Hydrochemical characteristics and hydraulic connection of shallow and mid-layer water in typical mining area: a case study from Sulin mining area in Northern Anhui, China Open Access

Water is the source of life. Groundwater has the advantages of stable water supply conditions and good water quality (Mukherjee & Singh 2020; Nurtazin et al. 2020). It is an important water source for industrial and agricultural production and domestic water. Especially in areas lacking surface water, groundwater has basically become the main local water source (Jhhan et al. 2019; Jiang et al. 2021). With the rapid development of society and economy, water pollution is widespread, and the issue of drinking water safety has attracted more and more attention from society (Wang et al. 2019). In view of the importance of groundwater resources, countries all over the world pay increased attention to the protection and research of groundwater resources. The 33rd and 34th international hydrogeology conferences focus on the environment problems related to groundwater. The action plan for prevention and control of water pollution issued by the State Council in April 2015 clearly pointed out that prevention and control of groundwater pollution, and regular investigation and assessment of regional environmental conditions of centralized groundwater type drinking water source supply area should be carried out, which has opened a new chapter in the protection of drinking water sources (Qiu 2019).

The water supply of the Huaibei coalfield mainly comes from the groundwater in the middle aquifer of the thick loose layer (referred to as mid-layer water), and the overlying shallow groundwater (referred to as shallow water) is connected to the surface environment of the mining area. In recent years, during the process of conducting hydrogeochemical research on water-filled aquifers in coal mines, it was discovered that due to the large amount of water supplied by industrial and mining enterprises and residents, the water level difference between mid-layer and shallow water increases (Qiu et al. 2021). Whether there is a hydraulic connection between them is a hot issue that the residents of the mining area are generally concerned about. Under the disturbance of coal mining, does the supergene environmental medium in the mining area (such as coal gangue, mine wastewater, etc.) have an impact on the mid-layer water source? What is the hydraulic connection between shallow and mid-layer water? So far, there is no authoritative answer.

The coal resources in Sunan and Linhuan mining areas were developed earlier and have the largest scale of development. The accumulated environmental problems are representative in the entire Huaibei coalfield (Gui et al. 2015; Gui 2016). Therefore, this paper chooses Sunan and Linhuan mining areas (hereinafter referred to as Sulin mining area) as the research bases. Based on the analysis of the geological and hydrogeological background of the study area, the shallow and mid-layer water samples were collected systematically, and the conventional components were tested. The objective of this research was to analyze the hydrochemical characteristics of shallow and mid-layer water, and understand the water quality difference and hydraulic connection between the shallow and mid-layer water with the help of the variation of TDS, F⁻, NO₃⁻ content.

The Sulin mining area is located in the north of Anhui Province, between 116°15′–117°12′ East longitude and 33°20′–33°42′ North latitude, with convenient transportation, and includes 19 coal mines (Figure 1(a)). It is distributed in the vast plain area, with flat terrain, high in the northwest and low in the southeast. The climate in this study area is monsoon temperate semi-humid with hot summers, mild spring and autumn, cold and windy winters, rainfall concentrated in summer, and four distinct seasons.

The sedimentary caprock in Sulin mining area is a loose layer of the Cenozoic Quaternary, with large thickness variations, mainly controlled by paleotopography (Huang et al. 2018; Gui et al. 2019), the cross section is shown in Figure 1(b). The Cenozonic aquifers (groups) can be divided into four aquifers from top to bottom (Lin et al. 2016; Qiu et al. 2021a, 2021b), in which three aquicludes are developed. The first aquifer located in the uppermost part is usually the so-called shallow water, which receives the recharge of atmospheric rainfall or surface water. The mid-layer is taken from the second aquifer or the third aquifer, which can be recharged through regional interlayer runoff or laterally from precipitation in the hillside area. Where the first aquiclude and the second aquiclude are thin, there may be cross flow between the three aquifers. The sketch map of groundwater circulation in the study area can be seen in Figure 2.

In August 2020, we carried out extensive field investigation and groundwater sample collection in Sulin mining area. Combined with the regional topographic and geomorphic characteristics, aquifer structure characteristics and residential water use, we selected the water source with environmentally sensitive points around for sampling, and the sampling points are evenly distributed throughout the mining area. We collected a total of 34 shallow water samples from pumps (depths of less than 30 m), and 19 mid-layer water samples from water source wells in Sulin mining area. The sampling container used polyethylene plastic bottles. Before sampling, the bottles were rinsed with the fresh groundwater 3–5 times. After sampling, the bottles were sealed and stored. In order to prevent possible loss during transportation and laboratory analysis, an additional water sample was collected at each sampling point. The specific flowchart of sampling and testing process is shown in Figure 3.

Water temperature (T), pH, total dissolved solids (TDS) etc. were measured in the field with a portable electrochemical analyzer, and all other parameters (Na⁺, Ca²⁺, Mg²⁺, Cl⁻, F⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, HCO₃⁻) were tested in the National Engineering Research Center of Coal Mine Water Hazard Controlling, China. Before testing, the water samples were filtered through a membrane with a pore-size of 0.45 μm. The content of Na⁺, Ca²⁺, Mg²⁺, Cl⁻, F⁻, SO₄²⁻, NO₃⁻ were determined by ion chromatography (ICS-600-900), and the content of CO₃²⁻ and HCO₃⁻ was measured by conventional acid-base neutralization titration. All experiments were carried out at normal temperature. At the same time, various monitoring methods such as blanks, duplicates, and reference materials (Ministry 2020) were adopted to guarantee quality control. During the test, each sample was measured repeatedly three times, and the average value was taken as the final data. The relative deviation between parallel samples was less than 5%, and the recovery was 85–115%.

IBM SPSS Statistics 20.0 was used to analyze the hydrochemical data. CorelDRAW 12 and OriginPro 8 were used for graphic processing and analysis. Surfer software was used for the interpolation of the analyzed data.

The statistical results of conventional components (pH, TDS, Na⁺, Ca²⁺, Mg²⁺, Cl⁻, F⁻, SO₄²⁻, NO₃⁻, CO₃²⁻, HCO₃⁻) of shallow and mid-layer water samples in the study area are shown in Table 1. Because the content of K⁺ was less and its chemical properties were similar to Na⁺, Na⁺ was used to replace Na⁺ + K⁺ approximately.

As a whole, the mean concentration of cations in shallow and mid-layer water was Na⁺ > Ca²⁺ > Mg²⁺, and Na⁺ > Mg²⁺ > Ca²⁺. Their anion content was HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻ > F⁻ >CO₃²⁻, and HCO₃⁻ > SO₄²⁻ > Cl⁻ CO₃²⁻ > NO₃⁻ > F⁻. Although Na⁺ and HCO₃⁻ were the most important cation and anion in both shallow and mid-layer water, the hydrochemial types of them are still different (Figure 4). The hydrochemial types of shallow water were more complicated, namely Na + Mg + HCO₃⁻ and Mg + Na + HCO₃⁻ water type, reflecting the characteristics of being severely affected by the supergene environment, while the water chemistry type of mid-layer water was relatively simpler than that of the shallow water, they were Na + Mg + HCO₃⁻ + SO₄²⁻ and Na + Mg + HCO₃⁻ water type, etc., and the main source of solute was carbonate dissolution.

The content of the major component was compared with the grade III standard limit in the Standard for Groundwater Quality of China (GB/T14848-2017) (General Administration 2017), the content of SO₄²⁻, Cl⁻, Na⁺ exceeded the standard limit in both shallow and mid-layer water. In addition, the coefficient of variation (CV) can reflect the degree of dispersion of element distribution. It can be seen from Table 1 that the CV value of HCO₃⁻ in both shallow and mid-layer water was the lowest, indicating that the distribution of HCO₃⁻ was relatively uniform. The CV values of F⁻, NO₃⁻, and SO₄²⁻ in shallow water were relatively high, and the highest value of F⁻ reached 502.86%, while the CV values of NO₃⁻, CO₃²⁻, and SO₄²⁻ in mid-layer water were relatively high, and the highest value of NO₃⁻ reached 179.65%, indicating that the distribution of these ions was uneven, and may be greatly affected by factors such as aquifer media, hydrological conditions, and human activities (Wan et al. 2016). Except for CO₃²⁻, the CV value of other ions in mid-layer water was lower than those in shallow water. It showed that because mid-layer was buried deeply, it was not easy to be disturbed by shallow water under normal conditions, and the distribution was relatively uniform.

In order to understand whether mid-layer water in Sulin mining area was affected by the overlying shallow water, a chemical connection analysis of shallow and mid-layer water was carried out. Through the statistical analysis of the conventional hydrochemical test data of shallow and mid-layer water (Table 1), we could understand the overall picture of the conventional components of them, and at the same time, the difference and connection of the conventional components in the two aquifers can be seen, which is an important basis for judging whether mid-layer water was affected by shallow water.

The pH value is an important comprehensive physical and chemical index to measure the pH of aqueous solution, and an important index for judging the formation conditions of water chemistry (Yu et al. 2020; Jiang et al. 2021a, 2021b). The statistical results showed that the pH ranges of shallow and mid-layer water were 6.55–7.80 and 7.04–7.76, respectively. According to the Standard for Groundwater Quality of China, all shallow and mid-layer water samples met the grade I standard limit of pH (6.5–8.5). Although both are neutral water, the pH value of shallow water varied significantly, and the coefficient of variation was 0.94% larger than that of mid-layer water. It suggested that shallow water was closely related to the supergene environment, while the pH value of mid-layer water was relatively stable, and under normal circumstances, it was less disturbed by shallow water or supergene environment (except for the thinned first aquiclude).

TDS refers to the sum of soluble components in water after removing dissolved gas and suspended solids, and it is also an important indicator to measure the quality of water (Chen et al. 2014). From the results of statistical analysis, the TDS content of shallow water was between 287.86 and 1879.16 mg/L (mean value 818.65 mg/L); the TDS value of mid-layer water was between 437.00 and 1864.78 mg/L (mean value 904.94 mg/L). According to the grade III standard limit of TDS (<1000 mg/L) in the Standard for Groundwater Quality of China, the TDS content of 23 shallow water samples and 11 mid-layer water samples did not exceed the corresponding limit value belonging to fresh water, and other water samples belonged to brackish water. In addition, the variation range of TDS in shallow water was larger than that in mid-layer water (the difference of CV value between them is 7.77%), which also suggested that shallow water in the study area was greatly affected by the supergene environment.

From the spatial distribution features of TDS in shallow water (Figure 5(a)), except for the northwest (Haizi and Qingdong mine areas) and southeast (Zouzhuang, Qidong, and Qinan mine areas), TDS content was relatively low, and other coal mine areas had a relatively high TDS content, the content of TDS was provided with the highest value in Tongting mine, reaching 1879.16 mg/L, followed by the Renlou mine. The reason was likely to be that mining activities last for a long period and there were many, and concentrated, suburban residents in these areas, which led to a relatively large scale of shallow water utilization. As a result, shallow water gathered to these areas from the northwest and southeast. The spatial distribution features of TDS in mid-layer water are shown in Figure 5(b). As a whole, TDS of mid-layer water gradually increased from the southeast to the northwest, indicating that mid-layer water had a tendency to migrate from the southeast to the northwest. The reason was likely to be that the thickness of the loose layer in the northwest was relatively thin, and the thickness of the second aquifer was also thin. In order to ensure good water withdrawal, the depth of the well goes deep into the third aquifer, and the TDS of the third aquifer was higher than that of the second aquifer. The comparative analysis showed that there were differences in the TDS change trends of shallow and mid-layer water, and they had their own migration directions. This further showed that mid-layer water had not been affected by the overlying shallow water in most areas.

The concentrations of F⁻ for shallow water ranged from 0.00 to 0.92 mg/L, with a mean value of 0.03 mg/L, which did not exceed the limit value of China's ‘Standard for Groundwater Quality’ for Category III water (≤1.0 mg/L), while for the mid-layer water, the concentrations of F⁻ exhibited a wide range, from 0.44 to 2.06 mg/L, with a mean value of 1.12 mg/L, and five samples had F⁻ concentrations which exceeded the limit value. The spatial distribution features of fluoride for mid-layer water are presented in Figure 6. The low concentration of fluoride (<1.0 mg/L) appeared in A, C, G, H, L, N, P, R mines, whereas other coal mines had a high concentration of fluoride (>1.0 mg/L). The highest and lowest concentration of fluoride appeared in O and G coal mines, respectively, and their values were 2.06 and 0.44 mg/L.

Previous researches have shown that excessive fluoride was likely related to high-fluorine waste discharge sites, such as coal mines and power plants (Wu et al. 2010; Rashid et al. 2018). However, the statistical results suggested that the content of F⁻ was provided with a high value in mid-layer water, while the F⁻ content of shallow water remained at a relatively low level, thus basically ignoring the influence of the high fluorine waste discharge. In addition, it indicated that the hydraulic connection between mid-layer water and overlying shallow water was weak in most areas of Sulin mining area. The source of fluoride in mid-layer water was worth exploring. According to the schematic diagram of groundwater circulation system in the study area (Qiu et al. 2018), in addition to the infiltration recharge of the first aquifer, mid-layer water receives the rainfall and lateral recharge of exposed carbonate aquifer in the mountainous slope zone. It can be seen from the F⁻ content of mid-layer water and limestone water in Table 2 that the limestone water in 83.33% of the water samples had a higher F⁻ content, which may make a certain ‘contribution’ to the high fluorine in mid-layer water through lateral recharge, therefore, the source of fluoride in mid-layer water may be due to geological reasons.

The concentration of NO₃⁻ for mid-layer water was graded from 0.00 to 8.51 mg/L, with an average value of 1.48 mg/L, which not exceed the limit value of China's ‘Standard for Groundwater Quality’ for Category III water (≤20.0 mg/L), whereas for shallow water, the NO₃⁻ content varied between 0.00 and 420.50 mg/L, with a mean value of 45.90 mg/L, and 12 samples had NO₃⁻ concentrations which exceeded the limit value, indicating the shallow water of the study area had a certain NO₃⁻ pollution level. The spatial distribution features of nitrate in mid-layer water are shown in Figure 7. The high concentration value areas of NO₃⁻ concentrated in E, J, O, R, Q, F, and H coal mines exceeded 20.0 mg/L. The highest concentration of NO₃⁻ appeared in H mine, reaching 420.50 mg/L. However, other coal mines were basically maintained at a relatively low level.

The reason for the excessive NO₃⁻ in shallow water was mainly due to the shallow burial depth, thus it was easily affected by supergene activities. Through on-the-spot investigation, H coal mine, with the highest NO₃⁻ content, had a pig farm, and the sampling point was a domestic well with a depth of 6–7 m about 20 m away from the pig farm, thus the leachate of pig farm was infiltrated by rainwater, resulting in a high NO₃⁻ content in shallow water. The NO₃⁻ content in F and O coal mine was also high. The sampling points of these two mines were close to the farmland, and this may cause non-point source pollution due to the use of nitrogen-containing fertilizers and pesticides.

With the increase of water supply intensity, the water level difference between shallow and mid-layer water in the study area increased. As a result, the hydraulic connection between them is becoming closer. Whether the solute in the shallow water environment migrated and transformed to the mid-layer water environment mainly depended on the thickness of the ‘first aquiclude’. From the difference of NO₃⁻ content between shallow and mid-layer water (Table 3), it is concluded that the mid-layer in most areas of the study area had not yet been polluted by shallow water, but in some areas such as E, F, J, P, and R coal mines, the NO₃⁻ content of mid-layer water was relatively high.

Figure 8 shows a comparison of the NO₃⁻ content of shallow and mid-layer water and the thickness of the first aquiclude of 19 coal mines in Sulin mining area. As can be seen from the figure, although shallow water had a high NO₃⁻ content in Taoyuan, Qidong, Xutuan, Zhuxianzhuang and Haizi coal mines, the NO₃⁻ content in mid-layer water was 0 mg/L. The reason may be that the thickness of the first aquiclude in these areas was greater than 10 m, its ‘barrier’ effect was significant, and there was almost no solute migration between shallow and mid-layer water. However, shallow and mid-layer water in Qinan, Wugou, Qianyingzi, Suntuan, and Yangliu all contained NO₃⁻. This may be because the thickness of the first aquiclude in these areas is thinner than 5 m, and the barrier function of the first aquiclude was weak. There was a possibility that shallow water solutes migrated to mid-layer water. Therefore, the surface environment in these areas should be strictly protected, farms with serious pollution emissions should not be established, and pesticides and chemical fertilizers in farmland should be prohibited or reduced, so as to prevent surface pollutants from entering the groundwater environment from the source, and ensure a safe water supply in coal mining areas.

This paper collected a total of 34 shallow, and 19 mid-layer water samples from Sulin mining area, and analyzed the hydrochemical characteristics and the water chemical connection. The results showed that among the anions in shallow and mid-layer water, HCO₃⁻ content is dominant, followed by SO₄²⁻ and Cl⁻; in cations, Na⁺ content is the highest, followed by Ca²⁺ and Mg²⁺. Besides, the content characteristic and spatial distribution features of F⁻ and NO₃⁻ in shallow and mid-layer water suggested that 55.56% of mid-layer water samples exceeded the F⁻ standard value due to geological origin, and the NO₃⁻ content in shallow water seriously exceeds the standard value due to human factors. Further analysis of the distribution of the first aquiclude found that when its thickness is less than 5 m, the barrier function of the first aquiclude is weak. There is a possibility that shallow water solutes may migrate to mid-layer water. Therefore, the surface environment in these areas should be strictly protected, no farms with serious pollution discharge shall be established, and pesticides and chemical fertilizers shall be banned or used less in farmland to prevent surface pollutants from entering the groundwater environment from the source.

This article was supported by the National Natural Science Foundation of China (41773100); the Natural Science Foundation of Anhui Province (2008085QD192); a doctoral research initiation fund project (2020BS010); the fourth batch of excellent academic and technical backbone projects of Suzhou university (2020XJGG02); the postdoctoral research initiation fund project (2021bsh001); Key research projects of Suzhou university (2019yzd05); Domestic visiting study project for outstanding young backbone talents in colleges and universities (gxgnfx2021154); the open research fund of Anhui key laboratory of detection technology and energy saving devices, Anhui polytechnic university (DTESD2020B05); Key Natural Science Research Projects of Anhui Provincial Education Department (KJ2020A0733); Suzhou University scientific research platform open project (2020ykf01); the academic support project for top-notch talents in disciplines (majors) of colleges and universities at Anhui Province (gxbjZD21081), the software engineering fundamental teaching and research section demonstration project at Anhui Province (2020SJSFJXZZ417).

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