Study Area

Wolonghu coal Mine is located in the Huaibei coalfield, in the north of Anhui Province, China (Fig. 1). The elevation of the mine ranges from 20 to 50 m, with a length of 8-9 km from north to south and a width of 3.5-4 km from east to west, and a total area of 28.9 km². Wolonghu coal mine started to production in June 2008 and has ceased mining in July 2018 in response to the national capacity withdrawal plan. I, and it is currently the only unclosed abandoned mine in the Huaibei coalfield. The study area belongs to the southern part of the North China Plate, and the surface is generally covered by loose sediments of the Quaternary System, and is tectonically at the intersection of the Fengxian-Guoyang Fault and the Subei Fault. Except for the Silurian and Devonian, strata in this area have been developed in different degrees from the Neoproterozoic to the Cenozoic. According to the characteristics of surrounding rocks and void types, the aquifers can be divided from shallow to deep into the Quaternary loose layer pore aquifer (LA), Permian coal-bearing sandstone fissure aquifer (CA), and Carboniferous Taiyuan limestone karst fissure aquifer (TA). Among them, the composition of the surrounding rocks in the CA is dominated by medium and fine- grained sandstone, followed by mudstone, siltstone, and magmatic rocks.

Sample Collection and Analysis

A total of five groundwater samples were collected from the roadway in the coal-bearing sandstone aquifer at Wolonghu coal mine, and each sample was filled with two pre-cleaned 1L polyethylene bottles, one of which was acidified to pH<2 to prevent precipitation of trace elements or adsorption by the container. The pH values of water samples were tested on site using a portable water quality detector, and laboratory tests included conventional water chemical ions (Na⁺+K⁺, Ca²⁺, Mg²⁺, Cl^-, SO₄^2-, HCO₃^-and CO₃^2-), isotopes (D, ¹⁸O, ¹⁸O-SO₄^2- and ³⁴S-SO₄^2-) and REEs (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu). HCO₃^- and CO₃^2- concentrations were tested by acid-base titration (the concentration of hydrochloric acid used was 0.05 mol/L), and the remaining conventional ion concentrations were tested using inductively coupled plasma spectrometry (ICP-OES, Thermo Fisher Scientific). D and ¹⁸O were tested using stable gas isotope mass spectrometry (Nu perspective), and ¹⁸O-SO₄^2-and ³⁴S-SO₄^2- were tested by Thermo Fisher 253 Plus, USA. Among them, D, ¹⁸O-SO₄^2-and ³⁴S-SO₄^2- results were expressed as relative Vienna Standard Average Seawater (δ, V-SMOW) thousandths, and ³⁴S-SO₄^2- was expressed as relative International Sulfur Isotope Standard (δ, V-CDT) thousandths. REEs were tested by inductively coupled plasma mass spectrometry (ICPMS), instrument model Jena Plasma Quant MS, Germany. In addition, seven sandstone water samples from the preclosure mine were collected from the mine’s historical ledger.

Major Ion Concentrations

The concentrations of conventional chemical components in groundwater before and after the closure of the Wolonghu coal mine are shown in (Table 1). The pH and TDS in the water samples after mine closure ranged from 8.12~8.9 (mean 8.6) and 2276~3241.7 mg/L (mean 2610 mg/L), respectively, reflecting the weakly alkaline and highly mineralized environment of groundwater. Among the cations, K⁺+Na⁺ concentration dominated significantly with a mean value of 759.3 mg/L, followed by Ca²⁺ and Mg²⁺ with mean values of 103.3 mg/L and 42.9 mg/L, respectively. Among the anions, SO₄^2- had the highest content with a mean value of 1205.1 mg/L, HCO₃^-, and Cl^– followed by mean concentrations of 491.2 mg/L and 231.7 mg/L, respectively, and CO₃^2- had the lowest mean concentration of 21.9 mg/L. Overall, the mean concentrations of conventional ions in the abandoned mine water samples were ranked from the highest to the lowest: SO₄^2->K⁺+Na⁺>HCO₃^-->Cl^->Ca²⁺ >Mg²⁺>CO₃^2-. Compared with the pre-mine closure, the concentrations of conventional chemical components in the groundwater after closure changed significantly, which was mainly characterized by the significant increase in Ca²⁺, Mg²⁺, SO₄^2-, and TDS concentrations accompanied by the significant decrease in HCO₃^- concentration.

Fig. 1.

Location of the Wolonghu coal mine.

To further explore the evolution of water chemistry in abandoned mines, a Piper trilinear diagram was drawn (Fig. 2). It can be seen that all samples are located in the “D” zone for cations, indicating that Na⁺ is the dominant cation in groundwater before and after mine closure, but WLH1 and WLH2 have a tendency to shift to the “B” zone. In the anion trilinear diagram, the water samples were located in zone “F” before closure, but in zone “E” after closure, and the samples before and after closure were located in the diamond-shaped zones “IV” and “III”, respectively. The samples before and after closure were located in the diamond-shaped areas “IV” and “III”, indicating that the transition trend from HCO₃^- dominated to SO₄^2- dominated anion formation in groundwater. It was shown that the dominant anion in water shifts from HCO₃^- to SO₄^2- and Cl^- from the recharge area to the discharge area [18]. In this study, the groundwater water chemistry type transitioned from HCO₃-Na type to SO₄-Na type before and after mine closure. A large amount of pumping and draining was required at the working face before mine mining, and groundwater recharge in the aquifer was frequently renewed with a short retention time, while the groundwater runoff environment became worse after mine closure and water-rock interaction was more adequate, which may be the main reason for the formation of highly mineralized groundwater dominated by SO₄^2- in the aquifer.

Isotopic Composition Characteristics

δD and δ¹⁸O

Hydrogen and oxygen stable isotopes (D and ¹⁸O) are relatively stable in groundwater when not subjected to high temperature water-rock interaction and strong evaporation conditions, so their direct relationship is often used in studies related to the water cycle patterns and evolution of groundwater [17, 18]. In this study, δD varied from -74.15~-71.07‰ with a mean value of -72.79‰, while δ¹⁸O varied from -9.99~-9.42‰ with a mean value of -9.74‰ (Table 1.). Comparing the fitted lines of D and ¹⁸O in groundwater with the global Precipitation line (GMWL) and China’s atmospheric Precipitation line (NMWL) equations can better reveal the recharge and transport mechanisms of groundwater [19]. If the groundwater sample points are located near the atmospheric precipitation line, it indicates that the aquifer is directly recharged by rainfall. As can be seen from Fig. 3a, the δD and δ¹⁸O of the seven groups of groundwater samples have a good linear fit (R² = 0.97) [20], but significantly deviate from the GMWL and NWML, indicating that the sandstone water is less likely to be recharged directly by atmospheric rainfall.

It can be found that the δ¹⁸O content is significantly enriched relative to δD. In general, this characteristic may be caused by the following two situations:

Fig. 2.

Piper diagram of groundwater samples before and after closure of the Wolonghu coal mine.

Table 2.

Characterization of multiple isotope compositions in groundwater from abandoned mine (‰).

Sample	δ18D	δ18O	δ18O-SO42-	δ34S-SO42-
WLH1	-71.33	-9.70	13.03	33.25
WLH2	-71.07	-9.42	13.37	33.73
WLH3	-73.42	-9.73	14.79	30.67
WLH4	-73.98	-9.87	15.39	31.20
WLH5	-74.15	-9.99	15.00	29.77

(1) affected by infiltration evaporation, since the preferential escape of light isotopes (δD) in water under evaporation will lead to the relative enrichment of heavy isotopes (δ¹⁸O), but such a situation usually occurs in surface water and shallow groundwater, and in this study, the depth of the sample collection is located in the depth of less than 500 m, and there are multiple water traps in the upper part of the coal sandstone aquifer in this area, basically denying the contribution to the hydroxide stable isotope content; (2) Influenced by the recharge of other water sources, through the analysis of the literature in the previous study area (Fig. 3a), it is found that the limestone aquifer water samples in this area are located in the fitted line equation of the δ¹⁸O and δD of sandstone water of the present study (y = 5.98x – 14.53) nearby, indicating that the two aquifers in this mine may be hydraulically connected.

δ¹⁸O-SO₄^2-and δ³⁴S-SO₄^2-

Sulfate is a common inorganic component of groundwater, and its content is influenced by a combination of factors that may be anthropogenic (e.g., agricultural activities, domestic sewage, and discharge of sulfur-containing industrial effluents) and natural (mainly dissolution of evaporites and oxidation of sulfides in aquifers) [21]. Despite the complexity of the sources, physicochemical effects, except for the action of sulfate-reducing bacteria under anaerobic conditions, do not usually cause significant fractionation of sulfur isotopes in groundwater. Numerous studies have confirmed that different sources of sulfate have different ranges of sulfur and oxygen isotope end-members, and thus the relationship between δ¹⁸O-SO₄^2- and δ³⁴S-SO₄^2- is commonly used to indicate the source of sulfate in water bodies [22, 23]. In this study, ¹⁸O-SO₄^2- and δ³⁴S-SO₄^2- contents in groundwater samples from abandoned mines were relatively stable, with variations ranging from 13.03 to 15.39‰ and 29.76 to 33.73‰, respectively, and mean values of 14.32‰ and 31.72‰, respectively (Table 2.). As can be seen in Fig. 3b), all samples are located within the evaporite mineral dissolution region, implying that the dissolution of evaporite (e.g. gypsum) is the main source of high sulfate concentration in groundwater in the study area.

Rare Earth Element Content Characteristics and Partitioning Patterns

ΣREEs in groundwater before and after the closure of the Wolonghu coal mine areis shown in Table 3. The variation of ΣREEs concentration in groundwater after closure ranged from 0.153~0.36 μg/L, with a mean value of 0.251 μg/L, which was lower than the mean value of Wworld Rriver (0.754 μg/L), but significantly higher than that before closure (0.08 μg/L). Compared to other studies on REEs in coal-based sandstone water in the Huabei coalfield, the mean value of ΣREEs after closure of the Wolonghu mine coal was found to be higher than that of Renlou mine coal (0.1035 μg/L) [24] and Xutong mine coal (0.1785 μg/L) [25]. Generally, the ΣREEs content in water is mainly related to pH, complexation pattern, and dissolution-adsorption interaction. As can be seen in Fig. 4a), the fitting coefficient of ΣREEs with pH reached 0.87, showing a good negative correlation. In an alkaline environment, the inorganic complexation form of REEs is mainly the carbonate complexation state, so the relationship between ΣREEs and HCO₃^- is plotted in Fig. 4b). It can be seen that the ΣREEs content shows a significant decrease with the increase inof HCO^– content, and the two also show a negative correlation at the significance level of ρ<0.01. , this This phenomenon indicates that the complexation of carbonate in anunder alkaline environment is an important controlling factor of ΣREEs in groundwater in the study area. In addition, the adsorption/desorption of Fe-Mn oxides/hydroxides with REEs under oxidizing/reducing conditions can affect the decrease/increase of ΣREEs in water [26]. The correlation between ΣREEs and Fe+Mn content is not reflected in Fig. 4c), indicating that the adsorption/ desorption haves a small effect on the REEs content in groundwater in the study area.

Fig. 3.

Isotope composition relationships in groundwater from the abandoned mine. a) Relationship between δD and δ¹⁸O, b) Relationship between δ¹⁸O-SO₄ ^2- and δ³⁴S-SO₄ 2-.

The partitioning patterns of REEs in groundwater normalized by Post-Archean Australian Shales (PAAS) are shown in Fig. 5. It can be seen that the PAAS partitioning curves of REEs in different water samples differ somewhat, but generally show an upward trend to the right. The parameters (La/Sm)_N and (La/Yb)_N reflect the partitioning of REEs in groundwater, where (La/Sm)_N varies from 0.85 to 1.03 with a mean value of 0.98, indicating that the partitioning between LREEs in groundwater is not obvious, while (La/Yb)_N of all water samples is less than 1 (mean value of 0.71), indicating that the groundwater as a whole is lightly In anthe alkaline environment, the HREEs are more abundant than the LREEs. In anthe alkaline environment, the solubility of heavy rare earths is greaterlarger, and the preferential precipitation of LREEs relative to HREEs may be the main reason for the relative enrichment of HREEs. Compared with before mine closure, (La/Sm)_N in groundwater increased after closure, reflecting the gradual decrease of the differentiation between LREEs. (La/Yb)_N usually increases with weakening hydrodynamic conditions, and the significant increase of (La/Yb)_N in groundwater after the closure of Wolonghu mine indicates that the differentiation between light and heavy rare-earth elements is related to the shift of the runoff environment caused by mine mining and decommissioning.

Fig. 4.

Relationship between ΣREEs and pH a), HCO₃ ^- b) and Fe+Mn c) in groundwater from the abandoned mine.

Fig. 5.

Distribution patterns of PAAS-standardized rare earth element content in groundwater samples.

Rare Earth Element Anomalies and Influencing Factors

Ce anomaly (δCe) and Eu anomaly (δEu) are important features of REEs, because the hydrogeochemical behaviors of Ce and Eu are more sensitive compared with other REEs and are susceptible to redox environments during migration [28]. Among them, Ce anomalies are mainly related to pH and redox conditions. The variation of δCe in groundwater after the closure of Wolong Lake mine ranged from 0.91~0.99, with a mean value of 0.96, and all samples showed a slight negative anomaly of Ce. If Ce anomalies in groundwater were controlled by pH, both of them would show a better correlation, which was not found in Fig. 6a. In an oxidizing environment, trivalent Ce in water can be oxidized to tetravalent Ce (e.g. CeO₂ and Ca(OH)₄) resulting in a Ce deficit. Therefore, the negative Ce anomaly in groundwater in the study area can be considered an effect of the redox environment.

Compared with before closure, δEu in the groundwater of the mine after closure was significantly lower, with variations ranging from 0.84~1.21 and a mean value of 0.98. Among them, WLH3 and WLH4 showed positive Eu anomalies, while the rest of the samples were characterized by negative Eu anomalies. Negative anomalies of Eu in deep groundwater are more common, while positive anomalies are generally considered the exchange of Eu²⁺ with Sr²⁺ and the dissolution of Eu-rich minerals (e.g., plagioclase) under redox conditions. It was pointed out that Eu exists as Eu²⁺ under reducing conditions, and due to the similar chemical properties of Eu and Sr elements, if groundwater is enriched with large amounts of Sr²⁺, the exchange between the two will cause Sr²⁺ to precipitate while Eu²⁺ continues to be enriched in water, resulting in positive Eu anomalies. Fig. 6b) shows the relationship between δEu and Sr in groundwater, and it can be found that the correlation between the two is not significant, implying that the exchange interaction between Eu²⁺ and Sr²⁺ has no effect or a small effect on the positive Eu anomaly in groundwater in this area. In addition, the supersaturation of carbonate phase water and the weathering dissolution of silicate minerals are also important influencing factors for the positive Eu anomaly. As shown in Fig. 6c), there is a significant positive correlation between Na⁺ and HCO₃^- in the groundwater of the mine before and after closure, and this feature usually indicates the dissolution of feldspar-like minerals with the following chemical equation: 2Na(AlSi₃)O₈ + 2CO₂ + 3H₂O → Al₂Si₅O₅(OH)₄ + 2Na⁺ + 4SiO₂. Combined with the actual hydrogeological situation, the coal aquifer enclosing rocks are characterized by clastic rock types composed of feldspar-like minerals [29]; therefore, the positive Eu anomalies in samples WLH3 and WLH4 can be considered to be caused by the dissolution of high Eu minerals (feldspar-like minerals).

Fig. 6.

Relationships between δCe and pH a), δEu and Sr b) and Na⁺ and HCO₃ ^- c) in groundwater from the abandoned mine.

In general, the quality of groundwater in the aquifer after mine closure has changed significantly due to oxidation-reduction and hydrodynamic conditions, and in the subsequent use of water resources, attention should be focused on the health risks to human beings from SO₄^2--dominated highly mineralized groundwater, and further remediation work can be carried out in the future to address the quality of groundwater in abandoned mines.