upper stems during axial transport, therefore potentially representing a globally significant, yet previously unaccounted for, methane sink.

Volatile organic compounds (VOCs) in solid waste landfill cover soil: Chemical and isotopic composition vs. degradation processes

Landfills for solid waste disposal release to the atmosphere a large variety of volatile organic compounds (VOCs). Bacterial activity in landfill cover soils can play an important role in mitigating VOC emission. In order to evaluate the effects of degradation processes and characterize VOCs composition in landfill cover soil, gases from 60 sites and along 7 vertical profiles within the cover soil were collected for chemical and isotopic analysis at two undifferentiated urban solid waste disposal sites in Spain: (i) Pinto (Madrid) and (ii) Zurita (Fuerteventura, Canary Islands). The CO₂/CH₄ ratios and δ¹³C-CO₂ and δ¹³C-CH₄ values were controlled by either oxidation or reduction processes of landfill gas (LFG). VOCs were dominated by aromatics, alkanes and O-substituted compounds, with minor cyclics, terpenes, halogenated and S-substituted compounds. Degradation processes, depending on both (i) waste age and (ii) velocity of the uprising biogas through the soil cover, caused (i) an increase of degradation products (e.g., CO₂, O-substituted compounds) and (ii) a decrease of degradable components (e.g., CH₄, alkanes, alkylated aromatics, cyclic and S-substituted compounds). Terpenes, halogenated compounds, phenol and furans were unaffected by degradation processes and only depended on waste composition. These results highlight the fundamental role played by microbial activity in mitigating atmospheric emissions of VOCs from landfills. Nevertheless, the recalcitrant behaviour shown by compounds hazardous for health and environment remarks the importance of a correct landfill management that has to be carried out for years after the waste disposal activity is completed, since LFG emissions can persist for long time.

Environmental baseline monitoring for shale gas development in the UK: Identification and geochemical characterisation of local source emissions of methane to atmosphere

Baseline mobile surveys of methane sources using vehicle-mounted instruments have been performed in the Fylde and Ryedale regions of Northern England over the 2016-19 period around proposed unconventional (shale) gas extraction sites. The aim was to identify and characterise methane sources ahead of hydraulically fractured shale gas extraction in the area around drilling sites. This allows a potential additional source of emissions to atmosphere to be readily distinguished from adjacent sources, should gas production take place. The surveys have used ethane:methane (C2:C1) ratios to separate combustion, thermogenic gas and biogenic sources. Sample collection of source plumes followed by high precision δ¹³C analysis of methane, to separate and isotopically characterise sources, adds additional biogenic source distinction between active and closed landfills, and ruminant eructations from manure. The surveys show that both drill sites and adjacent fixed monitoring sites have cow barns and gas network pipeline leaks as sources of methane within a 1 km range. These two sources are readily separated by isotopes (δ¹³C of -67 to -58‰ for barns, compared to -43 to -39‰ for gas leaks), and ethane:methane ratios (<0.001 for barns, compared to >0.05 for gas leaks). Under a well-mixed daytime atmospheric boundary layer these sources are generally detectable as above baseline elevations up to 100 m downwind for gas leaks and up to 500 m downwind for populated cow barns. It is considered that careful analysis of these proxies for unconventional production gas, if and when available, will allow any fugitive emissions from operations to be distinguished from surrounding sources.

Closing the methane mass balance for an old closed Danish landfill

In this study, a methane (CH₄) mass balance was established for Hedeland landfill. CH₄ generation rates were modelled using a multiphase first-order decay model (The Afvalzorg model) and determined at between 57 and 79 kg h^-1. The CH₄ emission rate was quantified at between 2 and 14 kg h^-1, using the tracer gas dispersion method and the CH₄ gas recovery efficiency was between 8 and 21%. At three places along the perimeter of the landfill, gas remediation systems have been installed to protect the residential houses from any risk of migrating landfill gas. About 0.76 kg h^-1 of CH₄ was extracted from these three remediation systems. Using a carbon mass balance for the lateral migrating landfill gas showed a fractional oxidation of about 78%, which corresponded to a CH₄ flux of 3.5 kg h^-1 from the three remediation systems, including the oxidised CH₄. The total lateral CH₄ flux (un-oxidised) from the total landfill perimeter was estimated at between 6.9 and 10.4 kg h^-1. CH₄ oxidation efficiency in the landfill cover soil, determined from stable carbon isotope analyses, was found to be between 12% and 92%. This resulted in an average CH₄ oxidation rate of 32 kg h^-1, using an average CH₄ emission rate of 8 kg h^-1. CH₄ surface screenings and surface flux measurements supported the hypothesis that oxidation efficiency was in the higher range and that oxidation could close the CH₄ mass balance.

Methodologies for measuring fugitive methane emissions from landfills - A review

Fugitive methane (CH₄) emissions from landfills are significant global sources of greenhouse gases emitted into the atmosphere; thus, reducing them would be a beneficial way of overall greenhouse gas emissions mitigation. In Europe, landfill owners have to report their annual CH₄ emissions, so direct measurements are therefore important for (1) evaluating and improving currently applied CH₄ emission models, (2) reporting annual CH₄ emissions and (3) quantifying CH₄ mitigation initiatives. This paper aims at providing an overview of currently available methodologies used to measure fugitive CH₄ emissions escaping from landfills. The measurement methodologies are described briefly, and the advantages and limitations of the different techniques are discussed with reference to published literature on the subject. Examples are given of individual published studies using different methodologies and studies comparing three or more methodologies. This review suggests that accurate, whole-site CH₄ emission quantifications are best done using methods measuring downwind of the landfill, such as tracer gas dispersion and differential absorption LiDAR (DIAL). Combining aerial CH₄ concentration measurements from aircraft or unmanned aerial vehicles with wind field measurements offers a great future potential for improved and cost-efficient integrated landfill CH₄ emission quantification. However, these methods are difficult to apply for longer time periods, so in order to measure temporal CH₄ emission changes, e.g. due to the effect of changes in atmospheric conditions (pressure, wind and precipitation), a measurement method that is able to measure continuously is required. Such a method could be eddy covariance or static mass balance, although these procedures are challenged by topography and inhomogeneous spatial emission patterns, and as such they can underestimate emissions significantly. Surface flux chambers have been used widely, but they are likely to underestimate emission rates, due to the heterogeneous nature of most landfill covers resulting in sporadic and localised CH₄ emission hotspots being the dominant emission routes. Furthermore, emissions from wells, vents, etc. are not captured by surface flux chambers. The significance of any underestimation depends highly on the configuration of individual landfills, their size and emission patterns.

Distribution and isotopic signature of deep gases in submerged soils in an island of the Lower Delta of the Paraná River, Argentina

Subsoil CH₄ and CO₂ concentrations, δ¹³C-CH₄ and δ¹³C-CO₂ signatures, total organic carbon (TOC) and δ¹³C-TOC, together with C/N ratio of organic matter, were evaluated throughout a soil profile up to the atmosphere to understand the dynamics of CH₄ and CO₂ in the waterlogged environment of an island of the Lower Delta of the Paraná River, Argentina. The analysis of the vertical profile showed that a significant fraction of CH₄ exists as gas trapped within the sediment column, compared to CH₄ dissolved in soil solution. CH₄ concentration measurements in sub-saturated soils showed that free CH₄ is 1 order of magnitude smaller than CH₄ recovered from soil cores by ultrasonic degassing. The highest concentrations of CH₄ occurred at the 90-120-cm layer. At this depth, δ¹³C-CH₄ values resulting from methanogenesis were around - 71‰, which is well within the range of CH₄ produced from CO₂ reduction, and δ¹³C values of the associated CO₂ were enriched (~ - 7‰). Isotope mass balance models used to calculate the fraction of oxidized CH₄ indicated that around 30% of the CH₄ produced was oxidized prior to atmospheric release. In contrast to methanogenesis, during oxidation processes δ¹³C-CH₄ shifts to more positive values. The mineralogical, textural, isotopic, and geochemical characterization of subsoil sediments with abundant organic matter, like Paraná Delta, demonstrated that CH₄ storage capacity of the soil, production, consumption, and transport are the main factors in regulating the actual flux rates of CH₄ to the atmosphere.

An assessment of the potential use of compost filled plastic void forming units to serve as vents on historic landfills and related sites

Much of the solid municipal waste generated by society is sent to landfill, where biodegrading processes result in the release of methane, a major contributor to climate change. This work examined the possibility of installing a type of biofilter within paved areas of the landfill site, making use of modified pervious paving, both to allow the escape of ground gas and to avoid contamination of groundwater, using specially designed test models with provision for gas sampling in various chambers. It proposes the incorporation of an active layer within a void forming box with a view to making dual use of the pervious pavement to provide both a drainage feature and a ground gas vent, whilst providing an active layer for the oxidation of methane by microbial action. The methane removal was observed to have been effected by microbial oxidation and as such offers great promise as a method of methane removal to allow for development of landfills.

Methodology to determine the extent of anaerobic digestion, composting and CH4 oxidation in a landfill environment

An examination of the processes contributing to the production of landfill greenhouse gas (GHG) emissions is required, as the actual level to which waste degrades anaerobically and aerobically beneath covers has not been differentiated. This paper presents a methodology to distinguish between the rate of anaerobic digestion (r_AD), composting (r_COM) and CH₄ oxidation (r_OX) in a landfill environment, by means of a system of mass balances developed for molecular species (CH₄, CO₂) and stable carbon isotopes (δ¹³C-CO₂ and δ¹³C-CH₄). The technique was applied at two sampling locations on a sloped area of landfill. Four sampling rounds were performed over an 18 month period after a 1.0 m layer of fresh waste and 30-50 cm of silty clay loam had been placed over the area. Static chambers were used to measure the flux of the molecular and isotope species at the surface and soil gas probes were used to collect gas samples at depths of approximately 0.5, 1.0 and 1.5 m. Mass balances were based on the surface flux and the concentration of the molecular and isotopic species at the deepest sampling depth. The sensitivity of calculated rates was considered by randomly varying stoichiometric and isotopic parameters by ±5% to generate at least 500 calculations of r_OX, r_AD and r_COM for each location in each sampling round. The resulting average value of r_AD and r_COM indicated anaerobic digestion and composting were equally dominant at both locations. Average values of r_COM: ranged from 9.8 to 44.5 g CO₂ m^-2 d^-1 over the four sampling rounds, declining monotonically at one site and rising then falling at the other. Average values of r_AD: ranged from 10.6 to 45.3 g CO₂ m^-2 d^-1. Although the highest average r_AD value occurred in the initial sampling round, all subsequent r_AD values fell between 10 and 20 g CO₂ m^-2 d^-1. r_OX had the smallest activity contribution at both sites, with averages ranging from 1.6 to 8.6 g CO₂ m^-2 d^-1. This study has demonstrated that for an interim cover, composting and anaerobic digestion of shallow landfill waste can occur simultaneously.

Determination of gas recovery efficiency at two Danish landfills by performing downwind methane measurements and stable carbon isotopic analysis

In this study, the total methane (CH₄) generation rate and gas recovery efficiency at two Danish landfills were determined by field measurements. The landfills are located close to each other and are connected to the same gas collection system. The tracer gas dispersion method was used for quantification of CH₄ emissions from the landfills, while the CH₄ oxidation efficiency in the landfill cover layers was determined by stable carbon isotopic technique. The total CH₄ generation rate was estimated by a first-order decay model (Afvalzorg) and was compared with the total CH₄ generation rate determined by field measurements. CH₄ emissions from the two landfills combined ranged from 29.1 to 49.6 kg CH₄/h. The CH₄ oxidation efficiency was 6-37%, with an average of 18% corresponding to an average CH₄ oxidation rate of 8.1 kg CH₄/h. The calculated gas recovery efficiency was 59-76%, indicating a high potential for optimization of the gas collection system. Higher gas recovery efficiencies (73-76%) were observed after the commencement of gas extraction from a new section of one of the landfills. A good agreement was observed between the average total CH₄ generation rates determined by field measurements (147 kg CH₄/h) and those estimated by the Afvalzorg model (154 kg CH₄/h).

Anaerobic methanotrophic communities thrive in deep submarine permafrost

Thawing submarine permafrost is a source of methane to the subsurface biosphere. Methane oxidation in submarine permafrost sediments has been proposed, but the responsible microorganisms remain uncharacterized. We analyzed archaeal communities and identified distinct anaerobic methanotrophic assemblages of marine and terrestrial origin (ANME-2a/b, ANME-2d) both in frozen and completely thawed submarine permafrost sediments. Besides archaea potentially involved in anaerobic oxidation of methane (AOM) we found a large diversity of archaea mainly belonging to Bathyarchaeota, Thaumarchaeota, and Euryarchaeota. Methane concentrations and δ¹³C-methane signatures distinguish horizons of potential AOM coupled either to sulfate reduction in a sulfate-methane transition zone (SMTZ) or to the reduction of other electron acceptors, such as iron, manganese or nitrate. Analysis of functional marker genes (mcrA) and fluorescence in situ hybridization (FISH) corroborate potential activity of AOM communities in submarine permafrost sediments at low temperatures. Modeled potential AOM consumes 72-100% of submarine permafrost methane and up to 1.2 Tg of carbon per year for the total expected area of submarine permafrost. This is comparable with AOM habitats such as cold seeps. We thus propose that AOM is active where submarine permafrost thaws, which should be included in global methane budgets.

Characterization of methane oxidation in a simulated landfill cover system by comparing molecular and stable isotope mass balances

Biological methane oxidation may be regarded as a method of aftercare treatment for landfills to reduce climate relevant methane emissions. It is of social and economic interest to estimate the behavior of bacterial methane oxidation in aged landfill covers due to an adequate long-term treatment of the gas emissions. Different approaches assessing methane oxidation in laboratory column studies have been investigated by other authors recently. However, this work represents the first study in which three independent approaches, ((i) mass balance, (ii) stable isotope analysis, and (iii) stoichiometric balance of product (CO₂) and reactant (CH₄) by CO₂/CH₄-ratio) have been compared for the estimation of the biodegradation by a robust statistical validation on a rectangular, wide soil column. Additionally, an evaluation by thermal imaging as a potential technique for the localization of the active zone of bacterial methane oxidation has been addressed in connection with stable isotope analysis and CO₂/CH₄-ratios. Although landfills can be considered as open systems the results for stable isotope analysis based on a closed system correlated better with the mass balance than calculations based on an open system. CO₂/CH₄-ratios were also in good agreement with mass balance. In general, highest values for biodegradation were determined from mass balance, followed by CO₂/CH₄-ratio, and stable isotope analysis. The investigated topsoil proved to be very suitable as a potential cover layer by removing up to 99% of methane for CH₄ loads of 35-65gm^-2d^-1 that are typical in the aftercare phase of landfills. Finally, data from stable isotope analysis and the CO₂/CH₄-ratios were used to trace microbial activity within the reactor system. It was shown that methane consumption and temperature increase, as a cause of high microbial activity, correlated very well.

Evaluating methane inventories by isotopic analysis in the London region

A thorough understanding of methane sources is necessary to accomplish methane reduction targets. Urban environments, where a large variety of methane sources coexist, are one of the most complex areas to investigate. Methane sources are characterised by specific δ¹³C-CH₄ signatures, so high precision stable isotope analysis of atmospheric methane can be used to give a better understanding of urban sources and their partition in a source mix. Diurnal measurements of methane and carbon dioxide mole fraction, and isotopic values at King’s College London, enabled assessment of the isotopic signal of the source mix in central London. Surveys with a mobile measurement system in the London region were also carried out for detection of methane plumes at near ground level, in order to evaluate the spatial allocation of sources suggested by the inventories. The measured isotopic signal in central London (−45.7 ±0.5‰) was more than 2‰ higher than the isotopic value calculated using emission inventories and updated δ¹³C-CH₄ signatures. Besides, during the mobile surveys, many gas leaks were identified that are not included in the inventories. This suggests that a revision of the source distribution given by the emission inventories is needed.

A mass balance model to estimate the rate of composting, methane oxidation and anaerobic digestion in soil covers and shallow waste layers

Although CH₄ oxidation in landfill soil covers is widely studied, the extent of composting and CH₄ oxidation in underlying waste layers has been speculated but not measured. The objective of this study was to develop and validate a mass balance model to estimate the simultaneous rates of anaerobic digestion (r_AD), CH₄ oxidation (r_OX) and composting (r_COM) in environments where O₂ penetration is variable and zones of aerobic and anaerobic activity are intermingled. The modelled domain could include, as an example, a soil cover and the underlying shallow waste to a nominated depth. The proposed model was demonstrated on a blend of biogas from three separate known sources of gas representing the three reaction processes: (i) a bottle of laboratory grade 50:50% CH₄:CO₂ gas representing anaerobic digestion biogas; (ii) an aerated 250mL bottle containing food waste that represented composting activity; and (iii) an aerated 250mL bottle containing non-degradable graphite granules inoculated with methanotrophs and incubated with CH₄ and O₂ to represent methanotrophic activity. CO₂, CH₄, O₂ and the stable isotope ¹³C-CO₂ were chosen as the components for the mass balance model. The three reaction rates, r (=r_AD, r_OX, r_COM) were calculated as fitting parameters to the overdetermined set of 4mass balance equations with the net flux of these components from the bottles q (= [Formula: see text] , [Formula: see text] , [Formula: see text] and [Formula: see text] ) as inputs to the model. The coefficient of determination (r²) for observed versus modelled values of r were 1.00, 0.97, 0.98 when the stoichiometry of each reaction was based on gas yields measured in the individual bottles and q was calculated by summing yields from the three bottles. r² deteriorated to 0.95, 0.96, 0.87 when using an average stoichiometry from 11 incubations of each of the composting and methane oxidation processes. The significant deterioration in the estimation of r_COM showed that this output is highly sensitive to the evaluated stoichiometry coefficients for the reactions. r² deteriorated further to 0.86, 0.77, 0.74 when using the average stoichiometry and experimental measurement of the composition and volume of the blended biogas to determine q. This was primarily attributed to average errors of 8%, 7%, 11% and 14% in the measurement of [Formula: see text] , [Formula: see text] , [Formula: see text] and [Formula: see text] relative to the measurement of the same quantities from the individual bottles.

Evidence of Sulfate-Dependent Anaerobic Methane Oxidation within an Area Impacted by Coalbed Methane-Related Gas Migration

We evaluated water quality characteristics in the northern Raton Basin of Colorado and documented the response of the Poison Canyon aquifer system several years after upward migration of methane gas occurred from the deeper Vermejo Formation coalbed production zone. Results show persistent secondary water quality impacts related to the biodegradation of methane. We identify four distinct characteristics of groundwater-methane attenuation in the Poison Canyon aquifer: (i) consumption of methane and sulfate and production of sulfide and bicarbonate, (ii) methane loss coupled to production of higher molecular weight (C₂₊) gaseous hydrocarbons, (iii) patterns of ¹³C enrichment and depletion in methane and dissolved inorganic carbon, and (iv) a systematic shift in sulfur and oxygen isotope ratios of sulfate, indicative of microbial sulfate reduction. We also show that the biogeochemical response of the aquifer system has not mobilized naturally occurring trace metals, including arsenic, chromium, cobalt, nickel, and lead, likely due to the microbial production of hydrogen sulfide which favors stabilization of metals in aquifer solids.

Isotopic insights into methane production, oxidation, and emissions in Arctic polygon tundra

Arctic wetlands are currently net sources of atmospheric CH4 . Due to their complex biogeochemical controls and high spatial and temporal variability, current net CH4 emissions and gross CH4 processes have been difficult to quantify, and their predicted responses to climate change remain uncertain. We investigated CH4 production, oxidation, and surface emissions in Arctic polygon tundra, across a wet-to-dry permafrost degradation gradient from low-centered (intact) to flat- and high-centered (degraded) polygons. From 3 microtopographic positions (polygon centers, rims, and troughs) along the permafrost degradation gradient, we measured surface CH4 and CO2 fluxes, concentrations and stable isotope compositions of CH4 and DIC at three depths in the soil, and soil moisture and temperature. More degraded sites had lower CH4 emissions, a different primary methanogenic pathway, and greater CH4 oxidation than did intact permafrost sites, to a greater degree than soil moisture or temperature could explain. Surface CH4 flux decreased from 64 nmol m(-2)  s(-1) in intact polygons to 7 nmol m(-2)  s(-1) in degraded polygons, and stable isotope signatures of CH4 and DIC showed that acetate cleavage dominated CH4 production in low-centered polygons, while CO2 reduction was the primary pathway in degraded polygons. We see evidence that differences in water flow and vegetation between intact and degraded polygons contributed to these observations. In contrast to many previous studies, these findings document a mechanism whereby permafrost degradation can lead to local decreases in tundra CH4 emissions.

Winter precipitation and snow accumulation drive the methane sink or source strength of Arctic tussock tundra

Arctic winter precipitation is projected to increase with global warming, but some areas will experience decreases in snow accumulation. Although Arctic CH4 emissions may represent a significant climate forcing feedback, long-term impacts of changes in snow accumulation on CH4 fluxes remain uncertain. We measured ecosystem CH4 fluxes and soil CH4 and CO2 concentrations and (13) C composition to investigate the metabolic pathways and transport mechanisms driving moist acidic tundra CH4 flux over the growing season (Jun-Aug) after 18 years of experimental snow depth increases and decreases. Deeper snow increased soil wetness and warming, reducing soil %O2 levels and increasing thaw depth. Soil moisture, through changes in soil %O2 saturation, determined predominance of methanotrophy or methanogenesis, with soil temperature regulating the ecosystem CH4 sink or source strength. Reduced snow (RS) increased the fraction of oxidized CH4 (Fox) by 75-120% compared to Ambient, switching the system from a small source to a net CH4 sink (21 ± 2 and -31 ± 1 mg CH4  m(-2)  season(-1) at Ambient and RS). Deeper snow reduced Fox by 35-40% and 90-100% in medium- (MS) and high- (HS) snow additions relative to Ambient, contributing to increasing the CH4 source strength of moist acidic tundra (464 ± 15 and 3561 ± 97 mg CH4  m(-2)  season(-1) at MS and HS). Decreases in Fox with deeper snow were partly due to increases in plant-mediated CH4 transport associated with the expansion of tall graminoids. Deeper snow enhanced CH4 production within newly thawed soils, responding mainly to soil warming rather than to increases in acetate fermentation expected from thaw-induced increases in SOC availability. Our results suggest that increased winter precipitation will increase the CH4 source strength of Arctic tundra, but the resulting positive feedback on climate change will depend on the balance between areas with more or less snow accumulation than they are currently facing.

Carbon isotope fractionation reveals distinct process of CH4 emission from different compartments of paddy ecosystem

Carbon isotopic fractionations in the processes of CH4 emission from paddy field remain poorly understood. The δ(13)C-values of CH4 in association with production, oxidation and transport of CH4 in different pools of a paddy field were determined, and the stable carbon isotope fractionations were calibrated to assess relative contribution of acetate to CH4 production (fac) and fraction of CH4 oxidized (fox) by different pathways. The apparent isotope fractionation for CO2 conversion to CH4 (αapp) was 1.041-1.056 in the soil and 1.046-1.080 on the roots, indicating that fac was 10-60% and 0-50%, respectively. Isotope fractionation associated with CH4 oxidation (αox) was 1.021 ± 0.007 in the soil and 1.013 ± 0.005 on the roots, and the transport fractionation (εtransport) by rice plants was estimated to be -16.7‰ ~ -11.1‰. Rhizospheric fox was about 30-100%, and it was more important at the beginning but decreased fast towards the end of season. Large value of fox was also observed at the soil-water interface and soil and roots surfaces, respectively. The results demonstrate that carbon isotopic fractionations which might be different in different conditions were sensitive to the estimations of fac and fox in paddy field.

Soil Methane Sink Capacity Response to a Long-Term Wildfire Chronosequence in Northern Sweden

Boreal forests occupy nearly one fifth of the terrestrial land surface and are recognised as globally important regulators of carbon (C) cycling and greenhouse gas emissions. Carbon sequestration processes in these forests include assimilation of CO2 into biomass and subsequently into soil organic matter, and soil microbial oxidation of methane (CH4). In this study we explored how ecosystem retrogression, which drives vegetation change, regulates the important process of soil CH4 oxidation in boreal forests. We measured soil CH4 oxidation processes on a group of 30 forested islands in northern Sweden differing greatly in fire history, and collectively representing a retrogressive chronosequence, spanning 5000 years. Across these islands the build-up of soil organic matter was observed to increase with time since fire disturbance, with a significant correlation between greater humus depth and increased net soil CH4 oxidation rates. We suggest that this increase in net CH4 oxidation rates, in the absence of disturbance, results as deeper humus stores accumulate and provide niches for methanotrophs to thrive. By using this gradient we have discovered important regulatory controls on the stability of soil CH4 oxidation processes that could not have not been explored through shorter-term experiments. Our findings indicate that in the absence of human interventions such as fire suppression, and with increased wildfire frequency, the globally important boreal CH4 sink could be diminished.

Comparison of first-order-decay modeled and actual field measured municipal solid waste landfill methane data

The first-order decay (FOD) model is widely used to estimate landfill gas generation for emissions inventories, life cycle assessments, and regulation. The FOD model has inherent uncertainty due to underlying uncertainty in model parameters and a lack of opportunities to validate it with complete field-scale landfill data sets. The objectives of this paper were to estimate methane generation, fugitive methane emissions, and aggregated collection efficiency for landfills through a mass balance approach using the FOD model for gas generation coupled with literature values for cover-specific collection efficiency and methane oxidation. This study is unique and valuable because actual field data were used in comparison with modeled data. The magnitude and variation of emissions were estimated for three landfills using site-specific model parameters and gas collection data, and compared to vertical radial plume mapping emissions measurements. For the three landfills, the modeling approach slightly under-predicted measured emissions and over-estimated aggregated collection efficiency, but the two approaches yielded statistically equivalent uncertainties expressed as coefficients of variation. Sources of uncertainty include challenges in large-scale field measurement of emissions and spatial and temporal fluctuations in methane flow balance components (generated, collected, oxidized, and emitted methane). Additional publication of sets of field-scale measurement data and methane flow balance components will reduce the uncertainty in future estimates of fugitive emissions.

Methanogenic pathway and fraction of CH(4) oxidized in paddy fields: seasonal variation and effect of water management in winter fallow season

A 2-year field and incubation experiment was conducted to investigate δ¹³C during the processes of CH₄ emission from the fields subjected to two water managements (flooding and drainage) in the winter fallow season, and further to estimate relative contribution of acetate to total methanogenesis (F_ac) and fraction of CH₄ oxidized (F_ox) based on the isotopic data. Compared with flooding, drainage generally caused CH₄, either anaerobically or aerobically produced, depleted in ¹³C. There was no obvious difference between the two in transport fractionation factor (ε_transport) and δ¹³C-value of emitted CH₄. CH₄ emission was negatively related to its δ¹³C-value in seasonal variation (P<0.01). Acetate-dependent methanogenesis in soil was dominant (60–70%) in the late season, while drainage decreased F_ac-value by 5–10%. On roots however, CH₄ was mostly produced through H₂/CO₂ reduction (60–100%) over the season. CH₄ oxidation mainly occurred in the first half of the season and roughly 10–90% of the CH₄ was oxidized in the rhizosphere. Drainage increased F_ox-value by 5–15%, which is possibly attributed to a significant decrease in production while no simultaneous decrease in oxidation. Around 30–70% of the CH₄ was oxidized at the soil-water interface when CH₄ in pore water was released into floodwater, although the amount of CH₄ oxidized therein might be negligible relative to that in the rhizosphere. CH₄ oxidation was also more important in the first half of the season in lab conditions and about 5–50% of the CH₄ was oxidized in soil while almost 100% on roots. Drainage decreased F_ox-value on roots by 15% as their CH₄ oxidation potential was highly reduced. The findings suggest that water management in the winter fallow season substantially affects F_ac in the soil and F_ox in the rhizosphere and roots rather than F_ac on roots and F_ox at the soil-water interface.

Assessing methane oxidation under landfill covers and its contribution to the above atmospheric CO₂ levels: the added value of the isotope (δ¹³C and δ¹⁸O CO₂; δ¹³C and δD CH₄) approach

We are presenting here a multi-isotope approach (δ¹³C and δ¹⁸O of CO₂; δ¹³C and δD of CH₄) to assess (i) the level(s) of methane oxidation during waste biodegradation and its migration through a landfill cover in Sonzay (France), and (ii) its contribution to the atmospheric CO₂ levels above the surface. The isotope approach is compared to the more conventional mass balance approach. Results from the two techniques are comparable and show that the CH₄ oxidation under the landfill cover is heterogenous, with low oxidation percentages in samples showing high biogas fluxes, which was expected in clay covers presenting fissures, through which CH₄ is rapidly transported. At shallow depth, more immobile biogas pockets show a higher level of CH₄ oxidation by the methanotrophic bacteria. δ¹³C of CO₂ samples taken at different heights (from below the cover up to 8m above the ground level) were also used to identify and assess the relative contributions of its main sources both under the landfill cover and in the surrounding atmosphere.

Effect of biocover equipped with a novel passive air diffusion system on microbial methane oxidation and community of methanotrophs

A novel biocover with passive air diffusion system (PADS) was designed in this study. Its effect on landfill gas components in the macrocosms of simulated biocover systems was also investigated. The results show that O2 concentration increased in the whole profile of the macrocosms equipped with PADS. When simulated landfill gas (SLFG) flow rate was no more than 40 mL min(-1), the methane oxidation rate was 100%. The highest CH4 oxidation capacity reached to 31.34 mol m(-3) day(-1). Molecular microbiology analysis of the soil samples taken from the above macrocosm showed that the growth of type I methanotrophs was enhanced, attributable to enhanced air diffusion and distribution, whereas the microbial diversity and population density of type II methanotrophs were not so affected, as evidenced by the absence of any difference between the biocover equipped with PADS and that of the control. According to a phylogenic analysis, Methylobacter Methylosarcinafor type I, and Methylocystis, Methylosinus for type II, were the most prevalent species in the macrocosm with PADS.

Structure and function of methanotrophic communities in a landfill-cover soil

In landfill-cover soils, aerobic methane-oxidizing bacteria (MOB) convert CH(4) to CO(2), mitigating emissions of the greenhouse gas CH(4) to the atmosphere. We investigated overall MOB community structure and assessed spatial differences in MOB diversity, abundance and activity in a Swiss landfill-cover soil. Molecular cloning, terminal restriction-fragment length polymorphism (T-RFLP) and quantitative PCR of pmoA genes were applied to soil collected from 16 locations at three different depths to study MOB community structure, diversity and abundance; MOB activity was measured in the field using gas push-pull tests. The MOB community was highly diverse but dominated by Type Ia MOB, with novel pmoA sequences present. Type II MOB were detected mainly in deeper soil with lower nutrient and higher CH(4) concentrations. Substantial differences in MOB community structure were observed between one high- and one low-activity location. MOB abundance was highly variable across the site [4.0 × 10(4) to 1.1 × 10(7) (g soil dry weight)(-1)]. Potential CH(4) oxidation rates were high [1.8-58.2 mmol CH(4) (L soil air)(-1) day(-1) ] but showed significant lateral variation and were positively correlated with mean CH(4) concentrations (P < 0.01), MOB abundance (P < 0.05) and MOB diversity (weak correlation, P < 0.17). Our findings indicate that Methylosarcina and closely related MOB are key players and that MOB abundance and community structure are driving factors in CH(4) oxidation at this landfill.

Can a breathing biocover system enhance methane emission reduction from landfill?

Based on the aerothermodynamic principles, a kind of breathing biocover system was designed to enhance O(2) supply efficiency and methane (CH(4)) oxidation capacity. The research showed that O(2) concentration (v/v) considerably increased throughout whole profiles of the microcosm (1m) equipped with passive air venting system (MPAVS). When the simulated landfill gas SLFG flow was 771 g m(-3) d(-1) and 1028 g m(-3) d(-1), the O(2) concentration in MPAVS increased gradually and tended to be stable at the atmospheric level after 10 days. The CH(4) oxidation rate was 100% when the SLFG flow rate was no more than 1285 g m(-3) d(-1), which also was confirmed by the mass balance calculations. The breathing biocover system with in situ self-oxygen supply can address the problem of O(2) insufficient in conventional landfill covers and/or biocovers. The proposed system presents high potential for improving CH(4) emission reduction in landfills.

Mitigation of methane emission from Fakse landfill using a biowindow system

Landfills are significant sources of atmospheric methane (CH(4)) that contributes to climate change, and therefore there is a need to reduce CH(4) emissions from landfills. A promising cost efficient technology is to integrate compost into landfill covers (so-called "biocovers") to enhance biological oxidation of CH(4). A full scale biocover system to reduce CH(4) emissions was installed at Fakse landfill, Denmark using composted yard waste as active material supporting CH(4) oxidation. Ten biowindows with a total area of 5000 m(2) were integrated into the existing cover at the 12 ha site. To increase CH(4) load to the biowindows, leachate wells were capped, and clay was added to slopes at the site. Point measurements using flux chambers suggested in most cases that almost all CH(4) was oxidized, but more detailed studies on emissions from the site after installation of the biocover as well as measurements of total CH(4) emissions showed that a significant portion of the emission quantified in the baseline study continued unabated from the site. Total emission measurements suggested a reduction in CH(4) emission of approximately 28% at the end of the one year monitoring period. This was supported by analysis of stable carbon isotopes which showed an increase in oxidation efficiency from 16% to 41%. The project documented that integrating approaches such a whole landfill emission measurements using tracer techniques or stable carbon isotope measurements of ambient air samples are needed to document CH(4) mitigation efficiencies of biocover systems. The study also revealed that there still exist several challenges to better optimize the functionality. The most important challenges are to control gas flow and evenly distribute the gas into the biocovers.

Scaling methane oxidation: from laboratory incubation experiments to landfill cover field conditions

Evaluating field-scale methane oxidation in landfill cover soils using numerical models is gaining interest in the solid waste industry as research has made it clear that methane oxidation in the field is a complex function of climatic conditions, soil type, cover design, and incoming flux of landfill gas from the waste mass. Numerical models can account for these parameters as they change with time and space under field conditions. In this study, we developed temperature, and water content correction factors for methane oxidation parameters. We also introduced a possible correction to account for the different soil structure under field conditions. These parameters were defined in laboratory incubation experiments performed on homogenized soil specimens and were used to predict the actual methane oxidation rates to be expected under field conditions. Water content and temperature corrections factors were obtained for the methane oxidation rate parameter to be used when modeling methane oxidation in the field. To predict in situ measured rates of methane with the model it was necessary to set the half saturation constant of methane and oxygen, K(m), to 5%, approximately five times larger than laboratory measured values. We hypothesize that this discrepancy reflects differences in soil structure between homogenized soil conditions in the lab and actual aggregated soil structure in the field. When all of these correction factors were re-introduced into the oxidation module of our model, it was able to reproduce surface emissions (as measured by static flux chambers) and percent oxidation (as measured by stable isotope techniques) within the range measured in the field.

Observations on the methane oxidation capacity of landfill soils

The objective of this study was to determine the role of CH(4) loading to a landfill cover in the control of CH(4) oxidation rate (gCH(4)m(-2)d(-1)) and CH(4) oxidation efficiency (% CH(4) oxidation) in a field setting. Specifically, we wanted to assess how much CH(4) a cover soil could handle. To achieve this objective we conducted synoptic measurements of landfill CH(4) emission and CH(4) oxidation in a single season at two Southeastern USA landfills. We hypothesized that percent oxidation would be greatest at sites of low CH(4) emission and would decrease as CH(4) emission rates increased. The trends in the experimental results were then compared to the predictions of two differing numerical models designed to simulate gas transport in landfill covers, one by modeling transport by diffusion only and the second allowing both advection and diffusion. In both field measurements and in modeling, we found that percent oxidation is a decreasing exponential function of the total CH(4) flux rate (CH(4) loading) into the cover. When CH(4) is supplied, a cover's rate of CH(4) uptake (gCH(4)m(-2)d(-2)) is linear to a point, after which the system becomes saturated. Both field data and modeling results indicate that percent oxidation should not be considered as a constant value. Percent oxidation is a changing quantity and is a function of cover type, climatic conditions and CH(4) loading to the bottom of the cover. The data indicate that an effective way to increase the % oxidation of a landfill cover is to limit the amount of CH(4) delivered to it.

Seasonal greenhouse gas emissions (methane, carbon dioxide, nitrous oxide) from engineered landfills: daily, intermediate, and final California cover soils

Compared with natural ecosystems and managed agricultural systems, engineered landfills represent a highly managed soil system for which there has been no systematic quantification of emissions from coexisting daily, intermediate, and final cover materials. We quantified the seasonal variability of CH, CO, and NO emissions from fresh refuse (no cover) and daily, intermediate, and final cover materials at northern and southern California landfill sites with engineered gas extraction systems. Fresh refuse fluxes (g m d [± SD]) averaged CH 0.053 (± 0.03), CO 135 (± 117), and NO 0.063 (± 0.059). Average CH emissions across all cover types and wet/dry seasons ranged over more than four orders of magnitude (<0.01-100 g m d) with most cover types, including both final covers, averaging <0.1 g m d with 10 to 40% of surface areas characterized by negative fluxes (uptake of atmospheric CH). The northern California intermediate cover (50 cm) had the highest CH fluxes. For both the intermediate (50-100 cm) and final (>200 cm) cover materials, below which methanogenesis was well established, the variability in gaseous fluxes was attributable to cover thickness, texture, density, and seasonally variable soil moisture and temperature at suboptimal conditions for CH oxidation. Thin daily covers (30 cm local soil) and fresh refuse generally had the highest CO and NO fluxes, indicating rapid onset of aerobic and semi-aerobic processes in recently buried refuse, with rates similar to soil ecosystems and windrow composting of organic waste. This study has emphasized the need for more systematic field quantification of seasonal emissions from multiple types of engineered covers.

Reporting central tendencies of chamber measured surface emission and oxidation

Methane emissions, concentrations, and oxidation were measured on eleven MSW landfills in eleven states spanning from California to Pennsylvania during the three year study. The flux measurements were performed using a static chamber technique. Initial concentration samples were collected immediately after placement of the flux chamber. Oxidation of the emitted methane was evaluated using stable isotope techniques. When reporting overall surface emissions and percent oxidation for a landfill cover, central tendencies are typically used to report "averages" of the collected data. The objective of this study was to determine the best way to determine and report central tendencies. Results showed that 89% of the data sets of collected surface flux have lognormal distributions, 83% of the surface concentration data sets are also lognormal. Sixty seven percent (67%) of the isotope measured percent oxidation data sets are normally distributed. The distribution of data for all eleven landfills provides insight of the central tendencies of emissions, concentrations, and percent oxidation. When reporting the "average" measurement for both flux and concentration data collected at the surface of a landfill, statistical analyses provided insight supporting the use of the geometric mean. But the arithmetic mean can accurately represent the percent oxidation, as measured with the stable isotope technique. We examined correlations between surface CH(4) emissions and surface air CH(4) concentrations. Correlation of the concentration and flux values using the geometric mean proved to be a good fit (R(2)=0.86), indicating that surface scans are a good way of identifying locations of high emissions.

Use of gas push-pull tests for the measurement of methane oxidation in different landfill cover soils

In order to optimise methane oxidation in landfill cover soils, it is important to be able to accurately quantify the amount of methane oxidised. This research considers the gas push-pull test (GPPT) as a possible method to quantify oxidation rates in situ. During a GPPT, a gas mixture consisting of one or more reactive gases (e.g., CH(4), O(2)) and one or more conservative tracers (e.g., argon), is injected into the soil. Following this, the mixture of injected gas and soil air is extracted from the same location and periodically sampled. The kinetic parameters for the biological oxidation taking place in the soil can be derived from the differences in the breakthrough curves. The original method of Urmann et al. (2005) was optimised for application in landfill cover soils and modified to reduce the analytical effort required. Optimised parameters included the flow rate during the injection phase and the duration of the experiment. 50 GPPTs have been conducted at different landfills in Germany during different seasons. Generally, methane oxidation rates ranged between 0 and 150 g m(soil air)(-3)h(-1). At one location, rates up to 440 g m(soil air)(-3)h(-1) were measured under particularly favourable conditions. The method is simple in operation and does not require expensive equipment besides standard laboratory gas chromatographs.

Assessment of the methane oxidation capacity of compacted soils intended for use as landfill cover materials

The microbial oxidation of methane in engineered cover soils is considered a potent option for the mitigation of emissions from old landfills or sites containing wastes of low methane generation rates. A laboratory column study was conducted in order to derive design criteria that enable construction of an effective methane oxidising cover from the range of soils that are available to the landfill operator. Therefore, the methane oxidation capacity of different soils was assessed under simulated landfill conditions. Five sandy potential landfill top cover materials with varying contents of silt and clay were investigated with respect to methane oxidation and corresponding soil gas composition over a period of four months. The soils were compacted to 95% of their specific proctor density, resulting in bulk densities of 1.4-1.7 g cm(-3), reflecting considerably unfavourable conditions for methane oxidation due to reduced air-filled porosity. The soil water content was adjusted to field capacity, resulting in water contents ranging from 16.2 to 48.5 vol.%. The investigated inlet fluxes ranged from 25 to about 100g CH(4)m(-2)d(-1), covering the methane load proposed to allow for complete oxidation in landfill covers under Western European climate conditions and hence being suggested as a criterion for release from aftercare. The vertical distribution of gas concentrations, methane flux balances as well as stable carbon isotope studies allowed for clear process identifications. Higher inlet fluxes led to a reduction of the aerated zone, an increase in the absolute methane oxidation rate and a decline of the relative proportion of oxidized methane. For each material, a specific maximum oxidation rate was determined, which varied between 20 and 95 g CH(4)m(-2)d(-1) and which was positively correlated to the air-filled porosity of the soil. Methane oxidation efficiencies and gas profile data imply a strong link between oxidation capacity and diffusive ingress of atmospheric air. For one material with elevated levels of fine particles and high organic matter content, methane production impeded the quantification of methane oxidation potentials. Regarding the design of landfill cover layers it was concluded that the magnitude of the expected methane load, the texture and expected compaction of the cover material are key variables that need to be known. Based on these, a column study can serve as an appropriate testing system to determine the methane oxidation capacity of a soil intended as landfill cover material.

Flux measurements of benzene and toluene from landfill cover soils

Carbon dioxide and CH(4), C(6)H(6) and C(7)H(8) fluxes from the soil cover of Case Passerini landfill site (Florence, Italy) were measured using the accumulation and static closed chamber methods, respectively. Results show that the CH(4)/CO(2), CH(4)/C(6)H(6) and CH(4)/C(7)H(8) ratios of the flux values are relatively low when compared with those of the 'pristine' biogas produced by degradation processes acting on the solid waste material disposed in the landfill. This suggests that when biogas transits through the cover soil, CH(4) is affected by degradation processes activated by oxidizing bacteria at higher extent than both CO(2) and mono-aromatics. Among the investigated hydrocarbons, C(6)H(6) has shown the highest stability in a wide range of redox conditions. Toluene behaviour only partially resembles that of C(6)H(6), possibly because de-methylation processes require less energy than that necessary for the degradation of C(6)H(6), the latter likely occurring via benzoate at anaerobic conditions and/or through various aerobic metabolic pathways at relatively shallow depth in the cover soil where free oxygen is present. According to these considerations, aromatics are likely to play an important role in the environmental impact of biogas released into the atmosphere from such anthropogenic emission sites, usually only ascribed to CO(2) and CH(4). In this regard, flux measurements using accumulation and static closed chamber methods coupled with gas chromatography and gas chromatography-mass spectrometry analysis may properly be used to obtain a dataset for the estimation of the amount of volatile organic compounds dispersed from landfills.

Landfill methane oxidation across climate types in the U.S

Methane oxidation in landfill covers was determined by stable isotope analyses over 37 seasonal sampling events at 20 landfills with intermediate covers over four years. Values were calculated two ways: by assuming no isotopic fractionation during gas transport, which produces a conservative or minimum estimate, and by assuming limited isotopic fractionation with gas transport producing a higher estimate. Thus bracketed, the best assessment of mean oxidation within the soil covers from chamber captured emitted CH(4) was 37.5 ± 3.5%. The fraction of CH(4) oxidized refers to the fraction of CH(4) delivered to the base of the cover that was oxidized to CO(2) and partitioned to microbial biomass instead of being emitted to the atmosphere as CH(4) expressed as a percentage. Air samples were also collected at the surface of the landfill, and represent CH(4) from soil, from leaking infrastructure, and from cover defects. A similar assessment of this data set yields 36.1 ± 7.2% oxidation. Landfills in five climate types were investigated. The fraction oxidized in arid sites was significantly greater than oxidation in mediterranean sites, or cool and warm continental sites. Sub tropical sites had significantly lower CH(4) oxidation than the other types of sites. This relationship may be explained by the observed inverse relationship between cover loading and fractional CH(4) oxidation.

Effectiveness of a Florida landfill biocover for reduction of CH4 and NMHC emissions

Methane-oxidizing "biocovers" were constructed at the Leon County Landfill (Florida). The primary goal was to determine if a biocover placed above the existing thin (15 cm) intermediate clay cover would be capable of mitigating CH(4) and nonmethane hydrocarbon (NMHC) emissions to the atmosphere in this subtropical environment. A secondary goal was to maximize the use of locally recycled materials for biocover construction. The biocovers consisted of 30 or 60 cm of ground garden waste placed over a 15 cm gas distribution layer (clean crushed recycled glass from discarded fluorescent lights). The deep biocover reduced methane fluxes relative to the controls during temporal monitoring over more than a year; in large part, these reductions were attributable to increased methane oxidation. Both the shallow and the deep biocover exhibited significant percentages of negative fluxes (uptake of atmospheric methane) relative to the nonbiocover controls which had consistently positive fluxes. The overall annual effectiveness/performance of the biocover was limited by seasonally high moisture contents and the thin gas distribution layer. For NMHCs, the deep biocover demonstrated substantial reductions for nonmethane hydrocarbon emissions with high percentages of negative fluxes for several hydrocarbon groups, especially the aromatics, alkanes, and lower chlorinated compounds. Ranges of measured NMHC emissions (10(-9) to 10(-3) g m(-2) d(-1)) were similar to previous studies in the literature. Conservative calculations based on field data for total NMHC emissions from the 60 cm biocover area indicate that current U.S. Environmental Protection Agency (EPA) regulatory methods overestimate emissions by more than 2 orders of magnitude, suggesting that improved field-validated methods are needed.

Microbial methane oxidation processes and technologies for mitigation of landfill gas emissions

Landfill gas containing methane is produced by anaerobic degradation of organic waste. Methane is a strong greenhouse gas and landfills are one of the major anthropogenic sources of atmospheric methane. Landfill methane may be oxidized by methanotrophic microorganisms in soils or waste materials utilizing oxygen that diffuses into the cover layer from the atmosphere. The methane oxidation process, which is governed by several environmental factors, can be exploited in engineered systems developed for methane emission mitigation. Mathematical models that account for methane oxidation can be used to predict methane emissions from landfills. Additional research and technology development is needed before methane mitigation technologies utilizing microbial methane oxidation processes can become commercially viable and widely deployed.

Degradation of C2-C15 volatile organic compounds in a landfill cover soil

The composition of non-methane volatile organic compounds (hereafter VOCs) in i) the cover soil, at depths of 30, 50 and 70 cm, and ii) gas recovery wells from Case Passerini landfill site, (Florence, Italy) was determined by GC-MS. The study, based on the analysis of interstitial gases sampled along vertical profiles within the cover soil, was aimed to investigate the VOC behaviour as biogas transits from a reducing to a relatively more oxidizing environment. A total of 48 and 63 different VOCs were identified in the soil and well gases, respectively. Aromatics represent the dominant group (71.5% of total VOC) in soil gases, followed by alkanes (6.8%), ketones (5.7%), organic acids (5.2%), aldehydes (3.0%), esters (2.6%), halogenated compounds (2.1%) and terpenes (1.3%). Cyclics, heterocyclics, S-bearing compounds and phenols are <or=1%. In the wells the VOC composition is characterized by higher concentrations of cyclic (7.6%) and S-bearing compounds (2%) and lower concentrations of O-bearing compounds. The vertical distribution of VOCs in the cover soil shows significant variations: alkanes, aromatics and cyclics decrease at decreasing depth, whereas an inverse trend is displayed by the O-bearing species. Total VOC and CH(4) concentrations at a depth of 30 cm in the soil are comparable, inferring that microbial activity is likely affecting VOCs at a very minor extent with respect to CH(4). According to these considerations, to assess the biogas emission impact, usually carried out on the sole basis of CO(2) and CH(4) emission rates, the physical-chemical behaviour of VOCs in the cover soil, regulating the discharge of these highly contaminant compounds in ambient air, has to be taken into account. The soil vertical distribution of these species can be used to better evaluate the efficiency of oxidative capability of intermediate and final covers.

Effects of compost biocovers on gas flow and methane oxidation in a landfill cover

Previous publications described the performance of biocovers constructed with a compost layer placed on select areas of a landfill surface characterized by high emissions from March 2004 to April 2005. The biocovers reduced CH(4) emissions 10-fold by hydration of underlying clay soils, thus reducing the overall amount of CH(4) entering them from below, and by oxidation of a greater portion of that CH(4). This paper examines in detail the field observations made on a control cell and a biocover cell from January 1, 2005 to December 31, 2005. Field observations were coupled to a numerical model to contrast the transport and attenuation of CH(4) emissions from these two cells. The model partitioned the biocover's attenuation of CH(4) emission into blockage of landfill gas flow from the underlying waste and from biological oxidation of CH(4). Model inputs were daily water content and temperature collected at different depths using thermocouples and calibrated TDR probes. Simulations of CH(4) transport through the two soil columns depicted lower CH(4) emissions from the biocover relative to the control. Simulated CH(4) emissions averaged 0.0gm(-2)d(-1) in the biocover and 10.25gm(-2)d(-1) in the control, while measured values averaged 0.04gm(-2)d(-1) in the biocover and 14gm(-2)d(-1) in the control. The simulated influx of CH(4) into the biocover (2.7gm(-2)d(-1)) was lower than the simulated value passing into the control cell (29.4gm(-2)d(-1)), confirming that lower emissions from the biocover were caused by blockage of the gas stream. The simulated average rate of biological oxidation predicted by the model was 19.2gm(-2)d(-1) for the control cell as compared to 2.7gm(-2)d(-1) biocover. Even though its V(max) was significantly greater, the biocover oxidized less CH(4) than the control cell because less CH(4) was supplied to it.

Methane oxidation in landfill cover soils, is a 10% default value reasonable?

We reviewed literature results from 42 determinations of the fraction of methane oxidized and 30 determinations of methane oxidation rate in a variety of soil types and landfill covers. Both column measurements and in situ field measurements were included. The means for the fraction of methane oxidized on transit across the soil covers ranged from 22 to 55% from clayey to sandy material. Mean values for oxidation rate ranged from 3.7 to 6.4 mol m(-2) d(-1) (52-102 g m(-2) d(-1)) for the different soil types. The overall mean fraction oxidized across all studies was 36% with a standard error of 6%. The overall mean oxidation rate across all studies was 4.5 mol m(-2) d(-1) +/- 1.0 (72 +/- 16 g m(-2)d(-1)). For the subset of 15 studies conducted over an annual cycle the fraction of methane oxidized ranged from 11 to 89% with a mean value of 35 +/- 6%, nearly identical to the overall mean. Nine of these studies were conducted in north Florida at 30 degrees N latitude and had a fraction oxidized of 27 +/- 4%. Five studies were conducted in northern Europe ( approximately 50-55 degrees N) and exhibited an average of 54 +/- 14%. One study, conducted in New Hampshire, had a value of 10%. The results indicate that the fraction of methane oxidized in landfill greater than the default value of 10%. Of the 42 determinations of methane oxidation reported, only four report values of 10% or less.

Effect of temperature and oxidation rate on carbon-isotope fractionation during methane oxidation by landfill cover materials

The quantification of methane oxidation is one of the major uncertainties in estimating CH4 emissions from landfills. Stable isotope methods provide a useful field approach for the quantification of methane oxidation in landfill cover soils. The approach relies upon the difference between the isotopic composition of oxidized gas at the location of interest and anaerobic zone CH4 and knowledge of alpha(ox), a term that describes the isotopic fractionation of the methanotrophic bacteria in their discrimination against (13)CH4. Natural variability in alpha(0x) in different landfill soils and the effect of temperature and other environmental factors on this parameter are not well defined. Therefore, standard determinations of alpha(ox), batch incubations of landfill cover soils with CH4, were conducted to determine alpha(ox) under a variety of conditions. When these results were combined with those of previous landfill incubation studies, the average alpha(ox) at 25 degrees C was 1.022 +/- 0.0015. alpha(ox) decreased with increasing temperature (-0.00039 alpha(ox) degrees C(-1)) overthe temperature range of 3-35 degrees C. alpha(ox) was found to be higher when determined after CH4-free storage and declined following CH4 pretreatment. alpha(ox) declined nonlinearly with increasing methane oxidation rate, Vmax.

Biotic systems to mitigate landfill methane emissions

Landfill gases produced during biological degradation of buried organic wastes include methane, which when released to the atmosphere, can contribute to global climate change. Increasing use of gas collection systems has reduced the risk of escaping methane emissions entering the atmosphere, but gas capture is not 100% efficient, and further, there are still many instances when gas collection systems are not used. Biotic methane mitigation systems exploit the propensity of some naturally occurring bacteria to oxidize methane. By providing optimum conditions for microbial habitation and efficiently routing landfill gases to where they are cultivated, a number of bio-based systems, such as interim or long-term biocovers, passively or actively vented biofilters, biowindows and daily-used biotarps, have been developed that can alone, or with gas collection, mitigate landfill methane emissions. This paper reviews the science that guides bio-based designs; summarizes experiences with the diverse natural or engineered substrates used in such systems; describes some of the studies and field trials being used to evaluate them; and discusses how they can be used for better landfill operation, capping, and aftercare.

Modelling of stable isotope fractionation by methane oxidation and diffusion in landfill cover soils

A technique to measure biological methane oxidation in landfill cover soils that is gaining increased interest is the measurement of stable isotope fractionation in the methane. Usually to quantify methane oxidation, only fractionation by oxidation is taken into account. Recently it was shown that neglecting the isotope fractionation by diffusion results in underestimation of the methane oxidation. In this study a simulation model was developed that describes gas transport and methane oxidation in landfill cover soils. The model distinguishes between 12CH4, 13CH4, and 12CH3D explicitly, and includes isotope fractionation by diffusion and oxidation. To evaluate the model, the simulations were compared with column experiments from previous studies. The predicted concentration profiles and isotopic profiles match the measured ones very well, with a root mean square deviation (RMSD) of 1.7 vol% in the concentration and a RMSD of 0.8 per thousand in the delta13C value, with delta13C the relative 13C abundance as compared to an international standard. Overall, the comparison shows that a model-based isotope approach for the determination of methane oxidation efficiencies is feasible and superior to existing isotope methods.

Atmospheric emissions and attenuation of non-methane organic compounds in cover soils at a French landfill

In addition to methane (CH(4)) and carbon dioxide (CO(2)), landfill gas may contain more than 200 non-methane organic compounds (NMOCs) including C(2+)-alkanes, aromatics, and halogenated hydrocarbons. Although the trace components make up less than 1% v/v of typical landfill gas, they may exert a disproportionate environmental burden. The objective of this work was to study the dynamics of CH(4) and NMOCs in the landfill cover soils overlying two types of gas collection systems: a conventional gas collection system with vertical wells and an innovative horizontal gas collection layer consisting of permeable gravel with a geomembrane above it. The 47 NMOCs quantified in the landfill gas samples included primarily alkanes (C(2)-C(10)), alkenes (C(2)-C(4)), halogenated hydrocarbons (including (hydro)chlorofluorocarbons ((H)CFCs)), and aromatic hydrocarbons (BTEXs). In general, both CH(4) and NMOC fluxes were all very small with positive and negative fluxes. The highest percentages of positive fluxes in this study (considering all quantified species) were observed at the hotspots, located mainly along cell perimeters of the conventional cell. The capacity of the cover soil for NMOC oxidation was investigated in microcosms incubated with CH(4) and oxygen (O(2)). The cover soil showed a relatively high capacity for CH(4) oxidation and simultaneous co-oxidation of the halogenated aliphatic compounds, especially at the conventional cell. Fully substituted carbons (TeCM, PCE, CFC-11, CFC-12, CFC-113, HFC-134a, and HCFC-141b) were not degraded in the presence of CH(4) and O(2). Benzene and toluene were also degraded with relative high rates. This study demonstrates that landfill soil covers show a significant potential for CH(4) oxidation and co-oxidation of NMOCs.