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Clarifying the water quality formation and evolution mechanism of mine water is the theoretical basis for groundwater pollution prevention and control in coal mining areas. The formation and evolution of water quality in mine water is very complex and controlled by multiple fields such as hydrodynamic field, hydrochemical field, microbial field and temperature field, with high inhomogeneity and spatial and temporal variability. At present, scholars at home and abroad have studied the hydrodynamic and hydrochemical fields of mine water, but the microbial and temperature fields of coal mine sites have not been adequately studied, especially the formation and evolution of water quality under the influence of microorganisms coupled with multiple fields have rarely been reported.
1 Definition of mine water contaminated sites
Coal mining activities inevitably affect the water environment under natural conditions. Unlike conventional shallow surface chemical, metal smelting, industrial compound pollution, and waste mine dumps, the impact of coal mining is usually wider in area, deeper, and covers more diverse strata. The definition of contaminated sites in coal mining areas is the basis for water quality investigation, environmental assessment, pollution interdiction, pollution load reduction and groundwater protection, therefore, the definition of contaminated sites is crucial. Due to the complex hydrogeological structure of coal mining areas in China, the formation of mine water and its safety impacts on coal mines accompany the whole life cycle of coal mines, and the discharge of mine water is necessary to ensure the safe production of coal mines. Under the influence of mining and the discharge of mine water, the original state of groundwater flow fields (recharge, runoff, discharge), microbial community structure and water chemical components in coal mining areas are changed. Under the influence of coupled physical-chemical-biological fields, the characteristics of mine water quality undergo complex evolution. It can be seen that the formation process of mine water quality mainly revolves around the mining space, and is mainly controlled by two factors: coal mining activities and hydrogeological structure, and the scope of contaminated sites in coal mining areas should be defined on the basis of these two factors.
(1) Water quality in coal mining areas under the influence of mining "three zones"
Mining activity is the most significant driver of water quality evolution in mines. Therefore, in most coal mining areas (except for open pit mines), without considering the influence of tectonic and special hydrogeological structures, the water quality characteristics of mines can be divided into "three zones" based only on the magnitude of the influence of coal mining activities.
The water quality of the coal mining area draped to the "three zones" zoning
The first zone: the area outside the water-conducting fracture zone of the mining top (bottom) plate. Its water-bearing medium and surrounding rocks are basically not involved in the formation of mine water quality, and the groundwater dynamic field is basically not affected by mining disturbances, in which surface water and groundwater participate in the natural cycle, but the water quality of shallow groundwater and surface water within this zone is susceptible to the influence of mine water discharge.
The second belt: above the coal mining face, all areas affected by the top (bottom) plate water-conducting fissure belt. Due to the existence of the hydraulic fracture zone, the groundwater of the aquifer disturbed by mining enters the mine working face, and the groundwater flow field is changed significantly. The altered groundwater runoff paths cause gradual changes in the hydrodynamic, hydrochemical, microbiological, temperature and media conditions of the native groundwater. Therefore, in this process, there is a great possibility of water-water mixing between different aquifers and part of the water-rock interaction, resulting in different degrees of change in groundwater quality characteristics, which in turn affects the formation of mine water quality, but these effects are relatively short, so that the water quality with more obvious characteristics of the primary aquifer water quality However, these effects were relatively short-lived, making this water quality with more obvious primary aquifer water quality characteristics.
The third zone: the area of water accumulation underground (including the back of the working face, the mining area, the roadway and the water warehouse, etc.) This water quality zone is the most frequently affected area by human activities. During the mining period, it often presents an oxic environment due to sufficient oxygen in mine ventilation, and gradually changes to anoxic environment after the mine pit is closed. The change of environment controls the type of complex biogeochemical reactions and the degree of action, which affects the evolution process of groundwater chemical components in the extraction area, therefore, this water quality zone is the key area for the formation of mine water quality. In this water quality zone, groundwater in the production workings and drainage channels is influenced by long-term mine drainage and has good hydrodynamic conditions, but mine water runoff is slow in the closed mining area. The groundwater from each water-filled aquifer undergoes a long period of mixing, water-rock (coal) action, microbial action and other coupling to form a poorly sensed mine "sewage" with a rich microbial community. The formation and evolution of water quality can continue for a long time even after the mine is closed.
(2) Typical hydrogeological structure and water quality formation mode
China's coal resources and coal-producing areas are mainly distributed in North China, Northwest and Southwest, accounting for 95.5% of the total coal resource holdings as well as 88.3% of the raw coal production in the country (2020). According to the distribution characteristics of the main coal producing areas in China, there are three typical hydrogeological structures in China's coal mining areas, which are: North China type, Northwest-Northeast type and South type coalfield hydrogeological structure. On this basis, focusing on the source and formation mode of mine water, the typical hydrogeological structure and water quality formation mode in China are initially classified into 3 categories, and their respective characteristics are as follows.
Structure of mine water formation pattern in major coal mining areas in China ① North China type. China's North China type coalfield with multiple aquifers, by the fault and trap column and other influences as a complex hydrogeological structure, and due to the existence of mining fissures between the aquifers generally exist in varying degrees of hydraulic and water quality links, the top and bottom aquifers may play an important role in the formation of mine water quality. Due to the differences in hydrogeological structure, the maximum mining depth of some mines has exceeded 1300m, the role of mine water quality formation in the vertical space is large, the degree of mine water pollution in different mines also has a large difference, the evolution mechanism of mine water quality after pit closure is also relatively complex. ②Northwest-Northeast type. China's northwest type coalfield main mining coal seam for the Jurassic coal seam, hydrogeological structure is generally simple, but different areas vary greatly. The main influence on the formation of mine water quality is the weakly cemented sandstone aquifer with loose and porous roof structure. Under the influence of arid-semi-arid climate, seasonal rivers often exist on the surface, with low rainfall, high evaporation and strong evaporation concentration, often characterized by highly mineralized groundwater and mine water, and the TDS of mine water in some mines even exceeds 40 g/L. The main mining coal seam in the northeast type coalfield in China is the North Late Jurassic Early Cretaceous coalfield, and the hydrogeological structure is similar to that of the northwest type coalfield, and the main water sources for water filling are The main water sources are surface water and coal seam roof fracture water, and the TDS of the mine water is also higher due to the arid-semi-arid climate and seasonal precipitation. Unlike the northwest-type coalfield, the northeast-type coalfield has an earlier mining history, and most of the coal mines have successively entered the old age stage. Therefore, the phenomenon of water accumulation in the mining area (old empty water) is common, and the abandoned mining space of coal strata has sufficient water-coal interaction, which leads to a large amount of suspended matter exceeding the standard in the mine water of the northeastern coalfield and the formation of "mine black water"; arsenic, phenol, cadmium, lead and other toxic components were detected in the mine water of a few coal mines. Arsenic, phenol, cadmium, lead and other toxic components were detected in mine water of a few coal mines. ③Southern type. The top (e.g. Changxing Group) and bottom (e.g. Mao Kou Group) plates of coal seams in the southern coalfield in China are all karst-strongly developed tuff aquifers, whose mine water quality is controlled by the water-rock action of the top and bottom tuff aquifers and their various associated minerals. Due to the large topographic relief, deep valleys, complex karst system and high background value of metallic associated minerals (partly containing toxic and harmful elements) in the south, the mine water quality often has the characteristics of strong acidity (pH <3 in some mining areas) and high iron and manganese. Especially in the southwest of China, where metal mineral resources are abundant, the mine water is often characterized by high content of cadmium, lead, mercury, chromium, arsenic, copper and other toxic and harmful heavy metals, which has an important impact on the local ecological environment. And the mine water in some mining areas can flow out of the surface into the river system and shallow groundwater, which has a greater impact on regional environmental pollution. The hydrogeological structure of the mine reflects the relative position of various water-filled sources, water-conducting channels and mining space, which determines the seepage conditions and main components of the water-supplying medium in the region, thus controlling the background values of groundwater quality characteristics and the main types of water-rock interaction in the mining space. The influence of the filling water source on the formation of mine water quality is reflected in its provision of water sources for the mining space, which is a prerequisite for the occurrence of water pollution, and its influencing factors mainly include the water pressure, water richness, water temperature, water chemistry and microbiological indicators of the filling water source. The water conductivity channels such as mining damage zones, faults, trap columns and boreholes connect the water source and the mining space, and are the links that lead to the coupling of multiple fields between the water source and the deep groundwater and the complex environment downhole, and at the same time, the water conductivity channels are also important ways for the diffusion, transport and enrichment of various water chemical components.2 Hydrodynamic field evolution and its role in the formation of mine water quality
(1) Evolution of the groundwater dynamic field
During the coal mining process, on the one hand, the formation of top and bottom rock destruction zones, fault activation and water-conducting boreholes and other influences make significant changes to the hydraulic channels between aquifers in the mine area; on the other hand, the long-term mine drainage, artificial dewatering of top and bottom aquifers, aquifer grouting transformation and curtain interception and other projects implemented to ensure safe mining are bound to have a significant impact on the groundwater flow field in the mine area, further affecting The formation and evolution of mine water quality. From the perspective of the whole life cycle of the mine, the evolution of the groundwater dynamic field in the mine area can be divided into three stages: natural equilibrium before mining, strong disturbance after mining and re-equilibrium after pit closure.
① Pre-harvest natural balance stage. Before the coal seam mining disturbance, the regional groundwater system replenishment, runoff and discharge conditions are in dynamic equilibrium, and the natural groundwater cycle is stable. The hydrogeological unit in the coal mine area is often in a natural groundwater equilibrium state, and the water-bearing (septic) layer, weakly permeable layer, fault, trap column, etc. make the groundwater become a semi-closed and semi-open system. Due to the existence of steady-state factors such as water volume, water pressure, seepage rate and media conditions in the aquifer before mining, the water chemistry and water ecological environment of the same aquifer are relatively stable; however, the lithology, mineral composition, storage conditions and geothermal gradient of different strata lead to differences in natural hydrodynamic conditions, water chemistry and microbial environment of different aquifers, which in turn lead to differences in macronutrients, trace amounts and even harmful and beneficial elements in each aquifer. element content of each aquifer has different degrees of difference. (2) Strongly disturbed mining stage. The mining disturbance of the coal seam will induce structural damage to the primary aquifer, and the integrity of the water barrier at the top and bottom of the coal seam will be destroyed, enhancing the hydraulic connection between the aquifers; the main manifestations are activation of the water barrier fault, karst trap column, water conductivity of the borehole, the damage zone of the top and bottom plate mining, and the water storage space of the mining area. Under the influence of mining disturbance, the hydrogeological structure of the coal mine area changes from a relatively steady-state system with one-way hydraulic connection of water-bearing (isolated) layers to a non-steady-state system with complex hydraulic connection, involving aquifers, water-conducting channels, water-bearing bodies/spaces, boreholes, roadways, and mining areas, which causes significant changes in the hydrogeological structure, groundwater dynamic conditions, and biochemical environment of the coal mine area, and the impact of coal mining disturbance on The impact of coal mining disturbance on the regional groundwater dynamic field is summarized as follows.
1) Changes in water conductivity channels. ① coal mining disturbance to form the top plate hydraulic fracture zone, the bottom plate mining damage zone, induced activation of the water barrier fault, trap column sudden (gushing) water, as well as some poorly closed borehole, etc. will become a new channel of water conduction. ② In order to reduce the generation of mine gushing water and reduce the occurrence of sudden water accidents at the working face, the identified primary water-conducting faults and trap columns are slurry blocked and other measures are taken to block the primary water-conducting channels.
2) Change of boundary conditions. For some water-rich, good recharge, hydraulic connection complex aquifer, coal mines to ensure safe production, sometimes using curtain grouting interception, aquifer grouting transformation and other works to reduce the mine gushing water formation, in essence, change the coal mine area one or more hydrogeological unit of vertical or lateral boundary conditions.
(3) The change of the water-containing medium. ① Coal seam mining changes the stratigraphic stress field, the fracture and pore structure changes caused by the destruction and sinking of the roof rock layer, and the newly formed fractures and pores become runoff channels and water storage spaces, which can lead to the groundwater in the coal mine area before flowing into the workings whose water quality is different from that before the well was built. ② In order to increase the water barrier capacity and integrity of the water barrier at the top and bottom of the coal seam, the water-filled aquifer at the top and bottom is modified by grouting or regional treatment, and the aquifer is transformed into a water barrier, which directly changes the media conditions, thickness and permeability coefficient of the aquifer. ③Artificial spreading and lowering, as a common water control measure, can spreading and lowering the pressurized head of the direct water-filled aquifer in the coal seam to below the safe water level, avoiding the formation of mine water from the source. The hydrophobic depressurization will reduce the water-richness and skeleton compression of the aquifer; secondly, the hydrophobic depressurization forms an obvious increase of hydraulic gradient, which accelerates the percolation rate in the aquifer area and causes some salt colloids to dissolve out, which indirectly leads to the development of dissolved pores and increase of voids in the aquifer, and then makes the aquifer permeability enhanced. Especially in recent years, the center of gravity of coal development in China has shifted to the west, and the top plate aquifers of coal seams in western mining areas are mostly characterized by weak cementation of salts, and the aquifer permeability coefficient has a tendency to increase after long-term evolution due to the superposition of mining disturbance and artificial thinning.
4) Change of hydraulic slope. In the process of coal mining, either artificial thinning and lowering or long-term mine drainage will form regional landing funnels, prompting the hydraulic slope to increase, accelerating the water circulation conditions of groundwater, and to a certain extent changing the background conditions for the formation and evolution of groundwater quality.
③ The rebalancing stage after mine closure. The groundwater dynamic field will tend to a new equilibrium state after the long-term evolution of the mine pit closure. The aquifer hydrodynamic field gradually returns to the pre-mining state after mining disturbance, artificial thinning and lowering, and bottom slurry transformation, while curtain slurry interception completely changes the groundwater seepage path, which is often difficult to recover. When the abandoned underground extraction space is full of water, some of these closed and complete underground water storage spaces with less water pressure will become stagnant areas, and mine water will be formed mainly in the active water area where water alternates frequently. From the macroscopic point of view of the regional groundwater system, after the closure of the pit abandoned underground extraction space due to the cessation of drainage, the mine water gradually accumulates, the regional hydrodynamic conditions gradually weaken, making the groundwater flow rate slow down and the water level rebound; the closed extraction area tends to be static (stagnant water), the water chemical balance continues to maintain a positive reaction, and the water quality tends to deteriorate in the short term (take a mine in Shandong as an example). With the recovery of the groundwater level, when the mining area is filled, there will be groundwater from the bottom up through the mining area to backfill the top plate aquifer, similar to karst trap columns, hydraulic conductivity fault fracture zone can communicate multi-layer aquifers. As the bottom aquifer is often pressurized, it will not backfill the bottom aquifer until the water level in the mining area returns to the height of the pressurized head, and will not cause pollution of the bottom aquifer; when the pressurized water level in the mining area is higher than the bottom aquifer, driven by the regional groundwater dynamic field, the water from the mine and the top and bottom aquifers will mix with each other and spread along the groundwater seepage path, resulting in cascading pollution between aquifers The contamination load will form a certain hydrodynamic dispersion range in the aquifer seepage pathway.
(2) Impact of hydrodynamic field evolution on mine water quality
The physical-chemical-biological effects that occur during the formation of mine water quality during the whole life cycle of the mine groundwater system from pre-mining equilibrium, mining disturbance, mine pit closure to post-mining equilibrium are mainly driven and influenced by the groundwater dynamic field.
①The effect of water chemistry equilibrium perturbation. Hydrodynamic field is the source of power to control the formation of mine water, groundwater flow rate directly determines the time of chemical reaction of water-rock (coal), water-gas, water-water mixing, and the equilibrium state of physical-chemical-biological reaction The longer the contact time, the fuller the reaction, and the more complex the water quality of the formed mine water. Before and after coal seam mining, the groundwater dynamic conditions in the coal mine area will change to a greater extent, thus affecting the reaction direction of the water-chemical equilibrium.
② String layer mixing action. There are often natural geological structures such as trap columns and faults in coal mining areas. Before coal mining, some water-conducting faults and karst trap pillars will communicate with multiple water layers, making the water quality of different water layers mix, forming a natural water-chemical balance. When the first coal seam workings are mined, there is a difference in water quality between the original aquifers, but after the formation of new water-conducting channels due to mining disturbances, the hydrodynamic conditions have changed, so that the water in different aquifers occurs in the water-water mixing reaction, which can form a new water quality in a very short time, that is, the initial water quality of mine water. For example, the shallow low-mineralization groundwater in some mining areas, after mining disturbance, string layer and a large number of mining space into the formation of mine water, the overall reduction in the mineralization of mine water; coal seam depth mining, coal seam top and bottom plate mining damage zone, exploration / in situ test of the borehole also become a string layer mixing of water guide channel; when mining disturbance is very violent, some water barrier fault may be activated into water guide fault, so that the initial mine water The water quality formation process is more complicated.
③ Self-cleaning effect of mining area. Although the water quality of the closed mining area has a tendency to deteriorate in the short term, according to the analysis of the author's team on the water quality characteristics of the mining area water in Xuzhou mine, Lushanan mine and some mines in Ordos basin, it is found that the mining area has a certain self-purification ability for some characteristic components after long-term evolution under appropriate conditions, and some chemical components show a decreasing trend on the long time scale after the deterioration stage and equilibrium stage.
Schematic representation of the change in concentration of characteristic pollutants in mine water 3 Evolution of water chemical field and main control factors in the formation of mine water quality
The formation of mine water chemical field is both closely related to and has different characteristics from the native chemical background of groundwater: on the one hand, the main source of mine water is groundwater, which inherits part of the background values of native groundwater; on the other hand, groundwater enters the mine from the native aquifer through the hydraulic conduit and is transported and converged in the process of underground, it contacts non-native rock layers and various media to different degrees and undergoes complex water-water mixing, water-rock (coal) interaction, etc., which may lead to important changes in the content of characteristic components in water.
(1) Background and characteristics of primary groundwater chemistry
The coal strata are mainly sedimentary strata, and the groundwater mainly originates from atmospheric precipitation and surface water infiltration, buried in different types of hydrogeological structures, and continuously interacts with its surroundings during the long geological history, evolving a non-homogeneous layered distribution of primary water chemical field. The continuous dissolution and filtration of groundwater to aquifers is the most important factor in the formation of water chemistry, from recharge area, runoff area to discharge area, water chemistry diffusion occurs under the influence of hydrodynamic field, microbial field, temperature field, concentration difference, etc., during which the ion alternate adsorption, concentration, water-water mixing and other effects, in different stages and regions, one or two effects may be in the In different stages and regions, one or two effects may be dominant.
(2) Source characteristics and main chemical effects
The drastically changing natural conditions and various anthropogenic factors after coal mining lead to the contact and interaction of primary groundwater with different rock-mineral media. The main chemical interactions are dissolution/precipitation, oxidation/reduction, adsorption/desorption and alternate ion adsorption. The non-primary media involved in chemical interactions are mainly classified as inorganic and organic sources.
The concept of multi-field coupling action in the formation and evolution of mine water quality
(3) General characteristics and evolutionary trends of mine water quality
Based on the field research, analysis and testing, and literature search of 201 coal mines in typical mining areas in 14 large coal bases of 100 million tons nationwide, the overall characteristics of mine water quality in typical mining areas nationwide are summarized as follows: most of the mines mainly have excessive conventional components, especially TDS, SO2-4and Cl-The mine water in some mines contains toxic and harmful substances (such as As, Pb, Cr, volatile phenols, etc.), but the content is small and the proportion is small; a few mine waters also contain beneficial elements.
The water quality of mine water in typical mining areas in China exceeds the III water standard in GB/T 14848-2017.4 Basic characteristics of microorganisms in mine water and their roles2～106The main microorganisms are bacteria and archaea, and a few fungi are also included. In groundwater systems, these microbial communities are important bearers of material cycling, energy conversion and information transfer, and are the main drivers of biogeochemical cycles. The specificity of microbial community composition can reflect and influence the chemical characteristics of groundwater environment. In the process of coal mining, the existence environment of microorganisms has changed dramatically after the groundwater enters the mine space, and its influence on the formation of mine water quality has been less studied, therefore, it is crucial to study the distribution characteristics of microbial communities in coal mining areas and their mechanisms of action on the evolution and management of mine water quality for the prevention and control of mine water pollution.
5 The role of temperature field in the evolution of mine water quality
The temperature field in the coal mine area is mainly controlled by the geological background, and the state is relatively stable before mining. During the mining process, temperature changes will affect the evolution of mine water quality by changing physical, chemical and biological effects. First, temperature changes affect the physical parameters of the rock, such as thermal conductivity, specific heat capacity, etc., while the permeability of the rock is also related to temperature. Secondly, appropriate temperature increase will accelerate some chemical reactions in water chemistry, which will affect the water chemistry field to some extent. Moreover, the life activities of microorganisms in mine water are also closely related to the temperature change.
Sun Yajun, male, born in 1963, Anhui Fuyang, second professor of China University of Mining and Technology, doctoral supervisor, academic leader of mine water damage prevention and control research team, deputy director of Mine Water Prevention and Utilization Professional Committee of Chinese Geological Society, secretary general of International Mine Water Association (IMWA) China National Committee, member of Water Prevention and Control Professional Committee of Coal Industry Technical Committee, recipient of National Science and Technology Progress Award. Head of National Key R&D Project. He has received 14 Science and Technology Progress Awards at provincial and ministerial level, including one National Science and Technology Progress Award, four Provincial First-class Awards, two Second-class Awards and seven Third-class Awards. He has published 3 monographs and more than 100 papers.
Mine water damage control and pollution prevention and control
He has long been engaged in the research and teaching of mine hydrogeology, coal mine water damage control and GIS application. He introduced GIS technology and multi-information method into the field of mine sudden water prediction for the first time in China, and proposed multi-information fitting prediction method for mine sudden water. He has presided over two national 973 projects, "Basic theory research on the mechanism and prevention of mine water breakout (2007CB209401)" and "Basic research on geological disaster prevention and environmental protection under high intensity mining of western coal (2013CB227901)". "He is now the project leader of the National Key R&D Program "Materials and Technologies for Groundwater Pollution Prevention and Control in Coal Mine Sites (2019YFC1805400)", and has participated in and completed 2 key projects of the National Natural Science Foundation of China, 5 other vertical projects, and more than 50 projects entrusted by enterprises. He is responsible for more than 50 projects entrusted by enterprises. He has made significant research achievements in the areas of mine water outburst prediction, safe coal mining under surface water bodies, coal mining on ultra-high pressure aquifers, water conservation coal mining in western water-scarce mines, and "green pit closure" in mines.
Sun Yajun, Zhang Li, Xu Zhimin, et al. Multi-field mechanism of water quality formation and evolution of mine water in coal mining areas and research progress [J]. Journal of Coal,2022,47(1):423-437.
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