Bacterial biogas production in coastal systems affected by freshwater inputs

The concentrations of two greenhouse gases, nitrous oxide (N₂O) and methane (CH₄), and the bacterial processes involved in their production (nitrification and denitrification for N₂O, and methanogenesis for CH₄), were determined in surface waters of two coastal areas under the influence of freshwater inputs, on one part in the Gulf of Lions and the Rhone River plume, in northwestern Mediterranean Sea, and on the other part in the inner Thermaikos Gulf, in Aegean Sea, eastern Mediterranean Sea. High concentrations of dissolved CH₄ and N₂O were recorded in the surface waters of Gulf of Lions and Gulf of Thermaikos, up to 1300 nM for CH₄, and 40 nM for N₂O. No direct relationship could be found between the concentration and production of the biogases, as they may also be produced in deep water or bottom sediment in shallow areas, or derived from anthropogenic activity or ship contamination in polluted areas. Irrespective of the origin of CH₄ and N₂O, the presence of extremely high concentrations of these two gases in superficial seawater implies that they can easily escape to the atmosphere; consequently, these nearshore waters enriched in greenhouse gases may play an important role in the increase in atmospheric concentration of both CH₄ and N₂O.     

内陆水体好氧甲烷氧化过程研究进展

内陆水体是全球碳循环的关键组成部分,是大气中甲烷(CH4)的重要来源,每年从内陆淡水与自然湿地排放进入大气的CH4约为185～357 Tg/a.通常,内陆水体中CH4主要由分布于水层底部的厌氧区或沉积层内的产甲烷菌介导产生,其向水层表面传输的过程中易被甲烷氧化菌所氧化.甲烷氧化菌可分为好氧甲烷氧化菌和厌氧甲烷氧化菌,有...     

Hydropower – A Green Energy? Tropical Reservoirs and Greenhouse Gas Emissions

Reservoirs are man‐made lakes that severely impact on river ecosystems, and in addition, the new lake ecosystem can be damaged by several processes. Thus, the benefits of a reservoir, including energy production and flood control, must be measured against their impact on nature. New investigations point out that shallow and tropical reservoirs have high emission rates of the greenhouse gases CO₂ and CH₄. The methane emissions contribute strongly to climate change because CH₄ has a 25 times higher global warming potential than CO₂. The pathways for its production include ebullition, diffuse emission via the water‐air interface, and degassing in turbines and downstream of the reservoir in the spillway and the initial river stretch. Greenhouse gas emissions are promoted by a eutrophic state of the reservoir, and, with higher trophic levels, anaerobic conditions occur with the emission of CH₄. This means that a qualitative and quantitative jump in greenhouse gas emissions takes place. Available data from Petit Saut, French Guinea, provides a first quantification of these pathways. A simple evaluation of the global warming potential of a reservoir can be undertaken using the energy density, the ratio of the reservoir surface and the hydropower capacity; this parameter is mainly determined by the reservoir's morphometry but not by the hydropower capacity. Energy densities of some reservoirs are given and it is clearly seen that some reservoirs have a global warming potential higher than that of coal use for energy production.     

Methane and its transformation processes in water of some tributaries of the Rybinsk Reservoir

Concentration of dissolved methane was determined in the water of some tributaries of the Rybinsk Reservoir subject to different anthropogenic impact; the features of its distribution and the extent of transformations in summer are shown. Its concentration in water of the mouth areas varies within the limits of 2.2 to 280 μl CH₄/l; the rate of methane oxidation is 0.01–230 μl CH₄/(l day). Methanogenesis processes with a rate of 15–28 μl CH₄/(l day) were recorded in surface waters of heavily polluted rivers. A correlation was found to exist between the characteristics of methane cycle and the ecological conditions of the water body.     

Supersaturation and evasion of CO2 and CH4 in surface waters at Mer Bleue peatland,Canada

Carbon dioxide (CO₂) and methane (CH₄) concentrations and evasion rates were measured in surface waters draining Mer Bleue peatland (Ontario, Canada) between spring and autumn 2005. All sites exhibit a consistent pattern of supersaturation throughout the year, which is broadly related to hydrological and temperature changes between spring snowmelt and autumn freezing. Both measurements and estimates of CO₂ and CH₄ evasion from open water to the atmosphere suggest that parts of the catchment (including beaver dams) are significant degassing hot spots. We present data showing how vertical gaseous carbon fluxes compare with lateral carbon fluxes and make an initial estimate of the importance to the overall carbon budget of CO₂ and CH₄ evasion to the atmosphere from water surfaces at Mer Bleue. Copyright © 2007 John Wiley & Sons, Ltd.     

Oxidation and emission of methane in a monomictic lake (Rotsee, Switzerland)

The build-up of methane in the hypolimnion of the eutrophic Lake Rotsee (Lucerne, Switzerland) was monitored over a full year. Sources and sinks of methane in the water column were characterized by measuring concentrations and carbon isotopic composition. In fall, high methane concentrations (up to 1 mM) were measured in the anoxic water layer. In the oxic layer, methane concentrations were much lower and the isotopic composition shifted towards heavy carbon isotopes. Methane oxidation rates peaked at the interface between oxic and anoxic water layers at around 8–10 m depth. The electron balance between the oxidants oxygen, sulphate, and nitrate, and the reductants methane, sulphide and ammonium, matched very well in the chemocline during the stratified season. The profile of carbon isotopic composition of methane showed strong indications for methane oxidation at the chemocline (including the oxycline). Aerobic methane oxidizing bacteria were detected at the interface using fluorescence in situ hybridization. Sequencing the responsible organisms from DGGE bands revealed that aerobic methanotrophs type I closely related to Methylomonas were present. Sulphate consumption occurred at the sediment surface and, only towards the end of the stagnation period, matched with a zone of methane consumption. In any case, the flux of sulphate below the chemocline was not sufficient to oxidize all the methane and other oxidants like nitrate, iron or manganese are necessary for the observed methane oxidation. Although most of the methane was oxidized either aerobically or anaerobically, Lake Rotsee was still a source of methane to the atmosphere with emission rates between 0.2 mg CH₄ m^?2 day^?1 in February and 7 mg CH₄ m^?2 day^?1 in November.     

Application of the Clean Development Mechanism in the Sanitation Sector: “Proof of Concept”

Greenhouse gas emissions from the waste sector account for only 4% of the total production, with wastewater management accounting for accurately 8 to 10% of this contribution. Wastewater disposal and treatment activities, mainly contributes to non‐CO₂ gases such as methane (CH₄) and nitrous oxide (N₂O). Capturing or avoiding these emissions is thus both a concern and an opportunity. The clean development mechanism (CDM) offers an instrument to internalize global climate concerns into the design of wastewater treatment facilities. Properly designed facilities could improve effluent quality and optimize the abatement of greenhouse gas emissions, thus ensuring additional revenues to pay for capital, operation and maintenance costs and possibly justify higher levels of wastewater treatment. This document summarizes the experience of the “Rio Frio CDM project” in Colombia, as an example of what is achievable through the CDM application in wastewater treatment upgrade in developing countries. This document summarizes the scope of the project, the methodology used to establish current greenhouse emissions and future reductions, and the estimated financial results.     

Methane Formation and Consumption Processes in the Littoral Zone of the Rybinsk Reservoir

Concentrations of CH₄ and the rates of processes of its transformation and oxidation are evaluated in the littoral zone of a large polimictic reservoir in the open-water period. The production of CH₄ in bottom sediment was found to vary within 0.07 to 17.32 mmol/(m² day) and attain its maximum in the late summer. The main factors to control the activity of methanogens were found to be the temperature and the availability of substrates (acetate and H₂/CO₂). The newly formed methane was oxidized in the top soil layer with a rate of 0.09–17.15 mmol/(m² day). The rates of processes of biogenic production and consumption of CH₄ in silts of screened coast were higher than those in sands of the open coastal zone. Methanotrophs were found to consume >87% of the methane produced. These bacteria accounted for 20 and 8% of the total oxygen consumption by bottom sediment in the screened and open littoral zones, respectively.     

Seasonal variation of fluxes and distributions of dissolved methane in the North Yellow Sea

The concentrations and sea-to-air fluxes of dissolved methane (CH₄) were investigated in the North Yellow Sea during August 2006, January, April and October 2007. Dissolved CH₄ concentrations showed obvious seasonal variation, with maximum values occurring in summer and lowest values occurring in winter. The saturations of dissolved CH₄ in surface waters ranged from 78.7% to 1679.7% with an average of 252.4%. The estimated atmospheric CH₄ fluxes using the Liss and Merlivat (LM86), and Wanninkhof formulae (W92) were (4.2±4.7), (11.6±10.3), (8.5±12.7) and (0.2±1.0), and (6.9±7.3), (14.6±22.3), (13.8±14.3) and (0.4±1.7) μmol·(m² d)⁻¹, respectively, for spring, summer, autumn and winter. Based on the average annual atmospheric CH₄ flux and the area of the North Yellow Sea, the annual CH₄ emission was estimated to be (2.4×10⁻²–4.2×10⁻²) Tg a⁻¹, which suggests that the North Yellow Sea was a net source of atmospheric CH₄.     

Spatial–temporal variations of methane emissions from the Ertan hydroelectric reservoir in southwest China

Methane emissions from hydroelectric reservoirs can comprise a considerable portion of anthropogenic methane. However, lack of data on CH₄ emissions in different geographical regions and high spatial‐temporal variability in the emission rates of reservoirs has led to uncertainties regarding regional emission estimates of CH₄. In the subtropical plateau climate region, we used the Ertan hydroelectric reservoir as a study area. The CH₄ flux at the air‐water interface was assessed by floating chambers and factors influencing emissions, including the distance from the dam, water depth, seasonal variation in wet and dry season, air‐water temperature gradient and wind speed, and was also studied through a year‐long systematic sampling and monitoring experiment. The results showed that the surface of the reservoir was a source of CH₄ during the sampling period and the annual average CH₄ flux was 2·80 ± 1·52 mg m^?2 d^?1. CH₄ flux (and its variation) was higher in the shallow water areas than in the deep‐water areas. CH₄ flux near the dam was significantly higher than that of other locations farther from the dam in the dry season. The seasonal variations of CH₄ emission in wet and dry seasons were minor and significant diurnal variations were observed in wet and dry seasons. Exponential relationships between the CH₄ flux and air‐water temperature gradient were found. Air‐water temperature gradient was an important factor influencing diurnal variations of CH₄ flux in the Ertan hydroelectric reservoir. These results indicate that systematic sampling is needed to better estimate CH₄ flux through coverage of the spatial variation of different water depths, measuring‐point distance from the dam, seasonal variation in wet and dry seasons and changes in climate factors (such as air‐water temperature gradient). Our results also provide a fundamental parameter for CH₄ emission estimation of global reservoirs. Copyright © 2010 John Wiley & Sons, Ltd.     

Neodymium isotopic variations in seawater

New data for the direct measurement of the isotopic composition of neodymium in Atlantic Ocean seawater are compared with previous measurements of Pacific Ocean seawater and ferromanganese sediments from major ocean basins. Data for Atlantic seawater are in excellent agreement with Nd isotopic measurements made on Atlantic ferromanganese sediments and are distinctly different from the observed compositions of Pacific samples. These results clearly demonstrate the existence of distinctive differences in the isotopic composition of Nd in the waters of the major ocean basins and are characteristic of the ocean basin sampled. The average ε_Nd(0) values for the major oceans as determined by data from seawater and ferromanganese sediments are as follows: Atlantic Ocean,ε_Nd(0) ? ?12 ± 2; Indian Ocean,ε_Nd(0) ? ?8 ± 2; Pacific Ocean,ε_Nd(0) ? ?3 ± 2. These values are considerably less than ε_Nd(0) value sources with oceanic mantle affinities indicating that the REE in the oceans are dominated by continental sources. The difference in the absolute abundance of¹⁴³Nd between the Pacific and Atlantic Oceans corresponds to ～10⁶ atoms¹⁴³Nd per gram of seawater. The correspondence between the¹⁴³Nd/¹⁴⁴Nd in seawater and in the associated sediments suggests the possible application of this approach to paleo-oceanography.Distinctive differences in ε_Nd(0) values are observed in the Atlantic Ocean between deep-ocean water associated with North Atlantic Deep Water and near-surface water. This suggests that North Atlantic Deep Water may be relatively well mixed with respect to Nd isotopic composition whereas near-surface water may be quite heterogeneous, reflecting different sources for surface waters relative to deep water. This suggests that it may be possible to distinguish the sources of water masses within an ocean basin on the basis of Nd isotopic composition.The Nd isotopic variations in seawater are used to relate the residence time of Nd and mixing rates between the oceans.     

Methan- und Sauerstoffhaushalt im mesotrophen Lungernsee

Prealpine Lake Lungern shows in spite of low primary production rates (120 g C/m². year) and full winter overturns a complete oxygen depletion in the deepest hypolimnion (65–70 m below surface) towards the end of summer stagnation. Periodical examinations of O₂- and CH₄-concentrations, CH₄-oxidation rates and temperature in the water column during 1975/76 enabled an O₂-balance of Lake Lungern. The direct measurement of the CH₄-flux at the sediment-water-interface and of the CH₄-concentrations in sediment cores as well as the determination of the age of methane bubbles lead to the conclusion, that the hypolimnic oxygen depletion is partly due to the oxidation of fossile methane penetrating the lake from below.      

Groundwater impact on methane emissions from flooded paddy fields

High methane (CH₄) fluxes emitted from paddy fields strongly contribute to the accumulation of greenhouse gases into the atmosphere, compromising the eco-compatibility of one of the most important world foods. A strong link exists between infiltration rates of irrigation water and CH₄ emissions. Since depth to the groundwater table affects infiltration rates, a relevant groundwater impact is expected on CH₄ emissions from paddy fields. In this work, a theoretical approach is adopted to investigate the aquifer effect on CH₄ dynamics in paddies. Infiltration rates are strongly affected by the development of different connection states between aquifer and irrigation ponded water. A strong reduction in infiltration rates results from a water table near to the soil surface, when the system is hydraulically connected. When the groundwater level increases, the infiltration rate reduction due to the switch from disconnected to connected state promotes a relevant increase of CH₄ emissions. This is due to a strong reduction of dissolved organic carbon (DOC) percolation, which leads to higher DOC availability for microbial CH₄ production and, consequently, higher CH₄ emissions. Our simulations show that CH₄ fluxes can be reduced by up to 24% when groundwater level is decreased and the aquifer is disconnected from ponding water. In paddies with shallow aquifers, lowering the water table with a drainage system could thus represent a promising CH₄ mitigation option.     

A Stream‐Based Methane Monitoring Approach for Evaluating Groundwater Impacts Associated with Unconventional Gas Development

Gaining streams can provide an integrated signal of relatively large groundwater capture areas. In contrast to the point‐specific nature of monitoring wells, gaining streams coalesce multiple flow paths. Impacts on groundwater quality from unconventional gas development may be evaluated at the watershed scale by the sampling of dissolved methane (CH₄) along such streams. This paper describes a method for using stream CH₄ concentrations, along with measurements of groundwater inflow and gas transfer velocity interpreted by 1‐D stream transport modeling, to determine groundwater methane fluxes. While dissolved ionic tracers remain in the stream for long distances, the persistence of methane is not well documented. To test this method and evaluate CH₄ persistence in a stream, a combined bromide (Br) and CH₄ tracer injection was conducted on Nine‐Mile Creek, a gaining stream in a gas development area in central Utah. A 35% gain in streamflow was determined from dilution of the Br tracer. The injected CH₄ resulted in a fivefold increase in stream CH₄ immediately below the injection site. CH₄ and δ¹³C_CH4 sampling showed it was not immediately lost to the atmosphere, but remained in the stream for more than 2000 m. A 1‐D stream transport model simulating the decline in CH₄ yielded an apparent gas transfer velocity of 4.5 m/d, describing the rate of loss to the atmosphere (possibly including some microbial consumption). The transport model was then calibrated to background stream CH₄ in Nine‐Mile Creek (prior to CH₄ injection) in order to evaluate groundwater CH₄ contributions. The total estimated CH₄ load discharging to the stream along the study reach was 190 g/d, although using geochemical fingerprinting to determine its source was beyond the scope of the current study. This demonstrates the utility of stream‐gas sampling as a reconnaissance tool for evaluating both natural and anthropogenic CH₄ leakage from gas reservoirs into groundwater and surface water.     

Role of water flow in modeling methane emissions from flooded paddy soils

Methane (CH₄) is a potent greenhouse gas that is emitted from paddy fields, and the large CH₄ fluxes represent a worldwide issue for the rice production eco-compatibility. In this work a model is proposed to investigate the role of water flows on CH₄ emissions from flooded paddy soils. The model is based on a system of partial differential mass balance equations of the chemical species affecting CH₄ fate, and water flows are modeled by the Darcy equation. Moreover, in order to properly model the dynamics of CH₄, a number of physico-chemical processes and features not included in currently available CH₄ emission models are considered: paddy soil stratigraphy; nutrient adsorption and root water uptake; gas transport and respiration within root aerenchyma compartment. The proposed model allows to simulate the spatio-temporal dynamics of chemical compounds within paddy soil as well as to quantify the influence of different processes on nutrient input/output budgets. Simulations without water flow have shown a considerable overestimation of CH₄ emissions due to a different spatio-temporal dynamics of dissolved organic matter (DOC – source of energy for CH₄ production). In particular, when water fluxes have not been modeled the overestimation can reach 54%, 41% and 67% of daily minimum, daily maximum, and total over the whole growing season CH₄ emission, respectively. Moreover, the model results suggest that roots influence CH₄ dynamics principally due to their nutrient uptake, while root effect on advective flow plays a minor role. Finally, the analysis of CH₄ transport fluxes has shown the limiting effect of upward dispersive transport fluxes on the downward CH₄ percolation.     

Hydrogeochemistry of thermal and mineral water springs of the Azores archipelago (Portugal)

Mineral and thermal water chemistry from the Azores archipelago was investigated in order to discriminate among hydrochemical facies and isotopic groups and identify the major geochemical processes that affect water composition. A systematic geochemical survey of mineral and thermal water chemistry was carried out, incorporating new data as well as results from the literature. The Azores are a volcanic archipelago consisting of nine islands and samples were collected at São Miguel, Graciosa, Faial, São Jorge, Pico and Flores islands. Hydrothermal manifestations show the effects of active volcanism on several islands. Discharges are mainly related to active Quaternary central volcanoes, of basaltic to trachytic composition, but also some springs are related to older dormant or extinct volcanoes.Multivariate analysis – principal component and cluster analysis – enables classification of water compositions into 4 groups and interpretation of processes affecting water compositions. Groups 1 and 2 discharge from perched-water bodies, and mostly correspond to Na–HCO₃ and Na–HCO₃–Cl type waters. These groups comprise of cold, thermal (27 °C–75 °C) and boiling waters (92.2 °C–93.2 °C), with a wide TDS range (77.3–27, 145.7 mg/L). Group 3 is made of samples of dominated Na–SO₄ from very acid boiling pools (pH range of 2.02–2.27) which are fed by steam-heated perched-water bodies. Group 4 is representative of springs from the basal aquifer system and corresponds to Na–Cl type fluids, with compositions dominated by seawater.Results are used to further develop a conceptual model characterizing the geochemical evolution of the studied waters. Mineral and thermal waters discharging from perched-water bodies are of meteoric origin and chemically evolve by absorption of magmatic volatiles (CO₂) and by a limited degree of rock leaching. Existing data also suggest mixture between cold waters and thermal water. Water chemistry from springs that discharge from the basal aquifer system evolves by mixing with seawater; although, processes such as absorption of magmatic volatiles (CO₂), rock leaching and mixture with hydrothermal waters are not excluded by the data because the actual composition of these waters deviates from that expected considering only conservative mixing between fresh and seawater.     

Excess of bottom-released methane in an Arctic shelf sea polynya in winter

Latent heat polynyas are regions generating strong ice formation, convection and extensive water mass formation. Here we report on the effects of these processes on resuspension of sediments and subsequent methane release from the seafloor and on the resulting excess methane concentration in surface water on a polar shelf during winter. The study is based on measurements of concentration and δ¹³C values of methane, water temperature, salinity, light transmission and sea ice data collected in March 2003 in Storfjorden, southern Svalbard. In winter, strong and persistent northeasterly winds create polynyas in eastern Storfjorden and cause ice formation. The resulting brine-enriched water cascades from the Storfjordbanken into the central depression thereby enhancing the turbulence near the seafloor. A distinct benthic nepheloid layer was observed reflecting the resuspension of sediments by the cascading dense bottom water. High concentrations of ¹³C-depleted methane suggest submarine discharge of methane with the resuspended sediments. As the source of the submarine methane, we propose recent bacterial methanogenesis near the sediment surface because of extremely high accumulation rates of organic carbon in Storfjorden. Convective mixing transports newly released methane from the bottom to the sea surface. This eventually results in an excess concentration in surface water with respect to the atmospheric equilibrium, and a sea-air flux of methane during periods of open water. When a new ice cover is formed, methane becomes trapped in the water column and subsequently oxidized. Thus, the residual methane is strongly enriched in ¹³C in relation to the δ¹³_CCH4${\delta }^{13}{\mathrm{C}}_{{\mathrm{CH}}_4}$ signature of atmospheric methane. Our results show that latent heat polynyas may induce a direct pathway for biogases like methane from sediments to the atmosphere through coupling of biogeochemical and oceanographic processes. Extrapolating these processes to all Arctic ocean polynyas, we estimate a transfer of CH₄ between 0.005 and 0.02 Tg yr⁻¹. This is not a large contribution but the fluxes from the polynyas are 20–200 times larger than the ocean average and the methane evasion process in polynyas is certainly one that can be altered under climate change.     

Inspirations from the scientific discovery of the anammox bacteria: A classic example of how scientific principles can guide discovery and development

Anaerobic ammonium oxidation(anammox) is a relatively new pathway within the N cycle discovered in the late 1990 s. This eminent discovery not only modified the classical theory of biological metabolism and matter cycling, but also profoundly influenced our understanding of the energy sources for life. A new member of chemolithoautotrophic microorganisms capable of carbon fixation was found in the vast deep dark ocean. If the discovery of the chemosynthetic ecosystems in the deep-sea hydrothermal vent environments once challenged the old dogma "all living things depend on the sun for growth," the discovery of anammox bacteria that are widespread in anoxic environments fortifies the victory over this dogma. Anammox bacteria catalyze the oxidization of NH_4~+ by using NO_2~- as the terminal electron acceptor to produce N_2. Similar to the denitrifying microorganisms, anammox bacteria play a biogeochemical role of inorganic N removal from the environment. However, unlike heterotrophic denitrifying bacteria, anammox bacteria are chemolithoautotrophs that can generate transmembrane proton motive force, synthesize ATP molecules and further carry out CO_2 fixation through metabolic energy harvested from the anammox process. Although anammox bacteria and the subsequently found ammonia-oxidizing archaea(AOA), another very important group of N cycling microorganisms are both chemolithoautotrophs, AOA use ammonia rather than ammonium as the electron donor and O_2 as the terminal electron acceptor in their energy metabolism. Therefore, the ecological process of AOA mainly takes place in oxic seawater and sediments, while anammox bacteria are widely distributed in anoxic water and sediments, and even in some typical extreme marine environments such as the deep-sea hydrothermal vents and methane seeps. Studies have shown that the anammox process may be responsible for 30%–70% N_2 production in the ocean. In environmental engineering related to nitrogenous wastewater treatment, anammox provides a new technology with low energy consumption, low cost, and high efficiency that can achieve energy saving and emission reduction. However, the discovery of anammox bacteria is actually a hard-won achievement. Early in the 1960 s, the possibility of the anammox biogeochemical process was predicted to exist according to some marine geochemical data. Then in the 1970 s, the existence of anammox bacteria was further predicted via chemical reaction thermodynamic calculations. However, these microorganisms were not found in subsequent decades. What hindered the discovery of anammox bacteria, an important N cycling microbial group widespread in hypoxic and anoxic environments? What are the factors that finally led to their discovery? What are the inspirations that the analyses of these questions can bring to scientific research? This review article will analyze and elucidate the above questions by presenting the fundamental physiological and ecological characteristics of the marine anammox bacteria and the principles of scientific research.     

Impacts of Methane on Carbon Dioxide Storage in Brine Formations

In the context of geological carbon sequestration (GCS), carbon dioxide (CO₂) is often injected into deep formations saturated with a brine that may contain dissolved light hydrocarbons, such as methane (CH₄). In this multicomponent multiphase displacement process, CO₂ competes with CH₄ in terms of dissolution, and CH₄ tends to exsolve from the aqueous into a gaseous phase. Because CH₄ has a lower viscosity than injected CO₂, CH₄ is swept up into a ‘bank’ of CH₄‐rich gas ahead of the CO₂ displacement front. On the one hand, this may provide a useful tracer signal of an approaching CO₂ front. On the other hand, the emergence of gaseous CH₄ is undesirable because it poses a leakage risk of a far more potent greenhouse gas than CO₂ if the cap rock is compromised. Open fractures or faults and wells could result in CH₄ contamination of overlying groundwater aquifers as well as surface emissions. We investigate this process through detailed numerical simulations for a large‐scale GCS pilot project (near Cranfield, Mississippi) for which a rich set of field data is available. An accurate cubic‐plus‐association equation‐of‐state is used to describe the non‐linear phase behavior of multiphase brine‐CH₄‐CO₂ mixtures, and breakthrough curves in two observation wells are used to constrain transport processes. Both field data and simulations indeed show the development of an extensive plume of CH₄‐rich (up to 90 mol%) gas as a consequence of CO₂ injection, with important implications for the risk assessment of future GCS projects.     

Effect of atmospheric pressure and temperature on entrapped gas content in peat

Entrapped biogenic gas in peat can greatly affect peatland biogeochemical and hydrological processes by altering volumetric water content, peat buoyancy, and ‘saturated’ hydraulic conductivity, and by generating over‐pressure zones. These over‐pressure zones further affect hydraulic gradients which influence water and nutrient flow direction and rate. The dynamics of entrapped gas are of global interest because the loss of this gas to the atmosphere via ebullition (bubbling) is likely the dominant transport mechanism of methane (CH₄) to the atmosphere from peatlands, which are the largest natural terrestrial source per annum of atmospheric CH₄. We investigated the relationship between atmospheric pressure and temperature on volumetric gas content (VGC) and CH₄ ebullition using a laboratory peat core incubation experiment. Peat cores were incubated at three temperatures (one core at 4 °C, three cores at 11 °C, and one core at 20 °C) in sealed PVC cylinders, instrumented to measure VGC, pore‐water CH₄ concentrations, and ebullition (volume and CH₄ concentrations). Ebullition events primarily occurred (71% of the time) during periods of falling atmospheric pressure. The duration of the drop in atmospheric pressure had a larger control on ebullition volume than the magnitude of the drop. VGC in the 20 °C core increased from the onset of the experiment and reached a fluctuating but time‐averaged constant level between experiment day 30 and 115. The change in VGC was low for the 11 °C cores for the initial period of the experiment but showed large increases when the growth chamber temperature increased to 20 °C due to a malfunction. The core maintained at 4 °C showed only a small increase in entrapped gas content throughout the experiment. The 20 °C core showed the largest increase in VGC. The increases in VGC occurred despite pore‐water concentrations of CH₄ being below the equilibrium solubility level. Copyright © 2009 John Wiley & Sons, Ltd.