Karst hydrogeological characteristics of Jindong large coal basin, northern China

Jindong coal basin is one of the 14 large coal basins planned and constructed by the state, and groundwater resources play an important role in supporting the sustainable development of the coal basin. To improve the understanding of deep karst hydrogeological characteristics of the coal basin, the combination of techniques (i.e., 1:50,000 and 1:100,000 surveys, geophysical prospecting, drilling, dynamic monitoring, hydrochemistry and isotopes, etc.) was used to characterize the hydrogeological structures of deep-buried aquifers and analyze the evolution characteristics of groundwater systems under the conditions of long-term and large-scale coal mining. Deep Cambrian Zhangxia formation oolitic limestone water-rich aquifer was newly discovered in this survey, which characterized by the development of karstic fissures and strong water-richness in the effective structural zone. The dissolubility of the Cambrian Zhangxia formation oolitic limestone is weaker than that of the Ordovician Majiagou formation and Carboniferous Taiyuan formation limestone, but stronger than that of the Ordovician and Cambrian dolomite. Controlled by Jinhuo fault zone, there are many large karst groundwater-bearing basins distributed on both sides, such as Jincheng basin, Yangcheng basin, Changzhi basin, etc., and water yield in the center of the basins can reach more than 10,000 m3/days. Main types of karst groundwater storage structures in study area are syncline basin type, fault fracture zone type and permeable-impermeable contact zone type. Affected by coal mining, the dynamic conditions of karst groundwater have changed significantly, mainly manifested in the movement of the boundary of the karst groundwater system, the decline of the groundwater level, the attenuation of karst springs flow, and the complex conversion of multi-source water. The variation characteristics of the spring flow can be subdivided into three stages, namely relatively stable stage, rapid decline stage and slow decline stage. The main controlling factors of these three stages are atmospheric precipitation, coal mining and karst water exploitation, respectively. The regional groundwater circulation pattern under coal mining can be divided into shallow groundwater flow system, deep groundwater flow system and local groundwater flow system. The local groundwater flow system was mainly affected by coal mining, which was manifested as the concentrated discharge of groundwater to goaf. The results of this study will provide scientific basis for groundwater exploration and exploitation and sustainable development of coal basins.


Introduction
Jindong large coal basin, as one of the 14 large coal bases planned and constructed by the state, is the largest anthracite supply base in China with a coal output accounting for more than 25% of total national coal output, which plays an important role in supporting the national energy security (Jiang et al. 2016;Li et al. 2021). In this work, the study area mainly includes two nationally planned mining areas, naming Lu'an mining area and Jincheng mining area, which are located in the 1 3 203 Page 2 of 15 southeast of Shanxi Province, and the administrative divisions belong to Changzhi city and Jincheng city.
The study area, located in the karst area of northern China, is a typical karst water deposit type. The scale of regional hydrogeological survey is 1:200,000, and the accuracy of survey and research is relatively low. With the increase of coal mining depth, regional survey and research work at medium and large scale is urgently needed to improve the understanding of deep-buried karst aquifers, overlying fractured aquifers, boundary conditions of karst groundwater systems, the superimposed relationship between coal seams and aquifers, and characteristics of groundwater controlled by geological structure, etc. In addition, affected by the long-term and large-scale coal mining, the regional groundwater system has undergone dramatic changes, and environmental and geological problems such as aquifer damage, water level decline, and the flow of karst spring attenuation have become prominent. Therefore, there is an urgent need to carry out study on the characteristics of groundwater dynamic changes to serve the protection of aquifers and ecological restoration of mining areas.
Domestic and foreign scholars have carried out a lot of researches in the study area on the division of karst groundwater systems, the structure and water-richness of Ordovician limestone aquifers (Han et al. 1990;Liang and Wang. 2010;Wang et al. 2020;Li et al. 2021). However, limited by technical conditions and survey methods, the characteristics of deepburied Cambrian limestone aquifers and the evolution law of karst groundwater system under coal mining are insufficiently understood and lack systematical research results.
Based on the 1:50,000 and 1:100,000 surveys in the Jindong coal basin carried out in the early stage, a series of results have been achieved by applying the comprehensive means of geophysical prospecting, drilling, dissolution tests of carbonate rocks, groundwater level simultaneous measurement and dynamic monitoring, hydrochemical isotopes test analysis to study the karst hydrogeological characteristics of Jindong large coal basin (Wang et al. 2019;Li et al. 2021;Wang et al. 2020;Zhang et al. 2020;Zhang et al. 2021aZhang et al. , 2021bZhang et al. 2022). The purpose of this study is to find new types of water-rich aquifers, characterize the hydrogeological structures of deep-buried aquifers, reveal the control law of regional structures on large water storage basins, and analyze the evolution characteristics of groundwater systems under the conditions of long-term and largescale coal mining.

Geologic and hydrogeologic setting of study area
Jindong large coal basin is located in the southeastern flank of the Qinshui basin, which lies in the middle of the North China Craton. The Qinshui basin is composed of a series of multiple synclines, formed after the coal-forming period of the Late Paleozoic, with the long axis extending along the NNE direction. The basin is adjacent to a series of faults and surrounding uplift belts, with Jiaocheng fault attached to Wutaishan uplift in the north, Henghe fault to Zhongtiaoshan uplift in the south, Taihangshan fault to Taihangshan uplift in the east, and Huoshan fault to Huoshan uplift in the west, forming a fault-depression structural pattern that runs through the whole Shanxi Province (Cao et al. 1998;Zhang et al. 2022).
The Qinshui Basin is a typical North China stratigraphic area with complete stratigraphic development, except for the Silurian, Devonian and Lower Carboniferous strata. The distribution of strata in the basin has typical syncline characteristics, the basin margin is surrounded by the widely exposed Paleozoic, and the newer strata are exposed in sequence from bottom to top in the basin. From bottom to top, the study area develops Precambrian, Cambrian, Ordovician, Middle-Upper Carboniferous, Permian, Triassic, Middle Jurassic, Neogene, and Quaternary strata. Due to frequent magmatic activities in the Qinshui Basin, different types of magmatic rocks have been formed since the Middle and Late Archean time, especially the Hercynian, Yanshanian and Himalayan periods which have important effects on the coal measure strata in the basin. The coal-bearing strata are mainly Carboniferous Taiyuan formation and Permian Shanxi formation.
The eastern part of the Qinshui Basin is bounded by Taihang Mountain fault zone, which is connected with Taihangshan Uplift. As one of the crustal faults with the most prominent geologic features of the North China Craton, Taihangshan fault zone is nearly parallel to Taihangshan uplift along the NNE direction, with a full length of about 350 km and a width ranging from 1 to 8 km. The fault zone is segmented from north to south, and the strata, structural forms and deformation intensity of different segments are obviously different (Fig. 1).
The Paleozoic strata in the study area are the Cambrian and Ordovician carbonate strata in the northern karst area, in which large karst spring areas such as Xin'an, Sangu and Yanhe spring area are developed. For the mining of coal seams, these spring areas constitute karst water-filling mine. There are mainly three types of groundwater in the study area, namely pore groundwater, fissure groundwater and karst groundwater. Karst water is the main water supply source for the coal basin. The water-bearing rock formations include Ordovician Middle-Lower Series and Cambrian Middle-Upper Series carbonate rocks, which are widely distributed in the area. Among them, the Middle Ordovician Fengfeng formation and the Upper and Lower Majiagou formation limestone, Middle Cambrian Zhangxia formation oolitic limestone are the main aquifer.

Hydrogeological characteristics of deep-buried Cambrian karst aquifer
This survey found that karst fissures were developed in the deep-buried Cambrian Zhangxia formation oolitic limestone of Jindong coal basin, and a strong groundwater-rich zone can be formed at effective structural sites, which is another strong groundwater-rich aquifer discovered in addition to the Ordovician Majiagou formation karst aquifer, so as to obtain new understandings on the structure of regional aquifers and the occurrence law of groundwater.

Structure of deep-buried Cambrian karst aquifer
The deep-buried Cambrian karst aquifer in the study area is mainly composed of the thick oolitic limestone of the Middle Cambrian Zhangxia formation, with a thickness of 130-350 m and a buried depth of 400-800 m. The aquifer is overlaid with the dolomitic limestones and dolomites of the Late Cambrian Sanshanzi formation, which constitute upper confining bed of the aquifer. There are Lower Cambrian mud shale and Changcheng sand shale as lower confining bed, which provide good conditions for groundwater enrichment. However, the spatial distribution and development degree of karstic fissure in the Cambrian aquifer show great heterogeneity, and typical vein-shaped karstic channels are concentrated along the structural fracture zone, and the karst development in the area with intact rocks is relatively weak.

Dissolution of oolitic limestones of the Zhangxia formation
Rock samples were taken from the main carbonate rock formations in the study area, and the mineral composition was tested. The results are shown in Table 1. The main mineral components of limestone are CaO, MgO, SiO 2 , Al 2 O 3 , Fe 2 O 3 , K 2 O, Na 2 O, TiO 2 , P 2 O 5 , MnO and acid-insoluble substances. The content of CaO accounts for 73%-75% in the Cambrian Zhangxia formation oolitic limestone, which is lower than that in the Carboniferous Taiyuan formation bioclastic limestone and the Ordovician Majiagou formation limestone, but higher than that in the Ordovician Fengfeng formation limestone and the Ordovician-Cambrian dolomite. Through the dissolution test of a complete hydrological year, it is found that the surface dissolution amount of different carbonate rock test pieces is far greater than the underground dissolution amount, which was 2-6 times of the underground dissolution (Fig. 2), indicating that the dissolubility of atmospheric precipitation and surface water is stronger than that of groundwater. In comparison, the dissolubility of the Cambrian Zhangxia formation oolitic limestone is weaker than that of the Ordovician Majiagou formation and Carboniferous Taiyuan formation limestone, but stronger than that of the Ordovician and Cambrian dolomite.

Water-richness of the Cambrian Zhangxia formation oolitic limestone aquifer
This survey found that deep-buried Cambrian Zhangxia formation oolitic limestone aquifer is a water-rich aquifer in the study area. which shows great heterogeneity in law of groundwater occurrence, and generally forms a strong water-rich section in the effective structural zone. The total thickness of the aquifer is 130-350 m, and the lithology is relatively simple, mainly are dark gray middle to thick oolitic limestone. Under the action of tectonic stress, it is easy to form wide and penetrating fissures. In addition, karst fissures are developed into water-containing fissure spaces due to water dissolution. The Lijinzhang drilled hole (ZK01) in Sangu spring area, with a depth of 452 m, developed many karstic fissures at the buried depth of 345 m and 410-450 m, with obvious water erosion and widespread pink iron staining. The phenomenon of water dissolution and pink ion infestation is obvious and common. The main aquifer is a 410-440 m vein-shape fracture development zone, and the groundwater hydrostatic level is 95 m, which is pressurebearing. The pumping test shows that the drawdown of water level was 3.80 m, with a water yield of 1032 m 3 /days and a standard well yield of about 2500 m 3 /days. The Beisanjia drilled hole (ZK02) in Xin'an spring area, with a depth of 800 m. Karstic fissures are developed at the buried depths of 664-673 m, 681.8-686 m and 705-710 m (Fig. 3). The pumping test shows that the drawdown of water level was 0.30 m, with a water yield of 1248 m 3 /days and a standard well yield of about 38,278 m 3 /days.

Groundwater control law of Jinhuo fault zone
Jinhuo fault zone, located in the eastern flank of Qinshui Basin, is the structural boundary between Qinshui basin and Taihangshan uplift. It is a regional fault zone developed on the basis of the weak basement zone, with obvious structural segmentation characteristics due to the differences in deformation intensity and uplift, denudation and reconstruction in the later period. The development and evolution of the Jinhuo fault zone plays an important role in controlling the occurrence and distribution of groundwater in the Jindong large coal basin (Fig. 4).

Controlling the distribution of karst groundwater-bearing basins
The segments from Shanxi Licheng to Jincheng Fenggou of the Jinhuo fault zone were characterized by thrust displacement from west to east in the Mesozoic era, with tilted-up strata on the west side of the fault, and the Paleozoic carbonate strata were exposed in the later period under the influence of denudation. In the Cenozoic era, the structure reversed by normal fault activity descending towards the west and ascending towards the east, so that the Paleozoic carbonate strata on the east side of the fault zone was also tilted up. This action form of thrusting followed by extension makes a series of secondary syncline structural basins formed on both sides of the fault zone.
The syncline structures of carbonate strata provide basic geological conditions for the formation of karst groundwaterbearing basins. Preliminary investigations show that there are many large karst groundwater-bearing basins on the east and west sides of Jinhuo fault zone, such as Jincheng basin, Yangcheng basin, Changzhi basin, etc. The water yield in these basins is mostly above 5000 m 3 /days, and the center of the basins can reach more than 10,000 m 3 /days, which are the main occurrence sites of karst groundwater in study area. Cenozoic structural inversion is the most significant in the Changzhi segment (Zhang et al. 2022). The Changzhi normal fault controls the formation of the Changzhi faulted basin on the west side and the Huguan basin on the east side, with rich karst groundwater amount. The understanding of this law also provides a scientific basis for the exploration of groundwater in coal basins and mine water damage control.

Controlling the boundary properties of karst spring area
The deformation intensity of the Jinhuo fault zone becomes weaker from north to south, and the characteristics of structural segmentation control the boundary properties of the karst spring area (Fig. 5). From Licheng city to Zhuangtou fault, the tectonic form is dominated by thrust faults, followed by oblique folds and deflections. The Cenozoic tectonic inversion is strong, which is the distinctive feature of this segment. The Changzhi normal fault develops in this section, which controls the formation and development of the Changzhi faulted basin, and is also the main enrichment area for karst groundwater in the Xin'an spring area.
The structural deformation intensity is weakened in the segment from Zhuangtou fault to Gaoping city. The fault zone only shows inclined folds formed by the continuous Upper Paleozoic strata. The northern boundary between Xin'an spring area and Sangu spring area was also formed in this segment. Since the aquifer is continuous, the boundary between the two springs area is a moving boundary of the underground watershed.
The thrust faults in the south of Gaoping city are exposed occasionally, and the thrust compression structure in the Jincheng city is relatively obvious. The asymmetric anticline in the hanging wall is a long and narrow mountain ridge in the landform. This segment also becomes the boundary between Sangu spring area and Yanhe spring area, which is a local unobstructed boundary owing to the discontinuity of faults.

Groundwater storage in syncline basin
Controlled by the north-south fold belt, a series of near north-south syncline basins are formed in the study area. In the center of the basins, karstic fissures of the Ordovician and Cambrian limestone aquifers are developed, forming the basins with good groundwater storage space and strong water-richness. The groundwater is recharged by the precipitation and surface water in karst exposed areas, or the shallow groundwater in structural zones. For instance, in Huguan county in the east of the Jinhuo fault-fold belt and in the syncline basin in the north, the drilled holes show that the Cambrian Zhangxia formation oolitic limestone aquifer is extremely rich in groundwater, and the water yield of a single well is more than 10,000 m 3 /days (Fig. 6a).

Groundwater storage in fault fracture zone
Karstic fissures are generally developed in the fault-affected zone, which constitutes a good space for groundwater storage, especially when the rock layer on one side of the fault is weak in permeability and acts as a confining bed, and a groundwater rich area can be formed in the strata with karstic fissures well developed. The horst and graben structures composed of faults were widely developed in the study area, and they are also karst groundwater rich areas. The groundwater in this area is mainly recharged by the precipitation, surface water and shallow groundwater along the structural zone (Fig. 6b).

Groundwater storage in contact zone of permeableimpermeable layers
The karst springs in study area are mostly of contact zone type, which mainly refers to the contact of the limestone aquifer with the underlying impermeable silty limestone and dolomite. The karstic fissures of the limestone aquifer in contact zone were well developed, forming a groundwater rich area. For example, the contact zone between the Upper Majiagou formation (O 2 s) limestone and the silty limestone at the bottom of O 2 s, and the zone between the Lower Majiagou formation (O 2 x) limestone and the Lower Sanshanzi formation (O 1 s and ∈ 3 s) dolomite formed a groundwater rich area under the water resisting effect of silty limestone and dolomite (Fig. 6c).

Evolution of groundwater system boundary
The boundaries of the Sangu spring area and the Xin'an spring area are movable groundwater watershed boundaries. In the 1980s, due to the small amount of groundwater exploitation, the underground watershed basically coincided with the surface watershed, which was located in the area from Setou to Xihuo town. Since the 1990s, the underground watershed began to move with the increase of coal mining and exploitation of groundwater. Previous studies have shown that the water levels of the Sangu spring area and the Xin'an spring area in the north are in a state of decline. However, due to the differences in the hydrogeological conditions, atmospheric precipitation, groundwater exploitation and coal mining intensity of the two spring areas, the water level decline of the Sangu spring area was much greater than that of the Xin'an spring area (Fig. 7), which leads to the In 1986, the water level in Setou town was 668.6 m, in Majiagou village in the south and Changzhi county in the north were 642.5 m and 660.3 m, respectively, and the watershed was located around Setou town (Fig. 7). In 1992, the water level in Setou town dropped sharply, and the groundwater watershed in the two spring areas moved to the area between Setou town and Changzhi city. According to the groundwater level contours map plotted by the survey in 2015, the groundwater system boundary between the two spring areas has moved northward from the surface watershed identified by predecessors due to the influences of karst groundwater exploitation and geological structure in the area, and now the northern edge of groundwater watershed should be the north of the intersection between Jinhuo fault zone and Zhuangtou fault. The underground watershed of Sangu spring area moved northward by 10 km, with a moving rate of 0.345 km/a.

Evolution of groundwater flow
The karst groundwater rich area of the Xin'an spring area is mainly located in the Shangdang-Changzhi-Lucheng area of the Changzhi basin. The aquifer is mainly the Ordovician Majiagou formation limestone, the karst groundwater level is 630-640 m and the water yield of a single well is more than 10,000 m 3 /days. The area around Longquan-Miaozhuang town in Huguan County, on the east side of the Jinhuo fault zone, is also rich in groundwater. The aquifer is the Cambrian Zhangxia formation oolitic limestone with water yield of single well greater than 10,000 m 3 /days and the karst groundwater level is 634-638 m.
Comparing the karst groundwater level of the Xin'an spring area in 2018 and 2004 (Fig. 8), the water level in the Changzhi Basin dropped by about 8-12 m as a whole, and the hydraulic gradient of groundwater flow became smaller. The karst groundwater level in the entire basin was between 632 and 635 m. The karst groundwater level was close to the discharge elevation of the Nanliu spring, but lower than the discharge elevation of the Xiliu Spring, which is one reason why Xiliu spring was dried up and the groundwater discharged only below the Nanliu spring in the drainage area. The karst groundwater level near Huguan county in the east of the spring area dropped by 10-15 m. In the north of the spring area, there are state-owned coal mines such as Zhangcun and Wuyang coal mine near Xiangyuan county, and the groundwater level dropped by about 10-15 m. The groundwater level near Xibaitu town dropped by about 14 m, and the change in the western part of the spring area was not significant, with a drop of 3-5 m.
From 1968 to 2017, the karst groundwater level in the Xin'an spring area has generally declined. According to the long-term monitoring well of the Martyrs Cemetery (Fig. 9a), the groundwater level dropped from 667.2 m in 1968 to 636.5 m in 2017. From 1976 to 2000, the decline rate of karst groundwater level gradually increased, with a decline of 23.2 m and an annual average decline of 0.93 m. After 2000, the groundwater level was in a slow decline stage, with an annual average decline of 0.3 m by 2017.
The karst well in the Zhaidian town, located in the runoff area of karst groundwater system and in the west of the Jinhuo fault zone, showed an overall downward trend of karst groundwater level from 2008 to 2017, with a decline rate of 8.55 m (Fig. 9b). From 2008 to 2010, the karst groundwater level was in a rapid decline stage, with a decline rate of 8.28 m. Since then, the karst groundwater level has fluctuated at a slow decline rate. From 2010 to 2016, the karst groundwater level dropped by only 0.75 m.
The richest areas of karst groundwater in the Yanhe spring area are mainly distributed in the Qinhe drainage zone between Xiahe spring and Heishui spring, with a standard water yield or spring flow of more than 10,000 m 3 /days. The lithology of karst groundwater-bearing aquifer is the Majiagou formation limestone and the Zhangxia formation oolitic limestone, which are exposed from north to south in order from the Xiahe spring, Yanhe spring, Jingetuo spring, Zhaoliang spring, Motan spring and Heishui spring. The Qinhe river valley cuts through the aquifer, and the groundwater level on both sides of the valley is buried at a shallow depth, generally within a few meters. The richer areas of karst groundwater are distributed around the richest areas, from Jiafeng county in the north, to Yangcheng county-Wulonggou village in the northwest.
The karst groundwater level generally declined from 2004 to 2016 of the Yanhe spring area (Fig. 10)

Dynamics of karst springs flow
The karst springs flow in the Jindong coal basin is gradually attenuated (Fig. 11). From 1956 to 2017, the Xin'an spring 1 3 203 Page 10 of 15 flow decreased from 14.4 m 3 /s to 3.6 m 3 /s, with a decline of 10.8 m 3 /s, and the attenuation rate reached 75%. According to the long-term dynamic observation of the Xin'an spring flow (Fig. 11a), the variation characteristics of the spring flow can be subdivided into three stages, namely relatively stable stage, rapid decline stage and slow decline stage. Although the spring flow fluctuated before 1976, it was relatively stable overall. From 1956 to 1976, the spring flow declined by only 0.47m 3 /s, with an average annual decline of 0.022 m 3 /a. The spring flow was in a rapid decline stage from 1976 to 2002, which decreased from 13.97 m 3 /s in 1976 to 4.64 m 3 /s in 2002, with a decline of 9.33 m 3 /s. The attenuation rate of spring flow reached 66.8%, and the average annual decline was 0.36 m 3 /a. The spring flow was in The Yanhe spring flow also has the characteristics of stage changes (Fig. 11b). The spring flow was in a relatively stable stage from 1956 to 1968. From 1968 to 2004, the spring flow rapid declined from 4.60 to 2.07 m 3 /s, with a decline of 2.53 m 3 /s. The attenuation rate of spring flow reached 55%, and the average annual decline was 0.07 m 3 /a. The spring flow was in a slow decline stage after 2004, which decreased from 2.07 m 3 /s in 2004 to 1.53 m 3 /s in 2016, with a decrease of 0.54 m 3 /s. The attenuation rate of spring flow reached 26%, and the average annual decline was 0.045 m 3 /a.
The dynamic change characteristics of the Xin'an spring and the Yanhe spring are generally consistent. The spring flow was relatively stable before the 1970s, declined rapidly from the 1970s to around 2004, and declined slowly after 2004.
The multi-factor correlation analysis between karst spring flow and atmospheric precipitation, mine water inflow, and karst groundwater exploitation shows that in the first stage, there is a positive correlation between the Xin'an spring flow slip 1a data and atmospheric precipitation, and the fitting coefficient is 0.225, which is significantly greater than that of coal mining, indicating that the fluctuation of spring flow in this stage is mainly controlled by atmospheric precipitation. In the second stage, the Xin'an spring flow has a good negative correlation with mine water inflow and karst groundwater exploitation, and the fitting coefficients are 0.791 and 0.713, respectively, indicating that the rapid decline of spring flow in this stage is mainly controlled by coal mining and karst water exploitation. In the third stage, there is a good negative correlation between spring flow and mine water inflow, and the fitting coefficient is 0.853. However, the spring flow is not well correlated with karst water exploitation and atmospheric precipitation (Table 2). Therefore, the slow

Conversion relationship between multi-source water
Based on the characteristic values of deuterium and oxygen isotopes (Fig. 12), the typical leakage section of surface water and karst window of the Quaternary in the Xin'an spring area were selected for calculating the mixing ratio of multi-source water. The results show that in the leakage sections of surface water such as Xiying town, Shangyao town, Beidanche town, and Machigou village, Xiangyuan county (Fig. 8), the proportion of surface water recharge to groundwater can reach 63.7%-84.5%. In the structural sites such as Wenwang mountain base and Ergang horst, surface water and pore groundwater recharge about 20% of groundwater.
In the direct contact area between Quaternary and Ordovician limestone, pore groundwater recharges 41.6%-66.7% of groundwater through karst window.
The main karst springs of the Yanhe Spring area were selected for calculating the mixing ratio of multi-source water using deuterium and oxygen isotopes. The results show that the mixing ratio of surface water in Yanhe spring is 41.8%, that of Heishui spring is 50.5%, and that of Zhaoliang spring is 62.1%.
The leakage of surface water and pore groundwater are important recharge sources of karst groundwater. Affected by coal mining, the surface runoff and pore groundwater level decreased, which will reduce the recharge of karst groundwater, resulting in the decline of the karst groundwater level and the attenuation of the karst spring flow.

Regional groundwater circulation pattern under the influence of coal mining
Combined with the hydrogeological conditions and groundwater flow system of Jindong large coal basin, the regional groundwater circulation pattern under coal mining can be divided into shallow groundwater flow system, deep groundwater flow system and local groundwater flow system (Fig. 13).
The shallow groundwater flow system mainly includes pore groundwater, shallow fissure groundwater and shallow karst groundwater system in mountains and basins. The groundwater is mainly recharged by atmospheric precipitation and surface water. The direction of groundwater runoff is controlled by topography, development conditions of weathered fissure zone, rock stratum inclination and structure. The groundwater is discharged to the river valley with the nearby river valley as the discharge base level. The groundwater in this system has a short runoff distance, active circulation and generally low TDS, with HCO 3 -Ca(Ca·Mg) as the main hydrochemical type.
The deep groundwater flow system is mainly a regional karst groundwater system. The groundwater recharge, runoff and discharge conditions are primarily affected by regional geological structure, strata lithology and other factors. The karst groundwater is mainly recharged by the infiltration of atmospheric precipitation in the karst exposed area, and the leakage recharge from the overlying aquifer in the covered area. The karst groundwater runoff zone is the main runoff channel of groundwater, and the karst springs discharge to downstream in a centralized way is the main discharge pattern. The aquifer in this system has a large burial depth, and the groundwater has a long runoff distance, slow circulation, stable dynamic, with HCO 3 -Ca·Mg as the main hydrochemical type.
The local groundwater flow system is mainly the groundwater flow system in the coal mining affected area. Under the influence of mining activities, "three zones" were developed in the mining area, and the groundwater flowed to the goaf through the water-conducting fracture zone. The strata damaged by mining activities are mainly Carboniferous, Permian and Quaternary. The groundwater mainly recharged by leakage of atmospheric precipitation, surface water and shallow

Conclusions
The Cambrian Zhangxia formation oolitic limestone aquifer is a newly discovered deep-buried groundwater rich aquifer in the Jindong coal basin. The dissolubility of the Zhangxia formation oolitic limestone is weaker than that of the Ordovician Majiagou formation limestone and Carboniferous limestone, but stronger than other types of soluble rocks in study area. The buried depth of the aquifer is 400-800 m, with a water yield of more than 10,000 m 3 /days. The groundwater storage structures are mainly of the syncline basin type and the fault fracture zone type. The Jinhuo fault zone plays a significant role in controlling the distribution of karst groundwater-bearing basins and the boundary properties of spring areas in the Jindong large coal basin. There are many large karst groundwater-bearing basins on both sides of the fault fracture zone, and the water yield is often more than 5000 m 3 /days. From the Zhuangtou fault to the Gaoping city, the structural deformation intensity is weakened, and the aquifer is continuous, making the boundary between the Xin'an Spring area and the Sangu Spring area to become the moving boundary of the underground watershed.
The hydrodynamic conditions of karst groundwater have evolved dramatically under the influences of climate change and coal mining. The boundaries of karst spring areas moved constantly, the karst groundwater level dropped rapidly, and the karst spring flow attenuated greatly. From 2004 to 2016, the regional karst groundwater level dropped by about 8-20 m. From the 1970s to 2017, the attenuation rate of karst spring flow was about 70%. Correlation analysis showed that in the initial stage of coal mining, karst groundwater level and karst spring flow mainly controlled by atmospheric precipitation, the coal mining growth stage is mainly controlled by coal mining and karst groundwater exploration, and the coal mining sharp increase stage is mainly controlled by coal mining.
Under the coal mining conditions, the regional groundwater circulation pattern of Jindong large coal basin can be divided into shallow groundwater flow system, deep groundwater flow system and local groundwater flow system. The local groundwater flow system is mainly affected by coal mining, which changes the original circulation pattern of fissure groundwater and pore groundwater. The system is characterized by the concentrated discharge of shallow groundwater to the goaf, forming a new confluence center.
The current research work lacks systematic understanding of the structure of deep aquifers larger than 500 m, especially the characteristics of deep groundwater circulation and the mechanism of water-rock reaction, and the follow-up work should be strengthened. Based on the investigation and research work, ecological environmental protection and restoration treatment should be strengthened according to the types of coal base, focusing on regional aquifer protection, karst spring restoration and surface deformation disaster management.