Uncontrolled coal fires and their environmental impacts: Investigating two arid mining regions in north-central China

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Applied Geography 27 (2007) 42–62 www.elsevier.com/locate/apgeog

Uncontrolled coal fires and their environmental impacts: Investigating two arid mining regions in north-central China Claudia Kuenzera,, Jianzhong Zhangb, Anke Tetzlaffc, Paul van Dijkd, Stefan Voigte, Harald Mehle, Wolfgang Wagnerb a

IPF of TU Vienna, Gusshausstr. 27-29, A-1040 Wien, Austria Institute for Photogrammetry and Remote Sensing (IPF), Vienna University of Technology, Vienna, Austria c Swedish Meteorological and Hydrological Institute (SMHI), Norrko¨ping, Sweden d International Institute for Geo-Information Science and Earth Observation (ITC), Enschede, Netherlands e German Remote Sensing Data Center, DFD of the German Aerospace Center, DLR, Oberpfaffenhofen, Germany b

Abstract Uncontrolled coal fires occur worldwide and pose a great threat to the environment. This paper introduces the problem of coal fires referring to two coalfields in north-central China. These areas were regularly investigated during numerous fieldwork campaigns between 2002 and 2005. Emphasis is put on the environmental impacts of the fires, such as atmospheric influences, land subsidence, landscape degradation, as well as the danger for water resources and human health. New approaches for coal fire research are undertaken in numerous national and multi-lateral projects. Research disciplines, addressing the problem of coal fires, include geography, geology, geo-physics, miningengineering, and remote sensing. In combination, they lead the direction towards a holistic approach to detect, monitor, quantify, and finally extinguish the coal fires. r 2006 Elsevier Ltd. All rights reserved. Keywords: Coal fires; Coal mining; Environmental degradation; Remote sensing; China

Introduction: uncontrolled coal fires Coal fires are an environmental and economic problem of international magnitude. They occur worldwide in countries like China, Russia, the USA, Indonesia, Australia and Corresponding author.

E-mail address: [email protected] (C. Kuenzer). 0143-6228/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeog.2006.09.007

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South Africa, amongst others (Boekemeier, Wang, Zhu, & Elleringmann, 2002; Deng, Wan, & Zhao, 2001; Glover, 1998; Prakash, Gens, & Vekerdy, 1999; Prakash & Gupta, 1998). The term coal fire refers to a burning or smouldering coal seam, coal storage pile or coal waste pile. The adsorption of oxygen at the outer and inner surface of coal is an exothermic reaction leading to an increase in temperature within the coal accumulation (Rosema et al., 1999). If the temperature of a coal accumulation exceeds approximately 80 1C the coal can ignite and starts to burn (Kuenzer, 2005). This process is referred to as ‘‘spontaneous combustion’’. It is—after human influences—the second-most common cause for coal fires of large extent (Chaiken et al., 1980; Kim & Chaiken, 1993; Van Genderen & Guan, 1997; Walker, 1999). Coal fires can also be started through thunderstorm lightning, and forest- or peat fires (Page et al., 2002; Ruecker, 2003). However, mining activities, mining accidents and careless human interaction, such as improper mining techniques, thrown away cigarettes, small coal fire for heating in winter or for daily cooking are the most common courses for coal fire ignition (Jia, 2002–2005; Kuenzer, Zhang, & Voigt, 2003). Depending on cause, location and age we can differentiate human-induced coal fires from natural coal fires, surface and sub-surface coal fires, coal seam fires and coal waste- or storage pile fires as well as recent and paleo coal fires (Zhang, Wagner, Prakash, Mehl, & Voigt, 2004). China is the leading country concerning coal production, consumption and export, whereas Russia and the USA rank first and second in terms of coal reserves on a global scale. Annual Chinese coal production averaged above 1.4 billion tons in 1999 (Walker, 1999) and already 1.9 billion tons in 2004. Despite the trends of oil import and the current set up of 19 new nuclear power plants (9 already exist) the energy supply for 1.3 billion Chinese people, of which over 28% live in cities, is mainly provided by coal. In 2003, coal still supplied 74% of the total energy consumption and it is expected to account for 770% for the next 20 years. The country at present accounts for 28% of the world’s coal use with projected requirements of 1632 million tons in 2010. China faces the problem of numerous uncontrolled burning coal fires in more than 11 of its provinces (Chen, 1997; Daniel, 1994; Walker, 1999). In historic documents coal fires in China were mentioned as early as 1000 before present in the travel report of Li Dao Yuan, who explored Northwest China during the ‘‘Northern Song Dynasty’’ (960–1280). Also Marco Polo (1254–1324), mentioned the ‘‘burning mountains along the silk road’’ in his travel documents (Gielisch, 2002). Nowadays, the main coal fire areas stretch along the coal-mining belt in China. This belt extends for 5000 km from East to West along the North of the country. In this area more than 50 coal fires areas of larger extent are known (Van Genderen & Guan, 1997). Area wise, China therefore faces the worldwide biggest problem of coal fires, including the negative impacts associated. It is only within the last years that the topic of coal fires experiences growing popularity also within the daily media. This is the result of several bilateral and multi-lateral research initiatives addressing the problem of coal fires. One of these initiatives is the geoscientific Sino-German Research Initiative on Coal Fire Research coordinated by the German Aerospace Agency, DLR. Within this and further projects the problem of coal fires is assessed from a geologic-, mining-engineering-, climatologic-, socio-economic- and remote sensing point of view. The overall goal of these projects is firstly to get a better understanding of the physical-chemical as well as mechanic processes of the fires. Secondly, projects aim at the set up of coal fire monitoring and early warning systems. Thirdly,

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Fig. 1. A typical coal fire in the mining area of Wuda in north central China. Note the cracks and fissures in the bedrock surface in the left photograph. Gaseous emission of the burning coal seam underground limit visibility. The main author is taking a photograph into one of the cracks, which occur due to volume loss underground. This picture can be seen on the right side. The area shown here is about 30 cm high and 40 cm wide. Temperatures measured exceeded 800 1C. Left photo: C. Hecker, 2003, right photo: Kuenzer, 2003.

projects aim at the support of Chinese extinction strategies to save the valuable resource coal. Overall goal is to reduce the greenhouse relevant gas emissions (CO2, CH4) released through this environmental hazard. The main aim of this paper is to introduce the problem of coal fires in a synoptic overview to a wide geo-scientific community. We therefore present the two arid mining areas of Wuda and Ruqigou–Gulaben, which were (and still are) a spatial focus for coal fire research in China within several bilateral and multilateral research projects. For these areas we especially demonstrate the disastrous environmental impacts coal fires have on their environment. Following, we present research and practical studies, currently pursued, to address the problem from various disciplines. These studies aim at a better understanding and thus a better monitoring and—finally—extinction of the fires (Fig. 1).

Geography of Wuda and Ruqigou–Gulaben in north central China Location of the study areas: Wuda, Gulaben, Ruqigou and adjacent regions Wuda is a coal-mining city located in Inner Mongolia Autonomous Region adjacent to a structural syncline hosting a large coal fire area investigated. The city lies on the western side of the Yellow River, not far from the border to Ningxia Hui Autonomous Region at 39.511 north and 106.601 east. Ten kilometres northwest of Wuda, the first dunes of the Badai Jaran Desert can be found. Altitude in the broader study area of Wuda varies from 1010 to 1980 m above sea level. Ruqigou–Gulaben, the second study area, is located within the northern part of the Helan mountain range in the border zone of Ningxia Hui Autonomous Region and Inner Mongolia Autonomous Region. The mining area is located at approximately 39.071 north and 106.121 east, 25 km west of ShizuishanDawukou City and 70 km northwest of Yinchuan City. Altitude in this mining area varies from 1400 to 2640 m above sea level. The general location of both study areas can be seen in Fig. 2.

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Fig. 2. Location of the study areas Wuda and Ruqigou–Gulaben in north-central China. The magnified area covers about 200 km from North to South and 220 km from East to West.

Fig. 3. General characteristics and topography of the study area Wuda as seen in Landsat 7 ETM+ satellite imagery from September 2002 combined with a digital elevation model.

Geology of Wuda and Ruqigou– Gulaben Wuda The north–south striking structural syncline located 5 km west of Wuda has a spatial extent of 35 km2 and holds a geological reserve of 630 million tons of coal. The coal layers originate from Upper Carboniferous and Lower Permian times. Mineable reserves are stated to be 27 million tons with seam thickness varying between 1 and 6 m. The coal layers are interbedded in different layers of coarse to fine-grained white to dark grey or yellowish sandstone and greyish-, brown- or green-yellowish shale. Quaternary alluvial layers of silt, shale and gravel cover small parts of the Permian and Carboniferous outcrops (Kuenzer,

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2005; Zhang, 2004). In the Wuda syncline the coal layers are mined within the three coal mining fields of Wuhushan in the South, Huangbaici in the East and Suhai-Tu in the Northwest. From these fields three different quality types of coal—fat coal, cooking coal and steam coal—are being extracted from the mineable of the overall 24 coal-seems (Jia, 2002; Kuenzer, 2005; Sun, 2003) (Fig. 3). Ruqigou– Gulaben The Ruqigou and Gulaben coalfields belong to an asymmetrical Jurassic synclinal basin located at the western rim of the Paleo-Mesozoic sedimentary Ordos Basin. The syncline with the Ruqigou coalfield covers an area of approximately 80 km2. Gulaben area is located at the western rim of Ruqigou syncline turning into Houlugou anticline. The 10 main coal seams of the two regions were deposited under lacustrine-fluvial conditions during the Middle Jurassic and are well exposed nowadays due to the Yanshanian uplift and erosion (Zhang, 1998). Average coal rank is high. It ranges from low volatile bituminous coal to high-quality Anthracite, resulting in a very good export market. Ruqigou and Gulaben area together hold a proven geologic reserve of 1.040 million tons with prospects of nearly 1 billion tons (Jia, 2002). The coal seams are interbedded in different layers of older Triassic (Yanchang formation) and younger Jurassic (Zhiluo formation) fine- to coarse-grained white to grey sandstones, purplish red mudstones, yellow-green siltstones and shale, occasional deposits of claystones and interbedded conglomerates (Chen, 1997) (Fig. 4).

Fig. 4. General characteristics and topography of the study area Ruqigou–Gulaben as seen in Landsat 7 ETM+ satellite imagery from September 2002 combined with a digital elevation model.

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Further geographic characteristics of the two study areas The climate of Wuda, Ruqigou–Gulaben and the surrounding area is a middle latitude strong continental semi-arid to fully arid climate, with large daily and seasonal temperature amplitudes influenced mainly by the East Asian Monsoon (Weischet, 1988; Xie, 2001). Winters are cold and long, summers can be very hot, precipitation is low and winds are strong. The climate station in Yinchuan—150 km south of Wuda and 50 km east of Ruqigou–Gulaben (1112 m above sea level) shows an average annual temperature of 8.6 1C, an average annual precipitation amount of 193 mm, and an average absolute daily minimum and maximum of 30.6 and 39.3 1C, respectively. Annual evaporation rates of above 2500 mm lead to a water shortage of the surface flow as well as underground (Chen, 1997; Wu & Zhang, 2003). Most fluvial features like creeks, wadis, and small rivers are seasonal. Due to the semi-arid to arid climate soil development progresses slow. Developed soils occur mainly on the irrigated river terraces of the Yellow River and on its actual floodplains as sandy, silty and loamy types. Vegetation cover is very sparse too. It dominated by dry desert shrubs with partially sclerophyllous leaves, which are commonly Tetranea mongolica, Reaumuria soongorica, Bassia dasyphylla and species of Artemisia (Jaeger, 2003). Economy wise the predominant source of employment is coal mining and related activities. Coal mining takes place in state-run, locally controlled and private mines. Coal mining related industry includes coal washing, coal cooking and coal power plants, respectively. Labour productivity is very low compared to other countries. The average coal output per employee is 163 ton per year. This is only 1/20th of a worker in South Africa (Daniel, 1994). On average, in China every year 6300 fatalities occur in mine accidents. They are also quite often coal fire related. Further, heavy industries in the area are limestone processing and related calcite—and cement factories, dolomite processing, brick burning and loess mining. A good transportation network of roads and railways has been developed, which allows the transportation of coal and other products to the neighbouring provinces and bigger cities. In addition to industry, the primary agricultural sector is the other main source of income for the local inhabitants. Commercially grown food products include corn, sunflowers, rice, potatoes, soybeans, vegetables and fruits. These activities are mainly restricted to the flood plains of the Yellow River’s, irrigated terraces and other irrigated areas. The coal fires in Wuda and Ruqigou–Gulaben Coal fire situation in Wuda Since mining in the Wuda syncline started commercially in 1958, 120 Mtons were mined until the year 2000. Mainly mechanized long wall methods are applied and the average mining depth of state run- or commercial mines is around 100 m but can reach up to 200–300 m. First coal fires in the area were discovered in 1961. The fires started due to spontaneous combustion in coal volumes, which were exposed through mining activities. Nowadays, the annual amount of coal burned by these fires is estimated to 200,000 tons. An acceleration of the process from year to year has been observed. It is assumed, that so far two million tons of coal were destroyed due to coal-fires. An even larger amount

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became inaccessible due to coal fire danger or danger of land subsidence. Today about 10% of the three mines in the Wuda syncline are affected by coal fires, which equals an area of 4 km2 (Jia, 2005). Small-scale private mines with low levels of mechanisation and poor safety standards pose the greatest threat concerning the development of new coal fires or the ventilation of existing fires. Private mining was permitted from 1980, as long as mine shafts were not drilled deeper than 300 m from the mine entrance into the strata. However, many of the formerly over 100 small-scale private mines scattered along the seam outcrops are abandoned today, because coal fires made mining too dangerous. Since mine entrances and ventilation systems were in most cases not properly sealed off, these abandoned private mines are still risk areas for spontaneous combustion of coal seams. Also remaining coal waste- and leftover coal storage piles are at danger of coal fire ignition. The coal of medium to low quality in the Wuda area is particularly prone to the catalytic effect of spontaneous combustion Coal fire situation in Ruqigou– Gulaben In Ruqigou and Gulaben governmental mining started in 1960. By this time many private small-scale mines already existed. In the year 2000, four million tons were mined in the three major mines of Ruqigou (Dafeng open pit mine, Ruqigou- and Baijigou underground mines). One million tons were produced in the seven mines of the Gulaben area, belonging to Inner Mongolia Tai Xi Anthracite Company, which exports 30% of the mined coal to the world market. Mining operations in the Ruqigou area are thus more advanced (better equipment, higher level of mechanization) than in the Wuda area. This can also be seen in Fig. 5. About 45 private mines still exist next to these major mines. Over the past years, the mines in Ruqigou were affected by over 20 coal fires. Over 15 of these fires have been extinguished in the past 5 years. In Gulaben area five active fires are reported, which are all related to former small-scale or present commercial mining. In Gulaben alone the total coal fire area is assumed to 1 km2 with 10 million tons of coal being affected. Here, 600,000 tons of coal was already lost. The overall area affected by coal fires is assumed to cover more than 5.4 km2. Since coal rank in Ruqigou and Gulaben is very high (anthracite coal) all coal fires are the result of mining influence or accidents. They are not the result of purely natural spontaneous combustion. Environmental impacts of coal fires in Wuda and Ruqigou–Gulaben Impact of coal fires on recent geology and geomorphology A local geologic phenomenon resulting from coal fires is the occurrence of pyrometamorphic rocks. These are rocks, which were baked or heated by coal fires resulting in texture- and colour changes due to oxidation and dehydration. The original rocks can be transformed into only slightly pyro-metamorphised baked rock or into heavily pyro-metamorphosed brecciated or molten rock, showing the characteristics of lava due to the increase of temperature (Zhang, 1996). Areas, where pyrometamorphic rock appears on the surface are between several decimetres (e.g. in Wuda as shown in Fig. 6) and up to several hundreds of metres in diameter as found in the Hongliang area within the Ruqigou basin.

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Fig. 5. Coal mining in Ruqigou. The strata dip with an angle exceeding 301. The coal seam is interbedded in banded light sandstones and shales. Dynamite blasting and caterpillars are used to remove the overburden and the coal. The coal is loaded onto large trucks, which serve the processing plants. The mountain on the opposite side of the small valley is about 150 m high. Photo: C. Kuenzer, 2002.

Furthermore, fumarolic minerals—as shown in Fig. 7—are newly generated through coal fire influence. The minerals in the original formation are disassociated by the heat of the fires and transferred along the cracks to the surface. Here, they re-crystallize due to the drop of temperature. In Wuda and Ruqigou–Gulaben study areas most occurrences of fumarolic minerals and related exhalation products are black tar oil, yellow sulphur (see Fig. 7), white calcite (CaCO3), white decomposition of carbonate rocks (CaO) or white salammoniac (NH4Cl). These fumarolic mineral deposits are indicators for the stage of oxidation of the coal and the coal chemistry. Black tar oil or yellow sulphur deposits on the surface along the cracks indicate, that the coal fire is located relatively deep underground and the near ground temperature is lower than roughly 100 1C, because sulphur is only stable below 100 1C—otherwise it would melt (depending on the mineral structure this number varies for 710 1C) (Stracher, 1995). Field observations show, that black tar oil is often related with sulphur. Salammoniac is stable above 100 1C and would not crystallise below this temperature (Schroeter, Lautenschlaeger, & Bibrack, 1992). Therefore, salmiac deposits indicate a higher surface temperature. The minerals occur around cracks and sinkholes and were found to cover an area between 10 cm and 5 m in diameter. An additional influence on the local geology, but also a coal fire-induced geomorphologic processes, is the residue of extensive ash-layers after a coal-seam has been burnt out. Ash layers result in a volume loss underground and thus often lead to land subsidence. Especially in the Wuda area the cap rock collapses, after the underground coal has burned. This results in sudden surficial cracks and slow trough-like land subsidence. The cracks are

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Fig. 6. Red pyrometamorphic rock in the Wuda syncline. Photo: C. Kuenzer, 2002.

perpendicular to the spreading direction of the underground coal fire. Thus a larger number of cracks resulting from on coal fire form a kind of ‘‘crack field’’ and the bedrock surface. In Ruqigou, the dip angles of the bedrock layers are steeped than in Wuda, sudden land subsidence of coal-covering bedrock occurs less often. This is because the coal layers do not support the overlaying bedrock as strongly as in more horizontal layering conditions. Coal fire-induced uncontrolled subsidence poses a great threat to local infrastructure and buildings. In the coal fire area of Centralia (Pennsylvania, USA) parts of a city had to

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Fig. 7. Sulphur minerals crystallising around a vent above a coal fire in Ruqigou. The GPS device acting as scale is about 15 cm long. Photo: C. Kuenzer, 2003.

be evacuated and moved due to the danger of subsidence and fire (Chaiken et al., 1980; Livingood, Winicaties, & Stein, 1999). Furthermore, cracks and vents developed within the overburden rocks provide ventilation paths for the air and exhaust pathways for the by products of coal combustion and hence accelerate the burning process (Zhang, 1998). The following Fig. 8 depicts the coal fire-related features such as pyrometamorphic rocks, fumarolic minerals, and subsidence phenomena as observed in both study areas investigated. Impact of gaseous coal fire emissions on atmosphere and human health Coal fires release heavy smoke, rich of carbon-monoxide (CO), carbon-dioxide (CO2), methane (CH4), sulphur-dioxide (SO2), nitrous oxides (NOx) and other green-house- or toxic gases (e.g. H2S, N2O) as well as fly ash from vents and fissures (Kuenzer, 2005). The heavy smoke above underground coal fires could already be observed in Fig. 1. Chemical calculations indicate, that complete burning (as in occurring e.g. in industry such as electricity generation through coal burning) of 1 ton of average quality coal leads to an emission of 1.17 ton of CO2 and 0.17 ton of methane, with the latter having a 23-fold higher greenhouse capability than CO2. This means that 1 kg of methane in the atmosphere contributes 23 times stronger to atmospheric warming (green house effect) than 1 kg of CO2. Since the burning process within coal fires is not as complete as in industry, these numbers have to be lowered. First estimates of the emissions released through coal fires in China in comparison with human-induced CO2 and similar emissions in Germany are

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15

13 16

14

10 9

12

3 17

6 1

5

4

8 7

11 2 Fig. 8. A schematic diagram of coal fires and their related features—cross-section through the surface. 1: Original rock; 2: Coal seam; 3: Soil; 4: Ash; 5: Molten rock; 6: Baked rock; 7: Coal mine area; 8: Pillar; 9: Underground coal mine fire; 10: Surface coal mine fire; 11: Underground natural coal fire; 12: Surface natural coal fire; 13: Fumarolic minerals; 14: Burnt trench; 15: Burnt pit. 16: Crack; 17: Subsidence. Source: Zhang et al. (2004).

shown in Table 1. More detailed quantifications on the amount of greenhouse relevant and toxic gases released by the coal fires are currently under investigation by the German company DMT and others. This research poses many challenges though, since for many coal fire areas it is already hard to assess how much coal is burning each year. However, taking into account that the amount of coal uncontrollably burnt in China averages between 15 and 20 million tons each year, the gaseous emissions are estimated to sum up to 0.1% of the global annually human-induced CO2 emissions. The visibility within the mining regions was even on sunny and clear days without any cloud coverage or high air moisture extremely low—sometimes less than 2 km. Thick smog covering the coal fire areas was observed from higher elevations. With increasing focus on global-certified emission trading within the framework of the Kyoto protocol and related international agreements (Victor, 2001), the problem of coal fire emissions, therefore, becomes a topic of international scientific and economic dimension. Concerning climatic trends in the regions investigated, analysis of meteorological data for the past half century of northern Ningxia and south-central Inner Mongolia indicate an increase in average annual temperature and a decrease in average annual precipitation (Wu, Lambin, & Courel, 2001; Wu & Zhang, 2003). To separate the influences of a general global warming pattern from the regional influences of mining-induced emission and surface heating can hardly be achieved. The local population suffers from carbon monoxide and other toxic fumes, which can seep into buildings, possibly asphyxiating people inside or causing long-term respiratory problems. It is common to see people wearing facemasks to protect themselves against the gaseous emissions and coal dust. Investigations of the health conditions of local inhabitants in the coal fire areas have shown a general decrease of immune system activity and an increased risk for illnesses of the respiratory tract, tuberculosis and lung cancer (Chen, 1997; Jia, 2002).

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Table 1 Estimation of coal fire induced gaseous emissions in China in comparison with human induced emissions in Germany Gaseous combustion products

Coal fires in China: estimated annual emissions (in thousand tons)

Annual emissions in Germany resulting from industry, households and traffic (in thousand tons)

Carbon dioxide, CO2 Carbon monoxide, CO Sulphur Dioxide, SO2 Methane, CH4 (CO2 equivalent)

16,380a 490c 150c 2380a (54,740d)

858,511b 4952b 831b 3271b (75,233d)

a Based on the assumption, that 14 million tons are burning per year, with 1 ton of coal burnt resulting in 1.17 ton of CO2 and 0.17 ton of methane (this would be in complete combustion). b Umweltbundesamt (Federal Environmental Office, Germany):Federal Emission report (2002). c Aerophotogrammetry and Remote Sensing Bureau of China Coal ARSC (2001). d The green house capability of methane is 23 fold higher than that of CO2.

Fig. 9. Vegetation deterioration due to toxic gasses released from coal fires. Also the heat in the root zone of the plants leads to the slow decay. Photos: Kuenzer, 2004.

Impacts of coal fires on vegetation Vegetation density was mapped within the Wuda coal-mining syncline during the fieldwork in 2002 and 2003. It was observed, that the vegetation density in coal fire areas is decreased. The toxic gasses and the underground heat are responsible for the deterioration of vegetation located above the underground fires. Here, vegetation density is often reduced to less then five percent, with no vegetation at all or only dead vegetation remaining within the close association of the fires (Kuenzer & Voigt, 2003) (Fig. 9). Further influences Apart from the gaseous emissions, coal dust and aerosols are being deflated and later deposited on agricultural fields, in housing areas and in the water resources of the region. Since the self purifying capacity of a water body is dependent on the water volume, the regional precipitation deficit raises the vulnerability of the area for water pollution through

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fluid mine wastes, coal ash and toxic compounds. Through the use of the sparse water resources for irrigation, the pollutants easily reach the food chain. Especially the Yellow River shows extremely high levels of pollution not only stemming from fertilizer and pesticide input but also from mining related activities (Chen, 1997; Jia, 2003; Xie, 2001). Furthermore, food crops in coal fire areas show higher contaminations with Copper (Cu), Fluoride (F), lead (Pb), Zinc (Zn) and Cadmium (Cd) than in other regions (Chen, 1997). An indirect strain on the water resources results from the increase in mining activities, which has lead to a steadily growing population in the studied regions—especially for the area of Wuda. This resulted in an increased demand for food produce, which can only be grown with intensive irrigation. The water level of the Yellow River has therefore declined dramatically within the past 50 years due to agricultural and industrial water withdrawal (Wu et al., 2001). Current and future approaches to combat coal fires On the Chinese side, the main motivation for coal fire fighting is an economic one, namely to save the valuable resource coal. Nevertheless, extinguishing coal fires is also of high relevance from an environmental point of view in order to reduce green house relevant emissions, protect land- and water resources, and to reduce the detrimental impacts on the local population. Possible extinction methods either aim at the removal of the combustible (coal), the cut off of the oxygen supply (ventilation of air), the lowering of the temperature (heat) or combinations of these. In both mining areas fire-fighting activities have already been undertaken. In the Ruqigou–Gulaben study area most of the coal was ignited through careless human interaction. Here, fire fighting already started in 1978. A subsection of the underground through mud injections was undertaken to split up the coal fires. The smaller burning patches were then extinguished with further injections of mud or inertial gasses. Also loess was applied to cover and seal surface fissures. Furthermore, burning seams were dug out and fully excavated. The burning material was transported away by trucks. All these attempts have been very successful in Ruqigou–Gulaben. Most of the coal fires there are nowadays extinct or under control. One reason for this success is of course the availability of governmental money for extinguishing activities. Since the anthracite coal in this area has a high world market value, extinction is still cheaper than the economic loss of coal. The fires in Wuda mainly ignited in abandoned private mines through spontaneous combustion due to the low quality of the coal. Coal in the Wuda area is highly prone to spontaneous combustion, and even two large coal waste piles in the area have caught fire. Until now, only small-scale coal fire-fighting activities at individual fires have taken place to hinder existing coal fires from extending into the present active coal-mining areas. Methods for extinction were the same as mentioned above for Ruqigou–Gulaben. However, the Wuda syncline has received a lot of public attention through a Sino-German coal fire research initiative, coordinated by the German Aerospace Centre, DLR, which started in 2003 and will last until the end of 2009. Within the course of this large scale project extinction activities in Wuda are foreseen from the Chinese side. Here, methods for the extinction of burning coal seams need to be adapted to the local situation of the mining

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areas. The methods should be as efficient as possible, given the very limited water supply, the limited financial possibilities of the local communities and the large spatial extent of the fires. To support fire-fighting activities it is necessary to understand coal fire ignition, the fires’ dynamics, and their physical and chemical characteristics. It is important to survey and monitor existing coal fires and observe not yet burning coal accumulations as well as mining areas on a regular basis. This will grant the early detection of new coal fires and, therefore, a better option to extinguish them at an initial stage. In the last 5–8 years the coal fires in Wuda and Ruqigou–Gulaben, respectively have been thoroughly investigated in situ. This included in base data generation activities such as the mapping of geology, geomorphology (including subsidence phenomena) and landcover. Furthermore, between 2000 and 2005 the coal fires Wuda were mapped with high detail using radiant thermometers, GPS and mobile GPS units. The mapping process of an underground phenomenon is relatively complex, since one cannot see the fire underground directly and has to map indirect indicators, such as hot gasses pouring from cracks and fissures, heated ground, degraded vegetation, pyrometamorphic rocks and fumarolic minerals. For example, in 2005 over 2000 temperature measurements were collected at the nowadays 20 coal fires in the Wuda syncline. Fig. 10 shows the result of such a coal fire mapping for the years 2003. The mapping process of coal fires is described in detail in Kuenzer et al. (2006). We could conclude from multi-year coal fire mapping that the individual fires show a high spatial dynamic. This means within on year an underground coal fire can easily spread for several tenths of metres. Also the intensity of the fires varies strongly—often within the range of weeks. The burning intensity mainly depends on ventilation conditions (influenced by subsidence and surface cracking), local weather conditions (wind, rain) and the availability of coal. Further fieldwork approaches included the collection of gas measurements and micro seismic. Concerning the first several gas samples were collected at individual cracks and fissures on the ground’s surface. Results indicate strongly varying gas emissions and strongly varying compositions depending on sampling time and location. So far the analysis of more or less punctual analysis have not lead to an overall estimation of gas emissions (amount or composition) for one coal fire area or even country-wide coal fire emissions. The latter, micro-seismic, is a technique, where several microphone devices are dug into the ground surrounding and coal fire. The underground burning of the coal as well as the related cracking in the bedrock surface can be recorded as acoustic signal. In this way geophysicists hope to locate the unknown underground centre of a coal fire. Such information would be very useful for a focussed injection of extinction fluids. Gas analysis and micro-seismic are still ongoing, with transferable results to be expected by 2009. The same applies to ground-based electromagnetic measurements. These measurements are conducted to locate underground rock layers, which have changed their magnetic behaviour though a partial melting due adjacent coal fire heat. At temperatures below the so-called Curie point the magnetic elements in rocks (ferromagnetic minerals) are aligned according to the geomagnetic north pole several thousands of years ago. As the temperature is increased, thermal fluctuations increasingly destroy this alignment, until the net magnetisation becomes zero. During the following cooling process (when the coal fire is burned out) the magnetic minerals align towards the nowadays magnetic pole. These changes can then be traced.

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Fig. 10. Wuda syncline as seen in panchromatic (black-and-white) 60 cm high-resolution Quickbird satellite imagery. Dark areas indicate coal accumulations, outcropping coal seams and coal-dust covered roads. The outskirts of the town of Wuda can be seen in the eastern part of the image. In the Northwest the first dunes of the Badain Jaran Desert can be found. The bright surface in the centre of the image is a plateau of white sandstone. The superimposed polygons in blue and yellow show the individual coal fire’s outlines as mapped in the year 2003. Blue regions have temperature below 150 1C, while yellow hot spot areas are well above 150 1C up to near 900 1C in some cases. Note the dashed red and white objects. These are thermal anomalies extracted from 60 m resolution thermal Landsat satellite data. Note that only the hottest (yellow) areas of the coal fires can be relocated in the thermal satellite images of lower resolution.

Next to the in situ oriented disciplines especially remote sensing has played and plays a crucial role in coal fire research. Next to the two coal fire areas investigated, at least 50 more coal fire areas are known in China alone. Field investigations of the fires are expensive, mining areas are often remote and the terrain is dangerous. The process of monitoring and early warning can, therefore, be supported by remote sensing techniques

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Fig. 11. Flow chart of remote sensing and related activities in coal fire research in China within the past 5 years. Completely finished tasks are marked in grey. Detection and monitoring of coal fires is ongoing (DLR). Field campaigns are also planned for the future. Here, geophysical investigations in the field of gas sampling, geomagnetic and micro seismic (German institutes BGR and GGA) will be continued. Reports, which show the results of the coal fire research, are already now delivered on a regular basis to the Chinese mining authorities. This will continue in the future. DEM—digital elevation model.

Fig. 12. This figure shows changes over time (1987–2002) in a small subset of Wuda study area for part of the Yellow River. Dark blue—deep water, light blue—shallow water. The land cover change was assessed from two supervised remote-sensing image classifications. The area is located 5 km northeast of Wuda city. The subset covers an area of around 3  3 kilometres. Yellow River water level decrease due to increased migration into the prospering mining areas, which lead to an increase in the demand of agricultural produce (—increased irrigation).

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(Knuth, Fisher, & Stingelin, 1968; Kuenzer, 2005; Kuenzer et al., 2003; Prakash et al., 1999; Rathore & Wright, 1993). Remote sensing can help to survey the regions with optical and thermal satellite data. So far optical, thermal and radar data of the satellites Landsat-7 ETM+, Landsat-5 TM, ASTER, MODIS, the DLR satellite BIRD, the radar satellite ERS-2 and optical high resolution QUICKBIRD data has been used to derive land coverand geologic maps of the mining environments, to assess land cover changes over time, to extract coal fire-related thermal anomalies based on image statistics, to assess land subsidence with radar differential interferometry, as well as to produce coal fire risk maps from the synergistic use of all the data (Hoffmann, Roth, Tetzlaff, & Voigt, 2003; Kuenzer, 2005; Kuenzer, Strunz, Voigt, & Wagner, 2004; Tetzlaff, 2004; Zhang, 2004). Fig. 11 shows, which tasks have been completed in spaceborne remote sensing research of coal fires in China. In general multi-temporal land cover studies the processes of increased mining activity and population growth in the Wuda region could be observed comparing Landsat-5 TM data derived land cover classifications of the Wuda region from 1987 with landcover maps

Agricultural area Desert Coal fire area

Wuda Desert / bare rocks

Desert / bare rocks

Artemisia shrubs

N

Fig. 13. Soil Adjusted Vegetation Index (SAVI) image on 21.09.2002. The SAVI calculates as: SAVI ¼ (rNIRrRed)/(rNIR+rRed+L) (1+L), where rNIR and rRed are the reflectances in the near infrared and the read channel respectively and L is a climate-dependant constant. The area shown covers about 20 km from north to south and 22 km from east to west. Dark areas indicate regions with no or little vegetation, while bright areas indicate an abundance of vegetation. Thus, the Yellow River appears completely black, while agricultural areas appear white. Note the very limited vegetation density in Wuda syncline. The area is similar to the one on shown Fig. 3. The white-circled mining syncline of Wuda is also presented in Fig. 10.

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derived from Landsat-7 ETM+ data of 2002. Within the defined Wuda study region (1400 km2), between 1987 and 2002 the area covered with coal increased from 8.7 to 16.6 km2, the settled area extended from 12.4 to 33.5 km2, while coverage of the Yellow River decreased from 25.1 to 17 km2. This can clearly be seen in Fig. 12 on the right side, where the river arm still existing in 1987 nowadays lies dry. The phenomenon of vegetation deterioration through coal fire heat and toxic gases can also be picked up in remote sensing data. The soil adjusted vegetation index (SAVI) for arid areas is a measure for vegetation density and vigour and can easily be retrieved from remote sensing data. Though areas with low vegetation density (a low SAVI) do not necessarily indicate a coal fire area, no coal fire areas will show a high SAVI. If coal fire areas have already been demarcated by other means, the search for local SAVI ‘‘troughs’’ can help to delineate possible coal fire areas from satellite data in more detail (Fig. 13). Only recently, within the last 5 years Zhang (2004) has developed an algorithm for the automatic extraction of coal fire-related thermal anomalies form Landsat and MODIS data. Kuenzer (2005) has developed a method to automatically delineate coal fire risk areas from Landsat and ASTER data. Both algorithms together have proven their synergistic effect multiple times. While automated thermal anomaly extraction from thermal daytime or nighttime data still leads to the extraction of false alarms through industry and biomass fires, coal fire risk area delineation can help to exclude such false alarms. Risk area

No coal fires Storage pile fires

No coal fire Subsurface coal fires

Industry

Solar effect no coal fire

N

Fig. 14. Newly detected coal fires based on land cover information and thermal anomaly detection in the two parallel valleys of Hulusitai and Shitanjing, 30 to 50 km northeast of Ruqigou–Gulaben and 50 km southwest of Wuda. (They are also depicted in Fig. 4 as ‘‘coal mining valleys’’.) The coal fires were not known outside of the local community. Extracted coal is displayed as a superimposed vector layer in light blue; thermal anomalies are displayed in yellow (within the demarcated risk area) and green (outside of the demarcated risk area); demarcated coal-fire (risk) areas are displayed in red. Centre coordinate: 611355E, 4342156N, UTM, Z48N. The background image is a panchromatic Landsat 7 ETM+ image.

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delineation can furthermore focus monitoring to areas, which do not yet show thermal anomalous expression, but which are likely to develop coal fires in the future. A further algorithm has been developed to quantify the thermally anomalous clusters, extracted from remote sensing data, regarding their temperature and energy release (Tetzlaff, 2004). It is hoped that the energetic values derived from an individual fire will—in the near future—allow the calculation of the amount of coal burnt in the fire. This again would have implications for the quantification of gas emissions from coal fires. A recent success could be achieved during the field campaigns in 2003 and 2004. Six thermal anomalies derived from thermal data, which were also located within delineated risk areas were checked in situ. The anomalies were located outside the study areas of Wuda and Ruqigou–Gulaben. The remote places were visited during the field campaigns. It turned out, that five of the six anomalies belonged to a subsurface coal fire, while the other one represented a large burning coal waste pile. The fires were to that date not known by the local mining authorities yet. This proves that remote sensing is a powerful tool to detect new or unknown coal fires. It also underlines the capabilities of a remotesensing-based large-scale monitoring system for coal fires in China (Fig. 14). Acknowledgements The authors thank the German Ministry for Education and Research (BMBF) for the funding of Sino-German Coal Fire Research Initiative. Furthermore we thank all German and Chinese project partners for their cooperation. Special thanks to our colleagues from ITC, Netherlands, for their support during two field campaigns. The reviewer’s comment have greatly improved the quality of the manuscript. References Boekemeier, R., Wang, H., Zhu, L., Elleringmann, S. (2002). Ho¨llenfahrt durch China. In: GEO, 09, 2003. Hamburg, Germany Chaiken, R. F., Brennan, R. J., Heisey, B. S., Kim, A. G., Malenka, W. T., Schimmel, J. T. (1980). Problems in the control of anthracite mine fires: A case study of the Centralia Mine Fire. In: Report of investigations 8799 of the United States Department of the Interior. Pittsburgh, USA. Chen, L. (1997). Subsidence assessment in the Ruqigou coalfield, Ningxia, China, using a geomorphological approach. Master thesis at the International Institute for Aerial Survey and Earth Sciences (130pp). ITC, Enschede, Netherlands. Daniel, M. (1994). Chinese coal prospects to 2010 (35pp). London: International Energy Agency, IEA on Coal Research. Deng, W., Wan, Y., & Zhao, R. (2001). Detecting coal fires with a neural network to reduce the effect of solar radiation on Landsat Thematic Mapper thermal infrared images. International Journal of Remote Sensing, 22, 933–944. Gielisch, H. (2002). Lo¨schung von Kohlebra¨nden—Statusbericht zum GTZ Projekt. Internal unpublished report of the DMT (German Montan Technology) on a project on coal fire extinction in China within the framework of a German GTZ project. Glover, L. (1998). Burning beneath the surface. Tribune Review May 3. Hoffmann, J., Roth, A., Tetzlaff, A., Voigt, S. (2003). Detecting coal fires differential interferometric synthetic aperture radar. Online proceedings of the fringe workshop, 01–05.12.2003, Esrin, Italy. Jaeger, E. (2003). Personal interviews in January 2003. E. Jaeger is Professor for botany at the University of Halle, Germany. He named the plant samples brought home from the field campaign in September 2002. Jia, Y. (2002–2005). Personal interviews during the fieldwork campaigns (Y. Jia is the chief engineer of the coal fire fighting team Wuda).

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