Study on Geological Environment of Coal Exploitation and Its Influence on Energy Development

First, the geological environment of coal occurrence

1. Geological survey

Geologically, Ordos basin refers to the Mesozoic (especially Permian and Jurassic) in the western part of China-Korea plate. For a long time, geologists thought it was an independent and self-sufficient Mesozoic sedimentary basin. The scope of Ordos energy base studied in this book is basically consistent with the geological Ordos basin, which is roughly 34 ~ 4120' north latitude and 0/05 30' ~130' east longitude. The specific geographical boundary is from Luliang Mountain in the east to Table Mountain, Helan Mountain and Liupanshan Mountain in the west. South to the northern slope of Qinling Mountain, north to the southern foot of Yinshan Mountain, spanning five provinces (regions) of Shaanxi, Gansu, Ningxia, Inner Mongolia and Shanxi. It covers an area of about 400,000 square kilometers.

Ordos basin is an unstable internal craton basin. After the formation of the basin basement, in the subsequent development and evolution of the caprock, it experienced many basin prototype evolutions, such as Aola trough-craton depression (inside and around)-intraplate multicycle continental basin and its forecourt-surrounding fault depression. The present Ordos basin is the superposition of the above basin prototypes (Sun et al., 1990). Since Mesozoic, the influence of basement strata on caprock is not obvious, and the surface folds in the basin are extremely undeveloped. Therefore, the occurrence of strata above Mesozoic in the basin is mostly gentle, with few fractures and fractures.

The basement rock series in Ordos basin can be divided into two types: one is Archean composed of granulite lithofacies (granulite, granulite, mixed granite and gneiss granite); The other is Mesoproterozoic, which is mainly composed of greenstone facies (greenschist, phyllite, marble and metamorphic pseudovolcanic rocks). The sedimentary cap rocks above the basement rock series are from Mesoproterozoic to Tertiary (Paleogene and Neogene), and the cumulative maximum thickness is over10000 m, in which the Great Wall sandstone and Jixian stromatolite siliceous limestone are deposited in the whole Mesoproterozoic basin, with the thickness of1500 m. In the early Paleozoic, 400 ~ 700 m carbonate marine sediments were deposited in the middle of the basin, and 4500m carbonate marine sediments were deposited in the southern and western margins at the same time. From the late Carboniferous to the early Early Permian, a unified coastal facies deposit characterized by coal measures strata was formed in this area, with a thickness of150 ~ 530m ~ 530m. Inland differential subsidence basins were formed in the basin of Late Triassic, including five distinct cycles of continental clastic rock deposition, namely Yanchang Formation of Late Triassic, Yan 'an Formation of Early Middle Jurassic, Zhiluo-Anding Formation of Middle Jurassic and Zhidan Group of Early Cretaceous (Sun, 138), which led to the middle Yanshanian movement at the end of Early Cretaceous between this area and the coastal Pacific region of eastern China from Late Cretaceous to Tertiary (Paleogene). Under this uplift background, the Cenozoic graben system around Ordos Mesozoic basin, represented by Fen, Wei, Yinchuan and Hetao, was formed, and graben deposits with a thickness of several thousand meters to ten thousand meters were deposited in it, mainly in Neogene. However, most of the strata from Late Cretaceous to Tertiary (Paleogene and Neogene) in the central part of the basin are missing.

Since the Quaternary, loess with large thickness has been deposited in most parts of central and southern Ordos Basin. However, due to uplift and erosion, there is no loess deposition in the northern part of it.

Most of the southern part of Ordos Basin is the Loess Plateau. The topography of the Loess Plateau is largely controlled by ancient landforms. The part of the flat basement that is not cut by running water is the loess plateau, while the high principle of strong erosion becomes a broken plateau. The platforms in southern Shaanxi and Longdong, Gansu are well preserved, such as the famous Luochuan platform and Dongzhi platform. Under the action of flowing water and gravity, loess strata and landforms with severely cut basement become loess beams and mounds. In addition, due to the erosion of flowing water, narrow loess gullies and wide and shallow loess gullies can be formed, which makes the ridge rise and fall, the gullies are vertical and horizontal and the terrain is fragmented. It is an area where human activities are frequent, vegetation destruction and soil erosion are the most serious.

Due to climate drought and long-term wind erosion, many crescent-shaped mobile sand dunes and semi-fixed and fixed sandy land have been formed in the high plains of northern Ordos uplift. Kubuqi desert in the north, Mu Us sandy land in the south and loess hills in the east. Kubuqi Desert is an east-west zonal desert extending on the south bank of the Yellow River, and most of the mobile and semi-mobile dunes have good moisture. Mu Us sandy land is mostly fixed and semi-fixed sand dunes with good water conditions, forming irrigated grassland between hills.

2. Geological environment of coal occurrence

Ordos basin is rich in coal resources, with proven reserves of nearly 400 billion t, accounting for 39% of the country's total reserves. Coal-bearing strata include Carboniferous, Permian, Triassic and Yanan Formation of Middle and Lower Jurassic.

(1) Jurassic coalfield

The coal-bearing series belongs to the middle and lower Jurassic Yan 'an Formation, which is composed of sand, mudstone and coal seam, of which mudstone and siltstone account for about 70%, and the water permeability is weak. The overlying Zhiluo Formation and the underlying Fuxian Formation are both weakly permeable rocks. The recharge and runoff conditions of groundwater in Jurassic strata are poor, mainly weathering fractures, and structural fractures are not well developed. The depth of weathering zone is about 40 ~ 60 m, and the water content of rock strata below weathering zone decreases rapidly. After reaching a certain depth, the mine water inflow will not only increase with the increase of mining depth, but also decrease. The groundwater below the weathering zone has slow runoff, poor water quality and high salt content. Hydrogeological types of mineral deposits generally belong to fissure water-filled type, with simple hydrogeological conditions. However, in areas where Quaternary loose sand layers (Salawusu Formation) are widely distributed and burnt rocks are distributed, the hydrogeological conditions often become more complicated, especially when mining shallow coal seams, which may cause serious hydrogeological and geological environment problems. According to the different mine water filling intensity and hydrogeological conditions, the Jurassic coalfield is divided into four hydrogeological zones: ① Mao Liang area on the Loess Plateau. Mainly distributed in the northern part of the basin. The terrain in the area is strongly cut, there is no loose rock cover or sporadic sand nests on the upper part, and the precipitation is small and concentrated, which is not conducive to the recharge and collection of groundwater. The water-rich rocks are weak, the water filling of the deposit is mainly atmospheric precipitation, the mine water inflow is very small, and the hydrogeological conditions of the deposit are simple. ② Distribution area of burnt rock. Along the strike of the main coal seam, it is distributed in a strip shape, and the depth is generally about 60 meters. The width is controlled by the number, spacing, dip angle and topography of coal seams. Rock fractures are developed, and its permeability depends on the recharge area and the cutting degree of valleys to aquifers. When the distribution area is large or covered with a wide Quaternary sand layer, the water permeability is strong, which has an impact on shallow coal seam mining and is often an important local water supply source. ③ Quaternary sand-covered area. The sand layer is exposed on the ground and widely covered on the coal measures, with a thickness of several meters to dozens of meters or even more. Although there is less atmospheric precipitation in this area, the infiltration condition of sand layer is very good, and the infiltration supply of nearby atmospheric precipitation can be obtained in a large scale, and then it will be collected in places with thick sand layer and low paleotopography and discharged in the form of spring water or evaporation. In the shallow coal seam of mine mining, it is often the most important water filling source, and there may be water inrush and sand outburst problems. The hydrogeological conditions of mining deposits in shallow coal seams in this area are medium to complex. Sand water and rock-burning water often have close hydraulic connection and contain valuable water resources, but improper coal mining and water intake will lead to the destruction of large-scale recharge areas, water pollution and deterioration of ecological environment. Therefore, in coal field development, coal mining, water source protection and ecological environment protection should be regarded as the unified planning of system engineering. ④ General area. Other areas without the above three hydrogeological divisions. The groundwater recharge conditions of coal measures strata in this area are not good and the water abundance is weak. The hydrogeological conditions of the deposit are simple, few of them are medium, and the mine water inflow is mostly 1m3 to dozens of cubic meters/hour.

(2) Triassic coalfield in northern Shaanxi

The coalfield is located in the loess Mao Liang area in the middle of the basin. Groundwater in Huangtuliang area receives a small amount of recharge from atmospheric precipitation and is discharged in the form of valleys. The runoff is shallow, the water quantity is small, and the water abundance of rock stratum is weak. The strata below the weathering zone are weak in water abundance and high in salinity, and the hydrogeological conditions are mostly simple. It belongs to fissure water-filled deposit.

(3) Carboniferous and Permian coalfields

Carboniferous-Permian coalfields are distributed in the east, south and west of the basin. The basement of coal measures is Ordovician and Cambrian limestone, which is a regional strong aquifer. The coal measures themselves are relatively weak in water and belong to fractured karst water-filled deposits. The complexity of hydrogeological conditions of the deposit depends on whether limestone water in the basement of coal measures becomes the water source of mine water filling and the ways and means of water filling. The current zoning is described as follows: ① Eastern region. Including Zhungeer coalfield and Hedong coalfield. The groundwater level of limestone aquifer under coal measures strata is deeply buried, usually under the exploitable coal seams in many mining areas. The water content of coal measures strata is weak and the hydrogeological conditions of the deposit are simple. Ordovician limestone water is the main water supply source in mining area. In the long run, when coal seam mining extends below Ordovician limestone water level, limestone water will threaten the mining of lower coal seam. ② Weibei coalfield in the south. The groundwater level elevation of Ordovician limestone water is about 380m, and the occurrence elevation of coal seam gradually increases from east to west. For example, the mining of coal seams in Taiyuan formation in the east is generally threatened by Ordovician limestone water, while most coal seams in Tongchuan mining area in the west are above limestone groundwater level. In Weibei coalfield, because the contact relationship between Ordovician limestone and coal measures is slow angle unconformity, the lithology and water abundance of coal measures underlying limestone in different areas are different, resulting in different hydrogeological conditions. Hydrogeological conditions of coal seams with water level above 380m are mostly simple to medium, while those of coal seams with water level below 380m are medium to complex. Ordovician limestone and CAMBRIAN limestone are exposed or concealed under Quaternary along the southern margin of coal field, and are directly or indirectly replenished by atmospheric precipitation. Limestone and strong runoff zone are also distributed in the shallow part of the southern margin of coal field. Therefore, when mining shallow coal seams, the mine water inflow is large, and the possibility of water inrush increases when mining deep coal seams, but the water inflow may decrease. In the north of Hancheng mining area, there is a certain hydraulic connection between the Yellow River water and limestone water. Limestone water is the most important water source for local industry and agriculture, so the comprehensive utilization of mine water and the combination of drainage and supply should be considered. ③ the western region. There is a thick weak aquifer between coal measures and Ordovician limestone, so Ordovician limestone water cannot enter the mine, and the water content in coal measures is weak. The hydrogeological conditions of the deposit are mostly simple to medium (Wang Shuangming, 1996).

The second is the change of geological environment in the process of coal development.

The geological environment problems caused by coal development are influenced by the characteristics of economic activities such as natural geographical environment, topography, stratigraphic structure, hydrometeorology, vegetation, mining types and development methods. At present, the geological environment problems of coal mines in Ordos Basin are very serious. Underground mining and open-pit mining have different effects on the geological environment of the mining area. The coal mines in this area are mainly underground mining, and its output accounts for about 96% of coal production. The geological environment problems caused by underground coal mining are the most serious. According to the results of geological environmental problems caused by coal mining, the main geological environmental problems can be classified into three types: resource destruction, geological disasters and environmental pollution (Table 3-2) (Xu, 2006).

According to the summary data and field investigation, combined with the actual situation of Daliuta mining area and Tongchuan mining area in key areas, the following five prominent geological environment problems are mainly introduced: ① land subsidence and ground fissures; ② Coal gangue occupies land and pollutes water and soil environment; ③ Destruction and pollution of groundwater system; ④ Soil erosion and land desertification; ⑤ Environmental deterioration in resource-exhausted mining cities.

1. Land subsidence and ground fissures

Ground subsidence and ground fissures caused by underground mining damage cultivated land, road collapse, railway track distortion, building cracks, groundwater seepage along cracks, leading to mine flooding and other accidents. In arid areas, the destruction of surface water system makes it difficult to use water for production, life and agriculture in mining areas. At the same time, it can also induce mountain cracking to form landslides.

Table 3-2 Main Geological Environment Problems of Coal Mining

Land subsidence and ground fissures are the most common and serious disasters in large and medium-sized underground coal mining areas. For example, Huating Coal Mine in Gansu, Shizuishan and Shitanjing Coal Mines in Ningxia, Weibei North City-Tongchuan and Shenfu-Dongsheng Coalfield Mining Areas in Shaanxi.

Because of the dense population in the Loess Plateau, the land destruction caused by land subsidence is mainly the destruction of farmland. The mining areas of Tongchuan, Hancheng, Bai Pu and Hecheng in Weibei area of Shaanxi Province are located in loess tableland, which is a high-quality agricultural production area in Weibei area of Shaanxi Province and a national high-quality apple production base. Decades of underground mining in these large and medium-sized state-owned old coal mining areas has led to ground subsidence, ground fissures and mountain cracks, which has become one of the most serious areas in northwest China where coal mining has damaged agricultural production. The total area of ground subsidence in mined-out areas in Shaanxi Province is about 1 10km2, which is mainly distributed in Weibei and northern Shaanxi coal mining areas. Incomplete accumulation, at the end of 1999, the ground subsidence in Tongchuan mining area was 63.82km2, accounting for 55.38% of the ground subsidence area in the province, of which 80% was cultivated land. The ground collapse in coal mining area is the most serious, because the thickness of coal seam is greater than that of metal ore body, the space of over-exploitation area is much larger than that of metal and other non-metal mines, and the overlying strata are mostly soft shale, siltstone and argillaceous rock. The range and depth of surface subsidence and ground fissures in coal mines are related to many factors, such as coal mining method, mining area of working face, recovery rate of mining area, coal seam occurrence and so on. Generally speaking, the shallower the buried depth, the larger the mining area, and the greater the range and depth of ground subsidence and cracks. 5# coal seam is mined in Dalian Yao Coal Mine, Shenfu Mining Area, Yulin, with a thickness of 4 ~ 6m and a buried depth of 90 ~ 12000m2 and 1992. On May 5, 2002, the ground subsidence occurred above the mine with a depth of 0.7m The mining area of Shizuishan Coal Mine in Shizuishan City, Ningxia was 5. 15km2, and the subsidence area reached 6.97km2, accounting for 135% of its mining area, forming a ground subsidence depression with a depth of 8-20m, and the crack width in some sections was1m. The railway transportation base in the mining area is 10 ~ 20m higher than the subsidence area, which makes the annual railway pad charge of mining enterprises as high as1000000 yuan, and the national road crossing the mining area 109 is forced to divert.

The total area of ground subsidence in coal mine goaf in Shaanxi Province is about 1 10km2 (Table 3-3), which is mainly distributed in Weibei and northern Shaanxi coal mining areas. Among them, the old mining area in Tongchuan City suffered serious ground subsidence due to early mining. By the end of 1999, the land subsidence was 63.82km2, accounting for 55.38% of the land subsidence area in the province, of which 80% was cultivated land. In recent years, the development of coal mines in Shenmu county has been increasing, and the degree of ground subsidence has intensified. As of 200 1, the land subsidence caused by township coal mines in this county has reached 5.32km2

Table 3-3 Ground Collapse in Coal Mining Area of Ordos Energy Base in Shaanxi Province

(According to Northwest Institute of Geology and Mineral Resources)

Ground subsidence, ground fissures and landslides occurred in Tongchuan, Huangling, Heyang, Baishui, Hancheng and other mining areas in Weibei coalfield of Shaanxi Province, Daliuta, Dalian kiln, carambola embankment, Shachuangou, Liuzhangou and Xinmin coalfield in Shenfu coalfield of northern Shaanxi Province to varying degrees, resulting in large-scale farmland destruction, house cracking, track distortion and road collapse. In July of 200 1 year, heavy rain caused ground subsidence and ground fissures in Group 3 1.2hm2 of Cangcun, No.5 mining area of Shaanxi Coal Mine in Huanglingdiantou. The widest ground fissure is 15m, and the settlement gap reaches 7.45m. 60% of the cultivated land cannot be reclaimed, and the farmland is abandoned, with an estimated economic loss of 2.7 million yuan. There are more than 5400 ground fissures in Tongchuan mining area. Taking Wang Shi 'ao Coal Mine as an example, more than 70 fractures were drawn on the topographic map of 1∶5000, with a total length of nearly 7000m. The coal seam in No.201working face of Daliuta Mine in Shenfu Mining Area is shallow, and mining started in July 1995. After caving, cracks were formed on the surface, and the measured crack area was 5742.5m2 After the completion of the first-phase mining plan, it is estimated that the future total mined-out area of Daliuta Mine will be 5.8hm2, and the total area where ground fissures may occur will be about 5.45hm2 At present, the subsidence area is 7.7km2. There are 20 ground collapses, with a total area of 443.54hm2. The area of ground collapses is 48.4% larger than that in 1980s, and the annual expansion rate is 14.47hm2. During the period of 10, many catastrophic geological accidents such as landslides caused by collapses occurred. The area of soil erosion was 449～550 hm2 in 1980s, and reached 663～720 hm2 in 1990s.

2. Coal gangue occupies land and pollutes water and soil environment.

Coal gangue is the waste produced in the process of coal mining and coal preparation, which usually accounts for 12% ~ 20% of coal mine output. It is one of the largest solid wastes in coal mines, and its accumulation will occupy land vegetation. Huanglingdiantou, Shaanxi Province is located in the Loess Plateau, and the forest vegetation in the small watershed area is good, but some coal gangue discharged from coal mines accumulates on the hillside, occupying a well-grown miscellaneous forest. The Yellow River wetland in Xiayukou, Hancheng, Shaanxi has dense reeds and a good ecological environment. However, the coal gangue discharged from Xiayukou Coal Mine fills the beach, which occupies and destroys the ecological resources and environment of the Yellow River wetland, and should be highly valued by the relevant departments. The continuous exploitation of coal resources in a large area has caused groundwater damage that is difficult to recover, and at the same time, it has led to a sharp drop in surface river flow and ecological environment damage. Since 1997 in Shenfu Coalfield Development Zone, Shaanxi Province, many rivers, including Kuye River, have been cut off.

Coal gangue accumulation occupies land for a long time. By the end of 2000, Tongchuan Mining Bureau had a total of 2 mines 12, and accumulated coal gangue12,649,900 tons, and there were more than 50 large and small coal gangue hills 150, of which 35 were over1100,000 tons, and the gangue pressure accounted for 2.37km2.

The piled waste hills are prone to spontaneous combustion, producing a large number of harmful gases such as hydrogen sulfide, which is very harmful to the health of surrounding villagers. According to relevant data, every square meter of gangue hill spontaneously burns for one day and night, which can emit CO 10.8kg, SO26.5kg, H2S and NO22kg. According to the national health standards, the maximum allowable concentration of harmful substances in the air environment of residential areas is 0. 15mg/m3, and the average daily concentration of H2S is 0.0 1mg/m3. Obviously, the air pollution in the spontaneous combustion area of coal gangue exceeds the national standard, which is bound to endanger the health of residents.

In the 13 mine under Tongchuan Mining Bureau, Shaanxi Province, there are six gangue hills that spontaneously ignite (Figure 3-2), and SO2, TSP and benzopyrene around the gangue hills seriously exceed the standard. According to relevant data, all employees who have worked around spontaneous combustion coal gangue piles for more than 5 years have different degrees of emphysema. The spontaneous combustion of Sangshuping Coal Mine in Hancheng, Shaanxi Province caused the SO2 and CO2 in the air to exceed the standard seriously, in which the SO2 concentration exceeded the standard by an average of 16 times and the CO2 concentration exceeded the standard by an average of 20 times. In this air environment, some workers even fainted in the garbage.

Figure 3-2 The smoking gangue hill in Wangshiao Coal Mine of Tongchuan Mining Bureau

Coal gangue will not only cause air pollution, but also the leaching water of coal gangue mountain will pollute the nearby surface water source, groundwater and underlying soil. The color of the leached water sample collected in Jin Huashan Coal Mine of Tongchuan Mining Bureau was black, and it was found to be acidic water, with a pH of 2.82, a COD of 8 12.5mg/L, a suspended matter content of 128.0mg/L, and heavy metal contents of mercury, cadmium, copper, nickel, zinc and manganese all exceeded the standard. The pH value of the leached water collected in Sanlidong Coal Mine is 1.77, the COD is 62 1.6mg/L, the TDS content is 160.658g/L, and the hydrochemical type is magnesium sulfate. The leaching water from these abandoned mountains will flow into the surface water or penetrate into the soil, causing certain pollution.

3. Destruction and pollution of groundwater system

Most coal mining areas in Ordos energy base are seriously short of water. Mine drainage causes the destruction of groundwater balance system, the groundwater level drops and the water quantity decreases. The well water with acidity and high salinity in coal mine caused groundwater pollution, which aggravated the water crisis. The continuous exploitation of coal resources in a large area has caused groundwater damage that is difficult to recover, and at the same time, it has led to a sharp drop in surface river flow and ecological environment damage. Since 1997, many rivers in Shenfu Coalfield Development Zone of Shaanxi Province have been cut off, such as 75d in 2000, 106d in 2006 and 1 in 2006. Due to cracks in the mined-out area of coal mine, the maximum width is more than 2 meters, and the ground in some areas drops by 2 ~ 3 meters. The Shuanggou River with the original flow of 7344 cubic meters per day has completely dried up, and more than 400 mu of paddy fields have become dry land, and poplar and other vegetation have died in a large area.

Tongchuan, Bai Pu, Hecheng and Hancheng coal mines in Weibei, Shaanxi are the main mine water inrush sites, and Tongchuan, Bai Pu, Hecheng and Hancheng, known as the "black belt" in Weibei, are also high gas mining areas. On May 1975, a serious gas and coal dust explosion occurred in Qianwei Mine of Jiaoping Coal Mine of Tongchuan Mining Bureau, killing people. In April, 20001year, two gas explosion accidents occurred in Tongchuan and Hancheng, which killed 86 people and had a very bad social impact.

Mine water inrush in Shaanxi Province mainly occurs in Tongchuan, Bai Pu, Hecheng, Hancheng and other coal mining areas in Weibei. 1989 there are 3 1 natural mines in the 27 coal mines of the above four mining bureaus, and the mines threatened by groundwater account for 32.3%. According to incomplete statistics, there were 36 mine water inrush accidents, including 29 Ordovician limestone geotechnical accidents from 1975 to 1982, accounting for 80.56% of mine water inrush accidents. The underground water disaster in this area mainly comes from karst water of Ordovician limestone and water accumulated in goaf of ancient kiln. 1960 19 years1October 19 The old kiln in Lijiata Coal Mine of Tongchuan Mining Bureau burst into water 53476m3, submerged 18 roadway, with a total length of 1880m, resulting in direct economic loss of 7/kloc-0. Before 1960s, the main mine roadways in this area were still located at +380m level. After 1970s, some pioneering roadways in new mining areas such as Bai Pu, Hancheng and Hecheng were located below +380m level. After 1974, there were 29 Ordovician limestone water inrush accidents in Xiangshan, Magougou, Sangshuping, Dongjiahe, Quanjiahe, No.2 Mine and Macun Mine, which flooded more than 10,000 meters of roadway, resulting in huge losses of forced suspension of production and re-excavation of roadway, and direct economic losses of nearly 20 million yuan.

In Shizuishan coal mine area of Ningxia, ground subsidence, staggered ground fissures, ground water and surface water entered underground tunnels along the fissures, causing many water inrush accidents in the mining area, causing casualties and huge economic losses (Table 3-4).

Table 3-4 List of Water Inrush in Shizuishan Coal Mine in Ningxia

There are more than a dozen individual small coal mines on both sides of Qushui River in Diantou, huangling county, Shaanxi. They mined recklessly under the river, forming four large subsidence areas within 8 square kilometers, all of which span the meandering river bed. The ground fissure is 20cm, and the maximum subsidence area exceeds 1000m2. Cultivated land collapsed in a large area, cracks appeared in houses, and the quantity and quality of drinking water wells changed. 1 September, 998 13 Niuwu Mine, an individual small coal mine, illegally exploited the security pillar of He Ju river bed, crossing the Qushui River connected with several individual small ditch kilns, which led to the mine flooding, and finally led to the flooding of the west mining area of Cangcun 1 inclined shaft, and in June 1999, Shaanxi Huangling Mining Company/kloc-0. The disorderly coal mining in small coal mines not only caused its own flooded wells to stop production, but also caused direct economic losses of 340 1 10,000 yuan and indirect economic losses of 3 1 10,000 yuan to Huangling Mining Company. At the same time, Qushui River enters the goaf of coal mine in the upstream, and then flows out from the wellhead of abandoned small coal mine in the downstream and discharges into Qushui River, which brings great difficulties to the production and life of residents. The mine water inrush accident induced by disorderly mining in Huangling individual coal mine once again shows that the development of mining industry must follow the principle of sustainable development, rationally arrange and strengthen the daily supervision and management of mining order, so that the whole mining industry can develop along a healthy track.

For a long time, due to the limitation of technical level and lack of understanding, mine water has been treated as a water hazard, and it has been discharged in vain without comprehensive utilization and protection. In 2000, the coal output of state-owned mines in northwest China was 37.85 million tons, with an average displacement of 65.438+0.3 t, while the coal output of other mines was 52.09 million tons, with an average displacement of 0.324t t. The coal mines in northwest China are mainly located in arid and semi-arid areas, and the mining areas are short of water resources. Uncontrolled drainage not only greatly destroys groundwater resources and increases the cost per ton of coal, but also leads to ground collapse, groundwater loss and water quality deterioration, and may also lead to sudden underground water seepage accidents.

Coal mine water is mostly acidic water, which is directly discharged without treatment, which aggravates the mine water crisis in arid areas. The pH value of some mine water in Shaanxi, Ningxia and Inner Mongolia is less than 6, and the pH value of Lijiata mine water in Tongchuan, Shaanxi is even lower, which is 3. The direct discharge of acidic mine water will destroy the living environment of aquatic organisms in rivers and inhibit the growth of vegetation in mining areas. Most of the mine water in Gansu, Ningxia, western Inner Mongolia and central and eastern Shaanxi is high salinity water, and the salinity is generally greater than1000 mg/L/L.

In July 2002, some mining bureaus in Weibei coal mining area of Shaanxi Province found that some mines in Baishui, Shaanxi Province directly discharged roadway wastewater into underground karst fractures, resulting in karst water pollution. This problem should be highly valued by relevant departments, and measures should be taken as soon as possible to protect karst water and prevent groundwater resources from being polluted.

4. Soil erosion and land desertification

Soil erosion caused by soil erosion is an important cause of ecological deterioration. The loess area and the transition area between loess and sandstorm have the largest soil erosion. Shaanxi Tongchuan, Hancheng and Shenfu coal mining areas; Shizuishan and Shitanjing coal mining areas in Ningxia; The soil erosion from Shaanxi and Shenfu in Inner Mongolia to Dongsheng in Inner Mongolia is very serious. According to the forecast of environmental report data, the average erosion modulus of Shenfu-Dongsheng mining area in Shaanxi Province is1.21000t/km2 a, with an area of 3024km2. The annual soil erosion is 36,590,400 t. According to the remote sensing data of several mining areas in different periods before and after development, as well as the comprehensive analysis and calculation of river, dam and sediment data, it is shown that the soil erosion after coal mining is generally about twice that before mining. In Wuda and other mining areas in Inner Mongolia, the erosion modulus is10000 ~ 30000t/km2 a, which is 3.0 ~ 4.5 times of the water and soil loss before mining. Soil erosion modulus was 500t/km2·a before mining in Huangling mining area of Shaanxi Province, and it reached1000t/km2 a in soil erosion modulus after five years of mining. With the development of mining area, the problem of soil erosion is becoming more and more serious, which not only destroys the ecological environment, but also directly threatens the safety of mining area. For example, in Shenmu Zhongji Coal Mine, Shaanxi Province, slag poured into the river, occupying 2/3 of the riverbed, blocking the river back in the rainy season in August of 1984, causing serious well flooding accidents.

The ground subsidence caused by coal mining causes the destruction of shallow groundwater system and the death of vegetation in the subsidence area, which provides conditions for the activation of land desertification. Secondly, the construction of open-pit coal mines, transportation and natural gas pipelines occupies a lot of cultivated land, destroys vegetation, loosens topsoil and activates some fixed and semi-fixed sand dunes. Coal mine waste residue accumulates in Gobi desert area, and weathering intensifies land desertification.

The large-scale development of Shenfu coalfield mining area in Shaanxi Province, as well as the indiscriminate excavation and mining by local people and individuals on both sides of the river ditch, destroyed vegetation, led to bare sand, and aggravated soil erosion and land desertification. Since the development in the mid-1980s, 666.7hm2 of cultivated land has been destroyed, more than 60 million tons of waste residue have been piled up, 4946.7hm2 of vegetation has been destroyed, and the Yellow River has added 2010.9 million tons of sediment. According to the forecast results provided by the Environmental Impact Report of Shenfu Dongsheng Mining Area, if necessary measures are not taken to prevent and control sand, when the production capacity of the mining area reaches 30 million tons, the desertification area will increase by 129.64km2, and the desertification area caused by coal mine development will be 1.53 times that of natural development, with an increase of 4.8 million tons of river sand, which is 650,000 more than that under the existing conditions.

5. Exhaustion of coal resources and deterioration of urban environment

Some existing coalfields in Ordos were developed earlier, dating back to 1950s and 1960s. At first, due to backward technology and waste of resources, many mining areas have reached their service life, and now there are no resources to recycle. Tongchuan Mining Bureau, for example, is a large coal enterprise developed on the basis of 1955 Laotongguan Coal Mine. There are 3,004 registered employees1person, 3,269 retirees1person, and about 216,000 employees' families. Because most of the production mines were built in the late 1950s and put into operation in the early 1960s, the coal reserves, mine field scope, production capacity and service life of the mines were all limited by the geological conditions and mining conditions at that time. Since 1980s, nine pairs of mines have been scrapped and closed, reducing the design capacity by 3.96 million tons. At present, there are 8 pairs of approved production capacity of 9.65 million t/a, and there are no associated mines. Some mines in the eastern region are exhausted, with heavy burden and high production cost, and are applying for the closure and bankruptcy project of national resource-exhausted mines. The problem of production development is increasingly prominent, and the survival and development of enterprises are facing severe challenges. The sustainable development of mining cities has attracted the attention of local governments and related scholars. The direct consequence of the exhaustion of coal resources is that mining cities are facing transformation, and a large number of problems need to be solved, such as personnel placement, environmental improvement, and finding new main businesses.

Three, the impact of geological environment problems caused by coal development on coal mining

Large-scale coal mining activities not only greatly destroyed the local geological environment and ecological environment, but also restricted the normal operation of coal mining activities to a great extent, mainly in the following aspects:

(1) Coal mining subsidence and ground fissures reduce water resources, pollute underground water bodies and affect the normal coal mining activities in mining areas.

The collapse of coal mining leads to the destruction of aquifer structure, which makes the phreatic water, which is mainly horizontal runoff, leak vertically along the water-conducting fracture and transform into mine water. In the process of mining drainage, water is discharged to the surface, which generally affects groundwater resources. Coal mining subsidence forms subsidence pits and through cracks, which makes the scarce local surface water and groundwater enter the pits and be polluted, which affects the quality of groundwater and the available resources of groundwater. For example, in Shenfu Dongsheng mining area, on the one hand, coal mining subsidence caused a large amount of groundwater and fine sand in the aquifer of Salawusu Formation to flood into the mine pit, resulting in underground water inrush and sand breakage accidents; On the other hand, mine drainage needs to discharge a large amount of groundwater, which not only wastes valuable water resources, but also destroys the water environment in the mining area (Zhang Fawang, 2007).

In addition, the collapse crack is the most important factor that affects the water environment due to coal mining collapse. Its existence not only increases the evaporation of water in vadose zone, resulting in the drying up of surface ditches, springs and rivers, but also increases the infiltration channels of pollutants, thus leading to the pollution of soil water and groundwater.

The water resources in the northwest coal mining area are already scarce, and the available water resources are reduced due to subsidence and ground fissures, which makes the mine water, coal washing plant water and domestic water in the mining area face severe challenges.

(2) Spontaneous combustion of coal seams and coal gangue not only wastes a lot of coal resources, but also affects coal mining.

In the Jurassic coalfield distribution area in the north of Ordos Basin, the shallow buried depth of coal seam is only 0 ~ 60m, the climate is dry, and the vegetation is scarce, which forms the climatic conditions conducive to large-scale spontaneous combustion of coalfields. Therefore, spontaneous combustion of coal seams and coal gangue is widely distributed, such as Wuhai coalfield and Shendong coalfield. Spontaneous combustion of coal seams and coal gangue will not only burn valuable coal resources, but also affect coal mining, pollute the air and cause huge economic losses.

(3) Mine water inrush accidents not only destroy surface water and groundwater resources, but also often flood mine roadways, seriously affecting coal mining and causing heavy casualties and economic losses.

In China, most Carboniferous-Permian coal mines will be threatened by abundant Ordovician limestone water. Because of the huge amount of water, fast flow rate and high water pressure, the water inrush accidents caused by Ordovician limestone water are often very huge. For example, Fangezhuang Coal Mine in Kailuan,1June 1984, the water inrush accident is rare in the world. The water inrush flooded the Fangezhuang coal mine in four days, and then it broke into the adjacent Lujiatuo coal mine, which was completely submerged, and penetrated into another adjacent Linxi coal mine. It took four months to complete the plugging work, causing economic losses. Carboniferous-Permian coal seams in Ordos Basin are mainly distributed in Tongchuan, Bai Pu, Hecheng and Hancheng, and there have been more than 40 mine water inrush accidents in history. For example, on 1960 65438+ 10/9, the old kiln in Lijiata Coal Mine of Tongchuan Mining Bureau was flooded, 18 roadway was flooded, and 14 people were killed.

A series of water inrush accidents occurred from September 65438 to March 1999 in a few small coal mines on both sides of Qushui River in Diantou, huangling county, Shaanxi Province, causing direct economic loss of 340 1 10,000 yuan and indirect economic loss of 3 1 10,000 yuan.