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Article

The Hydrochemical Characteristics and Formation Mechanism of Highly Mineralized Coal Mine Water in Semi-Arid Regions in Northwest China

by
Jian Yang
1,
Wei Zhao
2,*,
Xiangyang Liang
3 and
Feng Xu
3
1
College of Geology and Environment, Xi′an University of Science and Technology, Xi′an 710054, China
2
School of Water and Environment, Chang’an University, Xi’an 710054, China
3
Xi′an Research Institute of China Coal Technology & Engineering, (Group), Corp, Xi′an 710054, China
*
Author to whom correspondence should be addressed.
Water 2024, 16(16), 2244; https://doi.org/10.3390/w16162244
Submission received: 4 July 2024 / Revised: 29 July 2024 / Accepted: 6 August 2024 / Published: 8 August 2024
(This article belongs to the Special Issue Theory and Technology of Mine Water Disaster Prevention and Resource Utilization)
Browse Figures
Versions Notes

Abstract

:
The over-exploitation of groundwater and the deterioration of its quality have heightened the importance of non-traditional water resources, such as mine water. The study of the water’s chemical characteristics and the formation mechanism of high-salinity mine water in semi-arid regions holds significant importance for zero discharge and the resource utilization of mine water in Northwest China. In this study, a total of 38 groundwater and mine water samples were collected to examine the hydrogeochemical characteristics of high-salinity mine water using Piper diagrams and Gibbs diagrams, as well as isotope analyses and ion ratio coefficients. Additionally, the corresponding mine water treatment recommendations were put forward. The results show that the TDS content of groundwater increases with hydrographic depth. The average TDS concentration of Quaternary, Luohe, and Anding groundwater is 336.87, 308.67, and 556.29 mg/L, respectively. However, the TDS concentration of Zhiluo groundwater and mine water is 2768.57 and 3826.40 mg/L, respectively, which belong to high-salinity water. The Quaternary, Luohe, and Anding groundwater hydrochemical type is predominantly HCO3-Ca type, and the Zhiluo groundwater and mine water hydrochemical type is predominantly the SO4-Na type. Furthermore, there is minimal difference observed in δD and δ18O values among these waters. It can be inferred that the Zhiluo Formation in groundwater serves as the primary source of mine water supply, primarily influenced by the processes of concentration caused by evaporation. The high salinity of mine water is closely related to the high salinity of Zhiluo groundwater. The high salinity of groundwater has evolved gradually under the control of the concentration caused by evaporation and rock-weathering processes. The dissolution of salt rock, gypsum, along with other minerals, serves as the material basis for high-salinity groundwater formation. In addition, the evolution of major ions is also affected by cation exchange. The TDS concentration of mine water ranges from 3435.4 mg/L to 4414.3 mg/L, and the combined treatment process of nanofiltration and reverse osmosis can be selected to remove the salt. After treatment, mine water can be used for productive, domestic, and ecological demands.

1. Introduction

The role of groundwater is indispensable in fulfilling essential human needs, serving as a critical water source for over 1.5 billion individuals worldwide [1,2]. However, the over-exploitation of groundwater and the deterioration of its quality have heightened the importance of non-traditional water sources, such as mine water [3]. As the world’s largest coal producer, China is projected to produce 4.7 billion tons of raw coal in 2023, with demand expected to remain stable at approximately 4 billion tons per annum throughout the 14th Five-Year Plan period [4,5,6].
The shift in coal resource development to western China, driven by diminishing eastern reserves and increasing environmental constraints in central regions, has led to extensive groundwater seepage and the subsequent formation of mine water [7,8]. Each ton of raw coal extracted in China results in approximately 2.1 tons of mine water, with production projected to reach 800 million tons in 2023 [9]. This mine water often contains elevated levels of sulfate, fluoride, iron, manganese, and other contaminants, posing significant environmental and resource management challenges [10,11].
The ZYL Coal Mine in Yulin, Northwest China, serves as a case study to explore the fundamental formation mechanisms of high-salinity mine water. The primary source of mine water is derived from both surface water and groundwater, with high salinity primarily influenced by increased ion solubility [12,13,14]. Extensive research has been conducted on the distribution characteristics and change patterns of highly mineralized groundwater, with notable studies focusing on areas such as the Marcellus Basin, Ejina Basin, and Shiyang River Basin [15,16,17]. Researchers have also examined the distribution pattern of brackish water in northern China and the TDS concentration characteristics of high-salinity mine water in coal mining areas like Ningdong Coalfield and XinJulong Coalfield [18,19,20,21].
Analytical methodologies such as Piper’s trilinear diagram, the ion ratio relationship diagram, and Gibb’s semi-logarithmic diagram have been employed to study the hydrochemical characteristics and evolution processes of groundwater [22,23,24,25]. Additionally, advancements in the treatment of high-salinity mine water, including distillation, electrodialysis, and reverse osmosis, have been explored [26,27]. However, there has been limited focus on the chemical characteristics and mechanisms of high-salinity mine water.
This study aims to address this gap by investigating the ZYL Coal Mine in Yulin, Northwest China. The objectives are to (1) analyze the hydrogeochemical characteristics of high-salinity mine water, including the identification of major ions and trace elements, (2) elucidate the fundamental formation mechanisms of high-salinity mine water, encompassing geological, hydrological, and geochemical processes, and (3) propose effective treatment methods based on its hydrogeochemical characteristics and formation mechanisms, with a focus on enhancing water quality and minimizing environmental impact.
By providing a detailed investigation of the ZYL Coal Mine, this study contributes to a broader understanding of high-salinity mine water and offers insights applicable to similar contexts worldwide. The findings are expected to inform the development of more effective and sustainable management practices for high-salinity mine water in mining operations.

2. Method and Material

2.1. Study Area

The ZYL Coal Mine is situated in close proximity to the WS-Z Coal Mine and B-S1 Coal Mine to the south, as well as the K-G Coal Mine to the east (Figure 1). It is one of the major hundred-million-ton coal mines in the Northern Shaanxi Coal Mines, situated approximately 30 km northwest of Yulin City. The ZYL mining area covers a length of 14.7 km in the north–south direction and spans 12.5 to 23.3 km from east to west, encompassing an area of 265.63 km2. The area falls within the administrative jurisdiction of Xiaojihan Township and Bulanghe Township in Yuyang District, Yulin City. The mine possesses a recoverable reserve of 3209.42 Mt and boasts a production capacity of 15.0 Mt/a. Recoverable coal seams include No. 2, No. 3, No. 4-2, No. 5, No. 6, No. 8, and No. 9, with particular emphasis on the main recoverable seams of No. 2 and No. 3 coal. The study area is situated on the southern periphery of the Mu Us Desert, characterized by extensive desert flats and partially stabilized dunes, with desert coverage exceeding 70%. The terrain exhibits higher elevation in the northeast and lower elevation in the southwest, ranging from 1200 to 1327.30 m. The annual precipitation ranges from 279 to 541 mm, with an average of 410 mm and average annual evaporation ranging from 1720 to 2085 mm. The location falls within the continental climate zone of the arid temperate plateau.
The main strata in this region are arranged chronologically as follows: Quaternary, Neogene, Cretaceous, and Jurassic. The main aquifers are the pore aquifer of the Quaternary Salusu Formation, the pore (fissure) aquifer of the Cretaceous Luohe Formation, the pore (fissure) aquifer of the Anding Formation, and the fissure aquifer of the Zhiluo Formation, which are also the main direct or indirect water sources of the mine (Figure 1). The Quaternary aquifer is a phreatic layer located at a distance of 447.50~778.69 m from the No. 2 coal seam in the main mining area, exhibiting a permeability coefficient ranging from 0.479 to 8.42 m·d−1. The Cretaceous Luohe Formation aquifer is located at a distance of 148.42~412.43 m from the No. 2 coal seam in the main mining area. The aquifer has a thickness ranging from 80.91 to 397.83 m, with an average thickness of 273.42 m and a permeability coefficient ranging from 0.066 to 0.187 m·d−1. The Zhiluo Formation aquifer is located at a distance of 73.06~318.73 m from the main coal seam No. 2, exhibiting a thickness ranging from 38.60 to 256.19 m, with an average thickness of 106.19 m and a permeability coefficient ranging from 0.067 to 0.453 m·d−1.

2.2. Sample Collection and Testing

The hydrogeological and geochemical data of the ZYL Coal Mine are insufficient, and there is a lack of clarity regarding the water quality characteristics of each aquifer within the mining area. Thus, the hydrochemical characteristics of each aquifer in the minefield were investigated by collecting a total of 38 samples from the study area, including 4 samples of Quaternary groundwater, 7 samples of Luohe Formation water, 5 samples of Anding Formation water, 16 samples of Zhiluo Formation water, and 6 samples of mine water. The sampling containers were rinsed 3–5 times before collecting samples from each aquifer, and after sampling, the containers were promptly sealed without any air exposure. The sampling time and location were clearly marked on the sampling containers and subsequently dispatched to the laboratory for analysis. The tested parameters encompassed K+, Na+, Ca2+, Mg2+, SO 4 2 − , Cl−, HCO3−, total dissolved solids (TDSs), and δD and δ18O isotopes. The ion concentrations and TDS levels were determined using standard analytical methods, including ion chromatography (IC) for major anions and inductively coupled plasma mass spectrometry (ICP-MS) for cations.

3. Results and Discussion

3.1. Hydrochemical Characteristics of Groundwater and Mine Water

The main ion concentrations of water samples from the ZYL Coal Mine are presented in Table 1. Analysis of various water samples from the study area revealed the following:
(1)
The total dissolved solid (TDS) concentrations of Quaternary groundwater and the Anding Formation of Luohe Formation groundwater range from 216.99 to 911.767 mg/L, with an average of 393.10 mg/L.
(2)
The TDS concentration of Zhiluo Formation groundwater and the No. 2 coal seam ranges from 1615.67 to 5124.0 mg/L, with an average value of 3057.07 mg/L.
(3)
It can be inferred that the TDS content of groundwater increases with hydrographic depth.
The dominant cations of water samples within the study area were primarily Na+ + K+, exhibiting a concentration range of 1.14 to 1086.17 mg/L with an average concentration of 349.70 mg·L−1, followed by Ca2+ and Mg2+. The predominant anion is SO 4 2 − , with a concentration range of 20.37 to 3442.00 mg/L and an average concentration of 1224.66 mg·L−1, followed by HCO3− and Cl−.
From Table 1, the water samples in the study area are represented in a Piper diagram (Figure 2) [28]. The hydrochemical composition of the groundwater varies by formation: the Quaternary groundwater is of the HCO3-Ca type, the Luohe Formation is of the HCO3-Ca type and HCO3-Na type, the Anding Group is of the HCO3-SO4-Na·Ca type, and the Zhiluo Formation and the mine water are of the SO4-Na type. In addition, a Scholler diagram (Figure 3a) and a variable coefficient diagram (Figure 3b) are presented. The concentration of each ion in the fourth system, including Luohe and Anding groundwater samples, did not vary much. However, the SO 4 2 − , Na+ + K+, Ca2+, and Mg2+ concentration of Zhiluo groundwater and mine water is higher than that of Quaternary, Luohe, and Anding groundwater. In addition, the variable coefficients of the ions or TDS concentrations were calculated for quantitative dispersion of water quality (Figure 3b).
The variable coefficients of the ions or TDS concentrations were calculated for the quantitative dispersion of water quality (Figure 3b). The coefficient of variation positively correlated with the degree of data dispersion, indicating a higher fluctuation between the data points and suggesting a weak groundwater connection [29].
When combined with the Schoeller diagram, it became evident that SO 4 2 − and Cl− exhibit significant changes, resulting in different coefficients of variation. Conversely, HCO3− shows minimal change in the Schoeller diagram, leading to a low coefficient of variation. Generally speaking, Cl− remains relatively stable in groundwater and, thus, does not display noticeable changes on the Schoeller diagram. However, Zhiluo Formation groundwater demonstrates a relatively large coefficient of variation and exhibits similar hydrochemical characteristics to mine water, indicating a close connection between them.
This is consistent with the conclusion that there is a close hydraulic connection between the Quaternary aquifer and the Luohe Formation, suggesting no effective water barrier between the Quaternary groundwater and the Cretaceous groundwater. The Luohe Formation aquifer groundwater, recharged by atmospheric precipitation, seeps vertically into the aquifer below the Zhiluo groundwater, which then recharges the mine water.

3.2. Characteristics of High-Salinity Mine Water

This chapter will specifically analyze the characteristics of mine water. According to Table 1, the TDS concentration of 38 water samples from the study area ranges from 216.99 to 5124.00 mg/L. Among them, there are 22 samples of high-salinity water with a TDS value exceeding 1000 mg/L, all of which are sourced from the Zhiluo groundwater and mine water.
The correlation diagram depicts the relationship between the TDS and ion concentrations of 22 high-salinity water samples, as illustrated (Figure 4). There is a positive correlation between TDS and ion concentrations in all mine water samples, except for HCO3−. In Zhiluo Formation water samples, there is a positive correlation between TDS and ion concentrations, except for HCO3− and Cl−. The highest correlation is between TDS and SO 4 2 − , with a correlation coefficient of 0.998. The correlation coefficients of Na+, Mg2+, and Ca2+ with TDS are 0.792, 0.788, and 0.759, respectively.
The results indicate that mine water and Zhiluo Formation groundwater have significant salinity, particularly in 22 high-salinity samples with TDS values exceeding 1000 mg/L. This high salinity is primarily contributed to by the Zhiluo Formation and mine water. The strong positive correlation between TDS and most ions (except HCO3−) in mine water samples and between TDS and most ions (except HCO3− and Cl−) in Zhiluo Formation water samples highlights the influence of specific ions on overall salinity.
The exceptionally high correlation between TDS and SO 4 2 − (0.998) suggests that sulfate ions play a major role in the salinity of these water samples. This is further supported by the notable correlation coefficients of Na+ (0.792), Mg2+ (0.788), and Ca2+ (0.759) with TDSs in mine water samples. These findings indicate that sulfate, sodium, magnesium, and calcium are significant contributors to the high TDS levels observed.
Although the overall correlation between mine water and the Zhiluo Formation aquifer is relatively weak, the consistency in the composition of major ions indicates a hydrological connection between them. The correlation coefficients of Mg2+ (0.697) and Ca2+ (0.640) with TDS in the Zhiluo Formation aquifer further emphasize the importance of these ions in influencing the salinity.
In summary, the analysis reveals a clear pattern of ion contribution to the salinity in both mine water and Zhiluo Formation groundwater. The strong correlations between TDSs and specific ions, particularly SO 4 2 − , Na+, Mg2+, and Ca2+, suggest that these ions are key factors in determining salinity levels. The weak yet consistent overall correlation between the mine water and Zhiluo Formation aquifer composition suggests a significant hydrological interaction despite the relatively weak statistical correlation.

3.3. Formation of High-Salinity Mine Water

3.3.1. The Primary Source of Mine Water

The δD and δ18O values in the water remain largely unaffected by water–rock interactions, enabling the effective determination of distinct water sources and their origins [29]. The δD and δ18O isotopic values of Quaternary groundwater, the upper section of Zhiluo Formation groundwater, and the lower section of Zhiluo Formation groundwater and mine water are shown in Table 2. The stable isotope δD values of Zhiluo groundwater, Quaternary groundwater, and mine water exhibit a range from −83.6‰ to −56.3‰, while δ18O values range from −10.8‰ to −7.7‰. According to the literature [30], if δD < −400‰ or δD > 10‰, and −60‰ < δ18O < 0‰, atmospheric precipitation is identified as the primary source.
The average values of δ18O and δD of Quaternary groundwater, upper and lower sections of the Zhiluo Formation groundwater, as well as mine water, are plotted in a relationship diagram (Figure 5).
Due to the absence of local precipitation data, we rely on the equation of the atmospheric precipitation line in regions with analogous climatic conditions: δD = 7.1781 δ18O8.1151 [30]. The δD and δ18O values of Quaternary groundwater, upper Zhiluo Formation, lower Zhiluo Formation groundwater, and mine water closely align with the local atmospheric precipitation line, indicating that atmospheric precipitation serves as the primary source of recharge. The δD and δ18O values of Quaternary water samples exhibit significant deviations from the local atmospheric precipitation line, which is primarily influenced by atmospheric rainfall, thereby distinguishing them from other water samples. According to d (deuterium surplus) = δD-8δ18O, the deuterium surplus of Zhiluo Formation and mine water ranges from 1.6 to 2.9 ‰, which is significantly below the threshold of 10‰, indicating a slow runoff rate and prolonged retention time within the aquifer. Comparing the average values of δD and δ18O in Table 2, it has been determined that there is minimal disparity between the mean values of δD and δ18O in mine water and those of water samples from the Zhiluo Formation. These findings indicate that the Zhiluo Formation aquifer water serves as the primary source of recharge for mine water, aligning with the aforementioned analysis results on water chemical composition.

3.3.2. Water–Rock Interaction

According to the proportional relationship between Na+, Ca2+, Cl−, HCO3−, and TDS in water, the control factors of ion formation are determined by the Gibbs diagram [31,32]. The main control factors can be classified into three types: precipitation control type, rock-weathering control type, and concentration caused by evaporation. In the exploration of groundwater formation control, the graphical method has received a lot of attention from scholars at home and abroad. See Figure 6.
The TDS of shallow groundwater is below 1000 mg·L−1, with Na+/(Na+ + Ca2+) ratios ranging from 0.11 to 0.93 and Cl−/(Cl− + HCO3−) ratios ranging from 0.03 to 0.87, all situated within the rock-weathering control area, indicating that the primary process responsible for the formation of shallow groundwater is rock-weathering control. The deep groundwater and mine water exhibit a TDS concentration exceeding 1000 mg·L−1, with Na+/(Na+ + Ca2+) values ranging from 0.54 to 0.86 and Cl−/(Cl− + HCO3−) ratios ranging from 0.00 to 0.52, predominantly below 0.3. The hydrogeology of the deep ZhiLuo formation and mine groundwater is primarily influenced by processes of concentration caused by evaporation. The chemical composition of deep groundwater is predominantly characterized as SO4-Na type, exhibiting high mineralization and a relatively low concentration of Cl−, resulting in the majority of water samples being distributed beyond the boundaries defined by the three control types. From the Gibbs diagram, it can be inferred that there are two main factors leading to the formation of highly mineralized mine water: the lixiviation and concentration caused by evaporation. The surrounding rock contains a large amount of soluble minerals. During the long-term contact between groundwater and the surrounding rock, the minerals gradually dissolve. Through relevant chemical reactions, the ion content in groundwater gradually increases. In addition, the research area is located in an arid and semi-arid region with strong evaporation. The evaporation and concentration of groundwater further increase the concentration of various ions in the water, leading to an increase in TDS values.
Groundwater formed under different geneses or conditions often has a large difference in the content between components. Therefore, such ratio coefficients can be used to determine the source of groundwater components [31]. Among them, the ratio relationship between Na+ and Cl− can be inferred as the source of Na+. When the ratio of the two is about 1:1, it indicates that Na+ in groundwater mainly comes from rock salt dissolution [32]. As shown in Figure 7, most of the samples from the Zhiluo Formation and mine water in the study area are located above the 1:1 line, and the equivalent concentration of Na+ is significantly higher than that of Cl−. This discovery indicates that Na+ not only comes from the dissolution of rock salt but may also come from silica-aluminate minerals containing sodium or if there is a strong cation exchange interaction.
The main sources of Mg2+ and Ca2+ can be determined by analyzing the ratio relationship between (Ca2+ + Mg2+) and (HCO3− + SO 4 2 − ) [33]. The water samples from the Zhiluo Formation and mine in Figure 7b all fall below a 1:1 straight line, indicating that Mg2+ and Ca2+ in the groundwater mainly come from the weathering and dissolution of silicate and evaporite rocks rather than the weathering and dissolution of carbonate rocks. The approximate 1:1 ratio of Ca2+ to SO 4 2 − in groundwater indicates that the primary source of these ions is the dissolution of gypsum. However, the samples from the Zhiluo Formation and mine water in the study area are generally positioned below the 1:1 line (Figure 7c). This suggests that other chemical processes, such as cation exchange, also influence the Ca2+ content in groundwater.
In summary, the dissolution of rock salt, gypsum, and other minerals provides a material foundation for the formation of high-salinity groundwater. The chemical equations for these processes are as follows:
  • Dissolution of rock salt:
NaCl (rock salt) → Na+ + Cl−
2.
Dissolution of sodium feldspar:
2NaAlSi3O8 (Albite) + 2H2CO3 + 9H2O → Al2Si2O5(OH)4(kaolinite) + 2Na+ + 2 HCO3− + 4H4SiO4
3.
Dissolution of gypsum:
CaSO 4   ( gypsum ) → Ca 2 + + SO 4 2 −
4.
Solution of anorthite:
2CaAl2Si2O8 (anorthite) + 4CO2 + 6H2O → 2Al2Si2O5(OH)4(kaolinite) + 2Ca2+ + 4HCO3−

3.3.3. Cation Exchange

The ratio of (Ca2+ + Mg2+ − HCO3− − SO 4 2 − )/(Na+ − Cl−) serves as an indicator for assessing the impact of cation exchange on water quality [34,35,36]. When a linear relationship exists between the two parameters, with a slope of −1, it indicates that cation exchange is the primary process governing the hydrochemical evolution of water. As illustrated in Figure 8a, the ratio of (Ca2+ + Mg2+ − HCO3− − SO 4 2 − )/(Na+ − Cl−) demonstrates a linear correlation with a slope of −0.65 (R2 = 0.65), indicating the occurrence of cation exchange in groundwater during runoff. The analysis of major ion ratios and the aquifer rock composition mentioned earlier reveals that the concentrations of Ca2+ and Mg2+ are relatively low in both groundwater and surrounding rocks, thereby minimizing the impact of cation exchange.
The Chloro-Alkaline Index (CAI-1 and CAI-2) can be utilized to determine the process of ion exchange [37]. When the values of CAI-1 and CAI-2 are both less than 0, a reverse cation exchange occurs, where Ca2+ and Mg2+ undergo water exchange with Na+ and K+ on the mineral surface (Equation (7)). Otherwise, when the value is positive, a positive cation exchange occurs (Equation (8)), and the stronger the absolute value, the stronger the cation exchange [38].
The calculation formulas for CAI-1 and CAI-2 are as follows:
CAI-1 = [Cl− − (Na+ + K+)]/Cl−
CAI - 2 = [ Cl − − ( Na + + K + ) ] / [ HCO 3 − + SO 4 2 − + CO 3 2 − + NO 3 − )
The units of each index in the equation are meq/L.
The chemical reaction is represented as follows:
Ca2+ (Mg2+)(water) + Na+(K+)(rock) → Na+(K+)(water) + Ca2+(Mg2+)(rock)
Na+(K+)(water) + Ca2+(Mg2+)(rock) → Ca2+(Mg2+)(water) + Na+(K+)(rock)
The Chloro-Alkaline Index diagrams depicting water samples from various aquifers are illustrated in Figure 8b. The Chlor-Alkali Index for the Anding Formation, Zhiluo Formation, and mine water is less than 0, as observed from Figure 8b, indicating that a positive cation exchange process has occurred. In Quaternary Formation groundwater, 75% and 14.3% are located in Quadrant I, indicating a process of reverse cation exchange, as well as the Luohe Formation. On the other hand, 25% and 85.7% are both located in Quadrant III, indicating a positive exchange of cations. The complexity of the cation exchange processes in the Quaternary System and the presence of both positive and reverse reactions indicate Luohe Formation groundwater. However, the dominant process in the Zhiluo Formation and mine water is positive cation exchange, resulting in an increased concentration of Na+ in the water and the formation of highly mineralized groundwater.

4. Treatment and Utilization of High-Salinity Mine Water

High-salinity treatment technologies can be classified into three categories: chemical, thermal, and membrane separation methods [39,40,41]. Chemical methods primarily involve the use of reagents and ion exchange to remove salts from mine water through various chemical reactions. The reagent method typically uses limestone to reduce the high hardness levels in mine water. Ion exchange employs solid exchange agents to facilitate the ion exchange process, targeting ions such as calcium, magnesium, and chloride in mixed-type mine water. However, when the salinity exceeds 500 mg/L, the cost of this method increases significantly. Thermal methods use a heat source as the driving force for the separation of water and salts, thereby achieving demineralization. This method is suitable for mine water with total dissolved solids (TDSs) exceeding 3000 mg/L. Membrane Separation Demineralization Technology includes electrodialysis and reverse osmosis. Electrodialysis is suitable for treating mine water with a salt content between 500 mg/L and 4000 mg/L, whereas reverse osmosis is suitable for treating mine water with a salt content below 4000 mg/L and up to 10,000 mg/L [39,41].
Therefore, we chose reverse osmosis technology as the treatment method, which not only effectively reduced the salt content in water but, when combined with nanofiltration technology, also enhanced the purity of crystalline salt produced through evaporation crystallization.
Desalinated mine water can be used for industrial water supply for surrounding enterprises or for irrigation in nearby farmland, the domestic water needs of surrounding enterprises or residents, and ecological and environmental construction in mining areas [42,43]. This not only significantly reduces the discharge of mine water but also effectively alleviates the water shortage problem in mining areas. This helps to resolve the contradiction between water resource supply and demand in coal mining regions, achieving a harmonious balance between coal resource development and water resource protection and utilization.

5. Conclusions

Based on the comprehensive analysis of 38 groundwater and mine water samples using Piper diagrams, Gibbs diagrams, isotope analyses, and ion ratio coefficients, several key conclusions can be drawn. First, it was observed that the TDS concentration increases with depth, with the average TDS concentrations of Quaternary, Luohe, and Anding groundwater being 336.87 mg/L, 308.67 mg/L, and 556.29 mg/L, respectively. In stark contrast, the TDS concentrations of Zhiluo groundwater and mine water are significantly higher, at 2768.57 mg/L and 3826.40 mg/L, respectively, classifying them as high-salinity water. This pattern indicates a clear trend where deeper groundwater layers exhibit higher salinity levels.
The hydrochemical analysis revealed distinct types of groundwater within the study area. Quaternary groundwater is predominantly of the HCO3-Ca type, while the Luohe Formation presents both HCO3-Ca and HCO3-Na types. Anding Group’s groundwater is characterized by the HCO3-SO4-Na·Ca type, and the Zhiluo Formation, along with mine water, is primarily of the SO4-Na type. These classifications reflect the varied mineral compositions and geochemical processes affecting each groundwater source.
Isotope analysis further highlighted that the δD and δ¹⁸O values of all groundwater and mine water samples were close to the meteoric water line, suggesting that atmospheric precipitation is the primary recharge source. The high salinity of Zhiluo Formation groundwater, which is the main source of mine water, is closely related to the dissolution of rock salt and gypsum, as well as cation exchange processes. This finding underscores the significant role of geological formations in contributing to the salinity levels observed in mine water.
Given the high TDS concentration of mine water, ranging from 3435.4 mg/L to 4414.3 mg/L, the combined treatment process of reverse osmosis is recommended for effective salt removal. The treated mine water can be repurposed for various uses, including industrial, domestic, and ecological applications. This approach not only aids in resource utilization but also supports the goal of achieving the zero discharge of mine water in Northwest China.
In summary, this study offers valuable insights into the formation mechanisms and treatment strategies for high-salinity mine water. While the findings provide a robust foundation for managing mine water resources, there are inherent limitations due to the availability and quality of data. Potential gaps in long-term hydrogeochemical monitoring and the precision of analytical methods in detecting trace elements at extremely low concentrations may affect the comprehensiveness of the conclusions. Additionally, although the sample size was carefully selected, it may not fully capture the variability within the larger aquifer system. These limitations highlight the need for continued research and more extensive data collection to refine our understanding and management of high-salinity mine water.

Author Contributions

Conceptualization, J.Y. and W.Z.; methodology, W.Z.; software, W.Z. and J.Y.; validation, J.Y. and W.Z.; formal analysis, J.Y., W.Z. and X.L.; investigation, X.L. and F.X.; data curation, X.L. and F.X.; writing—original draft preparation, J.Y. and W.Z.; writing—review and editing, J.Y. and W.Z.; visualization, W.Z.; supervision, X.L. and F.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Natural Science Foundation of Shaanxi province (2023-JC-QN-0291) and National Key Research and Development Program of China (20l6YFC0501105).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

Authors Xiangyang Liang and Feng Xu were employed by the company Xi′an Research Institute of China Coal Technology & Engineering, (Group), Corp. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Water 16 02244 g001
Figure 1. Location, water sampling, and hydrogeological histogram of study area.
Figure 1. Location, water sampling, and hydrogeological histogram of study area.
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Figure 2. Piper diagram.
Figure 2. Piper diagram.
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Figure 3. Schoeller diagram and variable coefficient diagram of ions. (a) Schoeller diagram; (b) Variable coefficient diagram of ions.
Figure 3. Schoeller diagram and variable coefficient diagram of ions. (a) Schoeller diagram; (b) Variable coefficient diagram of ions.
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Figure 4. Correlation analysis chart of high-salinity water samples.
Figure 4. Correlation analysis chart of high-salinity water samples.
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Figure 5. δD-δ18O diagram illustrating water samples collected from the study area.
Figure 5. δD-δ18O diagram illustrating water samples collected from the study area.
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Figure 6. Gibbs diagram of water samples from study area.
Figure 6. Gibbs diagram of water samples from study area.
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Figure 7. Ion ratio diagram. (a) Na+ and Cl− relational graph; (b) Ca2+ and SO 4 2 − relational graph; (c) (Ca2+ +Mg2+) and ( SO 4 2 − +HCO3−) relational graph.
Figure 7. Ion ratio diagram. (a) Na+ and Cl− relational graph; (b) Ca2+ and SO 4 2 − relational graph; (c) (Ca2+ +Mg2+) and ( SO 4 2 − +HCO3−) relational graph.
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Water 16 02244 g008
Figure 8. (a) (Ca2+ + Mg2+ − HCO3− − SO 4 2 − )/(Na+ + K+ − Cl−) and (b) water Chlor-Alkali Index diagram.
Figure 8. (a) (Ca2+ + Mg2+ − HCO3− − SO 4 2 − )/(Na+ + K+ − Cl−) and (b) water Chlor-Alkali Index diagram.
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Table 1. Main ion concentrations and TDSs of water samples from the ZYL Coal Mine (mg·L−1).
Table 1. Main ion concentrations and TDSs of water samples from the ZYL Coal Mine (mg·L−1).
Water Sample TypeStatistical ItemNa+ + K+Ca2+Mg2+ SO 4 2 − HCO3−Cl−TDS
Quaternary SystemMaximum value26.4061.1012.6057.60199.2081.00358.17
Minimum value1.143.050.3020.37122.005.30292.00
Average value13.6137.325.3841.09177.9539.77336.87
Coefficient of variation0.760.660.960.440.211.010.09
Luohe FormationMaximum value67.0354.6510.90145.29201.4024.50408.95
Minimum value6.8119.955.2021.00133.981.15216.99
Average value35.9937.767.6151.11161.6214.57308.67
Coefficient of variation0.630.320.290.870.130.610.19
Anding FormationMaximum value211.78139.5212.20513.15210.5078.43911.76
Minimum value17.507.882.4028.8011.3613.15335.25
Average value100.5058.206.72230.96121.9138.00556.29
Coefficient of variation0.710.870.581.010.630.660.50
Zhiluo FormationMaximum value1086.17473.2448.603442.00146.4076.105124.00
Minimum value371.8080.204.90946.2055.680.001615.67
Average value566.99279.6018.821786.84103.9215.272768.57
Coefficient of variation0.310.530.740.400.271.390.36
Mine waterMaximum value819.40408.8071.702896.20142.1046.104414.30
Minimum value476.50124.3010.902555.0036.6019.803435.40
Average value567.95287.5528.322711.82111.4035.623826.40
Coefficient of variation0.250.400.880.060.360.240.10
Table 2. The average values of δD and δ18O for various types of water samples from the study area.
Table 2. The average values of δD and δ18O for various types of water samples from the study area.
Serial NumberAquifer PositionAverage Value of δD/‰Average Value of δ18O/‰
1Quaternary groundwater−56.3 −7.7
2Upper section of Zhiluo Formation−80.8 −10.3
3Lower section of Zhiluo Formation−83.6 −10.8
4Mine water−81.9 −10.6
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Yang, J.; Zhao, W.; Liang, X.; Xu, F. The Hydrochemical Characteristics and Formation Mechanism of Highly Mineralized Coal Mine Water in Semi-Arid Regions in Northwest China. Water 2024, 16, 2244. https://doi.org/10.3390/w16162244

AMA Style

Yang J, Zhao W, Liang X, Xu F. The Hydrochemical Characteristics and Formation Mechanism of Highly Mineralized Coal Mine Water in Semi-Arid Regions in Northwest China. Water. 2024; 16(16):2244. https://doi.org/10.3390/w16162244

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Yang, Jian, Wei Zhao, Xiangyang Liang, and Feng Xu. 2024. "The Hydrochemical Characteristics and Formation Mechanism of Highly Mineralized Coal Mine Water in Semi-Arid Regions in Northwest China" Water 16, no. 16: 2244. https://doi.org/10.3390/w16162244

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