Identified the hydrochemical and the sulfur cycle process in subsidence area of Pingyu mining area using multi-isotopes combined with hydrochemistry methods

Abstract

Groundwater serves as an important water source for residents in and around mining areas. To achieve scientific planning and efficient utilization of water resources in mining areas, it is essential to figure out the chemical formation process and the ground water sulfur cycle that transpire after the coal mining activities. Based on studies of hydrochemistry and D,¹⁸O-H₂O,³⁴S-SO₄ isotopes, this study applied principal component analysis, ion ratio and other methods in its attempts to reveal the hydrogeochemical action and sulfur cycle in the subsidence area of Pingyu mining area. The study discovered that, in the studied area, precipitation provides the major supply of groundwater and the main water chemistry effects are dominated by oxidation dissolution of sulfide minerals as well as the dissolution of carbonate and silicate rocks. The sulfate in groundwater primarily originates from oxidation and dissolution of sulfide minerals in coal-bearing strata and human activities. The mixed sulfate formed by the oxidation of sulfide minerals and by human activities continuously recharges the groundwater, promoting the dissolution of carbonate rock and silicate rock in the process.

Graphical abstract

Keywords

PCA / Ion ratio / Water chemistry / Sulfide minerals / Multi-isotopes / Subsidence area of mining area

Introduction

Coal is an important basic energy source and raw material in China (Tao et al. 2022). However, the process of coal mining is bound to cause damage and impact to groundwater in mining areas (Zhang et al. 2021; Sahoo et al. 2020), especially in northern China, where karst water often submerges in the coal measures, and high-intensity concentrated coal mining will greatly damage the original aquifer structure in the mining area. Large amount of water drainage in mine crates will also lead to large-scale ground subsidence or collapse in and around the mining area (Su et al. 2018), which in turn will change the original groundwater flow characteristics and hydrogeochemical processes around the mining area (Cha et al. 2022; Acharya et al. 2020). Groundwater functions as an important water source for residential areas around mining areas in northern China, and excessive SO₄²⁻ will greatly increase the salinity of water bodies, causing serious effect on human health and local ecological balance (Qu et al. 2022). Therefore, it is of great significance to study the hydrochemistry and laws of sulfur cycle of groundwater around mining areas so as to scientifically utilize the groundwater and improve its quality around the mining area, .

Groundwater hydrochemistry in mining areas is mainly influenced by a combination of natural effects such as endowment conditions and water-rock interaction, and anthropogenic activities such as coal production and human living (Acharya et al. 2020; Rinder et al. 2020), and ion ratio is one of the methods commonly used to analyze the hydro chemical evolution of groundwater (Zhang et al. 2021). Stable isotopes are sensitive tracers for tracing the origin of water bodies and ion sources (Moya et al. 2016). Since D and ¹⁸O-H₂O are important components of water molecules, the significant differences between D and ¹⁸O-H₂O from varied sources have paved way for of hydrogen and oxygen isotopes to trace the source of groundwater (Ansari et al. 2020). Sulfate is hardly fractionated during its formation, so ³⁴S- SO₄ can be used as a characteristic chemical "fingerprint" to demonstrate the sulfur cycling and the study of the migration and transformation process of sulfate can effectively analyze the sulfur cycling law in water bodies (Banks et al. 2020). In recent years, many Chinese and international scholars have applied a variety of methods combined with isotopes analysis to examine the sulfur cycle of groundwater in mining areas and other issues. For example, David et al. (2023) interpreted the solute and sulfur cycle in mine water by combining the stable isotope composition of D, ¹⁸O-H₂O and ³⁴S-SO₄ in water from abandoned and flooded mines with data of drilling holes and hydrochemistry. Wang et al. (2022) and Mao et al. (2022) respectively used the self-organization mapping function of artificial intelligence to analyze isotope and hydro chemical data of the mining area, studying the changes of hydro chemical characteristics before and after mining, which revealed the mechanism of hydrogeochemical evolution in the multi-layer aquifer before and after mining, as well as the sulfur cycle process of the mining area. Huang et al. (2023) employed multi-isotope and hydro chemical analysis to investigate the source of sulfate and the sulfur cycling process in groundwater in North-China-type coal mines surrounding area . Previous researches have, mostly focused on the sulfur cycle of groundwater in mining areas by means of hydrochemistry and multi-isotope analysis (Ágnes et al. 2022; Ren et al. 2021; Zheng et al. 2019), while little paid attention to the subsidence area around the mining area caused by mining.

Located in Xuchang City, Henan Province, the Pingyu mining area used to be in a region rich in groundwater resources with developed springs and surface rivers. In May 2010, the first crate of Pingyu Mine started to evacuate karst water on a large scale. After 11 years of mining, the karst water level has decreased from 120–130 m in May 2010 to 210–180 m in July 2021, while a number of rivers in the area have been cut off, Happiness Lake and other natural spring groups, which were originally formed by karst water recharge, have also dried up one after another. Apart from the annual supply from the South-to-North Water Diversion Project, there is no other drinkable surface water source in the questioned area. Rural areas, as well as industrial and agricultural production is highly dependent on the mixed mining of pore water and karst water. The large amount of groundwater pumping has led to the successive occurrence of ground subsidence with a total area of about 19 km² around the mining area, where some channels of the South-to-North Water Diversion Center Line have also suffered impacts of ground subsidence. The cumulative amount of ground subsidence reached 60.63–155.29 mm, with a continuous increase rate of 10–20 mm/a, seriously threatening the safe and stable operation of the channels in the long run (Su et al. 2023; Pan et al. 2011). Previous studies on this area have mainly focused on groundwater flow characteristics and karst water recharge sources (Zhang, 2017; Tang, 2011; Pan et al. 2011), but there has been no research related to groundwater environment. Therefore, in order to ensure the water safety for residents in and around the mining area, and to enhance the utilization rate of water resources around the mine, there is an urgent need to study the hydrochemistry and sulfur cycling process of groundwater (pore water and karst water alike) after large-scale evacuation of karst water in the ground subsidence area around the mine. In this study, a combination of multi-isotope, hydrochemistry and principal component analysis is applied to gain insights of the hydro chemical evolution of groundwater and to reveal the sulfur cycling mechanism in the ground subsidence zone formed by mining hydrophobicity. The main objectives of this study are as follows: 1) to describe the hydro chemical characteristics and major hydrogeochemical processes in the subsidence zone of Pingyu Mine through principal component analysis and ion ratio analysis; 2) to analyze the groundwater and its sulfate sources in the subsidence zone of the mining area using D, ¹⁸O-H₂O, ³⁴S- SO₄ multiple isotopes; 3) to elucidate the sulfur cycling mechanism in the subsidence zone under the influence of mining activities. This study aims to improve the understanding of groundwater evolution and sulfur cycling process in the subsidence zone of Pingyu Mine and provide scientific support for local groundwater management.

1 Study area

1.1 Natural features

The studied area is located in the northwest of Xuchang City, Henan Province, China. The overall landscape of the studied area leans from northwest towards southeast (Fig. 1). Except for the Guci Mountains in the north, other parts of the studied area are covered by the Quaternary alluvial plain. Pingyu mining area is located in the south of the studied area, covering a total area of 13.5 km². The studied area is exposed to continental monsoon climate, with an average annual temperature ranging from 13.0°C to 16.0°C, rainfall concentrated from July to September, and an average annual precipitation of 665 mm. The major rivers in the studied area are the Yinghe River and the Shiliang River, which belong to the Yinghe River system in the Huaihe River basin. Limited by topography, all the rivers in the studied area flow from northwest to southeast. are The middle route of the South-to-North Water Diversion Project functions as the main water conservancy facilities in this area, in addition to which are a number of drought relief reservoirs and subsidence lakes formed due to long-term coal mining activities around the mining area. is Densely populated and crowded with ore processing factories and machineryl processing factories, the studied area relies mainly on supply of groundwater and water from the South-to-North Water Diversion channel. Sewage, through underground pipeline networks, and the mine drainage processed by sedimentation treatment in the mining area are discharged into the neighboring natural rivers.

1.2 Geology

The tectonic unit of the studied area generally manifests a monoclinal structure, where Pingyu mining area is found in the northeast of the northern flank of Baisha syncline (Fig. 1). The exposed strata in the studied area consists of Songshan Group of Lower Proterozoic, Sinian System of Upper Proterozoic, Cambrian system of Lower Paleozoic, Carboniferous and Permian system of Upper Paleozoic, Paleogene system of Cenozoic, Neogene system and Quaternary system, among which Carboniferous-Permian system is the main coal-bearing strata, and 2₁ coal seam of Shanxi Formation of Lower Permian system is the exploitable coal seam in this area. The coal-bearing strata are rich in pyrite and other sulfide crystals, but contains no evaporite minerals such as gypsum. The northeastern part of the mining area is covered with folds in the east-west direction, accompanied by parallel fractures and a group of torsional fissures. Along the fractures are extrusion schistochemistry zones, crushing-influenced zones and fissure-intensive zones, which regulate the formation and distribution of karst water (Pan et al. 2011).

1.3 Hydrogeology

Except for the Guci Mountain area in the northern part of the research area, pore water is widely distributed in the Quaternary impact plain. The aquifer is composed of Neogene fluvial lacustrine sedimentary layer and Lower Pleistocene alluvial-lacustrine sedimentary layer, with the lithology of the Neogene fine sand, medium fine sand, coarse medium sand or sandstones, appear to be semiclastic, and with the mineral composition of quartz as dominant component. Pore water is replenished by atmospheric precipitation, surface water and lateral runoff from the west, with Yinghe River as the southern boundary of the discharge area and the line between unconsolidated strata and bedrock as the western part of the northern boundary, while the eastern part receives runoff recharge from the north. The lower interface of the pore water aquifer is located afore mountain, forming a water-repellent boundary by contacting sandstone and shale.

In the northwest and north part of the studied area bedrock and limestone belong to the exposed karst area are found exposed (Fig. 1), consisting of metamorphic rocks and intrusive rocks of the Proterozoic era, and of Paleozoic carbonates and clastic rocks. In the east and southeast direction, the area gradually falls under the cover of loose layers, and the burial depth gradually increases from more than 10 meters in the west to about 400 meters in the east. Previous borehole and geological data provided by predecessors (Pan et al. 2011) show that the karst water in the area is categorized as Cambrian limestone karst water with the main lithological compositions are dolomitic limestone and siliceous masses or bands. The karst water in the studied area is primarily supplied by atmospheric precipitation in the western mountain area (Pan et al. 2011). The karst water aquifer ends in the east with the former Shigu fault, with the Baisha syncline axis the southern boundary, both of which is water-repellent boundary, and its northern boundary is the Guci Mountains. Karst water runs off on steep slope from the western exposed karst area down to the east and southeast, gradually entering the areas where it is covered and then buried. The buried area finds its place between two layers of water-repellent boundaries, on is between the buried area and the upper sandstone, the other is the bottom interface of the limestone fromed by Cambrian sand shale.

Before the large-scale drainage of karst water in Pingyu Mining area, karst confined water in the thin carboniferous thin-bedded tuff and Cambrian thick-bedded tuff at the base of coal seam in the northern and northwestern mountain area in the mining area served as the major source of filling for mine water in Pingyu Mine. Karst water enters the coal mining face through karst fractures, fissures, faults, mining-induced fractures and poorly closed boreholes, threatening the safety of mine production. Besides, karst water also discharges naturally through the tectonic fractures in the northeastern part of the mine, where it supports and fills the loose strata and forms a cluster of karst springs (Fig. 2). For now, mine drainage in Pingyu Mine has completely altered the characteristics of local groundwater recharge and discharge. Although karst water in the northern and northwestern mountainous area is provided main supply of mine water in Pingyu mining area, the original natural discharge points of karst water has dried up one after another. Previously, the karst water supported and refilled the upper-layer pore water, while now the former has to receive cross-flow recharges from the latter (Su et al. 2023).

2 Data sources and test methods

2.1 Sample collection

Sampling principle: Taking into account both the types and lithology of aquifers in the studied area, drilling sampling constructions were carried out from July 2021 to August 2022 in areas showing relatively obvious influence of land subsidence (i.e. human gathering areas and important water conservancy facilities), collecting a total of 32 groups of groundwater and surface water samples of different aquifers (Fig. 1 for the locations of sampling spots).

In this study, a total of 14 groups of surface water samples were collected, including 2 groups of river water samples, 1 group of samples from the middle route channel of the South-to-North Water Diversion Project, 3 groups of subsidence lake water, 1 group of water from the Happiness Lake, and 7 groups of reservoir water. 15 groups of groundwater samples were collected, including 8 groups of pore water and 7 groups of karst water. For testing sulfur isotope, a total of 23 groups of samples were collected as samples, including 14 groups of surface water, 4 groups of pore water and 5 groups of karst water; while 19 groups of samples collected to test hydrogen and oxygen isotope, including 8 groups of surface water, 5 groups of pore water and 6 groups of karst water.

2.2 Test methods

In this study, all groundwater samples were sampled and tested on site within 0.5 h before the end of the pumping experiment. 500 mL brown bottles were used for sampling, which were moistened and washed with distilled water and water to be sampled for 3 times respectively before the samples were loaded and caps were tightly screwed under water. Portable multi-parameter water quality analyzers (HI 9828, HANNA) were employed to determine pH, EC and other indicators of water bodies. Conventional chemical components (including K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, HCO₃⁻, NO₃⁻, TDS, H₂SiO₃) were determined in the laboratory of Henan Geological Engineering Investigation Institute: K⁺, Na⁺ were measured by flame atomic absorption spectrophotometer, Ca²⁺, Mg²⁺, Cl⁻ and HCO₃⁻ were measured by titration method, SO₄²⁻ was tested by turbidimetric method, NO₃⁻ by ultraviolet spectrophotometry, TDS by gravimetric method, and metasilicicic acid by silico-molybdenum yellow spectrophotometry. Hydrogen and oxygen isotopes were measured by DELTA V Advantage Isotope Ratio Mass Spectrometer of Wuhan Nuclide Technology Co., LTD. The charge balance errors of anions and cations in the collected samples were all within ±5%; and the testing errors of δD, δ¹⁸O-H₂O and δ³⁴S-SO₄ are ±<0.5‰, ±<0.2‰ and ±<0.2‰, respectively.

3 Results

3.1 Hydrochemical characteristics

The hydrochemical results of different water samples are summarized in Table 1. As can be seen from Table 1, the pH value of the water bodied in the studied area ranged from 7.15 to 7.52. which was weakly alkaline in general. The TDS range of surface water in the studied area is 175.55–782.56 mg/L, and the TDS range of groundwater is 330.69–651.66 mg/L, both of which could be concluded as fresh water. Ranking from high to low, the average rates of concentration of the major cations in surface water were as follows: Ca²⁺＞Na⁺＞Mg²⁺＞K⁺, with their respective contents as: 74.11±87.90, 23.49±45.39, 22.77±13.81, 7.19±6.24.; the average rates concentration of major anions were as follows: HCO₃⁻＞SO₄²⁻＞Cl⁻＞NO₃⁻, with their respective contents as 199.01±115.26, 92.70±145.00, 63.47±201.34, 11.41±42.01. For ground water, ranking from high to low, the rates of the average concentrations of major cations were as follows: Ca²⁺＞Mg²⁺＞Na⁺＞K⁺, with their respective contents as 96.41±38.02, 27.94±15.62, 19.43±14.15, 3.72±4.05; The rates of the average concentration of major anions were as follows: HCO₃⁻＞SO₄²⁻＞Cl⁻＞NO₃⁻, with their respective contents as 325.76±54.42, 55.44±31.24, 47.45±112.07, 31.66±62.6. The saturation indices of calcite ad dolomite in the collected water samples, except few in surface water samples were over saturated with the saturation index over 0. The highest sulfate concentrations were found in surface water samples (13.5–237.7 mg/L, mean 92.7 mg/L), followed by that in pore water (23.68–86.68 mg/L, mean 86.68mg/L), and the lowest sulfate concentrations were in karst water (23.95–76.2 mg/L, mean 57.38 mg/L).

As can be seen from Piper's triplex diagram (Fig. 3), the cations in the studied area are mainly located in the lower left corner, while the anions are mainly located in the upper end of the triangle and the lower left corner, showing a high degree of consistency. The surface water hydrochemical types are relatively complex (Table 2), and were divided into 6 types, which were dominated by HCO₃-Ca·Mg (42.86%) and HCO₃·SO₄-Ca·Mg(28.57%). The hydrochemical type of pore water is relatively simple with primary type of HCO₃-Ca·Mg (75%). For karst water hydrochemistry there are three types, mainly the HCO₃-Ca·Mg (71.43%) type.

The principal component analysis of groundwater indexes (Table 3 and Table 4) shows that SO₄²⁻ is moderately correlated with Mg²⁺ and Ca²⁺ 0.5–0.75), with the correlation coefficients being 0.628 and 0.509, respectively. The first components affecting groundwater hydrochemistry are Cl⁻, NO₃⁻, Ca²⁺, Mg²⁺, SO₄²⁻ and Na⁺, with the respective loadings of 0.872, 0.850, 0.793, 0.723, 0.688 and 0.520. In general, Ca²⁺ and Mg²⁺ in groundwater mainly generated by the dissolution of silicates and carbonates in aquifers, while Cl⁻ and NO₃⁻ mostly comes from human activities (Jacob et al. 2023; Liu et al. 2020). In summary, the main factors affecting SO₄²⁻ and hydrochemical types in the studied area were water-rock interaction and human production activities.

3.2 Isotope characteristics

Hydrogen and oxygen isotopes can be employed totrace the source of water recharge in the studied area. According to Fig. 4(a-c), the highest average value of δD (67‰–28‰) was found in surface water in the studied area, which was −50.88‰; the δD value of pore water ranges from −72‰ to 59‰ with the average value of −62.8‰; and the value of δD in karst water (−68‰–61‰) has the lowest average value of −64.17‰. In the studied area, the value of δ¹⁸O-H₂O in surface water is the highest, which ranges from −9.5‰ to −2.4‰ with an average value of −6.93‰; the value of δ¹⁸O-H₂O in pore water ranges from −10.2‰ to −8.5‰ with an average value of −9.02‰; and the value of δ¹⁸O-H₂O in karst water ranges from −9.7‰ to −8.6‰ with an average value of −9.15‰. δ³⁴S-SO₄ values are all positive in the samples; the distribution of δ³⁴S-SO₄ values in surface water is more dispersed (7.6‰–15.4‰), with an average value of 10.02‰; pore water contained a more concentrated distribution (11.7‰–15.1‰), with an average value of 12.73‰; in karst water, δ³⁴S-SO₄ values ranged from 8.5‰ to 15.57‰ with an average value of 11.45‰. Comparatively, the value of δ³⁴S-SO₄ in pore water is relatively the highest.

Since the studied area and Zhengzhou are adjacent to each other and shared similar the natural conditions of rainfall and temperature, the rainwater precipitationline of Zhengzhou area (Wu et al. 2022) can be selected to represent the local rainfall line of Pingyu mining area. The isotope test results of different water bodies in the mining area were fitted and shown in Fig. 4d. Most of the water samples collected in the studied area are located between the local rainfall line and the global rainfall line, indicating that atmospheric precipitation is the primary recharge source of local water supply.

4 Discussion

4.1 Dissolution of carbonate and silicate rocks

The chemical composition of groundwater is governed by water-rock interactions (Zhang et al. 2021). Based on various ion ratio relationships, hydrochemical types of different origin can be qualitatively classified into different hydrochemistry genesis end-members. Gibbs diagram (Gibbs, 1970) is known as a valuable method of determining the source of the main chemical components in natural water by analyzing the relationship between Na⁺, Ca²⁺, Cl⁻, HCO₃⁻ and TDS, which is capable of dividing the chemical components of natural water into three end elements: Atmospheric precipitation, rock weathering, evaporation and crystallization. As is shown in Fig. 5a, except for sampling spot ZT, water samples in the study area are relatively concentrated, almost all of which fall in the category the rock weathering end element, signifying that the hydrochemistry of water bodies in the studied area is principally dominated by rock weathering effects. The ZT spot located in a sunken lake in the subsidence area formed by mine drainage and groundwater exposure to the surface. The water body at the ZT spot is basically static with evapotranspiration as its main drainage channel. Therefore, the water quality at this sampling point gradually fell under the controlled of evaporation and crystallization.

The molar ratio between Mg²⁺/Na⁺ and Ca²⁺/Na⁺ in groundwater is insusceptible to groundwater flow rate, dilution and evaporation, so the water-rock interaction process can be qualitatively identified by the Mg²⁺/Na⁺ and Ca²⁺/Na⁺ relationship (Huang et al. 2023). As Fig. 5b demonstrates, the water bodies in the study area mainly falls between carbonate and silicate end members, implying that the water body in this area is significantly affected by silicate and carbonate dissolution, whereas the dissolution of evaporite rocks (e.g. gypsum) has a lesser influence in the studied area.

Coal mining affects hydrogeochemical processes in mining areas (Cha et al. 2022; Acharya et al. 2020). Through the analysis of the interrelationships between multiple ions, it is possible to infer the hydrogeochemical processes occurring in aquifers (Zhang et al. 2021), which can then reveal the mechanism of sulfur cycle in groundwater. Drillings and previous research data show that siliceous masses or bands are widely present in karst aquifers in the studied area. Studies (Zou et al. 2022) that, under silicate-dominated environment, feldspar will react with carbonic acid to release HCO₃⁻, Mg²⁺ and Ca²⁺ into water, thus the concentration of Na⁺ and K⁺ in the solution will also increase. Therefore, when the mEq ratio of (Na⁺+K⁺)/Cl⁻ is over 1, it is mainly the dissolution of rock salt that occurs in groundwater; when the ratio is below 1, it can be deduced that the dissolution of silicate also takes place. When it comes to Ca²⁺, Mg²⁺, HCO₃⁻ and SO₄²⁻ in groundwater, they are mainly derived from the weathering and dissolution of carbonates such as calcites and dolomites and evaporites such as gypsum (Jacob et al. 2023), and thus, when the (Ca²⁺+Mg²⁺)/(HCO₃⁻+SO₄²⁻) mEq ratio is over 1, Ca²⁺ and Mg²⁺ in solution mainly come from the dissolution of carbonate.; if the ratio is below 1, it indicates that Ca²⁺ and Mg²⁺ are generated by the dissolution of silicates and evaporites. From Fig. 6a and Fig. 6b, it can be seen that most of the water bodies in the studied area are distributed below the equivalent ratio of Na⁺+K⁺/Cl⁻ mgEq to 1:1, and the mg equivalent ratio of (Ca²⁺+Mg²⁺)/(HCO₃⁻+SO₄²⁻) is much higher than 1, indicating that Ca²⁺ and Mg²⁺ in the area are mainly derived from the dissolution of carbonates. The dissolution of carbonates further promoted the dissolution of silicates, while the gypsum dissolution had relatively much smaller effect on SO₄²⁻ in water bodies this area.

4.2 Oxidation and dissolution of sulfide

If SO₄²⁻ in groundwater is mainly derived from the dissolved gypsum (Equation 1), then the molar ratio of (Ca²⁺+ Mg²⁺)/ SO₄²⁻ should be 1. If the Ca²⁺ and HCO₃⁻ in groundwater primarily comes from the dissolution of calcite, then the molar ratio between Ca²⁺ and HCO₃⁻ should be 1:2 (Equation 2). Similarly, if dolomite is the only source of Ca²⁺, Mg²⁺ and HCO₃⁻ in groundwater, the molar ratio between Ca²⁺ and HCO₃⁻ should be 1: 4 (Equation 3). The oxidation process of pyrite produces H₂SO₄, which is a strong carbonate weathering agent besides H₂CO₃. When the oxidation is also involved in karstification, the molar ratio of Ca²⁺/HCO₃⁻ should be between 1:1 and 2:1 (Equation 4-6) (Jiang et al. 2022). As is shown in Fig. 6c and Fig. 6d, the water samples collected in the studied area are all found at the upper left of the (Ca²⁺+Mg²⁺)/SO₄²⁻=1 control line, signifying that the ratio of Ca²⁺/HCO₃⁻ mole is between 1:1 and 2:1, that the (Ca²⁺+Mg²⁺)/HCO₃⁻ mg equivalent ratio in groundwater increased with the increase of SO₄²⁻/HCO₃⁻ mg equivalent ratio, and that SO₄²⁻ showed moderate correlation with Mg²⁺ and Ca²⁺ with correlation coefficients being 0.628 and 0.509, respectively. To summarize, the H₂SO₄ formed by the oxidation of pyrite was involved in the dissolution process in the studied area. In accord with Fig. 6e, Fig. 6f and Fig. 6g, calcite and dolomite in the water of the studied area are almost in a supersaturated state, and the concentrations of Ca²⁺, Mg²⁺ and HCO₃⁻ raise with the increase of saturation degree. The saturation of dolomite in groundwater is significantly greater than that of calcite. These findings declared that the dissolution of dolomite in the studied area is accelerated by H₂SO₄ formed by the oxidation of pyrite. While there existed an indirect correlation between SO₄²⁻ and Mg²⁺ and Ca²⁺, the oxidation and dissolution of pyrite in coal-bearing strata was the main source of SO₄²⁻ in the studied area.

(1)$ \begin{split} & {\rm{Ca(x)Mg(1-x)SO_{4}2H_{2}O}}=\\& {\rm{xCa^{2+}+(1-x)Mg^{2+}+ SO_{4}^{2-}+2H_{2}O}}\end{split} $

(2)$ {\rm{CaCO_{3}+ H_{2}O+ CO_{2}= Ca^{2+}+2HCO_{{3}}^-}} $

(3)$ {\rm{CaMg (CO_{3})_{2} + 2H_{2}O + 2CO_{2} = Ca^{2+}+Mg^{2+}+4HCO_{{3}}^-}} $

(4)$ {\rm{2FeS_{2}+7O_{2}+2H_{2}O=2FeSO_{4}+4H^++ 2SO_{4}^{2-}}} $

(5)$ {\rm{CaMg(CO_{3})_{2} +H_{2}SO_{4}^{2} = Ca^{2+}+Mg^{2+} + 2HCO_{{3}}^- + SO_{4}^{2-}}} $

(6)$ {\rm{2CaCO_{3} +H_{2}SO_{4} = 2 Ca^{2+}+2HCO_{{3}}^-+SO_{4}^{2-}}} $

4.3 Cation alternating adsorption

It can be seen from Fig. 6b and 6d that the (Ca²⁺+Mg²⁺)/( HCO₃⁻+ SO₄²⁻) mgEq ratio is well above 1:1, and sample from certain sampling points are located above the 1:1 molar ratio of Ca²⁺/HCO₃⁻, showing evident Ca²⁺ and Mg²⁺ surplus, which suggests that there could be reverse cation exchange in groundwater. The Schoeller index is applied here to assess the details of ion exchange between groundwater and aquifer medium. As is calculated using the Equation 1-2 below (milligram equivalent ratio). In general, Ca²⁺ and Mg²⁺ in groundwater will exchange Na⁺ and K⁺ in geotechnical particles on the surface of aquifer, which is a positive alternating effect, where the values of CAI-1 and CAI-Ⅱ are negative. Alternately, when Na⁺ and K⁺ increase in groundwater when supplemented by other water sources, Na⁺ and K⁺ will exchange Ca²⁺ and Mg²⁺ in the surface of geotechnical particles, which is a reverse alternating effect, where the values of CAI-1 and CAI-2 are both positive. The greater the absolute values of CAI-1 and CAI-2 are, the stronger the cation exchange is and the easier the cation exchange can occur.

(7)$ \mathrm{C}\mathrm{A}\mathrm{I}-1=\frac{{\mathrm{C}\mathrm{l}}^--{\mathrm{N}\mathrm{a}}^+-{\mathrm{K}}^+}{{\mathrm{C}\mathrm{l}}^-} $

(8)$ \mathrm{C}\mathrm{A}\mathrm{I}-2=\frac{{\mathrm{C}\mathrm{l}}^--{\mathrm{N}\mathrm{a}}^+-{\mathrm{K}}^+}{{\mathrm{S}\mathrm{O}}_{4}^{2-}+{\mathrm{H}\mathrm{C}\mathrm{O}}_{3}^-+{\mathrm{C}\mathrm{O}}_{3}^-+{\mathrm{N}\mathrm{O}}_{3}^-} $

As Fig. 6h demonstrates, most of the water samples fall in the first quadrant, and both values of CAI-1 and CAI-2 are positive, indicating that in most of the water samples in the studied area there exists a reverse cation exchange process, that is, Na⁺ and K⁺ from other sources exchange Ca²⁺ and Mg²⁺ out of the rock and soil matrix, causing the increase of Ca²⁺ and Mg²⁺ and meanwhile the decrease of Na⁺ and K⁺ in water bodies. It is worth noting that in Fig. 6a, still a small number of groundwater samples above the 1:1 mgEq ratio of (Na⁺+K⁺)/Cl⁻, but these points are located in the third quadrant of Fig. 6h, signifying that a small number of water bodies in the studied area are still subject to positive cation exchange, which lead to the increase of Na⁺ and K⁺ but the decrease of Ca²⁺ and Mg²⁺ in the water bodies.

4.4 Sulfate isotopes source identification and microbial desulfation

The sources of SO₄²⁻ in water can be divided into three major categories: Atmospheric deposition, oxidation and dissolution of sulfide minerals and/or gypsum, and input of pollutants. Except for the bacterial sulfate reduction (BSR), significant fractionations generally do not occur in natural sulfates (Huang et al. 2023). The relationship between the ulfate ³⁴S-SO₄ values and the SO₄²⁻ concentrates can, to a certain extent, be exploited to analyze the source of sulfate in water, and has been widely applied in the tracers for various sulfate sources in water, studying the hydrogeochemical processes (Wang et al. 2023; Mao et al. 2023).

Previous studies (Sabina et al. 2022; Zhou et al. 2016) have pointed out that, the values of ³⁴S-SO₄ generated by common natural and anthropogenic sources are as shown in Fig. 7a. Combined with Fig. 7b and Fig. 7c, it can be concluded that the distribution of SO₄²⁻ concentration in surface water is relatively dispersed, with a minimum value of 13.5 mg/L and a maximum value of 237.7 mg/L. Except samples taken at points XF, NT, ZT and HS, the ³⁴S-SO₄ value in surface water (7.6‰–9.9‰) is lower than that in groundwater, which is comparable to that of atmospheric precipitation (3‰–9‰), sewage water (8.7‰±2.4‰) and mine drainage (8.4‰±3.9), indicating that SO₄²⁻ in surface water in the studied area mainly derived from relatively complex sources and could be jointly affected by atmospheric precipitation, sewage and mine drainage at the same time. Larger values of ³⁴S-SO₄ appeared at the sampling spots XF (12.2‰), NT (15.4‰) and ZT (12.7‰), located in the sedimentation lake in the periphery of the mine, with their respective SO₄²⁻ concentrations of 73.2 mg/L, 61.28 mg/L and 98.12 mg/L, appearing to have identical concentration of SO₄²⁻ (23.68 mg/L–86.68 mg/L) and distribution range of ³⁴S-SO₄ (9.9‰–15.57‰) in groundwater. It can be concluded that SO₄²⁻ in the sedimentation lake is mainly supplied by groundwater.

Pore water and karst water in the studied area have the similar characteristics, taking form of low SO₄²⁻ concentration (23.95 mg/L–96.69 mg/L) and a high δ³⁴S-SO₄ value (8.5‰–15.57‰), but the value was much lower than that in evaporite rock (20‰–35‰). In the process of coal mining, the pyrite oxidation will form a large amount of mine water low in ³⁴S-SO₄ value, which is then discharged to the surface, constantly mixing with surface water high in ³⁴S-SO₄ value. Through the leaching and filtering effect of soil, the surface water is ceaselessly transformed into pore water, which then recharges the karst water through the fractions, fissures and holes at the bottom interface, resulting in the shared chemical characteristics of pore water and karst water. In accord with the condition of reduction, if oxidizable organic matter exists, bacteria can reduce SO₄²⁻ in groundwater to H₂S (Equation 7), causing large isotopic fractionation, bringing about the enrichment of ³⁴S-SO₄ and the decrease of SO₄²⁻ concentration (Huang et al. 2023). As Fig. 8a exhibits, the significant negative correlation between ³⁴S-SO₄ and SO₄²⁻ in karst water indicates that there may exist BSR fractionation process of ³⁴S-SO₄ in undamaged deep karst water.

(9)$ {\rm{SO_{4}^{2-}+2C+2H_{2}O=H_{2}S+2HCO_{{3}}^-}} $

Based on the relationship between ³⁴S-SO₄ value and 1/SO₄²⁻ in the water bodies of the studied area, this study divided the SO₄²⁻ source in the affected groundwater into three regions. As is shown in Fig. 7b, zone A showed high ³⁴S-SO₄ value and high SO₄²⁻ concentration which is mainly composed of ZT and NT sampling points of sedimentation lake, XF sampling points of Happiness Lake and some groundwater sampling points, mainly demonstrating characteristics of mine water. Region B features high ³⁴S-SO₄ value and low SO₄²⁻ concentration, which mainly consisted of river water sampling point HS, South-to-North Water Diversion point YL, pore water sampling point ZK16 and karst water sampling point ZK17, mainly representing the influence of SO₄²⁻ in atmospheric precipitation on the study area. Region C is characterized by low ³⁴S-SO₄ value and high SO₄²⁻ concentration, which is made up of surface water sampling points and some karst water sampling points, majorly manifesting the influence of human activities and sulfide mineral oxidation on SO₄²⁻ in the water bodies.

4.5 Sulfur cycle processes

The mining activities severely damages the original stratigraphic structure, making the sulfides such as pyrite in the original closed environment fully expose to groundwater and air, thus accelerates the oxidation and weathering process of sulfide minerals in coal seam (Mao et al. 2023) and increases the SO₄²⁻ in groundwater. Intensive mining has completely changed the circulation process of groundwater in the studied area, forming a huge falling funnel-like landscape centered on the mine in the mining area and its surrounding subsidence area (Fig. 8). Karst water's discharge channel has been transformed from spring drainage to artificial mining drainage (Su et al. 2023). Large amounts of sulfate formed by the sulfides oxidation was discharged to the surface along with acidic mine water, mixing with the sulfate produced by human life and production in the sedimentation lake. The mixture then recharged groundwater aquifers rainfall infiltrated the ground and abandoned mines, in turn accelerating the dissolution process of carbonate and increasing TDS in groundwater. Meanwhile, the BSR process of sulfate still exists in the karst water system out of reach of mining impacts, which causes the accumulation of ³⁴S-SO₄ and the continuous decrease of SO₄²⁻ in karst water.

5 Conclusions

(1) In the studied area, the water bodies in the subsidence zone of the mining area are weakly alkaline, and mainly recharged by atmospheric rainfall. The hydrochemical types of surface water in the area are dominated by HCO₃-Ca·Mg and HCO₃·SO₄-Ca·Mg, while the hydrochemical types of groundwater are primarily HCO₃-Ca·Mg, which are regulated by the oxidation and dissolution of carbonate rocks, silicate rocks and sulfides.

(2) Coal mining has severely damaged the original groundwater environment and changed the original hydrogeochemical process in the mining area and its surrounding subsidence area. Mine drainage accelerated the original water circulation around the mining area, resulting in the transformation of the groundwater environment from a reducing one to an oxidizing one. Sulfide minerals oxidized and dissolved on a large scale in the mining area produced large amounts of acidic mine water, accelerating the dissolution of carbonate rock (dolomite) and silicate rock. The hydrochemical processes in the studied area was also affected by positive and reversed cation exchange.

(3) The SO₄²⁻ in the groundwater of the sedimentation area of the mining area mainly was mostly generated by the sulfide minerals oxidation and dissolution and the wastewater formed by human activities. Sulfate formed by oxidizing and dissolving sulfide minerals was mixed with sulfate produced by production activities around the mining area, which contributed to the constant increase of TDS content in groundwater by continuously recharging the underground aquifer. In the deep, undisturbed karst water system of the subsidence area, there still existed BSR behavior, which led to the accumulation of ³⁴S- SO₄ and the decrease of SO₄²⁻ concentration.

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