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Explain that the water quality formation and evolution mechanism of mine water is the theoretical basis for groundwater pollution prevention and control in coal mine areas. The water quality formation and evolution process of mine water is very complex, and is controlled by multiple fields such as hydrodynamic field, water chemical field, microbial field and temperature field, and has high heterogeneity and space-time variability. At present, domestic and foreign scholars have conducted a lot of research on the hydrodynamic field and water chemical field of mine water, but the research on the microbial field and temperature field of coal mine sites is not sufficient, especially the research on the formation and evolution of water quality by multiple coupling effects under the influence of microorganisms is rarely reported.
1 Definition of mine water pollution sites
Coal mine mining activities will inevitably affect the water environment under natural conditions. Unlike conventional shallow surface chemicals, metal smelting, industrial composite pollution, waste mine yards, etc., the impact of coal mine mining is usually wider, has a larger depth, and covers more strata. The definition of polluted sites in coal mine areas is the basis for conducting water quality surveys, environmental assessments, pollution blocking, pollution load reduction and groundwater protection. Therefore, the definition of polluted sites is crucial. Due to the hydrogeological structure of my country's complex coal mine areas, the formation of mine water and its safety impact on coal mines are accompanied by the entire life cycle of coal mines. The discharge of mine water is a necessary measure 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 field (recharge, runoff, discharge), microbial community structure and water chemical components of the coal mine area have changed, and the characteristics of mine water quality have undergone complex evolution under the influence of multiple coupling fields such as physics-chemistry-biology. It can be seen from this that the formation process of mine water quality mainly revolves around mining space and is mainly controlled by two factors: coal mine mining activities and hydrogeological structure. The scope of polluted sites in coal mine areas should be defined based on these two factors.
(1) The "three belts" of water quality in coal mines under the influence of mining and mining
mining activities are the main driving force for the evolution of mine water quality. Therefore, in most coal mine areas (except open-pit mine areas), the water quality characteristics of the mine can be divided into "three belts" based on the size of the influence of coal mine mining activities only.
The water quality in the coal mine area is vertically "three belts" partition
First belt: the area outside the water conduction crack zone of the mining top (bottom) plate. Its aqueous medium and surrounding rock basically do not participate in the formation of mine water quality, and the groundwater dynamic field is basically not affected by mining disturbances. The surface water and groundwater in it participate in natural circulation, but the water quality of shallow groundwater and surface water within this zone is easily affected by the outflow of mine water.
Second belt: coal mining face or above, all areas affected by the water conduction crack zone of the top (bottom) plate. Due to the existence of the water conduction crack zone, the groundwater of the aquifer disturbed by mining enters the mine working surface, and the groundwater flow field undergoes important changes. The change in the groundwater runoff path causes the hydrodynamic field, water chemical field, microbial field, temperature field and medium conditions of the native groundwater to gradually change. Therefore, it is very likely that the water-water mixing and part of the water-rock action between different aquifers may occur in this process, resulting in different degrees of changes in the groundwater water quality characteristics, which in turn affects the formation of the water quality of the mine water. However, the time of these actions is relatively short, which makes this water quality with more obvious water quality characteristics of the native aquifer.
The third belt: underground water accumulation area (including the rear of the working surface, goaf area, tunnel and water silo, etc.). This water quality zone is the area with the most frequent human activities. During the mining period, due to the sufficient ventilation of the mine, the mine often presents an oxidative environment. After the mine is closed, it gradually turns into an oxygen-deficient environment. Environmental changes control the type and degree of action of complex biogeochemical reactions, thereby affecting the evolution of groundwater chemical components in goaf. Therefore, this water quality zone is a key area for the formation of mine water quality. In this water quality zone, the groundwater in the production work surface and drainage tunnel is affected by the mine's long-term drainage, and the hydrodynamic conditions are good, but the mine water runoff is slow in the closed goaf.Groundwater from each water-filled aquifer forms "sewage" of mines with poor sense and rich microbial communities after long-term coupling of mixing, water-rock (coal) action, microbial action, etc. Even after the mine is closed, the formation and evolution of its water quality can continue to proceed for a long time.
(2) Typical hydrogeological structure and water quality formation pattern
my country's coal resources and coal production areas are mainly distributed in North China, Northwest and Southwest, accounting for 95.5% of the total coal resource retention in the country and 88.3% of the raw coal production (2020). According to the distribution characteristics of my country's main coal-producing areas, there are three typical hydrogeological structures in my country's coal mine areas, namely: North China type, Northwest-Northeast type and Southern type coalfield hydrogeological structures. On this basis, based on the source and formation methods of mine water, the typical hydrogeological structure and water quality formation patterns in my country were initially classified into three categories. Their respective characteristics were as follows:
Mine water formation pattern structure in major coal mines in my country ①North China type. my country's North China-type coalfield is characterized by complex hydrogeological structures characterized by multiple aquifers, faults and fall columns, and due to the existence of mining fractures, there are common hydraulic and water quality connections between aquifers to varying degrees, and the top and bottom plate aquifers may play an important role in the formation of mine water quality. Due to the differences in hydrogeological structure, the maximum mining depth in some mining areas has exceeded 1300m. There is a lot of room for the formation of mine water quality vertically. The degree of mine water quality pollution in different mining areas also varies greatly. The evolution mechanism of mine water quality after the pit is closed is also relatively complex. ②Northwest-Northeast type. The main coal seam of the northwest coalfield in my country is the Jurassic coal seam. The hydrogeological structure is generally simple, but there are great differences in different regions. The main factors that affect the formation of mine water quality are weakly cemented sandstone aquifers with loose top plate structure and porous pores. Affected by the arid-semi-arid climate, seasonal rivers often exist on the surface, with little rainfall, large evaporation and strong evaporation and concentration. It is often characterized by groundwater and mine water with high mineralization degree . The TDS of mine water in some mining areas even exceeds 40g/L. The main coal seam of the Northeast-type coalfield in my country is the North Late Jurassic Early Cretaceous coalfield. The hydrogeological structure is similar to that of the Northwest-type coalfield. The main water-filled water sources are surface water and the top crack water of the coal seam. Due to the arid-semi-arid climate and seasonal precipitation, the TDS of its mine water is also higher. Unlike the Northwest-type coalfield, the Northeast-type coalfield has been mined earlier, and most coal mines have entered the aging stage one after another. Therefore, the phenomenon of water accumulation in goaf (old hollow water) is common, and the abandoned mining space of coal-based formations has sufficient water-coal effect, which leads to a large number of suspended water in mines in Northeast-type coal fields exceeding the standard, forming "mine black water"; toxic components such as arsenic, phenol, cadmium, and lead are detected in the mine water of a few coal mines. ③Southern type. The top (such as Changxing Formation) and bottom (such as Maokou Formation) of coal seams in southern my country are both limestone aquifers with strong karst development. The water quality of the mine is also controlled by the water rock action of the limestone aquifers on the top and bottom plate and the various associated minerals in it. Due to the undulating terrain in the southern region, deep valleys, complex karst systems, and high background values of metal-related minerals (some containing toxic and harmful elements), the water quality of its mines is often highly acidic (the pH of some mines is <3),>
(1) The evolution of the groundwater dynamic field
(1) In the coal mining process, on the one hand, the formation of the top and bottom slate rock layer damage zone, fault activation and water guide drilling and other influences, causing major changes in the hydraulic channels between the aquifers in the mine area; on the other hand, the long-term mine drainage, artificial drop of the top and bottom plate aquifers, grouting transformation of the aquifers and curtain interception projects implemented in order to ensure safe mining will inevitably have a significant impact on the groundwater flow field in the mine area, further affecting the formation and evolution of the mine water quality. From the perspective of the entire life cycle of the mine, the evolution of the groundwater dynamic field in the mine can be divided into three stages: natural balance before mining, strong mining disturbance and rebalancing after closing the pit.
① natural balance stage before harvesting. Before the coal seam mining disturbs, the regional groundwater system replenishment, diameter and discharge conditions are in a dynamic equilibrium state, and the groundwater circulation is stable naturally. Hydrological geological units in coal mine areas are often in a natural underground water balance state, including (segment) water layers, weak permeability layers, faults, sinking columns, , etc., making groundwater a semi-closed and semi-open system. Due to the existence of steady-state factors such as water volume, water pressure, seepage velocity, and medium conditions before mining, the water chemistry and water ecological environment of the same aquifer are relatively stable; however, the lithologic properties, mineral composition, distribution conditions, ground temperature gradient and other factors of different formations lead to differences in the natural hydrodynamic conditions, water chemistry conditions, and microbial environment of different aquifers, which in turn leads to different degrees of differences in the content of constant, trace or even harmful and beneficial elements of each aquifer. ② mining strong disturbance stage. The disturbance of coal seam mining will induce damage to the structure of the native water-containing layer, and the water-blocking integrity of the coal seam top and bottom plate water-blocking layer is damaged, enhancing the hydraulic connection between the aquifers; it is mainly manifested in the activation of water-displacement faults, karst sinking column protrusions, drilling water conduction, top and bottom plate mining failure zones, goafs, etc. becoming the advantageous channels or water storage spaces for groundwater. Under the influence of mining disturbances, the hydrogeological structure of the coal mine area has changed from a relatively stable and unidirectional hydraulic connection system with (separated) water layers to a non-stable and complex hydraulic connection system, involving aquifers, water conduit channels, water bodies/spaces, drilling, tunnels, goafs and other aspects, causing major changes in the hydrogeological structure, groundwater dynamic conditions, and biochemical environment of the coal mine area. The impact of coal mining disturbances on the regional groundwater dynamic field is summarized into the following points:
1) Changes in the water conduit channels. ① Coal seam mining disturbances form a top plate water conduction crack zone and a bottom plate mining failure zone, which induces the activation of water barrier faults, collapse column protrusions (influxes), and some closed and poor drilling holes will become new water conduction channels. ② In order to reduce the occurrence of in the mine and reduce the occurrence of water inrush accidents on the working surface, the grouting and sealing of the identified native water conduction faults and fallen columns have been blocked, and the original water conduction channels have been blocked.
2) Change of boundary conditions. For some aquifers with strong water-rich, good rechargeability and complex hydraulic connections, in order to ensure safe production, coal mines sometimes use curtain grouting interception, aquifer grouting transformation and other projects to reduce the formation of water inrush in the mine, which essentially changes the vertical or lateral boundary conditions of one or more hydrogeological units in the coal mine area.
3) Changes in aqueous medium. ① Coal seam mining changes the formation stress field, and the structure of cracks and pores caused by the damage and sinking of the top slab rock layer. The newly formed cracks and pores become runoff channels and water storage spaces, which will lead to the difference between the water quality of groundwater in the coal mine area before the inflow work and the water quality before the well construction.② In order to increase the water barrier capacity and integrity of the coal seam top and bottom plate water barrier, grouting transformation or regional treatment of the top and bottom plate water-filled aquifer is carried out, and the aquifer is transformed into a water barrier, which directly changes the medium conditions, thickness, permeability coefficient, , etc. of the aquifer. ③Article diversion is a common water control measure. It can reduce the pressure-bearing head of the coal seam directly filled with water aquifer to below the safe water level, and avoid the formation of mine water from the source. Hydrophobic pressure reduction will reduce the water-richness of the aquifer and compress the skeleton; secondly, hydrophobic pressure reduction will form a significant increase in hydraulic gradient, which accelerates the seepage speed of the aquifer area, causing some salt cement to dissolve, indirectly lead to the development of aquifer pores and the increase of voids, which in turn will enhance the permeability of the aquifer. Especially in recent years, the focus of my country's coal development has shifted to the west. Most of the aquifers on the top plate of coal seams in western mining areas have weak salt cementation characteristics. Due to the impact of mining disturbances and artificial diversion and dropping, the permeability coefficient of the aquifer has increased tendency after long-term evolution.
4) changes in hydraulic slope . During the coal mining process, whether it is artificial drainage or long-term mine drainage, an area landing funnel will be formed, which will increase the hydraulic slope, accelerate the water circulation conditions of groundwater, and to a certain extent change the background conditions for groundwater water quality formation and evolution.
③Rebalancing stage after the mine is closed. After the coal mine is closed, the groundwater dynamic field will tend to a new equilibrium state after a long-term evolution. After mining disturbance, manual dropout, and bottom plate grouting transformation, the hydrodynamic field of the aquifer gradually returns to the pre-machining state, while curtain grouting interception completely changes the groundwater seepage path and is often difficult to recover. When the underground waste mining space is filled with water, some of the closed and complete underground water storage spaces with low water pressure will become retention areas, and the mine water is mainly formed in areas with frequent alternation of water. From the macro perspective of regional groundwater system, after the pit is closed, the abandoned mining space underground is gradually accumulated due to the suspension of drainage, and the regional hydrodynamic conditions gradually weaken, causing the groundwater flow rate to slow down and the water level to rebound; the closed goaf area tends to static (stagnant water), the water chemical balance continues to maintain a positive reaction, and the water quality tends to deteriorate in the short term (taking a mine in Shandong as an example). As the groundwater level recovers, when the goaf is filled, groundwater will appear after the goaf is filled from bottom to top to repair the roof aquifer. Similar to karst sinking columns and water-guiding fault breaking zones, it can communicate with multiple aquifers. Since the bottom plate aquifer often has pressure bearing properties, it will not reverse the bottom plate aquifer before the goaf water level returns to the height of the pressure head, nor will it cause pollution of the bottom plate aquifer; when the pressure water level in the goaf is higher than the bottom plate aquifer, driven by the regional groundwater power field, the mine water and the water of the top and bottom plate aquifers each other and then move and diffuse along the groundwater seepage path, causing contamination between the aquifers, and the pollution load will form a certain hydrodynamic dispersion range on the aquifer seepage path.
(2) Effect of hydrodynamic field evolution on mine water quality
During the entire life cycle of groundwater system in the mine area from pre-mining equilibrium, mining disturbance, mine pit closing to post-mining equilibrium, physical-chemical-biological effects occurring in the mine water quality formation process are mainly driven and affected by the groundwater dynamic field.
① Water chemical equilibrium disturbance effect. The hydrodynamic field is the source of power to control the formation of mine water. The groundwater flow rate directly determines the time of the chemical reaction between water-rock (coal), water-gas, and water-water, and has different impacts on the equilibrium state of physical-chemical-biological reactions. Generally, the longer the contact time, the more sufficient the reaction, and the more complex the water quality of the mine formed. Before and after coal seams mining, the groundwater dynamic conditions in the coal mine area will change to a greater extent, which will affect the reaction direction of water chemical equilibrium.
② string layer mixing effect. Coal mine areas often have natural geological structures such as fall columns and faults. Before coal seam mining, some water conduction faults and karst sinking columns will communicate with multiple aquifers, so that the water quality of different aquifers is mixed to form a natural water chemical balance.When the coal seam working surface is first mined, there are differences in water quality between each aquifer. However, after the new water conduction channel is formed due to mining disturbances, the hydrodynamic conditions change, causing the water in different aquifers to undergo a water-water mixing reaction, which can form a new water quality in a very short time, that is, the initial water quality of the mine water. For example, after the shallow low-mineralization groundwater in some mining areas, after mining disturbances, the series layer enters the mining space and forms mine water, which overall reduces the mineralization degree of mine water; when the coal seam is deeply mined, the drilling of the top and bottom plate mining failure zones of the coal seam and the exploration/in-situ test also becomes a water guide channel for the mixed series layer; when the mining disturbances are very severe, some water barrier faults may become water guide faults, making the initial water quality formation process more complicated.
③Gao self-purification effect. Although the water quality of closed goafs tends to deteriorate in the short term, according to the actual measurement analysis of the water quality characteristics of the goafs in Xuzhou mining area, southwestern Shandong mining area and some mining areas in the Ordos Basin of , it was found that the goafs have a certain self-purification ability for certain characteristic components after long-term evolution under appropriate conditions, and some chemical components show a decreasing trend on a long-term scale after experiencing the deterioration stage and the equilibrium stage.
Changes in the concentration of characteristic pollutants in the mine water characteristics 33m4 Water chemical field evolution and main control factors
The formation of the mine water chemical field is closely related to the original chemical background of groundwater and has different characteristics: on the one hand, the main source of mine water is groundwater, which inherits part of the background value of native groundwater; on the other hand, groundwater enters the mine through the water guide channel and moves and converges from the original aquifer and moves and gathers downhole, and encounters complex water-water mixing and water-rock (coal) effects, which may lead to important changes in the content of characteristic components in the water.
(1) Groundwater native chemical background and characteristics
coal-based strata are mainly sedimentary strata. Groundwater mainly originates from atmospheric precipitation and surface water infiltration, is buried in different types of hydrogeological structures. It continuously interacts with its surrounding environment during a long geological historical period, and evolves a native water chemical field with heterogeneous layered distribution. The continuous filtration effect of groundwater on the aquifer is the most important factor in the formation of water chemical components. From the recharge area, runoff area to the discharge area, the water chemical components diffuse under the influence of hydrodynamic field, microbial field, temperature field, concentration difference, etc., and undergo alternating ion adsorption, concentration, water-water mixing and other effects. At different stages and regions, one or two effects may be dominant.
(2) source characteristics and main chemical effects
The violently changing natural conditions after coal mining and various human factors cause native groundwater to come into contact with different rock mineral media and interact with each other. The main chemical effects include dissolution/precipitation, oxidation/reduction, adsorption/desorption and alternating ion adsorption. Non-native media involved in chemical action are mainly divided into inorganic sources and organic sources.
Multi-field coupling effect concept of formation and evolution of mine water quality
(3) Overall characteristics and evolutionary trends of mine water quality
Based on on-site research, analysis and testing, literature search, etc. of 201 coal mines in typical mining areas of 100 million tons in 14 countries in the country. To summarize the overall characteristics of the water quality of mines in typical mining areas across the country as: most mining areas mainly have conventional components that exceed the standard, especially TDS, SO2-4 and Cl-. Emissions that fail to meet the standards will have a certain impact on the ecological environment; some mining areas contain toxic and harmful substances (such as As, Pb, Cr, volatile phenols, etc.), but have a small content and a small proportion; a few mine waters also contain beneficial elements.
The water quality of mines in typical mining areas in my country exceeds the standard of Class III water in GB/T 14848-2017 4 Basic microorganisms of mine water and their functions 2~106 microorganisms, mainly bacteria and archaea , and also a small number of fungi.In groundwater systems, these microbial communities are important bearers of matter circulation, , energy conversion and information transmission, and are the main driving factors of biogeochemical cycle. The specificity of microbial community composition can reflect and affect the chemical characteristics of groundwater environment. During the coal mine mining process, after groundwater enters the mine space, the existence environment of microorganisms has undergone tremendous changes, and the impact on the formation of mine water quality is still relatively small. Therefore, studying the distribution characteristics of microbial communities in coal mine areas and their mechanism of action on the evolution and management of mine water quality is crucial to the prevention and control of mine water pollution.
5 Temperature field effect of water quality evolution of mine
The temperature field in the coal mine area is mainly controlled by geological background, and the state is relatively stable before mining. During mining, 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 rocks, such as thermal conductivity, specific heat capacity, , etc., and the permeability of rocks is also related to temperature. Secondly, proper heating will accelerate some chemical reactions in water chemistry and affect water chemistry field to a certain extent. Moreover, the life activities of microorganisms in the mine water are also closely related to temperature changes.
Author profile
Sun Yajun , male, born in 1963, from Woyang, Anhui, second-level professor of , China University of Mining and Technology, doctoral supervisor, academic leader of the mining water damage prevention and control research team, deputy director of the China Geological Society Mining and Water Prevention and Utilization Professional Committee, International Mine Water Association (IMWA) China National Committee, member of the Water Prevention and Control Professional Committee of the Coal Industry Technical Committee, winner of the National Science and Technology Progress Award, and head of the National Key R&D Project. He won 14 scientific and technological progress awards at the provincial and ministerial levels and above, including 1 second prize in national scientific and technological progress, 4 first prizes in provincial and ministerials, 2 second prizes, and 7 third prizes. He has published 3 monographs and published more than 100 papers.
Research direction
Mine water damage prevention and pollution prevention and control
Main achievements
Hundred years of engaged in research and teaching work in mine hydrology, coal mine water damage prevention and control and GIS application. For the first time in China, GIS technology and multivariate information method were introduced into the field of mine water inrush forecasting, and a multivariate information fitting prediction method for mine water inrush is proposed. He has successively presided over the 2 "Basic Theoretical Research on Water Mechanism and Prevention and Control of Coal Mines (2007CB209401)" in the National 973 Program" and "Basic Research on Geological Disaster Prevention and Control of Geological Disaster Prevention and Control of and Environmental Protection under High-intensity Coal Mining in Western China (2013CB227901)". He is currently the project leader of the National Key R&D Plan "Materials and Technology for Groundwater Pollution Prevention and Control of Coal Mine Areas (2019YFC1805400)" project. He has participated in and completed 2 key projects of the National Natural Science Foundation and 5 other vertical fund projects, and is responsible for more than 50 projects entrusted by the enterprise. Remarkable research results have been achieved in mine water induction prediction, safe coal mining under surface water bodies, coal mining on ultra-high pressure aquifers, water-keeping coal mining in western water-scarce mining areas, and "green closed pits" in mines.
Source:
Sun Yajun, Zhang Li, Xu Zhimin, et al. Multi-field action mechanism and research progress of water quality formation and evolution of mines in coal mines [J]. Acta Coal , 2022, 47(1): 423-437.
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