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Pollution Characteristics and Health Risk Assessment of Fluoride and Potentially Toxic Elements in Coal Mine Water of Shanxi Province, North China: A Comparative Analysis of Raw Mine Water and Mine Drainage.
| Toxics | Pei Y, Luo J, Ma C, Ma P, Lin X, Li W, Yan J, Sun X. | 14(7):553.
China Coal mine water Health Risk Assessment
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Abstract

Original full-text study online at
https://www.mdpi.com/2305-6304/14/7/553

 

Highlights

What are the main findings?
  • Fluoride concentrations in raw mine water from several mines exceeded the WHO guideline limit of 1.5 mg/L, whereas those in mine drainage were below the WHO standard.
  • Among heavy metals, arsenic induced a moderate ecological risk in raw mine water, while mine drainage bore low overall risk.
What is the implication of the main finding?
  • The main finding highlights that water quality as well as fluoride-/heavy-metal-linked ecological and health risks differ between raw mine water and mine drainage; thus, long-term monitoring and targeted pollution management are essential for safe mine water utilization.

Coal mining critically affects Shanxi’s economy and national energy security in China, whereas mine water significantly influences regional water quality and ecological stability. However, studies on pollution characteristics and health risks of fluoride and potentially toxic elements (PTEs) remain limited, especially comparative analyses between raw mine water and treated mine drainage. This study comprehensively analyzed the pollution characteristics of fluoride and PTEs, along with water quality evaluation, ecological risks, and human health risks associated with raw mine water and mine drainage. Fluoride concentrations in raw mine water from several mines exceeded the WHO guideline limit of 1.5 mg/L, whereas those in mine drainage were below the WHO standard. The total hazard index (THI) of fluoride in both water types was unacceptable (THI > 1). For PTEs, only arsenic in raw mine water exceeded the Grade III groundwater standard, while all PTEs in mine drainage met standards. Total health risk of PTEs in raw water was approximately one order of magnitude higher than in mine drainage, and both exceeded acceptable levels, mainly contributed by carcinogenic elements, particularly arsenic. These results underscore continuous monitoring and targeted control of arsenic are still required for safe utilization of coal mine water.

Keywords: coal mine water; fluoride; potentially toxic elements; health risk assessment

Graphical Abstract

1. Introduction

Mine water refers to various water inflows generated during the extraction of mineral resources [1], including raw mine water and mine drainage [2]. It usually contains a variety of contaminants [3], such as excessive fluoride (F) [4] and PTEs, which directly affect its widespread use [5,6]. F– is an important component of natural water [7] and the World Health Organization (WHO) recommends an acceptable F– concentration of 1.50 mg/L in drinking water [8]. Excess fluoride in water causes health hazards to the natural environment [9]. PTEs are non-biodegradable pollutants that enter the water and sediment phases of aquatic ecosystems and accumulate in organisms [10].The uncontrolled discharge of coal mine water poses substantial risks of potentially toxic elements contamination to proximal aquatic systems, soil and plants [11,12,13,14]. Therefore, evaluating the mine water quality is significant to the sustainable development of water resources in China [15,16].

Coal has remained the dominant energy source in China [17], China is currently the largest coal-producing country in the world [18], with annual production nearly 3.36 billion tons in 2016 [2]. At present, domestic and international research on raw mine water and mine drainage mainly focuses on the source and mechanism analysis of PTE pollution, as well as investigations into water quality evolution and treatment technologies. For example, most investigations have concentrated on the sources of PTE contamination in surface water [19], groundwater [5,20], and sediments [21] adjacent to mining areas. Meanwhile, several other studies have explored the application of synthetic materials for the removal of PTEs from water samples [6,22,23,24,25], considering both metal species and concentrations [26,27,28,29]. Generally, raw mine water are unfit for directly used, but mine drainage, rainfall, and infiltration cause PTE enrichment, adversely affecting the groundwater and harming human health [6]. Researchers investigated the environmental pollution and human health risks of surface water samples collected from coal mines in Sunan Coal Mine, and the results revealed that chromium is the major carcinogenic factor in the study area, contributed to 95.45% of the total health risk [6], and authorities in this region must closely monitor three PTE—Fe, Mn, and Cr [6]. Therefore, conducting assessments on pollution status and human health risks of raw mine water and mine drainage can provide fundamental data support for local environmental supervision, and has a significance for the protcetion of aquatic environment surrounding mining areas, especially for the adjacent river basins.

As one of China’s key coal-producing regions [30], Shanxi Province reported a total treated mine water volume of 21.5 billion cubic meters from its 589 coal mines in 2021, corresponding to a utilization rate of 68%. The Zhuozhang River is one of the headwaters of the Haihe River Basin [31], originating in Changzhi City, Shanxi Province. In recent year, active coal mining operations along the Zhuozhang River was appeared, and anthropogenic activities possible accelerated the pollution status of the river [32,33]. However, the pollution and health risk of PTEs of coal mine water along the Zhuozhang river during coal exploration and other processes in mining areas was still limited. To better understand mine water quality characteristics and the difference between raw mine water and mine drainage, this study investigated the environmental quality and health risks of fluoride and PTEs for raw and mine drainage from five mining areas in the upper reaches of the Zhuozhang River, providing scientific recommendations for regional water conservation and water security.

2. Materials and Methods

2.1. Study Area

Zhuozhang River is an important water system in Shanxi Province and runs through two cities—Jinzhong and Changzhi—and thirteen counties [34]. The sampling sites lies within the core area of the Qinshui Coalfield, where stratigraphic units—including the Ordovician, Carboniferous, Permian, Triassic, and Cenozoic (Neogene and Quaternary) systems—are arranged from east to west in order of increasing geologic age [35]. These areas is a major coal-producing region, with coal-bearing areas covering approximately 829.7 km (71.7% of the county’s total area). The county has 7.58 billion tons of proven coal reserves and an approved annual production capacity of 16.1 million tons, hosting multiple coal mining enterprises.

According to the distribution of mining areas along the western tributary of the Zhuozhang River, sampling was conducted at mines S1, S2, S3, S4, and S5 (Figure 1). For mines S1, S2, and S3, both raw water (RW) and effluent water (EW) were collected, while only effluent water was sampled for mines S4 and S5. The raw-versus-effluent comparison samples with differnet treatment method (reverse osmosis treatment and chemical treatment) were collected from S1. All the samples were collected in August 2024.

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Figure 1. Study area and sampling points.

EXCERPTS:

3. Results and Discussion

3.1. Fluoride Ion Distribution Characteristics of Mine Water and Health Risk Assessment

The F– concentrations in the raw water from Mine S1 (3.6 mg/L) and Mine S3 (4.4 mg/L) exceed the Chinese agricultural irrigation standards (GB 5084-2021) [48], which cannot directly used for irrigation. However, after reverse osmosis treatment, the F– concentrations of raw water from Mine S1 (0.28 mg/L) meet China’s agricultural irrigation standards. The effluent water from coal Mines has F– concentrations below the WHO recommended range (1.5 mg/L) for drinking [49], but higher the maximum permitted F– concentration in drinking water (1.00 mg/L) in China, which indicated the effluent water unsuitable for oral ingestion [15].

Compared with previous studies, the F– concentration in coal mine effluent in this study (0.38–1.4 mg/L, average 1.04 mg/L) is close to that in India (0.1–2.3 mg/L, average 1 mg/L) [50] but lower than that in Shaanxi coal mines (0.16–12.75 mg/L, average 6.1 mg/L) [51], indicating regional differences in the spatial distribution and hydrogeochemical processes of fluoride in coal mine water. Numerous studies indicate that the enrichment and depletion of F– are closely related to the hydrogeochemical and alkaline conditions of groundwater [52,53]. In this study, except for the drainage water from mine S3 and S4, F– concentration in mine water is exceeded the maximum permitted value (1 mg/L), and the pH of the mine water was in the range of 6.54 to 8.38, indicating that weakly alkaline conditions possibly facilitate fluoride dissolution and migration [54]. The possible reason is that under alkaline conditions, the concentration of hydroxide ions is relatively high, reducing the activity of Ca2+ and inhibiting the formation of CaF. Meanwhile, F– in fluorine-bearing minerals is prone to ion exchange and fluoride primarily exists as free F- ions [55].

As shown in Figure 2a,d, the log (HQD) in raw water and effluent water was both lower than zero, which means that the health risk of F– from oral ingestion is negligible for different age of groups. However, for adult females, adult males, teenagers, children and infants, the log (HQO) values in raw water ranged from 0.87 to 1.39 (mean: 1.19), 0.96 to 1.49 (1.28), 1.1 to 1.63 (1.42), 1.04 to 1.57 (1.36), and 1.44 to 1.97 (1.76), respectively. In Figure 2b,e, the log (HQO) in effluent water showed a similar trend to that in raw water. These results reveals that the health risk of F– from dermal contact should not be ignored in different age groups.

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Figure 2. The logarithm of HQD, HQO and total hazard index (THI) values of F– in raw water (a–c) and effluent water (d–f) from coal mine. Note: The dotted line represents the position where log(HQD), log(HQO), and log(THI) equal zero.
It is worth mention that the log(THI) in raw water and effluent water was both higher than zero (Figure 2c,f), which reveals that the total risk posed by the F– to the human health is at unacceptable level. Among the five categories, the average value of log (THI) in raw water are in the following order: Infants (1.76) > Teenagers (1.42) > Children (1.36) > female (1.28) > male (1.19). A comparison of log (THI) values between raw and effluent coal mine water revealed a lower value in the effluent, thereby demonstrating that the treated water is cleaner than the raw water in coal mine.

3.2. Distribution Characteristics of Typical PTEs

Typical 11 PTEs indicators were analyzed in raw and effluent water from the upstream mining area of the Zhuozhang River. As shown in Table 1 , the average concentration of arsenic in raw mine water is 51 ug/L (range: 12–98 ug/L), which exceeds the Class III limit (10 ug·L-1) of the National Groundwater Quality Standards (GB/T 14848-2017) by 5.1 times. Furthermore, raw mine water from the S1 Mine was treated by chemical treatment and reverse osmosis membrane processes. The arsenic concentration in the raw mine water was reduced from 98 ug/L to 0.63 ug/L and 0.27 ug/L, with removal efficiencies as high as 99.4% and 99.7%, respectively. This demonstrates that the treatments can substantially reduce arsenic levels in polluted mine water and mitigate the associated environmental pollution.

Table 1. Comparison of PTE concentrations in mine water of the Zhuozhuang River with those in other rivers of China. (Note: —— = not detected).

A comparison of PTE concentrations between raw mine water and treated mine effluent showed that, with the exception of Cu and Hg, the concentrations of Pb, As, Cd, Zn, Ni, Al, Mn, Fe, and Co in the mine drainage were lower than those in the raw mine water (Table 1). Notably, among all raw mine water samples, only arsenic exceeded the Class III standard limit, while the concentrations of the other ten metal pollutants (Cu, Pb, Hg, Cd, Zn, Ni, Al, Mn, Fe, Co) were within the permissible range. In contrast, all eleven PTEs in the mine drainage were below the Class III limits of the National Groundwater Quality Standards, with no exceedances observed. This indicates that environmental supervision and management in the mining areas along the Zhuozhang River Basin are relatively effective, leading to a high compliance rate for mine drainage.

To clarify the pollution level of PTE in mine water from coal mining areas in the Zhuozhang River Basin at the national scale, this study compared and analyzed it with the PTE concentrations in mine water from other coal mines across China, with the results presented in Table 1. Compared with coal mine drainage from the Yudong River in Guizhou Province, the concentrations of Cu and Pb in the mine drainage investigated in this study were lower, while those of Cd, Zn, Ni, and Co were also significantly lower by approximately one to two orders of magnitude. Similarly, Mn, Fe, and Al were significantly lower by three to four orders of magnitude. Compared with groundwater from the Southern Jiangsu coal mine in Suzhou, the concentrations of Cu and Pb in the mine water of this study showed no statistically significant difference, whereas those of Zn, Mn, and Fe were significantly lower by approximately one to three orders of magnitude (Table 1). In comparison with raw mine water from coal mines in Inner Mongolia, Pb and As concentrations in the raw mine water of this study were at a comparable order of magnitude, while Zn, Mn, and Fe were roughly two orders of magnitude lower. By comparing with PTE concentrations in mine water from other coal mines across China, it can be concluded that the PTE pollution level of coal mine water in the Zhuozhang River Basin is relatively low (Table 1).

Further comparison between PTE levels in mine effluent and surface water within the basin demonstrated that PTE concentrations in mine effluent were 1–3 orders of magnitude lower than those in the Baisha River, Guizhou (Table 1). Relative to surface water of the Fenghe River within Shanxi Province, the ratios of PTE concentrations in mine drainage to those in Fenghe surface water were as follows: Cu (approximately 2.0), Pb (0.5), As (1.0), Cd (nearly equal), Zn (0.38), Ni (1.69), Mn (0.06), and Fe (0.03). Overall, the concentrations of several PTEs (Pb, Zn, Mn, Fe) in the mine drainage in this study were lower than those in the surface water of the Fenghe River Basin (Table 1). The reason is that the spatial variations in PTE concentrations among different basins are mainly attributed to the combined effects of geological background, pollution source composition, wastewater treatment efficiency, and hydrogeochemical conditions.

3.3. Mine Water Quality Evaluation

In this study, single-factor index and comprehensive pollution index were employed to assess PTE pollution in mine water from mining areas in the upper reaches of the Zhuozhang River, with the Class III standard of the Standard for Groundwater Quality as the reference (Table S1). As presented in Table S1, water pollution levels were classified into five grades—no pollution, slight pollution, light pollution, moderate pollution, heavy pollution—based on the Pi: <1, 1–2, 2–3, 3–5, and >5, respectively. The results of the single-factor pollution index analysis are presented in Figure 3. As shown, analysis of Pi values of seven PTEs in raw mine water and treated mine effluent revealed that only arsenic in raw mine water had a Pi value > 1, failing to meet the Class III groundwater standard. Specifically, Pi values of arsenic in raw mine water were 9.8 (S1, severe pollution), 4.2 (S2, moderate pollution), and 1.3 (S3, slight pollution) (Figure 3). Since the Pi values of all other PTEs (Cu, Zn, Cd, Hg, Zn, and Ni) were < 1, arsenic was identified as the primary pollutant in raw mine water, which may be associated with the background As content in the mining area and hydrochemical processes of raw mine water. In addition, the order of the pollution index for PTEs in raw mine water was As > Ni > Pb > Hg > Zn > Cd > Cu, whereas in mine drainage it followed the sequence As > Hg > Ni > Pb > Cd > Zn > Cu. This discrepancy is mainly attributed to the differences in geochemical occurrence forms and the concentration of PTEs between raw mine water and mine drainage, together with the differential removal effects of conventional treatment processes on various PTEs [59].

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Figure 3. The single pollution index of PTEs in raw mine water and mine drainage in the research area. Note: The dotted line represents the position where Pi equal one.

Further comprehensive pollution index evaluation of raw mine water and treated effluent near the Zhuozhang River revealed that raw mine water at S1 had a PN value of 7.0 (severe pollution), raw mine water at S2 had a PN value of 3.0 (moderate pollution), and mine drainage at S2 had a PN value of 0.64 (slight pollution). No PTE pollution was detected in raw mine water or mine drainage from other mining areas. In conclusion, PTE pollution in raw mine water and mine drainage in the study area is generally slight.

4. Limitation

This study systematically investigated the pollution characteristics of raw mine water and mine drainage, and further assessed their associated human health risks. Nevertheless, several limitations of this work should be acknowledged. First, the pretreatment and advanced purification processes for mine water at mining sites are largely regarded as confidential proprietary technologies. Only general categories of pretreatment methods are publicly available, while detailed operational procedures and technical parameters remain undisclosed. Accordingly, a comprehensive comparative analysis of diverse mine water treatment technologies could not be implemented in the present study. Second, raw mine water and mine effluents are rarely utilized directly as domestic drinking water; hence, this research merely serves as a fundamental exploratory investigation. Future studies can focus on in-depth exploration of pollutant source identification mechanisms and the performance evaluation of pretreatment processes, so as to enhance the practical applicability and scientific reference value of relevant research in this field.

5. Conclusions

This study systematically analyzed the pollution characteristics of fluoride and PTEs, along with water quality evaluation and human health risks associated with raw mine water and mine drainage in the upper Zhuozhang River Basin. Fluoride concentrations in raw mine water exceeded the WHO standards, whereas no exceedance was observed in mine drainage. Although the oral exposure risks of fluoride [61] were negligible, dermal contact risks [62] could not be ignored, and the total hazard index for all age groups was unacceptable.

For PTEs, only arsenic in raw mine water exceeded the Grade III groundwater standard, making it the primary pollutant. The discrepancy in pollution index sequences between raw and treated water was attributed to geochemical forms and the differential removal efficiency of treatment processes. Total health risks of PTEs in raw water were much higher than in drainage. Thus, long-term monitoring and targeted pollution management are essential for safe mine water utilization.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/toxics14070553/s1, Text S1 The detailed process of PTE human health risk assessment; Table S1 Evaluation criteria for the single-factor pollution index and the Nemerow pollution index; Table S2 Parameters for health risk assessment of fluoride in males, females, teenagers, children, and infants; Table S3 Parameters for health risk assessment of PTEs in adults and children; Table S4 Values of Pc, Sf and RfD.

Author Contributions

Y.P.: Investigation, Data curation, Formal analysis, Writing—original draft. J.L.: Investigation, Data curation, Formal analysis. C.M.: Sampling, Data curation. P.M.: Writing—review and editing. X.L.: Formal analysis. W.L.: Formal analysis. J.Y.: Conceptualization, Writing—review and editing. X.S.: Funding acquisition, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Shanxi Province Science and Technology Fundamental Research Program General Project (NO:202503021211069) and the program for the Young Scholar Grant of Wenying at Shanxi University (X.J. Sun, 2024).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.
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