Hydrogeochemistry mechanism contrasts between low and high sulfate in limestone aquifers at the massive coalfield in Anhui province, China

Mine groundwater is of worldwide concern as it severely deteriorates the quality of nearby water resource, as well as the safety of communities, soil and air in surrounding regions (Gao et al. 2018; Bordash et al. 2020). Under natural conditions, atmospheric precipitation, surface water and groundwater are in natural dynamic equilibrium. However, mining activities change the transformation relationships and natural flow field, resulting in groundwater pollution and forming a large amount of mine groundwater (Shokry & Hamad 2019). Among them, high sulfate groundwater is the most common type with sulfate concentration above 250 mg/l, widely distributed at a global scale, especially at mine areas of Australia, China, German and Scotland (Sibrell & Tucker 2012; Liu et al. 2019).

Numerous studies have been carried out to study the hydrogeochemical characteristics, distribution, pollution risk, water quality of mine groundwater (Galloux et al. 2015). The ions and isotopes tracking has been utilized to assess the source, migration, mechanism and transformation of pollution factor, as Sb, F⁻ and Pb (Khan et al. 2019; Hao et al. 2020). However, the hydrogeochemistry characteristics and mechanism of high sulfate mine groundwater remains unclear. High sulfate groundwater without treatment can cause equipment corrosion, soil acidification and crop wilting. Most research on high sulfate groundwater have focused on its removal, such as coagulant adsorbent, immersion ultra-filtration and packed bed anaerobic bioreactor (Wu et al. 2018; O'Niell & Nzengung 2019), but did not understand its formation mechanism. To identify the formation mechanism can promote its treatment. It is generally believed that high sulfate groundwater is mainly derived from two sources. One is weathering from the oxidized pyrite contained in coal seams during and after mining activities; the other is from dissolution of halite and gypsum in typical aquifers (Sun et al. 2020). Hence, elucidating the behavior and mechanism of sulfate in mine groundwater will enhance our comprehensive understanding of hydrogeochemical process and and provide theoretical support for mine wastewater treatment.

In this study, we investigated the difference in hydrogeochemistry characterization between high sulfate groundwater and low sulfate groundwater to study water environment for high sulfate mine groundwater. We further determined the mechanism leading to high sulfate concentrations in mine groundwater. Results of this study will inform treatment and recycling of mine groundwater, and help in securing safe water resources for local communities.

The deposit is located at the southern margin of the North China plate, influenced by Indosinian cycle and Yanshan cycle. The EW and NNE trending faults have arranged in a crisscross pattern, forming network fault block structures, and dividing into South and North structural areas (Figure 1(a)). Most of the area is covered by Cenozoic strata with a thickness of 280 m, and the main minable coal bearing strata are Permian, mainly consisted of sandstone and mudstone. The karst aquifers below the coal measure strata are the primary water inrush aquifers for mining, including Taiyuan formation aquifer (TA) and Ordovician aquifer (QA) (Figure 1(b)).

The TA is composed of twelve layers of limestone with much corrosion fractures, and has an average hydraulic conductivity of 0.98 m/d. Due to TA is close to coal seam, it could form huge water inrush pressure, and then pose a great threat to mining. The QA is characterized by pure brittle and super thick limestone, with hydraulic conductivity from 0.058 to 18.66 m/d. The Ordovician aquifer is in unconformable contact with the Taiyuan formation, with a total thickness of more than 500 m, super thick layered limestone, pure brittle, microcrystalline structure, karst fissure development, and permeability coefficient of 0.0058–18.66 m/d. Consideration far away from the coal seam, QA would not cause direct water damage to the mine. However, there are conducting structures (faults, collapse columns, etc.), water inrush disasters may occur (Gui et al. 2018).

A total of 17 groundwater samples from TA and 11 groundwater samples from QA were collected in Huaibei coalfield, as shown in Figure 1, during the July to August of 2015. Groundwater samples collected from surface drilling holes and underground boreholes. Before collection, sampling bottles were rinsed with distilled water for three times and moistened with water samples for three times. For each sample, three bottles of groundwater were obtained: a 100 ml bottle for hydrogen and oxygen isotope analysis, a 100 ml for cation and anion content analysis, and a 100 ml for pH and TDS analysis. Subsequently, the collection samples were immediately filtered with 0.45 μm filter paper, stored in ice-packed coolers and shipped to laboratory where stored at 2 °C.

The content of major cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) were determined by Ion chromatograph (ICS-600 USA), and major anions (Cl⁻, ⁠) were determined by Ion chromatograph (ICS-900, USA). The concentrations of carbonate and bicarbonate were determined by acid-base titration. The pH and total soluble solids (TDS) were obtained in the field by portable devices (ST20 and ST20T-B) from OHAUS (Shanghai, China). The isotope of δD and δ¹⁸O were measured by analyzer (LGR-LWIA-45EP, USA).

To ensure the accuracy of results, 15% of samples were kept as blind samples. Moreover, every analysis was carried out in triplicate. Each valid date was the average of three test results with relative standard deviation below 10%. Additionally, the analytical precision of measured ion contents were further verified by ionic balance error. The calculated ionic balance error of these samples were all below 5%. At last, 20% of samples were re-measured and the errors between the two results are less than 15%.

The major ions were used to study hydrogeochemistry characterization of high sulfate groundwater. The isotope of δD and δ¹⁸O were applied to analyse the recharging source. Data management and analysis were performed using SPSS 26.0. The piper diagram was designed with Aquachem 3.70. The saturation indice was calculated by Phreeqc Interactive 3.7. The graph drawing were performed using Origin 2018 and CorelDRAW X6.

The geochemical characterization of samples, including major chemical compositions, pH, Ec, total dissolved solids (TDS) and stable isotopes, are listed in Table 1. Due to apparent difference on the value of sulfate, the samples were divided into two groups of high sulfate (TA) groundwater and low sulfate (QA) groundwater. Both the high sulfate (TA) groundwater and low sulfate (QA) groundwater samples show wide sulfate ranges, varying from 140.23 to 1,654.12 mg/l (mean 658.47 mg/l) and 2.47 to 204.36 mg/l (mean 71.95 mg/l), respectively. Besides, 88% of high sulfate groundwater samples have sulfate value above 250 mg/l, which exceeds the world health organization's guidelines (WHO 1996). Therefore, the groundwater from TA belonged to high sulfate mine groundwater. High sulfate (TA) groundwater and low sulfate (QA) groundwater were all weakly alkaline with average pH values of 7.74 and 8.73, respectively. Also, the pH of TA samples was slightly lower than that of QA, which consist with previous studies (Zhang et al. 2019). The TDS of low sulfate (QA) groundwater samples range from 222.28 to 645.66 mg/l. The high sulfate (TA) groundwater has a wider TDS value range, ranging from 670.48 to 2,980.85 mg/l. The stronger the water rock interaction is, the more the components of surrounding rocks would enter the groundwater, forming groundwater with high TDS (Hao et al. 2020). The mean TDS content of TA is 3.83 times of than of QA, implying hydrochemical process have a stronger influence on high sulfate (TA) groundwater.

Table 1

Major ion content (mg/L), TDS (mg/L), pH, and stable isotopic (‰) of groundwater samples in study area

Type .	K+ + Na+ .	Ca2+ .	Mg2+ .	NH4+ .	Cl− .	.	.	.	TDS .	pH .	δD .	δ18O .
	mg/L .									– .	– .
High sulfate groundwater
min	119.16	12.08	12.27	0.00	0.00	140.23	53.70	0.00	670.48	7.20	−70.22	9.53
max	634.53	285.99	146.55	7.23	957.62	1,654.12	624.96	187.71	2,980.85	9.19	−60.05	−8.05
mean	308.13	124.46	61.77	2.55	231.03	658.47	401.93	12.31	1,599.68	7.74	−64.59	−8.81
SD	152.91	83.09	35.66	2.63	206.38	402.30	134.89	45.50	650.76	0.46	2.87	0.45
Low sulfate groundwater
min	11.82	2.43	2.95	0.00	24.42	2.47	3.56	0.00	222.28	7.10	−63.73	−8.69
max	154.30	119.01	47.62	5.20	197.33	204.36	490.84	81.09	645.66	10.45	−42.76	−5.40
mean	76.90	42.99	21.39	0.89	70.07	71.95	217.02	24.77	417.47	8.73	−55.89	−7.74
SD	54.14	44.68	13.37	1.71	57.47	82.20	171.10	28.36	124.91	1.18	6.28	1.00

Type .	K+ + Na+ .	Ca2+ .	Mg2+ .	NH4+ .	Cl− .	.	.	.	TDS .	pH .	δD .	δ18O .
	mg/L .									– .	– .
High sulfate groundwater
min	119.16	12.08	12.27	0.00	0.00	140.23	53.70	0.00	670.48	7.20	−70.22	9.53
max	634.53	285.99	146.55	7.23	957.62	1,654.12	624.96	187.71	2,980.85	9.19	−60.05	−8.05
mean	308.13	124.46	61.77	2.55	231.03	658.47	401.93	12.31	1,599.68	7.74	−64.59	−8.81
SD	152.91	83.09	35.66	2.63	206.38	402.30	134.89	45.50	650.76	0.46	2.87	0.45
Low sulfate groundwater
min	11.82	2.43	2.95	0.00	24.42	2.47	3.56	0.00	222.28	7.10	−63.73	−8.69
max	154.30	119.01	47.62	5.20	197.33	204.36	490.84	81.09	645.66	10.45	−42.76	−5.40
mean	76.90	42.99	21.39	0.89	70.07	71.95	217.02	24.77	417.47	8.73	−55.89	−7.74
SD	54.14	44.68	13.37	1.71	57.47	82.20	171.10	28.36	124.91	1.18	6.28	1.00

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As the properties of K⁺ is similar to Na⁺ and low concentration of K⁺ in groundwater, it is commonly merged with Na⁺, regarded as (K⁺ + Na⁺) (Zhang et al. 2020). In high sulfate (TA) groundwater and low sulfate (QA) groundwater, K⁺ + Na⁺ and Ca²⁺ are two dominant cations, followed by Mg²⁺ and NH₄⁺. Moreover, the mean (K⁺ + Na⁺ )content in high sulfate (TA) groundwater was about 4.0 times of that in low sulfate (QA) groundwater. The content of anions in the low sulfate (QA) groundwater present the following trend: (mean content: 217.02 mg/l) > (mean content: 71.95 mg/l) > Cl⁻ (mean content: 70.07 mg/l) > (mean content: 24.77 mg/l). However, with the content of and Cl⁻ increased obviously in high sulfate (TA) groundwater, the content of anions show different order: (mean content: 658.47 mg/l) > (mean content: 401.93 mg/l) > Cl⁻ (mean content: 231.03 mg/l) > (mean content: 12.31 mg/l). Under the interaction of water and rock, some substances in surrounding rock were transferred into groundwater, resulting in the addition of new components to groundwater (Zhang et al. 2019). In general, these results suggested that water-rock interactions in high sulfate (TA) groundwater are more complex and intense than that of low sulfate groundwater (Hao et al. 2020).

Indeed, the concentration of (K⁺ + Na⁺) and TDS in high sulfate (TA) groundwater are much higher than that of low sulfate (QA) groundwater samples, as shown in Figure 3(b) and 3(c). Furthermore, the relationship between Cl⁻, ⁠, ⁠, pH and are weak positive and negative, indicating anion and pH have no obvious inhibitory effect on enrichment. Therefore, Prominent high content of TDS, (K⁺ + Na⁺), Mg²⁺ and Ca²⁺ are water environment for high sulfate mine groundwater, which is consistent with the previous report (Chen & Gui 2018).

Meanwhile, the δ¹⁸O and δD of low sulfate groundwater exhibited a good linear relationship with the correlation coefficients (R²) above 0.9. Conversely, the data points of high sulfate groundwater does not appear to be a linear relationship with R² of 0.22, drifting towards right of δ¹⁸O. The result suggest high sulfate groundwater experienced evaporation after recharging from rainwater, while low sulfate groundwater did not.

If halite is the only dissolved mineral, the mole ratio between Na⁺ and Cl⁻ in groundwater shoule be 1:1 ; if the calcite is the only dissolved carbonate mineral, the mole ratio between Ca²⁺ and should be 1:2, and the dolomite is the only dissolved mineral, the mole ratio should be 1:4; if the gypsum is the only dissolved carbonate mineral, the mole ration between Ca²⁺ and should be 1:1 (Chen et al. 2017).

If ⁠, ⁠, Ca²⁺ and Mg²⁺ are dissolved from carbonate minerals, the mole ratio between + versus Ca²⁺ + Mg²⁺ should be 1:1. The deviation of point from the 1:1 dissolution line implies the existence of other controlling factors. Moreover, the farther away from the dissolution line, the more tense the other factor. As exhibited in Figure 5(d), all the data points of low sulfate groundwater locate around the 1:1 dissolution line, illustrating carbonate dissolution is the primary source of ⁠, ⁠, Ca²⁺ and Mg²⁺. For high sulfate mine groundwater, 94% of the samples appeared above the 1:1 dissolution line, suggesting the excessive (⁠⁠, ⁠) and other factors for chemical composition. Therefore, the result revealed that the water-rock interaction of low sulfate groundwater is controlled by carbonate (calcite and gypsum) dissolution, whereas the chemical composition of high sulfate groundwater is mainly controlled by halite, carbonate (calcite and gypsum), and silicate dissolution.

The relationship between /Na⁺ and Ca²⁺/Na⁺ and between Mg²⁺/Na⁺ and Ca²⁺/Na⁺ are usually to address whether carbonate dissolution, silicate dissolution and/or evaporation are occurred in the water interactions of a water sample (Berger et al. 2016). As shown in Figure 5(e) and 5(f), most data points of low sulfate groundwater samples locate carbonate dominance field, far away from silicate and evaporation dominance field, implying the carbonate dissolution is the main source of chemical composition for low sulfate groundwater. For high sulfate groundwater, the data points mainly fall silicate and carbonate dominance field, and partly appear at the evaporation dominance field, suggesting high sulfate groundwater is dominated by silicate dissolution, carbonate dissolution and fewer evaporation.

The Gibbs's ratios, Na⁺/(Na⁺ + Ca²⁺) versus TDS and Cl⁺/(Cl⁺ + ⁠) versus TDS, are usually used to distinguish the primary factors that influence chemical composition, such as rock dissolution, precipitation and/or evaporation (Gibbs 1970). The Gibbs's of samples are plotted in Figure 5(g) and 5(h). The data points of all samples in the study area mainly appear at rock dissolution dominance field, implying rock dissolution is the leading factor for chemical composition. Meanwhile, a few of data points of high sulfate groundwater locate at the evaporation dominance field, suggesting the evaporation effect involved in the geochemical process of high sulfate groundwater.

Figure 6(b)–6(e) show the relation between SI_Calcite, SI_Dolomite, SI_Halite, SI_Gypsum and content, which is in order to determine the key minerals for high sulfate mine groundwater. The values of SI_Calcite and SI_Dolomite for high sulfate groundwater and low sulfate groundwater are similar and not correlated with the ⁠, implying dissolution of calcite and dolomite are not the primary factor for the formation of high sulfate mine groundwater. The SI_Halite of low sulfate groundwater ranging from −7.97 to −6.08. For high sulfate groundwater, the SI_Halite has a narrower value, varying from −6.71 to −5.05. This indicate that the dissolution of halite in high sulfate groundwater is stronger than than in low sulfate groundwater, resulting in elevated Na⁺ content in high sulfate groundwater. As same with the SI_Halite, the SI_Gypsum values of high sulfate groundwater are higher than that of low sulfate groundwater, suggesting the dissolution of gypsum is more intense in high sulfate groundwater, and generating into elevated content. The results of SI illustrate that gypsum and halite were the key minerals for the formation of high sulfate mine groundwater.

As can be seen in Figure 7(a), the data points of high sulfate groundwater samples present a line with a slope of −1.1 and R² of 0.63, while the data points of low sulfate groundwater samples do not define a line with a slope of 0.05 and R² of 0.12. This implies ion-exchange apparently exists during the geochemical process in the high sulfate groundwater. In view of good liner relationship (R² = 0.63) in Figure 7(a), ion-exchange is also a main factor that influences the composition of high sulfate groundwater. However, the bad liner relationship (R² = 0.12) of low sulfate groundwater indicate the ion-exchange interaction is not the important factor.

The CAI-1 > 0 and CAI-2 > 0 represent (Na⁺ + K+) in groundwater was exchanged by (Ca²⁺ + Mg²⁺) in surrounding rock, while CAI-1 < 0 and CAI-2 < 0 indicate (Ca²⁺ + Mg²⁺) in groundwater was replaced by (Na⁺ + K⁺) in surrounding rock. Moreover, the larger the absolute values of CAI-1 and CAI-2, the more intense the exchange is. In addition, if the CAI-1 and/or CAI-2 values equal to 0, it implies ion-exchange does not take place (Kumar et al. 2017).

The CAI-1 values of low sulfate groundwater range from −2.71 to 0.52, with the average of −0.76, and CAI-2 values range from −0.74 to 0.07, with the average of −0.26. The high sulfate groundwater have lower values, varying from −2.78 to 0.37 for CAI-1, with the average of −0.99, and varying from −0.68 to 0.62 for CAI-2, with the average of −0.23. 94% of high sulfate groundwater and 91% low sulfate groundwater show negative value of CAI, suggesting (Ca²⁺ + Mg²⁺) in groundwater have been replaced by (Na⁺ + K⁺) in surrounding rock. Figure 7(c) is the plot of CAI-1 and CAI-2 of samples. The data points of higher sulfate groundwater has higher absolute values of CAI than those of low sulfate groundwater, illustrating ion exchange interactions are stronger in high sulfate groundwater.

The pH value of low sulfate groundwater ranges from 7 to 10.5. the high sulfate groundwater have a narrower and lower pH value range, varying mainly from 7 to 8 (Table 1). Figure 8(c) shows that the content in the samples have a apparent decreasing trend in terms of pH, which may related to competitive effect between and OH⁻. In view of belonging to strong acidity, it would inhibit its adsorption on the mineral surface and release into groundwater under relatively low pH (7–8) environment, resulting in elevated ⁠. Contrarily, the OH⁻ was released into the low sulfate groundwater and was adsorption on the minerals, leading to relatively high pH (7–10.5).

It is proposed that the initial in groundwater is mainly from the dissolution of carbonate (gypsum and calcite) and weak ion exchange. The undisturbed-mining hydrochemical facies of the limestone groundwater (QA) are Ca-Na-HCO₃, with low content of Na⁺ and TDS. During and after the mining, the coal mine was extracted and then oxygen entered into mined-out area, and the bearing minerals such as gypsum were oxidized. This led to elevated _, Na⁺, H⁺ and TDS content, and and Na⁺ had become the dominant factors, resulting in diverse hydrochemical facies (Na-SO₄-HCO_3, Na-Cl type).

The increase in Na⁺, H⁺ and TDS provide suitable conditions to form high sulfate water. Moreover, silicate weathering and stronger ion exchange effectively promoted the enrichment in TA groundwater. In addition, evaporation and competitive effect with OH⁻ also are important factors that contribute to the elevated content. All these factors related to enrichment in the groundwater go beyond permissible limits, generating high sulfate mine groundwater in TA.

The sulfate in mine groundwater have exceeded the permissible limit of the World Health Organization, posing a serious threat to water resources and the safety of residents, soil and air. Previous studies have paid attention to the risk, sources, and distribution of mine groundwater, where the hydrochemical process and mechanism of high sulfate groundwater are neglected. In this study, the hydrogeochemical characteristics and water-rock interactions of high and low sulfate groundwater are comparatively analyzed, aimed to reveal the mechanism of sulfate enrichment in mine groundwater at a massive coalfield. The main conclusions obtained from this study are summarized as follows:

• (1)

  In both high sulfate groundwater and low sulfate groundwater, K⁺ + Na⁺ and Ca²⁺ are two dominant cations, followed by Mg²⁺ and NH₄⁺. The contents of anions in the low sulfate groundwater follow the trend: > > Cl⁻ > whereas the high sulfate groundwater follows the order: > > Cl⁻ > ⁠. The hydrochemical facies of low sulfate groundwater is controlled by Ca-Na-HCO₃ type (82%) and Na-Cl type (18%), whereas the composition of hydrochemical facies of high sulfate groundwater is 88% of Na-SO₄-HCO₃ type and 12% of Na-Cl type.

• (2)

  The high content of TDS, (K⁺ + Na⁺), Mg²⁺ and Ca²⁺ are in the water environment for high sulfate mine groundwater. Results for hydrogen and oxygen isotopes revealed that the recharging source of groundwater in the study are mainly from atmospheric precipitation, and evaporation process existed in high sulfate groundwater.

• (3)

  Compared with low sulfate groundwater, the high sulfate mine groundwater possess more complicated and diverse geochemical process. The sulfate enrichment in mine groundwater is predominantly generated by silicate (halite) and carbonate (gypsum and calcite) weathering, strong ion exchange interaction and competitive effect, whereas low sulfate groundwater is mainly influenced by the carbonate weathering and weak ion exchange.

We sincerely thank the National Engineering Research Center of Coal Mine Water Hazard Controlling (Suzhou University, China) for providing the experimental site.

This research was funded by National Natural Science Foundation of China (41773100, 41872170), the Key scientific research projects of Anhui Provincial Department of Education (KJ2021A1117), Funding projects for research activities of academic and technological leaders of Anhui Province (2020D239), National Major Science and Technology Projects of China(2016ZX05044), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Fundamental Research Funds for the Central Universities (2020CXNL11) and the Key natural science research projects of Suzhou University (2020yzd03, 2020yzd07).

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

The authors declare there is no conflict.