Hydrogeochemical vertical zonation and evolution model of the Kongjiagou coalmine in Sichuan, China Open Access

In recent years, an increased depth of coal mining in China has resulted in an annual decline in groundwater levels and an increased risk of water-inrush disasters (Huang & Han 2016). Coal mining may affect the spatial and temporal evolution of groundwater, mainly due to the drainage of the aquifer, which reduces the available groundwater resources and alleviates the risk of water inrush (Wu et al. 2014), resulting an a cone of depression, changing the original groundwater flow system and even making a vertical flow system (Xu et al. 2018), and influenced hydrogeochemistry evolution. Qiao reported that the gradual increase in Na⁺ and decrease in Ca²⁺ concentration were the actions of cation exchange (Qiao et al. 2019). Li found that mine activities increased water solute concentration and changed hydrochemical types (Li et al. 2018). Mayo identified mine drainage chemical reaction pathways including pyrite oxidation, dissolution–precipitation, and ion exchange (Mayo et al. 2000).The oxidizing surface-water infiltration into an anoxic coalmine groundwater system led to a change in the original chemical condition and composition, and some led to the formation of acid mine drainage (AMD) (Akcil & Koldas 2006). So the groundwater of coalmines may be influenced by AMD, and generally characterized by a low pH, high total dissolved solids (TDS), and high hardness, as well as numerous heavy metal elements in some areas (Simate & Ndlovu 2014). Therefore, before environmental protection or treatment uptake, it is essential to investigate coalmine hydrogeochemistry characteristics, and study how vertical zonation formation and evolution occurs.

The groundwater system in coalmine areas is characterized by the specific geology environment, aquifer type, and groundwater circulation. Sandstone aquifers are commonly assumed to be porous media (Medici et al. 2016). But in T₃xj formation, fracture distribution is highly heterogeneous. There is bedding-plane fracture, structure fracture, tensile fracture, and mining fracture. Such aquifers provided preferential pathways for groundwater. The coal mining activity changed the groundwater media and circle (Prathap & Chakraborty 2019). Vertically, fractures are generally developed towards the goaf (Wang et al. 2017).

Rock weathering, mineral dissolution, cation-exchange, and anthropogenic activities were the primary processes governing the hydrogeochemical characteristics of the aquifer (Shishaye et al. 2020). The redox reaction of pyrite is a crucial reaction for the chemistry of coalmine (Acharya & Kharel 2020). The ion ratios in groundwater can represent the different hydro-geochemistry process, if the end members of different process can be distinguished, it will be simple to identified the main water–rock reaction (Stumm & Morgan 2012; Fetter 2018). Normalization prevent different units or abundance makes peaks or valleys in a distribution and to ensure that data will be the same graphically (Baird et al. 2017). N-score, Kaiser, Min–Max was the most used method in hydrogeochemistry characteristics assessment and quality evaluation (Tay et al. 2017; Karami et al. 2018). Geochemical heterogeneities in subsurface formations is often a leading factor controlling the chemistry of groundwater (Battistel et al. 2019). Previous studies have shown that groundwater chemistry can reflect water–rock interaction and often identified by PHREEQC (Christofi et al. 2020).

The main aspects studying the zonation characteristics of groundwater include the hydrogeological setting, hydrochemical and isotopic characteristics, ionic components, redox conditions or redox-sensitive elements (Fe²⁺, ⁠, /Cl⁻), mathematical statistics, and chemical simulation (Chidambaram et al. 2013; Chen et al. 2014; Jia et al. 2014; Lghoul et al. 2014; Peng et al. 2015). A hydrochemical simulation programs, such as PHREEQC, Geochemist's Workbench, MINTEQ were widely used in mineral phase transfer calculations and hydrochemical evolution mode analysis (e.g., for pore water, fractured water, and geothermal water) (Huan et al. 2011; Zhou et al. 2012; Jia et al. 2016), because they can solve element migration in groundwater and water–rock interactions (Teng et al. 2010). Some scholars have performed chemical simulations of deep groundwater circulation characteristics (Kebede et al. 2005).

Regionally, the chemical characteristics of groundwater exhibit both horizontal and vertical changes due to aquifer lithology, climate, and the seepage path length (Fetter 2018). Schoeller divided groundwater chemical zones into geological zones, vertical zones, and climatic zones. Geological zones correspond to the different mineral compositions of the aquifer media (Schoeller 1960). Gascoyne found that the geochemical evolution of groundwater presented similar trends with increased depth and with variations in the hydraulic gradient regardless of rock type (Gascoyne & Kamineni 1994). However, due to the complex groundwater system, sampling difficulty, and lack of significant chemical component changes, few studies have investigated the vertical zoning characteristics and evolution model of groundwater in coalmining areas.

Taking the Kongjiagou coalmine in Dazhu County as a research object, this study investigated the local vertical zoning characteristics and evolution model of the groundwater chemistry by determination of the hydrochemical characteristics, ion sources, and ratios of main ions, and the use of hydrochemical inverse simulation. The determination of the corresponding hydrogeochemical reactions in the coalmine vertical zone, especially the case in complex systems involving fractured-rock groundwater flow, plays a vital role in water resource management and protection (Tweed et al. 2005).

The Kongjiagou coalmine (107°21′48.29″E, 30°44′25.89″N) is located in Dazhu County, Sichuan Province, China. The mining area is ∼6,376 m long and 535 m wide and covers an area of 3.411 km². The inclined shaft-footrill combined exploitation method is used in the mine, with a mining elevation from 0 m to 350 m. The coal seams K₁₁ and K₉ in T₃xj⁷ and K₆ in T₃xj⁵ are mined, and the total sulfur content (S_{t, d}) of the coal dry base is approximately 0.56%.

The study area is within the subtropical monsoon climate zone and has an annual average temperature of 18 °C and annual average precipitation of 1,245.7 mm, whereby >60% of precipitation falls between May and September. There are four perennial flowing gullies in the study area: Xiaojiagou, Laohegou, Luanshijiaogou, and Kongjiagou. These are developed in the east–west direction from the south to the north, and all are tributaries of the Jialing River.

The study area is located in the monoclinal structure of the eastern wing in the northern part of the Tongluo Mountain (Zhongshan) anticline. The strata related to coal mining and water inrush are within the Lower Jurassic Zhenzhuchong Formation (J₁zh), which is mainly composed of mudstone and silty mudstone, with layers of sandstone and coal-bearing lines in the lower part. The Upper Triassic Xujiahe Formation (T₃xj) is characterized by sandstone, with layers of shale and coal seams. The 1st, 3rd, 5th, and 7th members of T₃xj are coal-bearing, and dominated by mudstone, interbed with thin layers of sandstone. while the 2nd, 4th, and 6th members are predominantly white and gray sandstone. The 6th and 7th members of T₃xj has mudstone, siderite, and coal enclosure, or lot of black siliceous rock debris. The Xujiahe sandstone main diagenetic processes were mechanical compaction, quartz cementation, carbonate cementation, authigenic clay mineralization, and dissolution (Liu et al. 2018). Calcareous cementation was the main rock-forming mineral of clastic rock. and the calcareous cementation was mainly consisting of carbonates, anhydrite, and fluorite (Tan et al. 2016). So, minerals in the surround rocks are dominated by calcite, dolomite, quartz, and kaolinite, and illite, followed by gypsum, anhydrite, siderite, and sulfide. The rock inclination direction is 101–116°, and the dip angle is 31–57°, with a steep slope zone in the north and a gentle slope zone in the south (Figures 1 and 2).

The primary groundwater type in the study area is fractured water. The sandstone in the second, fourth, and sixth members of the Xujiahe and Zhenzhuchong formations comprise the principal aquifers; they were recognized as possessing possibly two porosities: matrix and fracture porosity. While the shale and mudstone in the first, third, fifth, and seventh members of the Xujiahe and Zhenzhuchong formations act as aquicludes. the regional groundwater movement was controlled by the tectonic feature, from west to east, shallow to depth.

According to the local empirical formula, the maximum height of the water flowing fractured zone in K₁₁/K₉ coal seams is 83.3 m and in K₆ is 67.2 m (Figure 3). Under the influence of the goaf and water flowing fractured zone, an aquifer with a uniform hydraulic relation is formed in the fifth, sixth, and seventh members of the Xujiahe and Zhenzhuchong formations. Under the combined action of gravity and mine drainage activities, the groundwater moves along the monoclinal strata to depth following rainfall recharge and forms rapid infiltration channels at water-conducting cracks or a semi-closed water storage within the goaf. Where the aquifer is exposed at the roadway, groundwater is discharged to the surface through drainage facilities. The normal water inflow of the mine is approximately 230.19 m³/h, with a maximum of 1,080.24 m³/h.

A total of 24 representative coalmine groundwater samples from different depths was collected on May 1–2, 2020. The sample locations are shown in Figures 1 and 2. Groundwater sampling and storing were carried as per the methods of the International Organization for Standardization (ISO). All samples were collected in clean polyethylene bottles, which were pre-rinsed two to three times with sample water to avoid the influence of impurities. Samples were then transported in a shaded box and stored at 4 °C before chemical analyses. Physical parameters such as color, taste, and odor were determined and recorded in the field. The pH of each sample was measured in situ using a portable AP-700 multi-parameter meter (Aquaread), which was calibrated before sampling. The pH was recorded within 5 min of obtaining a stable reading. The sampling position was measured by compass and measuring tape according to the downhole traverse survey point.

The hydrochemical data and summarized statistical data for the 24 groups of water samples are shown in Table 1. The pH ranged from 6.70 to 7.90 (mean of 7.40). The TDS ranged from 235.35 to 1,718.09 mg/L (mean of 717.77 mg/L), and there was a gradually increasing trend with increased sampling depth. The TH (i.e., CaCO₃) ranged from 100.09 to 1,131.02 mg/L (mean of 369.84 mg/L). For cations, the ρ(Na⁺ + K⁺) was 0.25–371.50 mg/L (mean of 106.00 mg/L), ρ(Ca²⁺) was 20.04–314.63 mg/L (mean of 100.63 mg/L), and ρ(Mg²⁺) was 9.73–83.90 mg/L (mean of 28.83 mg/L). For anions, the ρ(Cl⁻) was 0.35–16.31 mg/L (mean of 1.61 mg/L), ρ(⁠⁠) was 102.80–1,048.00 mg/L (mean of 347.93 mg/L), and ρ(⁠⁠) was 85.42–579.66 mg/L (mean of 265.51 mg/L). The relative abundance of the main cations and anions in inrush water were ranked as: Na⁺ + K⁺ > Ca²⁺ > Mg²⁺ and > > Cl⁻. The groundwaters were classified into eight types: (i) Ca + Mg-SO₄ + HCO₃, 10 samples, (ii) Na-SO₄ + HCO₃, four samples, (iii) Ca + Mg-SO₄, three samples, (iv) Na + Ca-SO₄ + HCO₃, two samples, (v) Na-HCO₃ + SO₄, two samples, (vi) Ca + Na-SO₄ + HCO₃, (vii) Ca + Na- HCO₃ + SO₄, and (viii) Ca + Mg + Na- SO₄ + HCO₃, And the dominant types are Ca + Mg-SO₄ + HCO₃ water.

The Durov diagram is an improvement of the Piper diagram, whereby abnormal responses to pH and TDS are more obvious on the basis of reflecting the internal relationship between the principal components of groundwater. The 12 groups of water samples fell into two main categories in the Durov diagram (Figure 4). The cations tended to be dominated by Ca²⁺, and the anions tended to be dominated by ⁠. The range of TDS was large, although values were mainly <1,000 mg/L.

The partial pressure of CO₂ was between 10^−3.22 and 10^−1.19 atm (mean of 10^−2.21 atm), which is higher than the partial pressure of CO₂ in the atmosphere (10^−3.5 atm), thus indicating that the groundwater system in the mine area is an open system. The results showed that the CO₂ from the reaction of water and carbonate could be effectively supplemented, which is beneficial to the water–rock interactions in the mining area.

The milligram equivalent percentages of all ions and most ions in the water-burst area of the study area were >25% and reflect the main characteristic ions in the groundwater of the coalmine. Therefore, the sources of these ions could be determined by the Gibbs diagram and the chemical reaction equation.

A Gibbs diagram was plotted to analyze the formation of groundwater chemical components and ion sources (Luan et al. 2017). As shown in Figure 5, ρ(Na⁺ + K⁺)/ρ(Na⁺ + K⁺ + Ca²⁺) was >0.6, i.e., beyond the boundary. This is because the calculated mass concentration of Na⁺ + K⁺ in the hydrochemical analysis included other undetermined cation components. ρ(Cl⁻)/ρ(Cl⁻ + ⁠) was 0.001–0.062, and the TDS concentration was relatively high. This indicates that the water–rock interactions served as the main influencing factor that controlled the chemical characteristics of the sampled mine-water bursts. The TDS content of some inrush water (Figure 5) exceeded the boundary, which was plotted based on surface water composition in the Gibbs diagram (Sun et al. 2014). The groundwater was greatly affected by the seepage path's length and the mineral composition along the pathway.

Generally, groundwater constituents in the samples were derived from carbonate or silicate weathering, as determined from γ (Ca²⁺ + Mg²⁺)/γ(⁠⁠). The deviation towards the Ca²⁺ + Mg²⁺ axis indicates that silicate weathering and dissolution are dominated (Purushothaman et al. 2014). Figure 6(b) shows that most samples from mine-water bursts were close to the Ca²⁺ + Mg²⁺ axis, thus indicating that carbonate dissolution played a controlling role in groundwater formation, with silicate dissolution as the secondary factor. In the scatter plot of γ(Ca²⁺ + Mg²⁺)/γ(⁠ + ⁠) (Figure 6(c)), only some points are near the 1:1 line, which further indicates that silicate dissolution was the secondary source (Kamtchueng et al. 2014).

As indicated by Equations (12) and (13), if carbonate weathering was by carbonic and sulfuric acids only, the γ(Ca²⁺ + Mg²⁺)/γ(⁠⁠) should be exactly 1 and 2, respectively. Whereas if the γ(⁠⁠)/γ(⁠⁠) ratio of carbonate weathering is by carbonic and sulfuric acids only, it should be 0 and 1, respectively. Figure 6(d) shows that only a few samples were distributed between two end members, and more samples were around the sulfuric acid end member, indicating that sulfuric acid participated more in the weathering processes. In Figure 6(d), the gypsum dissolution line was referenced previously (Liu & Han 2020). Some samples were close to the gypsum dissolution line, so the contribution of gypsum dissolution cannot be ignored.

The silicate weathering as demonstrated in Equations (9)–(11) can produce ⁠, which will decrease the γ(Ca²⁺ + Mg²⁺)/γ(⁠⁠) value and the points will move downward. So, some samples γ(Ca²⁺ + Mg²⁺)/γ(⁠⁠) <1 occurred, which represented that silicate weathering was involved.

By taking each sampling point's burial depth as the foundation for vertical zoning, the shortest seepage distance of each water-inrush point could be calculated in the monoclinal stratum in combination with the stratigraphic dip (i.e., the straight-line distance from the surface recharge area to the water-inrush point).

We divided the mine inrush water into three zones for depth. From the ground-surface to the standard deviation the value of all sample’ depths was zone I, deeper than the mean of all sample depths was zone III, the middle was zone II. The mean values of six indicators (pH, TDS, Ca²⁺, Mg²⁺, ⁠, and ⁠) in each zone were calculated. The index of the mean values of each indicator was calculated using the minimum/maximum normalization method (Table 3 and Figure 7).

As can be seen from Table 3 and Figure 7, the index values in zone I were all <0.15, and the relative size of each index was ranked as: Mg²⁺ > pH > Cg²⁺ > > > TDS. The index values in zone II ranged from 0.18 to 0.38, and the relative size of each index was ranked as: pH > Cg²⁺ > Mg²⁺ > TDS = > ⁠. The index values in zone III ranged from 0.28 to 0.81, and the relative size of each index was ranked as: pH > > TDS > > Cg²⁺ > Mg²⁺. Thus, the TDS, and in the water-burst samples varied significantly with burial depth.

The six indicators showed overall increasing trends, which could be subdivided into three main categories according to the change between the zones. pH and increased slowly at first and then rapidly; Ca²⁺ and Mg²⁺ quickly increased and then decreased slowly; TDS and quickly increased and then slowed. In the process of infiltration, oxygen-rich groundwater first reacted with pyrite to generate H⁺ and ⁠, and the resultant low pH water immediately reacted with carbonate in the water-bearing medium, which could subsequently rapidly increase the concentrations of TDS, ⁠, Ca²⁺, and Mg²⁺. In contrast, the pH and did not decrease, which indicated that the amount of H⁺ ions was still low. In the process of continuous groundwater seepage, the oxygen content and H⁺ ions gradually decreased. The redox effect weakened to zero, thereby leading to (i) a rapid rise in pH and ⁠, (ii) a slow rise in TDS and ⁠, (iii) the continuous dissolution of carbonate to saturation (versus the formation of precipitate), and (iv) slow declines in Ca²⁺ and Mg²⁺ concentrations.

The milligram equivalent ratio analysis of the main ions (Table 4) showed that the γSO₄/γHCO₃ ratio in zone II was >2 and that of the γCa/γSO₄ ratio was <1, thus indicating that originated from sulfide oxidation (Li et al. 2000). It can also be seen that from zone I to zone III (i.e., with increased depth), the ratios of main ions exhibited three main characteristics as follows. (1) The ratios of γCa/γSO₄, γMg/γSO₄, and γ(Ca + Mg)/γSO₄ showed gradually decreasing trends, which indicated a corresponding gradual increase in ⁠. It could be inferred that the redox reaction of pyrite continued simultaneously. (2) The γSO₄/γ(SO₄ + Cl) ratio was unchanged, indicating that the vertical change of the concentration in groundwater from zone I to zone III was always much higher than that of Cl⁻. (3) The trend of first increasing and then decreasing could be divided into two sub-classes as follows. (a) The γCa/γHCO₃ and γ(Ca + Mg)/γHCO₃ ratios in zone III were less than those in the zone I. This suggests that low pH groundwater could promote carbonate dissolution, which was characterized by increased Ca²⁺ and Mg²⁺ in zone II. The dissolution of carbonate in zone II could produce CO₂ that was soluble in water, which would have resulted in an increased concentration in zone III. (b) The ratios of γSO₄/γHCO₃ and γSO₄/γ(HCO₃ + Cl) in zone III were greater than those in zone I, thus indicating that the total concentration in inrush water increased relative to the concentrations of and Cl⁻.

Both CAI positive values indicated that reverse ion exchange occurred between Na⁺ or K⁺ in the groundwater with Ca²⁺ or Mg²⁺ in the rocks. Similarly, both values were negative when there was an exchange between Ca²⁺ or Mg²⁺ in the water and Na⁺ or K⁺ in the rocks (Pisciotta et al. 2018). If CAI is equal to zero, it means there is no exchange between water and rocks (Koffi et al. 2017). While the absolute value of CAI largely illustrates the high degree of cation exchange (Wang et al. 2015).

We found all samples CAI-I and CAI-II were below zero, indicating that Na⁺ and K⁺ was released from the aquifer material through the process of cation exchange. The mean values of CAI-I in zones I, II and III were found to be −5.72, −128.18 and −292.41, the mean values of CAI-II in zones I, II, and III were −0.01, −0.10 and −0.50, respectively. This trend shows an increasingly stronger degree of cation exchange in zones I, II and III, which corresponds to shallow, medium and deep groundwater buried. The results of the Durov diagram also support the fact of ion exchange and the trend (Figure 4).

According to the hydrochemical data, PHREEQC was used to calculate the amount of groundwater transferred for deep mineral-phase seepage. The model was also used to characterize water–rock interactions’ strength to indicate the vertical evolution of groundwater in the studied mine. According to groundwater circulation characteristics in the study area and the sampling point locations, the following typical flow path was selected as the simulation path: S14 → S12 → S11 → S06 (from a shallow depth to the deep part of the mine) (Figure 1).

The depths at which hydrochemical samples were taken were all >30 m; hence, the influence of evaporation/concentration was not considered. The strata in the mining area mainly include sandstone, mudstone, and shale. The main minerals include quartz, feldspar, mica, and rock debris, and the cementing substances are mainly calcium, clay impurities, and iron. When the Na⁺ concentration was high (>3.5 mEq/L), the effect of cation exchange on the TH was particularly obvious, especially the cation exchange reaction between Na⁺ and exchangeable Ca²⁺ and Mg⁺ in clay minerals (Huan et al. 2011).

According to the hydrochemical characteristics, main mineral composition, and groundwater circulation conditions, six aspects were selected for the simulation paths as follows: four mineral phases (calcite, dolomite, gypsum, and halite), CO₂, and cation exchange (i.e., NaX and CaX₂). According to the hydrochemical determination results, K⁺ + Na⁺, Ca²⁺, Mg²⁺, ⁠, ⁠, and Cl⁻, were determined as the constraint variables. The calculation results of the mass equilibrium reaction model are presented in Table 5.

The overall hydrochemical evolution from a shallow depth (zone I) to the deep part of the mine (zone III) was represented by S14 → S06, whereby one model was used for the calculation. The most massive transfer in the simulation occurred for NaX and CaX₂, suggesting that cation exchange plays a significant role in groundwater chemistry at the studied mine. Calcite and gypsum were dissolved along the simulated groundwater path, indicating that mineral dissolution is also an essential factor in the groundwater chemistry. A large amount of dissolved CO₂ (g) indicated that the groundwater system in the study area is an open system, consistent with the partial pressure of CO₂ that was calculated in Table 1. Dolomite and halite were secondary factors affecting the groundwater hydrochemistry.

The evolutionary process of zone I to zone II was represented by S14 → S12 and included two calculation models, which were both reasonable. The difference between the two models was a small amount of halite dissolution. The dissolution of dolomite, gypsum, and halite occurred along the flow path. A large amount of gypsum dissolution could have caused the Ca²⁺ ions to increase significantly, whereas dolomite dissolution could only have slightly increased the Mg²⁺ ions. The amount of calcite precipitation was 1.63 mmol/L, which hardly changed the Ca²⁺ content; hence, the cation exchange was weak.

The path S11 → S06 represents the continuous change in depth within zone III and included one model for the calculation. Cation exchange was significantly enhanced, whereby Na⁺ increased as Ca²⁺ was removed from groundwater, thus indicating that the groundwater circulation in the deep mining area was weakened (Ge et al. 2000). The results showed that CO₂ dissolved in groundwater at a concentration of 3.29 mmol/L. Calcite continued to dissolve, and gypsum dissolved to a concentration of 1.52 mmol/L.

The ion index changes, ratios of main ions, and mineral phase transfer in the groundwater from a shallow to the deep part of a mine are important indicators of the vertical geochemical evolution of groundwater in coal mining areas.

In the ideal model, rainfall (or surface water) infiltration and recharge of the groundwater in the shallow area (zone I), will result in dissolution of the aquifer medium to form a Ca-HCO₃ water type with a low degree of mineralization. At mid-depths (zone II), weak weathering, and the pyrite content due to coal mining is high and not oxidized. The artificial groundwater circulation characteristics that form after mining increase the groundwater oxygen content, which can promote pyrite's oxidation; hence, a large amount of and H⁺ ions are formed and produce a Ca + Mg-SO₄ + HCO₃ water type with a low pH and high salinity. Due to the decreased dissolved oxygen in the groundwater in the deep area (zone III), the redox of pyrite is weakened, and acidic water accelerates the dissolution of minerals; thus, a large amount of Ca²⁺, Mg²⁺, and ions is generated. Also, the weakening of the groundwater circulation leads to enhanced cation exchange, and the final formation of a neutral pH, high-salinity Ca + Mg-HCO₃ + SO₄(or Na-HCO₃ + SO₄) type water. The above model is summarized in Table 6 and Figure 9.

The drainage of groundwater in the process of coalmine production can result in significant changes to the groundwater circulation. After mining, the groundwater in the mining area is replenished by rainfall and surface water. Groundwater discharge occurs at the roadway where primary fissures, goaf, and mined-out water flowing fractures are found. This water returns to the surface through the mine drainage system, thus forming the groundwater circulation system via artificial interference. A dramatic decline in the groundwater level can be seen in the coal seam (Figure 2). Such variations in the groundwater circulation can change a coal mining area from a reducing environment (before mining) to an oxidizing environment (after mining). Infiltrated water can contain enough oxygen to accelerate a series of reactions, including pyrite oxidation and mineral dissolution. Therefore, the degrees of dissolution and redox depend on the length of the seepage path and pyrite and groundwater's oxygen content. Accordingly, under the combined influence of these factors, groundwater and vertical zoning's basic chemical characteristics are formed.

Water–rock interactions are the main factor influencing the hydrogeochemical characteristics. CAI indicated that Ca²⁺ and Mg²⁺ in water were exchange by Na⁺ and K⁺ in aquifer material. Sulfate originating from gypsum dissolution and pyrite oxidation. Sulfuric acid participates more in the weathering processes, and involved carbonic acid, gypsum dissolution, silicate weathering.

Based on buried depth, three zones were classified according to mean and standard deviation value. It generally displays an enhanced normalization index, CAI, TDS, and pH with increasing depth, while ion ratio exhibited three complexes tendencies. According to PHREEQC-based inverse modeling, gypsum, halite, NaX, calcite and CO₂ were dissolution, while dolomite and CaX₂ were precipitation, from zones I to III. Water type and indicator variation were obvious controlled by main hydrogeochemical reactions.

The hydrochemical vertical zonation and evolution was due to drainage, which makes a vertical flow system, and leads to more oxygenic water infiltration into the aquifer, and changed the groundwater to oxidizing conditions, and dissolution occurred in zone I (shallow); after that the reaction was accelerated and complicated, redox appeared in zone II (middle), re-dissolution and cation exchange came out in zone III (deep).

We hereby express our deep gratitude to the 3D Technology Institute and Coalmine Safety Institute of Sichuan Academy of Safety Science and Technology, Sichuan Dazhou New Energy Development Co., Ltd for their enthusiastic help during the fieldwork at the Kongjiagou coalmine. We would also like to thank the reviewers and editors for their considerate work in preparing this paper for publication.

This research was funded by the 2018 planning project of the Sichuan Provincial Department of Science and Technology (2018SZ0290, 2018JY0425).

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