Methane exchange between coal-bearing basins and
the atmosphere: the Ruhr Basin and the Lower Rhine
Embayment, Germany
Thomas Thielemann
a,b, Andreas LuÈcke
a, Gerhard H. Schleser
a, Ralf Littke
b,*
aInstitut fuÈr ErdoÈl und Organische Geochemie (ICG-4), Forschungszentrum JuÈlich, D-52425 JuÈlich, GermanybLehrstuhl fuÈr Geologie, Geochemie und LagerstaÈtten des ErdoÈls und der Kohle, RWTH Aachen, D-52056 Aachen, Germany
Abstract
A precise knowledge of methane exchange processes is required to fully understand the recent rise of atmospheric methane concentration. Three of these processes take place at the lithosphere/atmosphere boundary: bacterial con-sumption of methane and emission of bacterial or thermogenic methane. This study was initiated to quantify these processes on a regional scale in the Ruhr Basin and the Lower Rhine Embayment. Since these areas are subject to bituminous coal and lignite mining, natural and anthropogenically-induced methane exchange processes could be stu-died. The methane emission and consumption rates and their carbon isotope signal were measured at the lithosphere/ atmosphere boundary using ¯ux chambers. On most of the soils studied, methane consumption by bacteria was iden-ti®ed. Thermogenic methane was released only at some of the natural faults examined. In active and abandoned bitu-minous coal mining areas methane emissions were restricted to small areas, where high emission rates were measured. The carbon isotope composition of methane at natural faults and in mining subsidence troughs was typical of ther-mogenic methane (ÿ45 toÿ32%d13C). Methane exchange balancing revealed that natural methane emissions from these two basins represent no source of atmospheric importance. However, methane release by upcast mining shafts dominates the methane exchange processes and is by about two orders of magnitude greater than methane consump-tion by bacterial oxidaconsump-tion in the soils.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Methane balance; Coal bed methane; Methane emissions; Methane consumption; Stable carbon isotopes; Flux chamber; Ruhr Basin; Lower Rhine embayment
1. Introduction
Methane, after carbon dioxide and water, is the third most important atmospheric trace gas, contributing about 23% to the anthropogenic part of the greenhouse eect (Houghton, 1997). Methane concentration reached 1.72 ppmV in 1994 (global average) with an average annual increase of 0.6% (10.3 ppbV, Houghton et al., 1995). Human activities such as rice farming, the keeping of ruminants, biomass burning, land®ll instal-lations, petroleum exploration and coal mining are
important methane sources (Cicerone and Oremland, 1988). They are decisive for an average rise in atmo-spheric methane concentration with time, as indicated by the positive, strong correlation between increase in human world population and atmospheric methane concentration within the last 300 years (Stauer et al., 1985; Blake and Rowland, 1988; Khalil et al., 1989). The overall methane sources range between 400 and 640 Mt per year (Cicerone and Oremland, 1988), with 90% of this gas being oxidized by photochemical and bac-terial processes within the same year (Lelieveld et al., 1993). The annual increase rate of methane is extremely irregular but principally is slowing down (Houghton et al., 1995). In 1992 for a few months this rate even reached negative values (Khalil and Rasmussen, 1993), which might correlate with the economic breakdown of
0146-6380/00/$ - see front matter#2000 Elsevier Science Ltd. All rights reserved. P I I : S 0 1 4 6 - 6 3 8 0 ( 0 0 ) 0 0 1 0 4 - 2
www.elsevier.nl/locate/orggeochem
* Corresponding author. Tel.: 241-80-5748; fax: +49-241-88-88152.
the former Soviet Union and with less emissions from natural gas production and distribution (Houghton, 1997). The precise extents of methane sources and sinks still remain unknown. A better database is needed. Methane emissions from coal mining on a global scale are estimated to be 4 to 11% of the sources (Cicerone and Oremland, 1988). In Germany emissions via upcast mining shafts are known (SchoÈn et al., 1993), but across the earth's surface have never been measured before. They are balanced here.
German hard coal pits are obliged to measure methane emissions via upcast mining shafts. The results are published for single years (Treskow, 1978; 1985; Fitzner, 1989; Zimmermeyer and Seeliger, 1989; Bundesumweltministerium, 1997). As recent gas con-tents of hard coal vary between 0 and 15 m3/t (Freu-denberg et al., 1996) only estimations exist about methane emissions from stored coal and from abandoned mines. Stored coals are assessed to emit 0.8 m3methane/t coal (Bundesumweltministerium, 1997). Eicker and Heû-bruÈgge (1984) calculated that three quarters of the abandoned mines release 98 Mill. m3 of methane per year and project this number to 120 Mill. m3 for all abandoned bituminous coal mines in the Ruhr Basin. Data of degassing from abandoned bituminous coal mines in the Lower Rhine Embayment have not been published. Lignite mining may release about 0.1 m3 methane/t into the atmosphere (SchoÈn et al., 1993).
Water-unsaturated soils are known to bear methane-consuming (methanotrophic) bacteria (Kaserer, 1906; Keller et al., 1983). These bacteria either oxidize atmo-spheric methane which diuses into the soil, or methane which is migrating from a source below (e.g. gas ®eld, coal, land®ll, wetland) into the atmosphere (Whalen and Reeburgh, 1990; Whalen et al., 1990). This self-cleaning potential of soils towards methane emissions must eventually be determined on a global scale. Estimations vary from 1% of the global methane sinks (Born et al., 1990) to 6% (DoÈrr et al., 1993) and 12.5% (Houghton et al., 1995). The reason for such drastic deviations is the small number of measurements on a global scale, which in dierent ecosystems present dierent results. Methane consumption rates on native soils in the tem-perate zone range from 0.08 mg/(m2d) (Koschorreck and Conrad, 1993) to 6.79 mg/(m2d) (Tyler et al., 1994). More precise estimations of the global role of methane consumption by bacteria require a better database with a high resolution in space and time. In this context, new data are provided here for the Ruhr Basin and Lower Rhine Embayment.
In order to balance methane exchange, areas with bacterial and thermogenic methane emissions, as well as methane consumption were considered. Their main characteristics are listed in Table 1. In the Lower Rhine Embayment (LRE) the localities vary in thickness of post-Carboniferous rocks (0 to 900 m). They contain
either Late Carboniferous bituminous coal seams or Tertiary lignite or both in the subsurface. Flux chamber experiments were concentrated at locality LRE 2 (Table 1) to study seasonal variabilities in methane exchange processes in detail. In the Ruhr Basin (RB) localities have been chosen, where post-Carboniferous sediments are 0±1260 m thick above bituminous coal bearing Car-boniferous beds. Lignite deposits do not exist in the Ruhr Basin. Underground mining activities were either continuing or abandoned or absent, as indicated in Fig. 1. Locations RB 7, RB 13 and RB 14 were placed on natural normal faults, to examine their gas exchange characteristics. Locations RB 11 and RB 12 were sam-pled intensively as they showed the most variable methane exchange patterns in the Ruhr Basin.
2. Sediment-, soil- and methane characteristics
In Germany, the Ruhr Basin and Lower Rhine Embayment contain the most important bituminous coal and lignite mines, respectively. They both contain clastic Upper Carboniferous sediments of up to 3.5 km thickness on top of older Paleozoic deposits. This thick sequence contains more than 100 coal seams (Littke, 1987). The Variscan structural evolution of this area is delineated by Drozdzewski (1993), Strack (1989) and Zeller (1987). The subsidence history of both basins from Carboniferous times to Present has been recon-structed by BuÈker et al. (1995) and Karg (1998). The western part of the basin contains Permian, Cretaceous and Cenozoic deposits, in some regions accompanied by Triassic and Jurassic sediments. In the eastern part of the Ruhr Basin, Cretaceous marine sediments uncon-formably overlie the Carboniferous and increase in thickness northward. Quaternary glacial and ¯uvial deposits overly the Cretaceous in some areas. In the southern part of the Lower Rhine Embayment, a Mio-cene peat accumulation led to lignite seams of up to 100 m thickness (Hager and PruÈfert, 1988). Possible methane source rocks are the Upper Carboniferous bituminous coal-bearing strata of both the RB and LRE as well as Tertiary lignite deposits in the LRE.
Exploration wells in both basins indicate that methane is accumulated in the coal seams (99% of all methane) and that the Carboniferous clastic rocks and post-Carboniferous rocks are mainly gas-empty. The only exceptions are Cretaceous sediments which cover the Carboniferous coal-bearing strata in the eastern Ruhr Basin. There, local gas shows have been reported since the beginning of exploration activities (MuÈller, 1904; Wegner, 1924a,b). According to data by Colombo et al. (1970) and TeichmuÈller et al. (1970), most of the
List of seven sampling locations in the Lower Rhine Embayment, 15 locations in the Ruhr Basin and their characteristics. Geological information was taken from Drozdzewski et al. (1982) and Hager and PruÈfert (1988), mining situation from Wrede et al. (1983), Wrede and Zeller (1988) and Hilden et al. (1995)
No. Post-Carboniferous
covering rock thickness (m)
Coal type below
Mining activity
Number of ¯ux chamber experiments
Methane exchange process
Exchange range [mg/(m2d)]
Remarks
LRE1 0 Lignite Abandoned 10 Bacterial consumption 0 toÿ0.31 Lignite outcrop
LRE2 800±900 Lignite None 402 Bacterial consumption ÿ0.15 toÿ3.32 Farmland meadow, forest,
natural fault
LRE3 300-320 Lignite/Hard coal Abandoned 30 Bacterial consumption ÿ0.4 toÿ1.02 Meadow, farmland
LRE4 500±550 Lignite/Hard coal None 8 Bacterial consumption ÿ0.46 toÿ1.6 Farmland
LRE5 340±360 Hard coal Abandoned 5 Bacterial consumption ÿ0.25 toÿ1.26 Farmland
LRE6 200±220 Lignite/Hard coal None 24 Bacterial consumption ÿ0.12 toÿ1.27 Farmland
LRE7 180±220 Lignite/Hard coal None 6 Bacterial consumption ÿ0.67 toÿ1.82 Meadow (fen)
RB1 0 Hard coal None 4 Bacterial consumption ÿ0.30 toÿ0.66 Forest, hard coal outcrop
RB2 0 Hard coal Abandoned 10 Thermogenic emission 354 to 28,607 Day fall, water
RB3 0 Hard coal Abandoned 6 Bacterial consumption ÿ0.24 toÿ0.72 Quarry, hard coal outcrop
RB4 3 Hard coal Abandoned 22 Bact. consum./thermo.emiss. ÿ0.32 toÿ1.64/1.10 to 13.62 Farmland
RB5 60 Hard coal Abandoned 8 Bact. consum./thermo.emiss. ÿ0.40 toÿ1.25/0.69 to 3.82 Meadow
RB6 450±470 Hard coal None 8 Bacterial consumption ÿ0.46 toÿ1.85 Meadow, farmland
RB7 600±620 Hard coal Active 23 Bacterial consumption ÿ0.25 toÿ1.79 Farmland, natural fault
RB8 940±960 Hard coal None 34 Bacterial consumption ÿ0.36 toÿ3.67 Farmland, forest
RB9 480±490 Hard coal None 5 Bacterial consumption ÿ0.21 toÿ0.90 Farmland
RB10 450±470 Hard coal Abandoned 7 Bacterial consumption ÿ0.43 toÿ1.35 Meadow
RB11 500±600 Hard coal Active 84 Bact. consum./thermo.emiss. ÿ0.28 toÿ3.45/16.0 to 78,640 Farmland
RB12 550±650 Hard coal Abandoned 115 Bact. consum./thermo.emiss. ÿ0.54 toÿ4.96/4.3 to 99,830 Meadow, water
RB13 820 Hard coal None 12 Thermogenic emission 6.57 to 274 Farmland, natural fault
RB14 870 Hard coal None 12 Thermogenic emission 482 to 8,803 Farmland, natural fault
RB15 1250±1300 Hard coal None 15 Bacterial emission 0.5 to 98.6 Moor (bog), wetland
T.
Thielemann
et
al.
/
Organic
Geochemistry
31
(2000)
1387±1408
methane. The isotopic data show a distinct depth cor-relation, with methane becoming lighter towards the top of the Carboniferous and towards fault zones, indepen-dent of coali®cation patterns (TeichmuÈller et al., 1970). The coal gas is characterized by low concentration ratios of methane over higher-molecular-weight hydro-carbons (< 1000, mainly < 300), which are typically thermogenic features (Bernard, 1978). The isotopic composition of methane in overlying Cretaceous rocks ranges from ÿ30 to ÿ65 %. Lommerzheim (1994) interprets this methane as thermogenic gas plus bacterial admixture. Hence, the isotopic composition of coal bed methane indicates the majority of gas to be of thermo-genic origin. The 13C-depleted values at the top of the Carboniferous as well as in the overlying Cretaceous rocks are either due to adsorption/desorption processes or indicate bacterial methanogenesis (Wingerning, 1975; Freudenberg et al., 1996).
In general, soils in the Lower Rhine Embayment are silt-dominated, whereas they are more sandy in the Ruhr Basin. In the Lower Rhine Embayment the sedi-ments at the ground's surface predominantly are Qua-ternary glacial, eolian and ¯uvial deposits. About 74% of the area is covered by luvisols (loess, loess loam) and over 13.5% by gleyic luvisols. Cambisols account for about 5% and podzols for 7.5% of the area (Heide, 1988). Sediments at the surface of the Ruhr Basin are mainly Cretaceous marine sands and marls and Qua-ternary glacial and ¯uvial sands. Forty four percent of the area is covered by podzols on sands, 18% by luvisols on loess, 17% by gleyic luvisols on top of marls and moraines, 15% by cambisols on calcareous rocks and 6% by gleysols in riverbeds (Dahm-Ahrens, 1995). Wetlands cover less than 0.05% of the area in both basins. Undeveloped entisols on outcropping coal seams can be neglected.
3. Methods
To determine methane exchange rates between litho-sphere and atmolitho-sphere, ¯ux chambers were used. They consist of a metal ring which was driven 4±8 cm into the soil. A plastic, translucent cap on top of the ring sealed the ¯ux chamber from the surrounding atmosphere, as described by Cramer (1997). The volume of ¯ux cham-bers varied between 15 and 20 l. Gas samples were taken from ¯ux chambers at variable time intervals by ®lling them into evacuated metal containers of 69.2 ml and were analysed gas chromatographically in the labora-tory. A Hewlett Packard GC (HP5890, Series II) was used to perform the measurements. Gas separation proceeded on a 10 m Poraplot Q-capillary column. Six methane standards (2.146 vpm, 9.81 vpm, 200 vpm, 490 vpm, 2.01%, 99.5%) were used to calibrate the GC data, balanced against the standard no. 1658 of the National Bureau of Standards (USA). For calculations see Appendix A and B. To ensure comparability of the methane data, all concentrations were converted into values at standard temperature and pressure conditions (STP: 0C, 101.325 kPa). Methane exchange rates are
given as negative values if they represent a sink of atmospheric methane and as positive values if they represent a source.
Soil temperatures were measured with a Hg-thermo-meter, which was pushed into the soil to a depth of 3 cm. The depth was controlled with a yard-stick. The temperature was read about 5 min after the Hg-ther-mometer had been inserted into the soil to ensure equi-libration of the mercury temperature with soil temperature.
Undisturbed soil samples of 300 and 600 cm3volume were cored and the moist soil samples weighed to cal-culate the bulk soil density (see Appendix C). After-wards, the samples were dried for three days at 105C
and weighed again. The dierence between dry and moist weight gave the gravimetric and volumetric water content of the samples. 100 to 150 g of dried soil were ground and its grain density measured with a Quanta-chrome helium-pyknometer. With the bulk soil density and its grain density, the bulk porosity was determined (Appendix C). From this and the volumetric water con-tent, the gas-®lled portion of soil volume was calculated according to Appendix C.
The stable carbon isotope measurements of methane in atmospheric concentrations (1.8 vpm and lower) have been conducted with a Precon-C-IRMS unit (Micro-mass, UK). The preconcentration interface (Precon) contains a combustion furnace and is connected to an isotope ratio mass spectrometer (IRMS) described by Brand (1995). The central part in the Precon-unit is a 450 mm ceramic tube ®lled with two catalytic wires (Pt, Ni) and an oxidation wire (Cu, 0.1 mm each). The tube is placed in a combustion furnace. The latter was heated
overnight to 1000C and conditioned with clean oxygen
(2 ml/min) to burn o residual carbon and to oxidize the Cu-wire. The oxygen ¯ow was stopped before mea-suring. Gas samples in 150 ml-containers were ¯ushed with a continuous ¯ow of helium across three chemical traps to purify the gas by removing water, carbon diox-ide and carbon monoxdiox-ide. This gas was then passed into the ceramic tube, where methane was converted into carbon dioxide completely. This carbon dioxide was cryofocussed, sent across 25 m of a Poraplot Q-capillary column to focus the carbon dioxide peak and measured in the IRMS (continuous ¯ow mode). Isotope values are given in the d-notation relative to the internationally adopted PDB standard. Minimum amount of methane needed was 5 nmol. The measurements showed a stan-dard deviation of 0.15±0.4%(n=5).
4. Results
4.1. Bacterial methane emissions
measured methane emission rates average 42.3 mg/(m2d). Data spread unequally across the year. The warmer period was slightly overrepresented. The non-weighted annual average emission rate was 35.7 mg/(m2d).
Fig. 3 presents the results of a ¯ux chamber experiment at locality RB 15. A linear rise in methane concentration
indicates a methane emission rate of 44.1 mg/(m2d). The isotopic composition of methane changed exponentially from data typical of atmospheric methane (ÿ47 %) to aboutÿ60%, which proves the bacterial methane emis-sion to be the reason for this isotope trend. The isotopic shift became faster with increasing emission rate.
Fig. 2. Emissions of bacterial methane and soil temperature in a bog, northern Ruhr Basin (RB 15). The dashed line traces the most likely seasonal variation of methane emission. The inlet presents the temperature dependance of this process.
Fens were developed on less than 0.03% of the area of both basins. One fen was examined near the river Niers in MoÈnchengladbach (LRE 7, Fig. 1). Grass was growing on Quaternary point bar deposits of this water. Due to water drainage systems installed for agriculture, the groundwater table today does not reach the surface and varies between 0.3 and 1.0 m below the surface. The results for the fen dier signi®cantly from the bog. Across the fen, ¯ux chamber experiments revealed a methane consumption instead of emission within the soil. Exchange rates of <1mg methane/(m2d) in winter times increase up to 1.82 mg/(m2d) in the summer.
4.2. Bacterial methane consumption
The process of bacterial methane consumption could be measured at all localities within the Lower Rhine Embayment and at many places in the Ruhr Basin. Typical results of ¯ux chamber experiments within a period of 2 years are presented for the Lower Rhine Embayment (Fig. 4a) and the Ruhr Basin (Fig. 4b). Data were taken from fertilized soils, where farmers annually add urea-nitrogen to the soil. The consump-tion rates ranged fromÿ0.12 toÿ1.85 mg/(m2d) and in most cases did not correlate well with these fertilized soils' temperature. However, this correlation improved with a coarsening of the soil, as the sand-dominated soils at localites LRE 4 (Fig. 4a), RB 6 and RB 7 (Fig. 4b) reveal. In general, the coarser (more sandy) a soil, the higher the seasonal variability and the higher the consumption. This observation shows the importance of soil type for the process of bacterial methane consump-tion.
A comparison of sites with identical soil type but dif-ferent soil utilization (Figs. 5a and 5b) revealed distinct dierences in range and seasonal variability of methane consumption between natural soils (e.g. in forests) and cultivated, fertilized soils (grass lands, farm lands). On a natural soil (Fig. 5a), the methane consumption rates ranged fromÿ0.63 mg/(m2d) in January to ÿ3.32 mg/ (m2d) in July, with an annual mean ofÿ1.38 mg/(m2d). At locality LRE 2, both the consumption mean and the range of consumption rates were about 300% higher in summer than in winter. On a cultivated soil (grassland in Fig. 5b), the methane consumption rates showed less annual cycling compared to the forest, with a smaller mean value ofÿ0.87 mg/(m2d). Due to regular mowing, the gas-®lled soil volume was reduced in the grassland soil by 10±20% compared to a natural soil (forest). These trends continued from grass land to farm land, where seasonal variability and mean consumption rates were smallest (Fig. 5b). Dierences in gas-®lled soil volume could not be found between grass land and farm land, although on farm lands a number of times per year urea-fertilizer was added. Similar results were obtained for the Ruhr Basin (RB 8). Hence, dierent types of soil
utilization have to be considered to evaluate and bal-ance methane consumption.
To check transferability of the result of one ¯ux chamber experiment for a wider area of identical soil type and utilization at one certain time, some localities were sampled with several ¯ux chambers simulta-neously. A dense network of measurement sites was established at locality LRE 2. To elucidate the spatial variability of methane consumption 25 ¯ux chamber experiments were conducted once every month within the year 1997 (Fig. 6). Locality LRE 2 is characterized by a homogeneous gleyic luvisol in a forest. Five ¯ux chambers were placed in one row with a distance of about 5 m in between. The spacing between the ®rst three rows was 5 m, between rows three and four 10 m and between rows four and ®ve 50 m (Fig. 6). This arrangement served to investigate whether a consump-tion pattern observed in a small area can be extra-polated to a wider area. The consumption rates are shown in Fig. 5a. The spatial variability according to Fig. 6 is low. The seasonal variability aects all sam-pling points simultaneously.
The results were subject to statistical analysis. The data (25 every month) from location LRE 2 were tested for a normal distribution (Shapiro±Wilk-test). They are normally distributed to a very high degree (89±96%). Hence, it is justi®ed to describe them with a mean and standard deviation. These data were calculated for every month of the year 1997. The data for 1998 and 1999 were much fewer in number compared to 1997 (45 instead of 300, see Fig. 5a), but presented a similar mean. Where data had been collected for more than two years, they presented annual means with a dierence of less than 5% from each other.
Those ¯ux chamber experiments which revealed a methane sink in the soil were characterized by a13 C-enrichment of the methane remaining in ¯ux chambers (Fig. 7).
4.3. Thermogenic methane emissions
Some coal beds degas when outcropping at the earth's surface (Creedy, 1993; Khalil et al., 1993). Also, methane escapes from opencast and underground coal mining areas (Patteisky, 1963; Hempel and Munke, 1981; Creedy, 1991). Hence, localities in this study were selected to cover examples of outcropping coal, of coal-bearing strata covered by dierent thicknesses of over-lying rocks and of active, abandoned and no mining activities. Additionally, natural fault zones were exam-ined for their emission characteristics.
bituminous coal outcrops (RB 1, RB 3) in the southern Ruhr Basin. Hence, methane emissions at outcropping coal beds in both basins equal zero.
Flux chambers have been placed across active normal faults in both basins. These were the localities LRE 2 at a fault activated in 1992 by an earthquake, and RB 7, RB 13 and RB 14 in the Ruhr Basin (Fig. 1). Only at RB 13 and RB 14 methane emissions were found (Table 1, Fig. 8). LRE 2 did not show any emissions. Carboni-ferous rocks are covered by 800±900 m of Cenozoic
unconsolidated sediments there. Measurements at exploration wells in August 1999 showed the Carboni-ferous to be hydrocarbon gas-free (Fig. 8). At RB 7 the top 300 m of Carboniferous coal beds are gas-free, but about 100 m deeper contain between 7 and 13 m3coal gas/t coal (Hinderfeld et al., 1993). On top of the Car-boniferous the foot wall of the normal fault at RB 7 consists of 600 m of Permian, Cretaceous and Tertiary sediments with a cover of 10 m glacial deposits of Qua-ternary age (Fig. 8). The hanging wall contains 300 m of
Cretaceous marine rocks with a 5 m cover of Qua-ternary glacial sediments (Wrede and Jansen, 1993). No methane emissions were measured at RB 7. The local-ities RB 13 and RB 14 dier obviously from RB 7 and LRE 2. The coal seams at the top of Carboniferous rocks contain 8±10.5 m3 coal gas/t coal. Gas contents decrease with increasing depth within the Carboniferous and also horizontally, because RB 13 and RB 14 are situated on top of a Carboniferous anticline (Hinderfeld et al., 1993). Late Cretaceous marine limestones and
marls cover the Carboniferous. They are of very low permeability (10ÿ17±10ÿ19m2, after Struckmeier, 1990) and are not overlain by a porous Quaternary unit (Fig. 8). Methane surface emissions at RB 13 and RB 14 reach up to 8.8 g/(m2d).
Spectacular methane emissions were measured in the eastern Ruhr Basin. At localities RB 11 and RB 12 ¯ux chamber experiments revealed methane emission rates from a few mg/(m2d) up to about 100 g/(m2d) (Table 1). Fig. 9 shows one example from RB 12. Thermogenic gas
emissions move the isotopic composition of methane within the ¯ux chamber in opposite direction compared to bacterial methane emissions (Fig. 3). These emissions were restricted to mining subsidence troughs in active
and abandoned coal mining areas. In the grain ®elds the rims of these subsidence troughs could be traced by their morphological edges with a height of up to 80 cm. At a few places inside and at the rim of these troughs agricultural
Fig. 6. Annual variation of bacterial methane consumption in a gleyic luvisol at locality LRE 2. Results of 25 ¯ux chamber experiments every month are presented for 1997. A small spatial but pronounced seasonal variation of bacterial methane oxidation is observed.
Fig. 7. At locality LRE 2 the concentration of methane decreased with time in the ¯ux chamber, which demonstrated bacterial methane consumption. The isotopic composition of methane remaining in the chamber was depleted in12CH
vegetation (grain) showed a stunted growth, with leaves turning from green to yellow in colour. In some of these areas the reason for this growth anomaly could be attributed to heavy rain falls and a temporary ¯ooding of the ground. However, more than 80% of the stunted growth was related to methane emissions originated
from coal, as ¯ux chamber experiments revealed. Areas overgrown with green grass did not show methane emissions higher than 5 mg/(m2d). Hence, during the vegetation period the state of grass growth was a good indicator to screen an area for thermogenic methane emissions.
Fig. 9. Flux chamber experiment on a very intensive ebullition site of methane in an area of abandoned mining (RB 12). A constant admixture of methane into the ¯ux chamber resulted in an opposite trend of methane carbon isotopic values compared to Fig. 3. The results here (RB 12) prove emissions of thermogenic methane.
The emission patterns across an active mining ®eld (locality RB 11 in Fig. 1) and an abandoned one (local-ity RB 12 in Fig. 1) both vary by ®ve orders of magni-tude, but among one another do not show obvious dierences (Fig. 10). In the active mining area (RB 11) all emission rates recorded surpassed 10 mg methane/ (m2d), whereas across the abandoned mining ®eld (RB 12) a high number (22) of emission rates was smaller than 10 mg/(m2d). This may indicate a local and slow decrease in emission intensities at abandoned mining areas compared to active ones. During the two year-measurement cycle declining intensities could not be found. Areas with high methane emissions between 10 and 100 g/(m2d) constantly stayed at these high rates. Also a displacement of major emission points was not observed.
5. Discussion
5.1. Bacterial methane emissions
The carbon isotope composition of bacterial methane from bogs ranges around ÿ60%. This characterizes bacterial methane produced by acetate fermentation, a typical process in freshwater sediments (Whiticar et al., 1986). Temperature controls the bacterial activity to produce methane (Games and Hayes, 1976). Data by
Christensen et al. (1995) support the seasonal variability observed in this bog (locality RB 15). The emissions show a high temperature correlation (Fig. 2). The spa-tial variability of emissions from a bog should be low, and varies between 10 and 15% (Bubier et al., 1995). This justi®es projecting the ¯ux chamber data to the whole (small) bog area in the two sedimentary basins. Hence, average emission rates of bacterial methane vary between 30 and 40 mg/(m2d). For the Ruhr Basin 0.9± 1.2 t/a of bacterial methane emissions can be calculated. The emissions range between 0.15 and 0.2 t/a in the Lower Rhine Embayment.
Normally, fens have anaerobic soils and character-istically show bacterial methane emissions with a global average of around 57 mg/(m2d) (Davidson and Schimel, 1996). Dewatering can lead to aerobic conditions in the ground, which immediately ceases bacterial methane emission (Harriss and Sebacher, 1982). Anthropogenic water management lowered the groundwater table at LRE 7 and hence, most likely stopped bacterial methane production. As dewatering is common in both sedimen-tary basins it is assumed here that bacterial methane emissions from the few remaining fens are insigni®cant.
5.2. Bacterial methane consumption
In principle, four factors in¯uence bacterial methane consumption. These are the local climate, pedosphere,
biosphere and anthropogenic in¯uences. The ®rst factor, local climate, aects physical and biological processes by sunlight, temperature, air pressure and rainfall. Sun-light raises soil temperature and, hence, enzymic activ-ities of methane consuming bacteria (King and Adamsen, 1992; King, 1993). Quick air pressure varia-tions result in advective eects which accelerate the gas transport between soil and atmosphere (Conen and Smith, 1998). Rain fall increases the soil humidity. This may have antagonistic eects on methane consumption. Rain on a dry soil enables metabolic processes and may enhance bacterial activities. In contrast, in a wet soil additional water reduces the gas-®lled porosity, raises the groundwater table and impedes oxidation processes (Crill, 1991). The second factor is related to the pedo-sphere. The fabric of the soil particles physically deter-mines the gas transport (Scheidegger, 1974; Cunningham and Williams, 1980). A rise in temperature enhances diusion through soil and at the same time forces eva-potranspiration, which enlarges the gas-®lled porosity of a soil (Blake and Page, 1948). The biosphere is the third factor, as methane is oxidized by bacteria. They vary in number, metabolic activity and species consuming methane (Hanson, 1980; Hanson et al., 1993; Murrell et al., 1993; McDonald et al., 1995). The fourth factor comprises anthropogenic in¯uences on the pedosphere. The cultivation of soils, which compresses the upper decimeters of a soil, reduces its gas-®lled porosity (Teiwes, 1988). Fertilization changes chemical composi-tion of soil water (Steudler et al., 1989). As many of these factors are not independent of each other a deter-mination of the in¯uence of one single factor on bac-terial methane consumption is mostly impossible. A variety of dierent measurements must be combined to unravel the importance of some of these factors.
Three factors appear to control methane consumption: soil temperature, methane diusion and fertilization. The data reveal a linear correlation between gas-®lled soil volume and methane consumption (Fig. 11a) as well as between soil temperature and methane consumption (Fig. 11b). Also, the gas-®lled soil volume was not independent of soil temperature (Fig. 11c), as evapo-transpiration from a soil rises with temperature. These results may be interpreted as a dependence of methane consumption on temperature-controlled bacterial enzy-mic activity or on methane diusion, controlled by gas-®lled porosity. Hanson (1980) found a rise in bacterial enzymic activity with temperature in laboratory experi-ments. The examination of such a dependence in nature requires microbiological techniques to count living methanotrophic bacteria in soil samples. Qualitative methods are the analysis of chemical fossils like metha-notroph-typical hopanoids (Neunlist and Rohmer, 1985; Zundel and Rohmer, 1985) or polymerase chain reac-tions (Murrell et al., 1993; McDonald et al., 1995), which multiply species-speci®c DNA-sequences, but do
not dierentiate between dead and living bacteria. New genetic tools, such as molecular beacons, should very speci®cally stain only living cells of one species (Tyagi and Kramer, 1996; Scho®eld et al., 1997), but in ®eld studies failed to provide clear results (Dr. R. Wilhelm, personal communication). Hence, up to now, no precise method exists to quantify methanotrophic bacteria in soil samples. Also a variety of bacteria are able to carry out methane oxidation in a soil (Green, 1993). At pre-sent, only little is known about the bacteria associations in soils. This group of species might vary spatially in number of species and in quantity, hence bacterial methane consumption cannot be quantitatively corre-lated with its origin today. It can only be assumed that a rise in bacterial activity and number with temperature might aect the seasonal variation of methane con-sumption.
Methane oxidizing bacteria are negatively aected by fertilization. The average intensity of bacterial methane consumption was highest in natural soils (e. g. forests) and lowest in fertilized soils (farmlands). On a fertilized gleyic luvisol annual average methane consumption accounts only for 38% of that on a natural gleyic luvi-sol. The seasonal variability was higher in natural com-pared to fertilized soils. One of the reasons for smaller seasonal variations in farmlands compared to forests is the smaller variation of gas-®lled porosity with soil temperature (Fig. 11c), which changes the diusion coecient stronger in the forest than in the farmland. However, calculations of the eective diusion coe-cient from gas-®lled soil volume data indicate that this process cannot fully account for the measured impe-dance of bacterial methane-oxidizing activity on farm lands. Steudler et al. (1989) found that a nitrogen-input into forest soils reduced bacterial activity and the sea-sonal variability of bacterial methane consumption. Mosier et al. (1991) quanti®ed an average reduction of the consumption process of 0.2 mg/(m2d) when applying 45 g/m2of urea-nitrogen on cultivated grassland. This can be explained by the fact that the metabolism of nitrifying bacteria is much more energy ecient and directs electrons away from the methane oxidizing metabolism to denitri®cation reactions (Davidson and Schimel, 1996). Hence, the metabolism and growth of methanotrophic bacteria are impeded and a fertilized soil emits N2O instead of consuming CH4(Mosier et al., 1991). To balance the process of bacterial methane con-sumption for the Ruhr Basin and Lower Rhine Embay-ment, its spatial and temporal variability must be known. The data presented here indicate that both the soil type and the soil cultivation as well as the tempera-ture-dependent gas-®lled porosity control the temporal (seasonal) variation of methane consumption. The results can be extrapolated to wider areas (Fig. 6).
type distribution and soil utilization data. The char-acteristics of dierent methane consumption classes deduced from these data are listed in Table 2. Results of the Lower Rhine Embayment and the Ruhr Basin did not reveal dierent trends. This justi®ed grouping the results of the same soil type and same utilization toge-ther (Table 2). Fertilized soils were generally character-ized by reduced consumption rates compared to natural soils (26±45% reduction). The one exception are cambi-sols where the methane consumption is reduced by only 3%. This most likely re¯ects the small number of mea-surements on cambisols (Table 2). The relative errors of bacterial methane consumption rates ¯uctuate between 15 and 67%. At least in fertilized soils this variability depends on the grain size distribution. Sandy podzols vary by 67%, gleysols only by 34% (Table 2). Sand inhibits gas diusion less than silt and contains a gas-®lled pore volume of higher variability. The spatial extents of dierent methane consumption classes and the balancing of bacterial methane consumption for both basins are presented in Table 3. The soil utilization was drawn from satellite pictures (Landesvermessungsamt Nordrhein-Westfalen, 1991) and agricultural statistics (Landwirtschaftskammer Westfalen-Lippe, 1996; Land-wirtschaftskammer Rheinland, 1997). Around 2050 t of methane are consumed every year in the Lower Rhine Embayment, 2727 t of methane in the Ruhr Basin. This is a mean of 0.73 mg methane (10%)/(m2d) in the Lower Rhine Embayment and a mean of 0.88 mg methane (12%)/(m2d) in the Ruhr Basin. A dierence of 17.1% between both averages re¯ects the generally coarser (sandy) grained soils in the Ruhr Basin com-pared to the silt-dominated ground in the Lower Rhine Embayment.
Methane consumption by bacteria was always accompanied by an isotope fractionation. The methane remaining in a ¯ux chamber was enriched in13C by 0.6± 1%(Fig. 7). Falling soil temperatures strengthened this enrichment. Depending on the precise global extent of methane consumption by bacteria and on the contribu-tion of this process to all methane sinks the carbon iso-tope fractionation coupled to methane consumption could eect the isotopic composition of atmospheric methane.
5.3. Thermogenic methane emissions
No methane emissions were measured at outcropping coal beds in both basins. This corresponds to Patteisky (1955) who reported the uppermost 50 m of Carboni-ferous rocks in the southern Ruhr Basin to be gas-free. This does not include lignite now outcropping in open pit mines of the Lower Rhine Embayment, in which no measurements were performed. Pressure release by removal of up to 350 m of sediments might have induced methane emissions there.
Hydrocarbon surface emissions are used as one indi-cator of fossil fuel reservoirs below. They typically occur at tectonically weakened areas like natural faults (Klus-man and Saeed, 1996; Matthews, 1996). The positions of natural faults are well mapped in the Lower Rhine Embayment (Wrede et al., 1983; Wrede, 1985; Wrede and Zeller, 1988) and the Ruhr Basin (Drozdzewski et al., 1980; 1982; Wrede, 1987, 1992). Some of the natural faults have been proven active up to Quaternary times (Klostermann et al., 1998). Methane exchange mea-surements revealed that emissions of thermogenic methane are restricted to a few natural faults only.
Table 2
Soils of dierent type and utilization in the Lower Rhine Embayment and the Ruhr Basin were subdivided into dierent classes according to their bacterial methane consumption characteristics
Methane consumption by bacterial oxidation
Soil type Locality No. of
data
Gleyic Luvisol LRE 2 (part) 345 ÿ1.38 0.49 36
Luvisol LRE 3 (part) 10 ÿ0.92 0.14 15
Cambisol RB 5 8 ÿ0.94 0.3 32
Podzol RB 8 (part), RB 10, RB 12 (part) 77 ÿ1.69 0.85 50
Cultivated, fertilized soils
Gleysol RB 12 (part) 20 ÿ1.04 0.35 34
Gleyic Luvisol LRE 2 (part) 57 ÿ0.75 0.32 42
Luvisol LRE 3 (part), LRE 5, LRE 6, RB 4 95 ÿ0.68 0.29 43
Cambisol LRE 4, RB 9 13 ÿ0.91 0.43 47
Locality LRE 2 did not show any surface emissions, as the Carboniferous is gas-free. This locality lacks a source. Locality RB 7 also does not show any surface emissions. It either lacks source, as the top 300 m of Carboniferous are gas-free (Hinderfeld et al., 1993) or upward migrating gases are defocussed in the Qua-ternary porous cover. This could obscure the signal of emitted methane. As in none of the exploration wells near RB 7 gas reservoirs within the post-Carboniferous rocks have been reported, these sediments can be regar-ded gas-free.
In contrast, RB 13 and RB 14 showed methane sur-face emissions. Major dierences to localities LRE 2 and RB 7 are an accumulation of coal bed methane at the Carboniferous top and a low permeable Cretaceous caprock. The fault planes at RB 13 and RB 14 probably are the only migration pathways. Gas is not defocussed by (missing) Quaternary, porous sediments. According to these results measurable natural methane emissions across normal faults in both basins can only be expected where gas accumulations persist at the top of
Carboniferous and where the migrating gas is focussed during its ascent.
Thermogenic methane emissions in areas of bitumi-nous coal mining are restricted to the places of under-ground longwall coal mining. This type of mining distorted or destroyed the original rock fabric of (post-Carboniferous) sediments there. Its collapse generated mining subsidence troughs (Kratzsch, 1974) and new gas migration pathways. This is an obvious link between underground mining activities and surface methane emissions. Emission rates have never been measured there earlier, so that a tendency of weakening of emission intensities (within some years) cannot be proven. Loca-tions RB 11 and RB 12 show that methane emissions can be expected once coal mining starts and can last for decades after mining has ceased.
5.4. Balancing the methane exchange
Dierent sources of methane emissions from coal mining enter into a complete balance. These are surface
Table 3
Balance of bacterial methane consumption in the Lower Rhine Embayment and the Ruhr Basin. Soil type distribution and utilization were taken from Heide (1988), Landesvermessungsamt Nordrhein-Westfalen (1991), Dahm-Ahrens (1995), Landwirtschaftskammer Westfalen-Lippe (1996) and Landwirtschaftskammer Rheinland (1997)
Soil type Utilization Area (km2) Methane consumption (mean) (t/a)
Lower Rhine Embayment 7730 km2
Entisol Natural 1 0.124
Gleysol Natural 14 7.556
Cultivated/fertilized 56 21.258
Gleyic luvisol Natural 300 151.110
Cultivated/fertilized 634 173.558
Luvisol Natural 110 36.938
Cultivated/fertilized 5300 1315.460
Cambisol Natural 40 13.724
Cultivated/fertilized 325 107.949
Podzol Natural 120 74.022
Cultivated/fertilized 430 147.533
Ð Built-up/under seal 400 Ð
Sum for Lower Rhine Embayment 7730 2049.232
Ruhr Basin 8520 km2
Entisol Natural 1 0.124
Gleysol Natural 250 125.925
Cultivated/fertilized 216 81.994
Gleyic luvisol Natural 170 85.629
Cultivated/fertilized 1153 315.634
Luvisol Natural 210 70.518
Cultivated/fertilized 1202 298.336
Cambisol Natural 150 51.465
Cultivated/fertilized 1018 338.129
Podzol Natural 680 419.458
Cultivated/fertilized 2740 940.094
Ð Built-up/under seal 730 Ð
methane escapes, upcast mining shafts, mined bitumi-nous and brown coal degassing during storage and emissions from abandoned mines (SchoÈn et al., 1993; Pospischill, 1994). Methane escape across the surface of the Lower Rhine Embayment (outside of the open pit mines) is negligible, but occurs in a very irregular pat-tern in the Ruhr Basin (see Table 1). To balance this process for the Ruhr Basin, three questions have to be answered:
1. How to balance methane emissions at single emission points?
2. What is the area in which methane surface emis-sions occur in the Ruhr Basin?
3. How to balance methane emissions across the whole sedimentary basin?
(1) Areas of methane emission (RB 2, RB 4, RB 5, RB 11, RB 12, RB 13, RB 14) have been sampled precisely to gain a detailed idea of the spatial extent of main and subsidiary emission points. From these results, means of emission rates were deduced for areas of a few square meters up to 100 m2. This method accounted for the high spatial variability of emissions and permitted an estimation of the overall mass of methane emitted. It accounts for between 1.9 and 4.16 t/a (Table 4).
(2) The Ruhr Basin is densely populated. This guar-antees that sites of surface methane escapes become public, because these emissions adversely aect agri-culture or are a safety risk for buildings (e. g. Schmidt, 1931). Hollmann et al. (1978) and Hollmann and SchoÈne-Warnefeld (1982) summarized emission inci-dents from the 1950s to the 1970s. They found methane escapes to be restricted to the central and eastern Ruhr Basin, east of the cities Essen±Gelsenkirchen±Haltern
(see Fig. 1). Our ¯ux chamber experiments corroborate this. Hence, this study considers methane surface emis-sions to occur in the eastern half of the Ruhr Basin (4000 km2) only.
(3) Flux chamber experiments in the eastern Ruhr Basin (RB 11, RB 12) indicated methane emission sites to be restricted to only a few 100 m2at most. The adja-cent ®elds appeared to be emission-free. To scrutinize this idea, a building site was inspected where the soil within a ®eld of 5 ha was removed up to a depth of 3 m. Cretaceous marls were exposed to the surface. After heavy rain falls the fracture system of these marls was water-saturated. Gas bubbles of ascending coal gases could be easily detected by eye. They were con®ned to 0.1±0.2% of the exposed area. Methane emission rates ranged between 5 and 13 g/(m2d). As these emissions were highly focussed, bacterial methane oxidation in the soil could lessen them by not more than 0.1%. Hence, at the building site the average methane emission was assessed at 5±26 mg per hectar per day. Presuming this emission per hectar to be representative of the complete eastern Ruhr Basin (4000 km2), between 0.73 and 3.8 t of methane per year are released into the atmosphere outside of the emission sites balanced with ¯ux chamber experiments (Table 4).
A compilation of the most accurate data available is presented in Table 4 for the year 1993. Table 4 shows that upcast mining shafts add substantially to the methane ¯ux into the atmosphere in both basins. One upcast mining shaft contributed to these emissions in the Lower Rhine Embayment. In the Ruhr Basin eleven such mines were in operation in 1993. Table 4 reveals that methane surface emissions, either alongside natural faults, fracture planes or in mining subsidence troughs, are of minor importance. Methane consumption can
Table 4
Balance of natural bacterial emissions, thermogenic methane escapes and bacterial methane consumption for the Ruhr Basin and the Lower Rhine Embayment. Literature data were most complete for the year 1993. Own measurements reached from January 1997 to April 1999. Not considered are anthropogenically induced bacterial methane emissions from land®lls and ruminant's keeping
Ruhr Basin Lower Rhine Embayment
Methane emissions across the ground's surface 1997±1999 (t/a)a 1.9 to 4.16 Ð
Projected methane emissions outside major emission sites 1997±1999 (t/a)a 0.73 to 3.8 Ð
Methane emissions via upcast mining shafts 1993 (t/a)b 450,596.5 24,641.9
Methane emissions from stored hard coal 1993 (0.8 m3/t) (t/a)c 26,202.6 850.2
Methane emissions from abandoned mines 1993 (120 mill. m3)d 86,016 ?(>0)
Methan emissions from lignite mining 1993 (t/a)e Ð 7318.2
Bacterial methane emissions 1997±1999 (t/a)a 0.9 to 1.2 0.15 to 0.2
Bacterial methane consumption 1997±1999 (t/a)a ÿ2,727.3 ÿ2049.2
Sum (t/a) 560,090.89 to 560,094.62 30,761.3
a Own measurements/calculations.
easily compensate for these surface emissions, which only amount to between 0.01 and 0.22% of the methane consumption by bacteria in the Ruhr Basin. In pre-mining times both sedimentary basins acted as sinks for atmospheric methane. Coal mining (especially bitumi-nous coal mining) changed this situation completely, because large volumes of methane are released, for example, by mining shafts. At present, only 0.8% of the annual methane emissions related to former and active coal mining activities are consumed in the soils of both sedimentary basins (Table 4). Assuming the bacterial consumption rates in the study area to be representative for all over Germany, about 14.5% of methane emis-sions related to coal mining are oxidized in soils of this country. Hence, the Ruhr Basin and the Lower Rhine Embayment are evident examples of how anthropogenic coal mining activities turned the local methane balance from a sink into a source of this greenhouse gas and thereby contribute to the recent global increase of atmospheric methane.
6. Conclusions
Three processes of methane exchange take place at the lithosphere/atmosphere boundary: methane con-sumption as well as bacterial and thermogenic methane emission. They are characterized by dierent methane exchange rates and carbon isotope values at the soil surface. This allows separation of the dierent pro-cesses. Bacterial methane consumption rates are highest in natural soils such as forests and in coarser grained (sandy) soils. Consumption values decline with increas-ing anthropogenic utilization of the soil, in grass lands and farmlands. Reasons for this eect are a consolida-tion of soil which impedes gas transport and the appli-cation of nitrogen-fertilizer which slows down methanotrophic metabolism (Steudler et al., 1989). The seasonal variation of methane oxidation in natural soils might be controlled by gas diusive transport or by temperature-control of bacterial consumption activity. Decisive factors for the process of methane consump-tion by bacteria are soil type and soil utilizaconsump-tion. Bac-terial methane oxidation can be balanced across larger areas once their consumption characteristics and their geographic distribution are known. Within the Lower Rhine Embayment around 2050 t methane are con-sumed every year. In the Ruhr Basin, due to its larger area and coarser grained soils, these are 2727 t/a. Both numbers imply an uncertainty of20%.
The few remaining wetlands in the Lower Rhine Embayment and the Ruhr Basin are restricted to around 240 km2altogether. Only bogs still release bac-terial methane, whereas the former fens are widely dewatered. Methane emissions from bogs are highly
correlated with soil temperature and range between 0.15 and 0.2 t/a in the Lower Rhine Embayment and between 0.9 and 1.2 t/a in the Ruhr Basin.
Thermogenic methane emissions were found only in the Ruhr Basin. They are spatially restricted to around 4000 km2 in the eastern half of the basin, east of the cities Essen, Gelsenkirchen and Haltern. Within this area thermogenic methane is released by gas ebullition alongside some natural normal faults and in active and abandoned coal mining ®elds. Emissions across natural faults are restricted to anticline structures in Carboni-ferous sediments, which contain coal gas accumulations and are covered by low permeability Cretaceous rocks. In such a geological setting, migrating coal gases were focussed along fault planes and could be detected at the surface. In coal mining areas surface emissions during the vegetation period could be visually traced by stunted growth in grain ®elds. This permitted reliable screening of methane emission localities. Emission rates within some meters could change from a few mg up to around 100 g methane/(m2d). This emission pattern, though rather irregular in space, was very stable during 28 months of measurements. The emission localities sam-pled in the Ruhr Basin amounted to between 1.9 and 4.16 t of methane per year.
Emissions outside of these major emission sites are projected to range between 0.73 and 3.8 t of methane per year. Most important sources of methane in both basins are upcast mining shafts, which contribute about 80% to all lithosphere sources in this area (Table 4). The surface emissions of methane balanced here, contribute only 0.002% to these emissions in the Ruhr Basin and are absent in the Lower Rhine Embayment. Both basins represented a natural sink for atmospheric methane in pre-mining times. This situation was reversed by coal mining. Hence, the Lower Rhine Embayment and the Ruhr Basin are evident examples of the impact that coal mining activities have on the global atmospheric methane budget.
Acknowledgements
The study bene®tted from fruitfull discussions and continuous interest by Dr. B. Cramer, Dr. B. M. Krooss, Dr. R. G. Schaefer and Professor Dr. Dr. D. H. Welte. This and technical assistance by J. HoÈltkemeier and W. KnoÈrchen is greatly appreciated. In addition the manuscript bene®ted from constructive comments by two reviewers, Dr. C. J. Clayton and Dr. S. Inan.
Appendix A. Methane emission
in the chamber. A steady emission shows a linear increase of methane concentration. The emission rateQ
is given by:
Appendix B. Bacterial methane consumption
Bacterial methane consumption leads to an exponen-tial decrease of methane concentration with time within the ¯ux chamber. Assuming a diusional control of this process, the eective diusion coecient Deff of methane according to Bird et al. (1960) in a soil is given by:
Here DSTP is the standard diusion coecient of methane in air at 0C and 101.325 kPa, which according
to Katz et al. (1959) amounts to 1.96*10ÿ5 m2 sÿ1.T andPare the local temperature and atmospheric pres-sure conditions. Gis the portion of bulk soil volume ®lled with gas and is the tortuosity. The former is measured in undisturbed soil samples (Appendix C), the latter is deduced from the gas-®lled soil volume according to Lai et al. (1976):
1 3
Gÿ 3
Methane consumption by bacteria can be described as a ®rst order reaction (Koschorreck and Conrad, 1993). Its rate coecient, k, is de®ned by the ratio of the eective diusion coecientDeff to the product of the depth of maximum methanotrophic bacteria accumula-tion land the ratio of ¯ux chamber volume and area,
VFC
The methane consumptionQis given by:
Q ÿkVFC
catm is the atmospheric concentration of methane, cend the stable concentration of methane in the ¯ux chamber after a timet 1.
Appendix C. Soil porosity
The bulk soil density S is given by the ratio of the soil massmSand the soil volumeVS:
The mineral grain densityGresults from the dried soil mass md and the volume of mineral grains VG, deter-mined with a Helium±Pyknometer:
The bulk porosityof a porous medium is given by:
1ÿVG VS
ÿ 8
Part of the bulk porosity,, in soil may be occupied by water to an extent ofW. The gas-®lled porosityGas forms the remaining rest:
GasÿWÿ 9
References
Alber, B.E., Clements, A.P., Jablonski, P.E., Latimer, M.T., Ferry, J.G., 1993. Enzymology of the methanogenic fermen-tation of acetate by Methanosarcina thermophila. In: Mur-rell, J.C., Kelly, D.P. (Eds.), Microbial Growth on C1 Compounds. Intercept, Andover, pp. 163±170.
Bernard, B.B., 1978. Light Hydrocarbons in Marine Sediments. Thesis. Texas A&M University, 144 p.
Bird, R.B., Stewart, W.E., Lightfoot, E.N., 1960. Transport Phenomena, Madison, John Wiley & Sons, 780 p.
Blake, G.R., Page, J.B., 1948. Direct measurement of gaseous dif-fusion in soils. Soil Science of America Proceedings 13, 37±42. Blake, D.R., Rowland, S., 1988. Continuing worldwide increase
in tropospheric methane, 1978 to 1987. Science 239, 1129±1131. Born, M., DoÈrr, H., Levin, I., 1990. Methane consumption in
aerated soils of the temperate zone. Tellus 42B (1), 2±8. Brand, W.A., 1995. Precon: a fully automated interface for the
pre-GC concentration of trace gases in air for isotopic ana-lysis. Isotopes Environ. Health Studies 31, 277±284. Bubier, J.L., Moore, T.R., Juggins, S., 1995. Predicting
methane emission from bryophyte distribution in northern Canadian peatlands. Ecology 76 (3), 677±693.
BuÈker, C., Littke, R., Welte, D.H., 1995. 2D-modelling of the thermal evolution of Carboniferous and Devonian sedimen-tary rocks of the eastern Ruhr Basin and northern Rhenish Massif, Germany. Z. dt. Geol. Ges. 146, 321±339.
Christensen, T.R., Jonasson, S., Callaghan, T.V., Havstrom, M., 1995. Spatial variation in high-latitude methane ¯ux along a transsect across Siberian and European tundra environments. Journal of Geophysical Research 100 (D10), 21035±21045.
Cicerone, R.J., Oremland, R.S., 1988. Biogeochemical aspects of atmospheric methane. Global Biogeochemical Cycles 2 (4), 299±327.
Colombo, U., Gazarrini, F., Gon®antini, R., Kneuper, G., Teich-muÈller, M., TeichTeich-muÈller, R., 1970. Carbon isotope study on methane from German coal deposits. Org. Geochem. 10, 1±26. Conen, F., Smith, K.A., 1998. A re-examination of closed ¯ux
chamber methods for the measurement of trace gas emissions from soils to the atmosphere. European Journal of Soil Sci-ence 49, 701±707.
Cramer, B., 1997. Methan im noÈrdlichen Westsibirischen Becken.- Bildung, LagerstaÈttendynamik und Austausch mit der AtmosphaÈre. Berichte des Forschungszentrums JuÈlich 3412, 187 p.
Creedy, D.P., 1991. An introduction to geological aspects of methane occurrence and control in British deep coal mines. Quarterly Journal of Engineering Geology 24, 209±220. Creedy, D.P., 1993. Methane emissions from coal related
sour-ces in Britain: development of a methodology. Chemosphere 26 (1±4), 419±439.
Crill, P.M., 1991. Seasonal patterns of methane uptake and carbon dioxide release by a temperate woodland soil. Global Biogeochemical Cycles 4 (4), 319±334.
Cunningham, R.E., Williams, R.J.J., 1980. Diusion in gases and porous media, Plenum Press, La Plata, 275 p.
Dahm-Arens, H., 1995. Boden und Bodennutzung. In Hilden, H. D. (Ed.), Geologie im MuÈnsterland. Krefeld, pp. 106±117. Davidson, E.A., Schimel, J.P., 1996. Microbial processes of production and consumption of nitric oxide, nitrous oxide and methane. In: Matson, P.A., Harriss, R.C. (Eds.), Bio-genic Trace Gases. Measuring Emissions from Soil and Water. Blackwell, Oxford, pp. 327±357.
DoÈrr, H., Katru, L., Levin, I., 1993. Soil texture para-meterization of the methane uptake in aerated soils. Chemo-sphere 26 (1±4), 697±713.
Drozdzewski, G., 1993. The Ruhr coal basin (Germany): structural evolution of an autochthonous foreland basin. Int. J. Coal Geol. 23, 231±250.
Drozdzewski, G., Bornemann, O., Kunz, E., Wrede, V., 1980. BeitraÈge zur Tiefentektonik des Ruhrkarbons. In: Geol. Survey Northrhine-Westfalia. Krefeld. 85 p. ISBN 3-86002-127-3. Drozdzewski, G., Jansen, F., Kunz, E., Pieper, B., Rabitz, A.,
Stehn, O. et al., 1982. Geologische Karte des Ruhrkarbons 1:100.000, dargestellt an der Karbonober¯aÈche. Map, Geol. Survey Northrhine-Westfalia, Krefeld. ISBN 3-85078-181-5. Eicker, H., HeûbruÈgge, F.W., 1984. Beherrschung von
Gasaus-tritten uÈber Tage aus stillgelegten Gruben. GluÈckauf 120 (23), 1553±1558.
Fitzner, G., 1989. Das Absaugen von Methan im deutschen Steinkohlebergbau in den Jahren 1987/88. GluÈckauf 125 (19/ 20), 1236±1240.
Freudenberg, U., Lou, S., SchluÈter, R., SchuÈtz, K., Thomas, K., 1996. Main factors controlling coalbed methane dis-tribution in the Ruhr District, Germany. In: Gayer, R., Harris, I. (Eds.), Coalbed Methane and Coal Geology. Geo-logical Society, London, Vol. 109, pp. 67±88.
Games, L. M. Hayes, J. M., 1976. On the mechanisms of CO2
and CH4production in natural anaerobic environments. In:
Proceedings of the 2nd International Symposium on envir-onmental biogeochemistry. Butterworth, Vol. 1., pp. 51±73. Green, P.N., 1993. Overview of the current state of
methylo-troph taxonomy. In: Murrell, J.C., Kelly, D.P. (Eds.), Microbial Growth on C1Compounds. Intercept, Andover,
pp. 253±265.
Hager, H., PruÈfert, J., 1988. Das TertiaÈr der Niederrheinischen Bucht. In: Geol. Survey Northrhine-Westfalia (Ed.), Geolo-gie am Niederrhein. Krefeld. pp. 32±40. ISBN 3-86090-040-9. Hanson, R.S., 1980. Ecology and diversity of methylotrophic
bacteria. Adv. Appl. Microbiol. 26, 3±39.
Hanson, R.S., Bratina, B.J., Brusseau, G.A., 1993. Phylogeny and ecology of methylotrophic bacteria. In: Murrell, J.C., Kelly, D.P. (Eds.), Microbial Growth on C1 Compounds.
Intercept, Andover, pp. 285±302.
Harriss, R.C., Sebacher, D.I., 1982. Methane ¯ux in the great dismal swamp. Nature 297, 673±674.
Heide, G. 1988. Boden und Bodennutzung. In: Geol. Survey Northrhine-Westfalia (Ed.), Geologie am Niederrhein. Kre-feld. pp. 73±78. ISBN 3-86090-040-9.
Hempel, D., Munke, K., 1981. Auftreten von Grubengas im untertaÈgigen Braunkohlenbergbau der DDR- Notwendigkeit und Umfang von Schlagwetterschutzmaûnahmen. Neue Bergbautechnik 117, 393±397.
Hilden, H.D., Drozdzewski, G., Hiû, M., Lehmann, F., Michel, G., Skupin, K. et al., 1995. Geologie im MuÈnsterland. In: Geol. Survey Northrhine-Westfalia, Krefeld. ISBN 3-86029-922-0.
Hinderfeld, G., Kunz, E., Ophale, M., Stengel, H., 1993. Die GasfuÈhrung des Ruhrkarbons in ihrer raÈumlichen Verteilung und Entstehung zur Prognose der Ausgasung. In: DMT-Berichte aus Forschung und Entwicklung, Vol. 14, pp. 1±85. ISBN 3-92063-681-4.
Hollmann, F., SchoÈne-Warnefeld, G., 1982. Methan (CH4).
AusstroÈmungen an der Erdober¯aÈche und Vorkommen im ober¯aÈchennahen Untergrund im westfaÈlischen Teil des Ruhrgebietes und im MuÈnsterland. Bergbau 8, 418±424. Hollmann, F., HuÈlsmann, K.H., Schmidt-Schleicher, H.,
SchoÈne-Warnefeld, G., 1978. Die Ausgasungen an der Erdo-ber¯aÈche im niederrheinisch-westf. Steinkohlenrevier (Ruhr-gebiet) als ingenieurgeologisches und bautechnisches Problem. Bergbau 5, 211±219.
Houghton, J., 1997. Globale ErwaÈrmung. Fakten, Gefahren und LoÈsungswege. Springer, Berlin, 230 p. ISBN 3-540-61844-9.
Houghton, J.T., Filho, L.G.M., Callander, B.A. Harris, N. (Eds.), 1995. Climate Change 1995 Ð The Science of Climate Change. Cambridge University Press, Cambridge.
Karg, H., 1998. Numerische Simulation der thermischen Geschichte, Subsidenz und Erosion des westlichen Rechtsrheinischen Schiefergebirges, des Ruhrbeckens und des PalaÈozoikums der Niederrheinischen Bucht. Berichte Forschungszentrum JuÈlich No. 3618. 456 p.
Kaserer, H., 1906. Ueber die Oxydation des Wasserstoes und des Methans durch Mikroorganismen. Zentralblatt fuÈr Bak-teriologie. Parasitenkunde und Infektionen 15, 573±576. Katz, D.D., Cornell, D., Vary, J.A., Kobayashi, R., Elenbaas, J.R.,
Keller, M., Goreau, T.J., Wofsy, S.C., Kaplan, W.A., McEl-roy, M.B., 1983. Production of nitrous oxide and consump-tion of methane by forest soils. Geophysical Research Letters 10 (12), 1156±1159.
Khalil, M.A.K., Rasmussen, R.A., 1993. Decreasing trend of methane: unpredictability of future concentrations. Chemo-sphere 26 (1±4), 803±814.
Khalil, M.A.K., Rasmussen, R.A., Shearer, M.J., 1989. Trends of atmospheric methane during the 1960s and 1970s. Journal of Geophysical Research 94 (D15), 18279±18288.
Khalil, M.A.K., Shearer, M.J., Rasmussen, R.A., 1993. Methane sources in China: historical and current emissions. Chemosphere 26 (1±4), 127±142.
King, G.M., 1993. Ecophysiological characteristics of obligate methanotrophic bacteria and methane oxidation in situ. In: Murrell, J.C., Kelly, D.P. (Eds.), Microbial Growth on C1
Compounds. Intercept, Andover, pp. 303±313.
King, G.M., Adamsen, A.P.S., 1992. Eects of temperature on methane consumption in a forest soil and in pure cultures of the MethanotrophMethylomonas rubra. Applied and Envir-onmental Microbiology 58 (9), 2758±2763.
Klostermann, J., Kremers, J., RoÈder, R., 1998. Rezente tekto-nische Bewegungen in der Niederrheinischen Bucht. Fortschr. Geol. Rheinld. u. Westf. 37, 557±571.
Klusman, R.W., Saeed, M.A., 1996. Comparison of light hydrocarbon microseepage mechanisms. In Schumacher, D., Abrams, M. A. (Eds.) Hydrocarbon migration and its near-surface expression. AAPG Memoir 66, 157±168.
Koschorreck, M., Conrad, R., 1993. Oxidation of atmospheric methane in soil: measurements in the ®eld, in soil cores and in soil samples. Global Biogeochemical Cycles 7 (1), 109±121. Kratzsch, H., 1974. Bergschadenkunde, Springer, Berlin, 346 p. KuÈhl, H., 1996. Katalytische Vernichtung des Methans in
Gru-benabwettern. Technische Forschung Kohle, Luxembourg, EUR 17181, 43 p.
Lai, S.H., Tiedje, J.M., Erickson, A.E., 1976. In situ measure-ment of gas diusion coecient in soils. Soil Science Society of America Journal 40, 3±6.
Landesvermessungsamt Nordrhein-Westfalen, 1991. Satelli-tenbildkarte Nordrhein-Westfalen 1:500.000. Map, Bonn. ISBN 3-89439-553-2.
Landwirtschaftskammer Rheinland, 1997. Agrarstatistische UÈbersichten 1997. Bonn. ISSN 0375-6262.
Landwirtschaftskammer Westfalen-Lippe, 1996. Zahlen zur Land-wirtschaft in Westfalen-Lippe 1996. MuÈnster. ISSN 0375-4921. Lelieveld, J., Crutzen, P.J., BruÈhl, C., 1993. Climate eects of
atmospheric methane. Chemosphere 26, 739±768.
Littke, R., 1987. Petrology and genesis of Upper Carboniferous seams from the Ruhr region, West Germany. Int. J. Coal Geol. 7, 147±184.
Lommerzheim, A.J., 1994. Die Genese und Migration der Erdgase im MuÈnsterlaÈnder Becken. Fortschr. Geol. Rheinld. u. Westf. 38, 309±348.
Matthews, M. D., 1996. Migration Ð a view from the top. In: Schumacher, D., Abrams, M. A. (Eds.), Hydrocarbon Migration and its Near-surface Expression. AAPG Memoir, vol. 66, pp. 139±155.
McDonald, J.R., Kenna, E.M., Murrell, J.C., 1995. Detection of methanotrophic bacteria in environmental samples with the PCR. Applied and Environmental Microbiology 61 (1), 116±121.
Mosier, A., Schimel, D., Valentine, D., Bronson, K., Parton, W., 1991. Methane and nitrous oxide ¯uxes in native, ferti-lized and cultivated grasslands. Nature 350, 330±332. MuÈller, G., 1904. Der Gasausbruch auf der Tiefbohrung
Ascheberg III bei Ascheberg. GluÈckauf 43, 1361.
Murrell, J.C., McGowan, V., Cardy, D.I.N., 1993. Detection of methylotrophic bacteria in natural samples by molecular probing techniques. Chemosphere 26 (1-4), 1±11.
Neunlist, S., Rohmer, M., 1985. Novel hopanoids from the methylotrophic bacteria Methylococcus capsulatus and Methylomonas methanica. Biochem. J. 231, 635±639. Patteisky, K., 1963. Grubengas- und Schlagwetterkunde, C.
Th. Kartenberg, Herne, 135 p.
Patteisky, K. 1995. Grubengaskarte des Rheinisch-WestfaÈ-lischen Steinkohlenbezirks. 1:25,000. WestfaÈlische Bergge-werkschaftskasse, Map, Bochum. ISSN 3-86031-379-4. Pospischill, H., 1994. Die Methanemissionen der vorgelagerten
Kohle- und Erdgasprozeûkette. Energiewirtschaftliche Tages-fragen 44 (3), 112±116.
Rice, D.D., 1993. Biogenic gas: controls, habitats, and resource potential. In Howell, D.D. (Ed.), The Future of Energy Gases. US Geol. Surv. Prof. Pap. No. 1570, pp. 583±606. Scheidegger, A.E., 1974. The physics of ¯ow through porous
media, University of Toronto Press, Toronto, 353 p. Schmidt, W., 1931. GasausbruÈche im oÈstlichen Teile des
west-faÈlischen Karbons. GluÈckauf 49, 1525±1526.
SchoÈn, M., Walz, R., Angerer, G., BoÈhm, E., Hiessl, H., 1993. Emissionen der Treibhausgase Distickstooxid und Methan in Deutschland, Umweltbundesamt, Berlin, 145.
Scho®eld, P., Pell, A.N., Krause, D.O., 1997. Molecular beacons: trial of a ¯uorescence-based solution hybridiza-tion technique for ecological studies with ruminal bac-teria. Applied and Environmental Microbiology 63 (3), 1143±1147.
Stauer, B., Fischer, G., Neftel, A., Oeschger, H., 1985. Increase of atmospheric methane recorded in Antarctic ice core. Science 229, 1386±1388.
Steudler, P.A., Bowden, R.D., Melillo, J.M., Aber, J.D., 1989. In¯uence of nitrogen fertilization on methane uptake in temperate forest soils. Nature 341, 314±316.
Strack, AÈ., 1989. Stratigraphie in den ExplorationsraÈumen des Steinkohlenbergbaus. Mitt. Westf. Berggewerkschaftskasse 62, 210±214.
Struckmeier, W., 1990. Wasserhaushalt und Hydrologische Systemanalyse des MuÈnsterlaÈnder Beckens. Landesamt fuÈr Wasser und Abfall (LWA) Schriftenreihe 45, 72 p.
TeichmuÈller, R., TeichmuÈller, M., Colombo, U., Gazzarrini, F., Gon®antini, R., Kneuper, G., 1970. Das Kohlensto-Iso-topen-VerhaÈltnis im Methan von Grubengas und FloÈzgas und seine AbhaÈngigkeit von den geologischen VerhaÈltnissen. Geol. Mitt. 9, 181±206.
Teiwes, K., 1988. Ein¯uû von Bodenbearbeitung und Fahrver-kehr auf physikalische Eigenschaften schlureicher Ack-erboÈden. Thesis. GoÈttingen University. 183 p.
Treskow, A.v., 1978. Auswertung der fuÈr den deutschen Stein-kohlenbergbau vorgeschriebenen Wetterberichte 1977. GluÈckauf 114 (23), 1038±1040.
Tyagi, S., Kramer, F.R., 1996. Molecular beacons: probes that ¯uoresce upon hybridization. Nature Biotechnology 14, 303± 308.
Tyler, S.C., Crill, P.M., Brailsford, G.W., 1994.13C/12C
frac-tionation of methane during oxidation in a temperate fores-ted soil. Geochim. Cosmochim. Acta 58 (6), 1625±1633. Umweltbundesamt, 1995. Jahresbericht 1995. Berlin.
Wegner, T., 1924a. Das Auftreten von Kohlenwasserstoen im Bereiche des westfaÈlischen Karbons. (Beginn). GluÈckauf 60 (30), 631±642.
Wegner, T., 1924b. Das Auftreten von Kohlenwasserstoen im Bereiche des westfaÈlischen Karbons. (Schluû). GluÈckauf 60 (31), 659±664.
Whalen, S.C., Reeburgh, W.S., 1990. Consumption of atmo-spheric methane by tundra soils. Nature 346, 160±162. Whalen, S.C., Reeburgh, W.S., Sandbeck, K.A., 1990. Rapid
methane oxidation in a land®ll cover soil. Applied and environmental microbiology 56 (11), 3405±3411.
Whiticar, M.J., 1990. A geochemical perspective of natural gas and atmospheric methane. Org. Geochem. 16, 531±547. Whiticar, M.J., 1996. Isotope tracking of microbial methane
formation and oxidation. Mitt. Internat. Verein. Limnol. 25, 39±54.
Whiticar, M.J., Faber, E., Schoell, M., 1986. Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation Ð Isotope evidence. Geo-chimica et CosmoGeo-chimica Acta 50, 693±709.
Wingerning, W., 1975. Die AÈnderung des natuÈrlichen Koh-lensto-IsotopenverhaÈltnisses im Methan von Grubengasen
auf Grund physikalischer VorgaÈnge. Thesis. RWTH Aachen 113 p.
Wrede, V., 1985. Tiefentektonik des Aachen-Erkelenzer Stein-kohlengebietes. In: Drozdzewski, G., Engel, H., Wolf, R., Wrede, V. (Eds.), BeitraÈge zur Tiefentektonik westdeutscher SteinkohlenlagerstaÈtten. Geol. Survey Northrhine-Westpha-lia, Krefeld. pp. 9±104. ISBN 3-86090-001-8.
Wrede, V., 1987. Einengung und Bruchtektonik im Ruhrkar-bon. GluÈckauf-Forschungshefte 48, 116±121.
Wrede, V., 1992. StoÈrungstektonik im Ruhrkarbon. Zeitschr. angew. Geol. 38 (2), 94±104.
Wrede, V., Jansen, F., 1993. Nachweis quartaÈrzeitlicher Bruchtektonik im Ruhrgebiet. N. Jb. Geol. PalaÈont. Mh. 1993 (12), 733±748.
Wrede, V., Zeller, M., 1988. Geologie der Aachener Steinkohlen-lagerstaÈtte (Wurm-und Inde-Revier). Geol. Survey Northrhine-Westphalia, Krefeld. 77 p. ISBN 3-86090-001-9.
Wrede, V., Zeller, M., Josten, K. H., 1983. Geologie der Stein-kohlenlagerstaÈtte des Erkelenzer Horstes. Geol. Survey Northrhine-Westphalia, Krefeld. 62 p. ISBN 3-86090-001-5. Zeller, M., 1987. Das produktive Karbon am Niederrhein.
Natur am Niederrhein 2 (2), 55±61.
Zimmermeyer, J. Seeliger, H., 1989. Methanemissionen bei der Steinkohlengewinnung. Stellungnahme fuÈr die Enquete-Kommission GluÈckauf Publishers, Essen. 15 p.
Zundel, M., Rohmer, M., 1985. Hopanoids of the methylo-trophic bacteriaMethylococcus capsulatus and Methylomo-nas sp. as possible precursors of C29 and C30 hopanoid
