Introduction

Beixinyao Coal Mine is a new coalmine of Tongmei Group, located at the northern end of Ningwu Coalfield and southeast of the Shuonan Mining Area [1]. It has widths of 11.09 km from east to west and 12.59 km from north to south, for an area of 53.29 km², and has a production capacity of 400 Mt/a [2,3]. Due to tectonic developments, its geological and hydrogeological conditions are complicated, and water damage from the top and bottom aquifers poses a threat to coal seam mining (Figure 1). Significant hydrogeological exploration has been undertaken, along with investigation of the hydrogeological conditions in which the mine is located and between the major aquifers [4]. The aquifer’s hydraulic connection and lithology, thickness, and water resistance still need to be investigated and assessed. Objective evaluation shows that the mine is susceptible to water inrush backplane from Ordovician limestone water [5]. Sedimentary strata in the mining field, from old to new, include the Ordovician Middle and Upper Majiagou Formations (O2s + x), the Carboniferous Benxi Formation (C2b), the Upper Taiyuan Formation (C3t), the Permian Shanxi Formation (P1s), a lower stone box group (P1x), an upper stone box group (P2s), a middle and upper update system of the fourth system (Q2 + 3), and a new system (Q4). The aquifer in the well field, from new to old, is divided into quaternary loose layer pore aquifer, upper and lower Shihezi Formation and Shiqianfeng Formation bottom sandstone fracture aquifer, Shanxi formation sandstone fracture aquifer, Taiyuan sandstone fracture aquifer, and limestone karst-fractured aquifer in the Upper Ordovician Middle Majiagou Formation [6].

During excavation of the return wing lane of the south wing system from north to south, a sudden gush of water appeared on the palm surface and gushing water volume gradually increased to 316 m³/h, then stayed steady at 300 m³/h. During excavation of the south wing auxiliary transportation lane from north to south, water inrush also occurred on the floor and gushing water volume reached 300 m³/h, after which the water volume stayed steady at 215 m3/h. To effectively control water damage in a timely manner, the source of water inrush needs to be identified [7-9].

This article comprehensively analyzes the reasons for water inrush from the area of the water outlet and the structure of coalmine fields, as well as changes in the roadway coal, rock formations, hydrographic and water quality data [10-12]. C¹⁴ isotope analysis of water samples, trace element measurements, combined with underground verification. The kilometer grouting drilling data of the construction indicates that the source of water inrush is tectonic water and provides accurate data for the next steps of scientific and engineering treatments.

Materials and Methods

Beixinyao Minefield Regional Structure

Beixinyao coalfield is located in the north of the Ningwu coalfield (Figure 1). Based on analysis of regional remote sensing data, the regional structural location is northwest of the Ningwu-Jingle syncline, which shows a north-northeast long axis distribution; the strata at the edges of the east and west wings have large inclination angles of up to 40-80° and some are even upsidedown. The internal structure of the basin is unevenly developed, with the north more complex in the Ningwu-Yangfangkou area [8,9]. A series of large-scale near west or northeast east eastward tensile faults were developed, including the Wangwanzhuang fault, Dingjialiang fault, and Ningwu fault. Accompanied by secondary folds, such as the Wang Wan Zhuang fault spanning the east and west of the minefield, a series of large-scale north south extensional faults, such as the Motioning fault zone, are divided into two parts: The tectonic structure is the western boundary of the Shuonan Coalfield, and the Mayi fault is the eastern boundary of the Shuonan Coalfield, along with a series of northwestward nappe structures, the Anziping-Shihuling fault.

Wellfield Structure

Affected by regional structure, faults and folds developed in the coal well field. The stratum as a whole runs northeast, leaning southeast or northwest, with an inclination angle of 4°-25°. There are 26 normal faults in the field, including nine faults exceeding 100 meters (Figure 2).

In the Beixinyao minefield, the topography is generally high on the east and west sides and low in the middle. Ordovician limestone is exposed on both the east and the west sides, and the area is large, directly supplying the Ordovician limestone formation. Groundwater runoff is generally from south to north and from west to east.

From the perspective of the regional structure and the geological structure in the well field, a large nappe structure was formed in a northwestward direction. Early compression caused the formation to be inverted on the right side, with the limestone rising, and later stretching, forming a limestone collapse zone. This shows that the three lanes are located on the east flank of the north south structure, cutting the northwest trending nappe and tension structure, while meeting the northeast direction structure.

Identification of Water Source of Water Inrush Points

Structural Analysis of Water Inrush Points

The water outlet is located in the east of the north-south structure (i.e., the Huihe channel) in the north of the minefield (Figure 3). There are two east-west faults: F7, the Wangwanzhuang fault in the regional structure and the other in the middle of the F8 cutting block, the northwestward thrusting structure in the southern limestone fracture zone.

Left Wing of South Wing Auxiliary Transport Lane

Figure 4 shows a cross-sectional view of the left gangway drainage ditch of the south wing auxiliary transportation lane. It can be seen that the south wing auxiliary transportation lane is severely squeezed on both sides and is almost upright, which indicates that the stratum near the water outlet is squeezed by external forces, even as the stratum in the south stretches to form a limestone breccia fracture zone.

Water Sample for Water Chemical Laboratory Analysis

The following is a statistical table of water quality types of water outlets and nearby boreholes (Table 1):

By processing the data, a piper three-line diagram is generated Figure 5:

The piper chart shows that the water quality at the water outlet point differs from the water quality of the Taiyuan Formation and the Ordovician limestone water. At present, the water volume of the two outlet lanes is essentially stable and has not changed for nearly a month, indicating that the Taiyuan Formation and the Ordovician limestone water aquifer have lateral recharge for the water outlets of the excavated alleys. Based on the geological structure of the area and the flow direction of the runoff zone (southwest to northeast), the F8 fault is believed to conduct water from the Huihe River from north to south. The structure has a certain hydraulic connection.

Analysis of Water Quality Types of Hydrological Pores

It can be seen from the preceding figure that after the water gushing from each of the hydrological observation holes in the south wing alley, drilling water levels have different depths Table 2, with the Taiyuan Group G6 observation hole in the same hydrological unit having the greatest indicating that the water layer in the water point recharges the coal seam. The bottom sandstone aquifer is relatively large [13-15]. The water quality types of the South Wing Backwind Lane and the South Wing Auxiliary Transport Lane are both HCO₃—Ca · Mg, and the water volume at both water outlets is currently steady at 300 m³/h; both are characterized by mixed water.

Water Temperature Discrimination

Table 3 shows changes in water temperature with depth of some hydrological boreholes in the area of Beixinyao Minefield. As depth increases, water temperature in the boreholes gradually increases. The temperature of the water outlet point was 11°. We know that the water temperature of the Carboniferous in Ningwu Shuonan coalfield generally exceeds 20°. Water temperatures at the water outlet point and in the Ordovician limestone water are nearly 10°: The water at the water outlet is not pure Ordovician limestone water, and the aquifer in the isothermal zone has a large recharge.

Determination of Radioisotopes in Water Samples

In isotope hydrogeology, groundwater formed before 1953 is usually referred to as “paleowater” and groundwater formed after 1953 is referred to as “new water”. After 1963, due to reductions in nuclear testing, tritium concentrations also decreased. Accordingly, groundwater formed after 1963 is called “recent water” or “modern water”. Qualitative estimations of groundwater age are based on tritium content (mainland region) [16-18]. Shown as (Table 4).

The 17 samples taken from each aquifer in Beixinyao Minefield were analyzed in the laboratory of the School of Environmental Science in the China University of Geosciences in Beijing, producing the results shown in Table 5.

From Table 5 and the histogram of tritium content in different samples Figure 5, it can be seen that the tritium content of the Huihe River water sample is 3.9 ± 0.21Tu, indicating that the mix of groundwater and surface water in this area is sub-modern water. The tritium content of Shentou Spring is higher than that of the Huihe River, indicating that Shentouquan has atmospheric precipitation recharge. The tritium content of the other 9 aquifer samples showed that the Shanxi mining group No. 1 in the first mining face had a large value of 7.3, indicating the presence of fresh water or tap water. This water is modern water, consistent with the afore-mentioned deuterium and oxygen analysis, and is allowed to have tap water mixed with it during processing. The tritium content of sample No. 3 in the Shihezi Formation of Huifeng Lane, South Wing is more than 5 Tu, corresponding to modern water; the remaining 7 samples have a tritium content of 1.5-4.3 Tu, corresponding to sub-modern water.

The ¹⁴C isotope test of the foregoing 17 samples is also performed in the isotope liquid scintillator laboratory of the School of Environmental Science in the University of Geosciences in Beijing, using an ultra-low liquid scintillation spectrometer (Table 6). If the ¹⁴C age is less than 1 ka, it can be treated as modern water.

According to Table 6 and the histogram of the ¹⁴C age value of each sampling point, the groundwater age of Shentou Spring is 0.34 ka B.P., which corresponds to modern water, indicating that the water sample taken by Shentou Spring is surface water. The Huihe water sample represents the mixed water of groundwater and surface water in well fields and thus is sub-modern.

During the long-term pumping process, the ¹⁴C age of the groundwater in the aquifer of the South Wing Belt Lane (Shanxi Formation) keeps increasing, and its age is very different from the ¹⁴C age of the Taiyuan Formation aquifer groundwater collected during the same period, indicating that the groundwater is being replenished by old groundwater.

This indicates that there is a mixture of groundwater in the aquifers of the Taiyuan and Shanxi formations. The ¹⁴C groundwater in the BKS27 Taiyuan Formation is older, indicating the presence of old groundwater of Ordovician aquifers. Differences in groundwater age between the BK16 (Shanxi and Taiyuan Formation) and BK16 (Ordovician) water samples within the margin of error are extremely small, further indicating a hydraulic connection between the Taiyuan Formation and the Ordovician aquifer.

In summary, there is a hydraulic connection between the Shihezi aquifer and the Cenozoic, and the Shanxi, Taiyuan, and Ordovician aquifers are mixed with one another. Moreover, water-conducting fault tectonic networks not only provide runoff channels and storage sites for groundwater in Ordovician limestone water and the Taiyuan and Shanxi formations, but are also the main controlling factors and prerequisites for runoff in various aquifers.

Trace Element Measurements of Water Samples

Geochemical studies of groundwater trace elements and isotopes in the well field have found that the Ordovician and Taiyuan Formation aquifers in the well field have high quality groundwater. It is initially determined that they have reached the standard for drinking natural mineral water, featuring high strontium (Sr) mineral water of great potential value.

Analysis of the laboratory testing results of 16 samples collected in 2018 and 2019 Table 7 revealed lower Li, Zn, Se, metasilicic acid, and free CO2 content than those specified in ‘National Food Safety Standard Drinking Natural Mineral Water’ (GB8537-2018). The local sampling lithium (Li) meets the standard’s requirements, and the strontium (Sr) content of all samples meets the requirements of the specification (≥ 0.2mg/L), so water quality in this area has reached the limit for strontium mineral water.

Verification and Grouting of Thousands of Holes downhole

To allow smooth development of the three main roads in the south wing of Beixinyao Coal Mine and ensure safe production for the mine Figure 6, construction of directional water exploration boreholes in the three main roads in the south wing of Beixinyao Mine is needed to facilitate detection of faults for water gushing conditions, providing a strong basis for the next step in water treatment.

Discussion

In this study, we investigated a case study in the Beixinyao Coal Mine to compare with several drilling data for the validation. In South-wing return Air Lane, there were two drilling holes: HF-1# and HF-4#. For HF-1#, the footage was 459 m and the Ordovician limestone was 102 m, while the faults F8 and F9 were found at 270 m and 416 m, respectively. There were seriously collapsed holes with abnormal return of water and slag occurred at 60-80 m. It is inferred that the fissures developed there. The water gushing in the hole of 80-240 m section increased gradually to 40m³/h from 30 m³/h, and the surge began to decrease after 270 m. The final drilling hole was stabilized at l0 m³/h. In comparison, for HF-4#, the footage was 210 m and the Ordovician limestone was 108 m. The water inflow (about 20 m³/h) appeared at 42 m. It is inferred that the fissures were developed nearby. There is no obvious change in the water inflow in the final drilling hole (stable at about 20 m³/h); and the final hole was stable at 45 m³/h.

In South- wing Belt Lane, there was one drilling hole: PD1#, in which the footage is 513 m, the Ordovician limestone is 67 m, while the faults F8 and F9 were found at 333 m and 448 m, respectively. The water inflow (about 30 m³/h) appeared at 54m. It is inferred that the fissures were developed, but there is no obvious change in the water inflow in the final drilling hole (stable at about 30 m³/h); and the final drilling hole was stable at 60 m³/h [19-21].

In South-wing Auxiliary Lane, there was one drilling hole: FY-1#. The footage was 522 m and the Ordovician limestone is 48 m, while the faults F8 and F9 were found at 341 m and 431 m, respectively. The water inflow was appeared at 54 m. It is inferred that the fissures were developed. Till 120 m deep, it was gradually increased to 100 m³/h; when stopped drilling, the water inflow in the drilling hole was stable at 40 m³/h; while the drilling hole was deep to 168 m, the water inflow was about 90 m³/h, then dropped to be stable at 65 m³/h, and slightly increased to be stable at 75 m³/h; and finally the water [22,23]. In flow in the drilling at 90 m³/h after the end hole was retreated.

Based on the construction situation for kilometer drilling, some new findings can be made:

1. The drilling water output of the three main lanes, from east to southwest wing back wind lane, south wing belt lane, and south wing auxiliary transport lane, increased, indicating that the source of recharge water came from the western Huihe River water diversion structural belt.

2. When the borehole passes through the F8 fault, rock is severely broken and water volume increases. After drilling is completed, water volume stayed steady, indicating that the F8 fault conducts water of certain water richness: When it passes through the F9 fault, borehole water volume is not obvious. The increase indicates poor conductivity of the F9 fault.

3. When grouting at the water outlet of the south wind wing of the east wing, the water outlet is completely stopped after grouting the Ordovician limestone, coal seam roof, and bottom sandstone aquifer. Accordingly, the presence of water outlet for the south wing auxiliary transportation road in the west will significantly reduce gushing, showing that the water outlets of the three roadways conduct water from northeast and northwest to the structure.

Conclusions

This study investigated and analyzed the water-inrush source of mining underground water in Beixinyao Coal Mine in Datong, Shanxi Province, China. Fieldwork featuring sample collection informs data analysis based on laboratory testing when seeking to investigate underground water inrush sources in coalmines and to accurately determine causes of water outflow in mining wells that feature water gushing, allowing to timely formulation of measures for effective controlling water damage and ensuring safe production, as at Beixinyao Coal Mine.

Certain conclusions may be drawn about similar conditions in other coalmines:

1. Based on multi-parameter measurement and analysis of water outlets and observation holes of Beixin Kiln, water discharge, quality, and temperature of outlets of the South Wing Backward Lane, South Wing Belt Lane, and South Wing Auxiliary Transport Lane are similar. Analysis of water, isotopes and trace elements shows strong correlation with structure.

2. The source of the water outlet is the replenishment of Huihe structural fault water from north to south. The Huihe structural fault water cuts the top, floor sandstone and Ordovician coal seam of Taiyuan Formation through the northeastward F8 fault, the northwestward nappe structure and the secondary structure and the Ordovician Limestone, recharging all fractured water aquifers and laterally recharging the middle and east of the minefield. Thus, prevention and control of coalmine water is focused on the northeast and northwest tectonic treatment of the flanks of the north-south structural water channel.

3. The north-south structural water in the west of the well field is strontium-rich mineral water. In preventing and controlling water in mines, the principles of coal and water coproduction should be combined to develop strontium-rich mineral water in coalmines, leading to potential local socioeconomic developments.

Author Contributions

Conceptualization, Y.G. and J.G. methodology, Y.G.; software, J.G.; validation, Y.G., J.C. and Y.Z.; formal analysis, Y.C.; investigation, Y.G.; resources, J.Y.; data curation, J.G.; writingoriginal draft preparation, Y.G.; writing-review and editing, Y.Z.; visualization, J.G.; supervision, Y.Z.; project administration, J.Y.; funding acquisition, Y.G.

Funding

This research was funded by the National Natural Science Foundation (U1901215), the Natural Scientific Foundation of Jiangsu Province (BK20181413), and the Marine Special Program of Jiangsu Province in China (JSZRHYKJ202007).

Acknowledgments

Water samples were analyzed by the laboratory of the School of Environmental Science in the China University of Geosciences in Beijing, China. We also thank the support from the National Natural Science Foundation (U1901215), the Natural Scientific Foundation of Jiangsu Province (BK20181413), and the Marine Special Program of Jiangsu Province in China (JSZRHYKJ202007).

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