Hydrogeochemical evolution of Taiyuan Formation limestone water under the disturbance of water inrush from karst collapse column in Taoyuan Coal Mine, China Open Access

China is one of the most serious coalmine water disasters in the world. The hydrogeological conditions of coal mines in China are complex, and the mining of coal resources is seriously threatened by water disasters. In recent years, with the extension of mining level and the increase of mining intensity, the mining conditions of coal mines are becoming more complex, the harm of karst water from the bottom of the coal seam is more serious, and the situation of safe mining is more severe (Wu 2014; Guo et al. 2020, 2021a).

To date, research on the chemical field of groundwater in Huaibei coalfield has mainly focused on the hydrochemical characteristics of primary aquifer, isotopic geochemical characteristics, groundwater cycle tracing, element pollution degree and health risk assessment (Gui & Chen 2016; Gui et al. 2016, 2017; Lin et al. 2016; Wang et al. 2019; Xu et al. 2022), while there are few studies on hydrogeochemistry under the disturbance of large-scale water inrush accidents and their treatment projects.

Based on the constant component data of Taihui water before and after the ‘2.3’ water inrush event, the chemical characteristics of Taihui water under the disturbance of water inrush were analyzed. The objectives of this study were: (1) systematically analyze the response characteristics of Taihui water to the ‘2.3’ water inrush event from the perspective of hydrochemistry; (2) clarify the impact of water inrush event on the chemical environment of limestone water; (3) provide certain hydrochemical basis for the identification of water inrush water sources and the prevention of water hazards in coal mines.

Taoyuan Coal Mine in Huaibei coalfield is located in the northern part of Anhui Province. It is 116° 59′ 22″–117° 02′ 33″ E and 33° 28′ 22″–33° 36′ 12″ N (Figure 1). The terrain in the area is flat and there are no large rivers, but there are many artificial ditches, which are small and medium-sized seasonal rivers. The flow of the ditches is controlled by atmospheric precipitation. The annual average rainfall is 766 mm, and the rainfall is mostly concentrated in July and August.

Taoyuan Coal Mine is a large coal mine in Huaibei coalfield, with an annual output of 1.75 million tons of coal. It is located in the west wing of Sunan syncline and the northern section of the east wing of Sunan anticline. The basement is Ordovician and Cambrian strata. The tectonic field and seepage field are special, and the geological and hydrogeological conditions are complex (Figure 1). The mine is cut into two parts by the F₂ fault, south and north, and the stratum strike has changed with the F₂ fault as the boundary. The north of F₂ fault is NNW, and the south is NNE. The mine is a monoclinic structure with a strike of nearly south-north and an inclination of east.

There are four water-filled aquifers in this minefield: Quaternary loose layer, Permian coal measure sandstone, Carboniferous Taiyuan Formation limestone, and Ordovician limestone. The limestone water of the Carboniferous Taiyuan Formation is the main water source for the mining of the lower group coal of Taoyuan Coal Mine. Under the influence of the water-conducting structure, it constitutes an important water hazard threat for mining activities.

There are 12 data in this study (Table 1), including three before ‘2.3’ water inrush (2011–2012) and nine after water inrush treatment and re-mining (2013–2016). The sampling points are taken from the boreholes of Taihui water exploration and drainage (Figure 1). Before sampling, the sampling barrel was washed with deionized water three times, and the sampling barrel was rinsed three times with the water sample when sampling. After collection, the samples were sent to the laboratory of the third exploration team of Anhui Coalfield Geological Bureau within 24 hours. After filtration with 0.45 μm filter membrane, they were stored in a 4 °C refrigerator for further testing.

Table 1

Conventional parameter values of Taihui water sample

No. .	Sampling time/year .	Na+ + K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	pH .	TDS .
		mg/L .	mg/L .	mg/L .	mg/L .	mg/L .	mg/L .	– .	mg/L .
1	2011	292.3	356.9	122.0	356.7	1,118.7	439.8	7.42	2467
2	2012	383.4	217.9	128.9	311.7	1,076.3	473.3	7.45	2355
3	2012	224.0	257.5	142.5	286.0	946.3	453.5	7.04	2083
4	2013	682.8	6.5	2.5	258.6	179.5	1,062.9	8.31	1661
5	2013	637.2	13.8	8.9	272.3	433.8	666.8	8.31	1699
6	2013	379.7	43.1	14.8	214.1	245.3	506.6	8.49	1178
7	2014	282.6	146.8	54.9	220.9	634.7	290.5	7.69	1485
8	2015	290.1	322.2	71.1	245.2	1,045.9	357.8	7.58	2197
9	2015	353.0	51.5	23.0	174.6	488.6	295.4	7.88	1257
10	2016	241.1	170.6	81.8	206.4	657.7	385.5	7.19	1583
11	2016	217.3	195.2	82.8	197.6	696.0	360.1	7.16	1599
12	2016	194.9	223.0	81.8	211.7	690.7	364.7	7.14	1615

No. .	Sampling time/year .	Na+ + K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	pH .	TDS .
		mg/L .	mg/L .	mg/L .	mg/L .	mg/L .	mg/L .	– .	mg/L .
1	2011	292.3	356.9	122.0	356.7	1,118.7	439.8	7.42	2467
2	2012	383.4	217.9	128.9	311.7	1,076.3	473.3	7.45	2355
3	2012	224.0	257.5	142.5	286.0	946.3	453.5	7.04	2083
4	2013	682.8	6.5	2.5	258.6	179.5	1,062.9	8.31	1661
5	2013	637.2	13.8	8.9	272.3	433.8	666.8	8.31	1699
6	2013	379.7	43.1	14.8	214.1	245.3	506.6	8.49	1178
7	2014	282.6	146.8	54.9	220.9	634.7	290.5	7.69	1485
8	2015	290.1	322.2	71.1	245.2	1,045.9	357.8	7.58	2197
9	2015	353.0	51.5	23.0	174.6	488.6	295.4	7.88	1257
10	2016	241.1	170.6	81.8	206.4	657.7	385.5	7.19	1583
11	2016	217.3	195.2	82.8	197.6	696.0	360.1	7.16	1599
12	2016	194.9	223.0	81.8	211.7	690.7	364.7	7.14	1615

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The pH values of samples were measured by portable instrument (model ST20, brand OHAUS, produced in Shanghai, China). The contents of Ca²⁺, Mg²⁺, Cl⁻ and were tested by ion chromatograph (model ICS-600-900, Symantec, California, USA), and Na⁺ + K⁺ and contents were determined indoors by acid-base titration. The total dissolved solids (TDS) were calculated by adding up the contents of various conventional components and subtracting half of the content.

In order to better support the mechanism analysis of water rock interaction, nine samples of limestone from the first layer to the fourth layer were collected from two boreholes in Taoyuan Coal Mine and sent to the laboratory of mineral characterization, School of Resources and Environmental Engineering, Hefei University of Technology. The mineral composition of rocks was tested by X-ray diffractometer (model DX-2700X, produced by China Dandong Haoyuan Instrument Co., Ltd).

The analytical results of groundwater were tested by calculating the ion balance error (IBE) (Lloyd & Heathcote 1985). The ion balance errors of all samples were less than 10%, and most of them were less than 1%. All sample data were analyzed by SPSS 20.0; Pearson correlation analysis was used to evaluate the correlation of water quality parameters; graph processing and analysis were completed by Origin 9.0, Aquachem 1.5, Coreldraw 2018 and Phreeqc software.

The statistical description of conventional parameters in Taihui water is shown in Table 2. Na⁺ + K⁺ was the main cation and was the main anion. Na⁺ + K⁺, Ca²⁺, Mg²⁺, and have large coefficients of variation, which indicated that they were disturbed by strong external interference, such as ‘2.3’ water inrush, grouting treatment of re-mining, mining activities, which were unstable in hydrochemical composition. The average of pH value was 7.6, mostly in the range of 7.1–8.3, belonging to weak alkaline water.

Table 2

Statistical description of water quality parameters before and after water inrush

Parameter .	Mean .	Standard Deviation .	Coefficient of Variation .	Minimum .	Maximum .
Na+ + K+/mgL−1	348.2	158.4	0.45	194.9	682.8
Ca2+/mgL−1	167.1	118.0	0.71	6.5	356.9
Mg2+/mgL−1	67.9	48.2	0.71	2.5	142.5
Cl−/mgL−1	246.3	53.0	0.22	174.6	356.7
/mgL−1	684.5	315.3	0.46	179.5	1118.7
/mgL−1	471.4	212.9	0.45	290.5	1062.9
pH	7.6	0.5	0.07	7.0	8.5
TDS/mgL−1	1764.9	416.4	0.24	1178.0	2466.5

Parameter .	Mean .	Standard Deviation .	Coefficient of Variation .	Minimum .	Maximum .
Na+ + K+/mgL−1	348.2	158.4	0.45	194.9	682.8
Ca2+/mgL−1	167.1	118.0	0.71	6.5	356.9
Mg2+/mgL−1	67.9	48.2	0.71	2.5	142.5
Cl−/mgL−1	246.3	53.0	0.22	174.6	356.7
/mgL−1	684.5	315.3	0.46	179.5	1118.7
/mgL−1	471.4	212.9	0.45	290.5	1062.9
pH	7.6	0.5	0.07	7.0	8.5
TDS/mgL−1	1764.9	416.4	0.24	1178.0	2466.5

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TDS value of Taihui water was 1178.0–2466.5 mg/L (average 1764.9 mg/L), which seems high. Because of the structural development, the limestone of Taiyuan Formation is connected with the coal bearing strata, and the coal mining leads to the transformation of the aquifer system from the closed environment to the open environment, and the oxidation of pyrite in the coal bearing strata increases TDS. In addition, due to the opening of the system, CO₂ in the limestone aquifer of Taiyuan Formation will escape, thus breaking the original water chemical equilibrium system. Only when the water rock interaction of Taiyuan Formation limestone aquifer is further strengthened can a new water chemical equilibrium be formed, and TDS will naturally rise.

The relationship between cation mass concentration before and after water inrush has not changed, which is Na⁺ + K⁺ > Ca²⁺ > Mg²⁺. The relationship between anions has changed, > > Cl⁻ before water inrush and > > Cl⁻ after water inrush. With the extension of time, the relationship between anions gradually returns to the state before water inrush. The pH value increased after water inrush, mostly between 8.3 and 8.5, which was presumed to be due to the influence of the precipitation water from the slurry with a larger pH value in the grouting treatment after water inrush (the pH value of slurry water is more than 12).

TDS was larger before water inrush and decreased after water inrush, and then gradually stabilized at 1600 mg/L with time. The results showed that the contents of Ca²⁺, Mg²⁺, were higher before the ‘2.3’ water inrush; after the event, Ca²⁺ and Mg²⁺ were depleted, while Na⁺ and contents were higher; then, with the increase of time, the parameter values were mostly between those before and after the event, and showed a trend of recovery to the state before the event.

After water inrush, ions fluctuated greatly, especially Na⁺, and ⁠, and then tended to be stable with the increase of time, which indicated that water inrush series events seriously disturbed Taihui water and the hydrochemical changes were complex. However, with the passage of time, the hydrochemistry of limestone in Taiyuan Formation gradually reflected its own characteristics.

Before the ‘2.3’ water inrush it was mixed water, mainly including SO₄•Cl-Ca and SO₄•Cl-Na; after the event it mainly included HCO₃ -Na and SO₄•Cl- Na; then with the increase of time the quality of Taihui water changed to SO₄•Ca•Na, SO₄• Na• Ca, SO₄•Cl-Ca and SO₄•Cl-Na, showing a trend to change to that before the event. There were three reasons for the change of Taihui water quality after the event. Firstly, the source of ‘2.3’ water inrush was Ordovician limestone aquifer, that is, during the water inrush process, Ordovician limestone water entered the mine through Taihui water, which would inevitably affect the quality, which was one of the reasons for the water quality change of Taihui water. Secondly, the ‘2.3’ water inrush caused the mixing of loose layer water, coal measures water, Taihui water and Ordovician limestone water, which greatly promoted the flow of water between aquifers, and the overlying pores or fissures water with low TDS containing CO₂ and O₂ continuously infiltrated, which enhanced the dissolving ability of the original Taihui water, so that the rock and soil components continued to transfer to the water. The higher the CO₂ content in the water, the stronger the ability to dissolve carbonate and silicate (Zhang et al. 2011), which was the second reason for the change of Taihui water quality. During the grouting process of plugging the inrush site, the strong alkaline precipitation water from cement slurry with higher Na⁺, K⁺, Ca²⁺ content was mixed into the Taihui, which was the third reason for the change of Taihui water quality. It can also be seen from Figure 3 that the variation characteristics of pH value and TDS value were consistent with the above.

The analysis of the above abundance characteristics of conventional parameters and hydrochemical types has a certain reference role, especially for the identification of aquifer water source after the treatment of mining resumption.

According to Figure 4, the hydrochemical process of Taihui water in Taoyuan Coal Mine was dominated by water rock interaction. Since Na/(Na + Ca) has a good indicator effect on ion exchange (Guo et al. 2021a), some water sample points fell outside the solid line after the ‘2.3’ event, indicating that there were other functions that control the chemical composition of groundwater, such as cation exchange, etc., which may be related to the separated water from grouting slurry. No water sample points fell in the precipitation area, indicating that there was no direct hydraulic connection between the Taihui water and the atmospheric precipitation in the study area.

The chlor-alkali index is often used to indicate the intensity of cation exchange in groundwater (Zhang et al. 2021). If Mg²⁺ or Ca²⁺ in groundwater exchanges with Na⁺ in aquifer minerals, the chlor-alkali index is negative and the groundwater is dominated by Na⁺. When the Na⁺ exchanges with Mg²⁺ or Ca²⁺ in the aquifer minerals, the chlor-alkali index is positive, and the groundwater is dominated by Mg²⁺ or Ca²⁺, and the greater the absolute value, the greater the ion exchange intensity (Zhu et al. 2017). In Figure 5, the chlor-alkali index CAI-1 and CAI-2 values of all samples were negative, indicating that the cation exchange process in the study area was that Ca²⁺ in the Taihui water exchanged Na⁺ in the aquifer minerals, and finally the Taihui water Ca²⁺ was depleted and Na⁺ enriched (Yang et al. 2019; Zhang et al. 2021). After the event (2013–2014), the chlor-alkali index of some sample points was large. It is known that during the implementation of grouting treatment accidents, the cement slurry contains more Na⁺, K⁺ and Ca²⁺. Therefore, grouting treatment promoted cation exchange.

In order to verify the influence of cation exchange on groundwater composition, the relationship between (Na⁺ + K⁺)-Cl⁻ and (Ca²⁺ + Mg²⁺)-(⁠ + ⁠) is shown in Figure 5. (Na⁺ + K⁺)-Cl⁻ characterizes the increase or decrease of Na⁺ + K⁺ in addition to the dissolution of rock salt and potassium salt; (Ca²⁺ + Mg²⁺)-(⁠ + ⁠) characterizes in addition to the dissolution of gypsum, dolomite and calcite, Ca²⁺ + Mg²⁺ increase or decrease. If cation exchange reaction is the main cause of groundwater mineralization, the relationship between these two parameters should be linear and the slope should be −1 (Qiu 2019). Figure 5 shows that the Taihui water sample points in the study area were basically distributed along the slope-1 line, indicating that Ca²⁺, Na⁺ and Mg²⁺ were basically involved in the ion exchange reaction (Farid et al. 2013).

In conclusion, the hydrogeochemical processes of Taihui water included cation exchange, carbonate dissolution, pyrite oxidation, sulfate dissolution and silicate dissolution. Under the disturbance of water inrush and its treatment, the cation exchange effect fluctuated greatly and the cation exchange effect was strong.

Based on Gibbs diagram and analysis of ion combination ratio, it can be seen that the water-rock interaction in the Taiyuan limestone water is complex and there are many types of action, which also reflects the importance of water rock interaction in the formation of chemical components of the water. Therefore, if the water source identification model based on the chemical composition of Taihui water is established, it can be used for reference.

Correlation analysis of water quality parameters in water is helpful to understand the relationship between parameters and determine the source of each parameter (Li et al. 2018). At the same time, correlation analysis is often used to reveal the consistency and difference between ion sources (Meybeck 1987; Kaur et al. 2016). The main hydrochemical parameters of the Taihui water in the study area were analyzed, and the results are shown in Table 3.

Table 3

Correlation matrix of water quality parameters

.	Na+ + K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	TDS .
Na+ + K+	1
Ca2+	−0.695*	1
Mg2+	−0.665*	0.853**	1
Cl−	0.208	0.452	0.500	1
	−0.576	0.929**	0.888**	0.547	1
	0.855**	−0.506	−0.438	0.299	−0.512	1
TDS	−0.103	0.757**	0.725**	0.858**	0.843**	0.020	1

.	Na+ + K+ .	Ca2+ .	Mg2+ .	Cl− .	.	.	TDS .
Na+ + K+	1
Ca2+	−0.695*	1
Mg2+	−0.665*	0.853**	1
Cl−	0.208	0.452	0.500	1
	−0.576	0.929**	0.888**	0.547	1
	0.855**	−0.506	−0.438	0.299	−0.512	1
TDS	−0.103	0.757**	0.725**	0.858**	0.843**	0.020	1

*The correlation was significant at the 0.05 layer.

**The correlation was significant at the 0.01 layer.

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Halite (NaCl) is the potential source of dissolved Cl⁻, but Cl⁻ and Na exhibited a low coefficient value, 0.21, so the evaporite origin of Cl⁻ was questioned. Except for Cl⁻, all the observed values are typical for a sedimentary geological setting with the widespread presence of carbonates and evaporite mineral phases – including gypsum and anhydrite, not halite. Then, it concludes that halite has a minor role in releasing Cl⁻ in water supplies (Mahaqi 2021). Before and after the event, the TDS of the Taihui water had a good correlation with ⁠, Cl⁻, Ca²⁺ and Mg²⁺. There was a significant positive correlation between and Ca²⁺ and Mg²⁺ in the 0.01 layer, which indicated that the two sources were basically the same. For example, the acid produced by gypsum dissolution or pyrite oxidation further dissolves calcite, and the contents of and Ca²⁺ or Mg²⁺ increased simultaneously. Ca²⁺ and Mg²⁺ ions showed a significant positive correlation on the 0.01 layer, indicating that they may also came from the dissolution of dolomite. Na⁺ was negatively correlated with Ca²⁺ and Mg²⁺, indicating that they may had inhibitory effects, such as jointly participating in cation exchange, which is consistent with the above analysis results of conventional ion ratio.

The above correlation analysis shows the importance of host lithology in altering the groundwater chemical composition. This statement in line with the previous idea, based on Gibbs plot, that water-rock interaction plays the principal role in the groundwater system (Mahaqi 2021).

In the process of water rock interaction, it is necessary to analyze the equilibrium state between the limestone water and the water bearing media, that is, the discrimination of dissolution and precipitation. The ion activity product (K_IAP) can be calculated by known pH value, relevant conventional ion content and ionic strength, and the comparison with its solubility product (K_SP) can determine whether the groundwater and minerals are in equilibrium, dissolution or precipitation (Guo 2022). Then:

K_IAP/K_SP > 1, the solution is in supersaturation state;

K_IAP/K_SP < 1, the solution is in unsaturated state;

K_IAP/K_SP ＝ 1, and the solution is in equilibrium.

In fact, there is a long supersaturation stage in water rock interaction. Therefore, the saturation index SI is often used to judge whether the interaction between mineral rocks and water is the dissolution or precipitation state. Saturation index SI = lg(K_IAP/K_SP) (Guo 2022), then:

SI > 0, insoluble substances precipitate in groundwater;

SI < 0, insoluble substances dissolve in groundwater;

SI = 0, the dissolution and settlement of insoluble substances in groundwater are balanced.

Due to the errors in mineral equilibrium constants, water quality analysis, and ion activity calculations, the calculation results of mineral saturation index are inevitably uncertain. Therefore, it is generally believed that when SI changes within a certain range close to 0, groundwater and minerals are in equilibrium, and this variation range is usually 0 ± 0.5 (Wei et al. 2020).

The saturation index of gypsum is mostly less than 0.5, and some are close to or greater than 0.5. It is usually in dissolution or equilibrium state, so gypsum will not precipitate from Taihui water. When there is gypsum in the aquifer, it will dissolve and tend to equilibrium state. The saturation index of calcite is mostly greater than 0.5 or close to 0.5, which is usually in the state of precipitation or equilibrium, then calcite may precipitate from Taihui water. When calcite exists in the aquifer, it is less dissolved. The saturation index of dolomite is greater than 0.5. When dolomite exists in the aquifer, it will not dissolve. It can be seen that after water inrush, the crystallization of calcite and dolomite in Taihui water increased, while the dissolution of gypsum increased.

Since the treatment of water inrush, the pH value of Taihui water first increases and then decreases, and the degree of mineral dissolution first decreases and then increases. Therefore, the mineral saturation index increases first and then decreases. The saturation index of gypsum decreases first and then increases, mainly because the ‘2.3’ water inrush led to the mixing of coal measure water, Taihui water, Ordovician limestone water and even even the fourth loose layer water. A large number of prominent Ordovician limestone water diluted Taihui water (Taihui contains a thin coal seam, and its gypsum content is greater than Ordovician limestone water), which reduced its gypsum saturation index and gradually increased with time.

The water disaster accident and its treatment have a certain disturbance to the primary aquifer. Through the processing and analysis of the conventional parameters of Taihui water samples, it is understood that the hydrogeochemical evolution characteristics of Taihui water under the disturbance of water inrush are as follows:

• (1)

  Through the research on the abundance and change trend of constant components and the hydrochemical type of Taihui water, it is found that the ‘2.3’ water inrush event has significantly disturbed the chemical field of groundwater.

• (2)

  The combination analysis of ion combination ratio and its correlation has a good effect on revealing the hydrogeochemical process, and also verifies the correctness of Gibbs map results. The water-rock interaction in the Taiyuan limestone water is complex and there are many types of action, which also reflects the importance of water rock interaction in the formation of chemical components of the water.

• (3)

  The response characteristics of the saturation index of different minerals to the ‘2.3’ water inrush disturbance are different, and it is found that the saturation index is positively correlated with pH value and TDS, and has strong correlation with pH value.

This study clarified the characteristics of the water chemistry evolution of Taihui water under the coal mine water inrush and water prevention project, and realized that the groundwater responds obviously to the mining disturbance, which has certain guiding significance for the further water damage control of the coal mine and the identification of the water inrush water source.

We sincerely thank the National Engineering Research Center of Coal Mine Water Hazard Controlling (Suzhou University) and the third exploration team of Anhui Coalfield Geological Bureau for providing the experimental site. Also for the basic geological and hydrogeological information provided by Huaibei Mining Group Co. Ltd and Hydrological exploration team of Anhui Coal Geological Bureau.

This research was funded by the Open Fund Project of Shaanxi Key Laboratory of Prevention and Control Technology for Coal Mine Water Hazard (2021SKQN01), Humanities and Social Science research project of Education Department of Anhui Province(sk2018A0481), key scientific research project (2020yzd09, 2020yzd03, and 2020yzd07), the National Natural Science Foundation of China (41773100), funding projects for research activities of academic and technological leaders of Anhui Province’ (2020D239), Natural Science Foundation of Anhui Province (2008085QD175).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.