Study on hydrogeochemical connection and water quality assessment of subsidence lake and shallow groundwater in Luling coal-mining area of the Huaibei coalfield, Eastern China Open Access

The hydrogeochemical connection and water quality of groundwater and surface water is a key issue observed in the hydrologic cycle and water resource management, which plays an important role in maintaining the renewable water capacity and ecosystem stability (Abbasnia et al. 2019). The coal reserves of the Huaibei Coalfield are abundant and the annual production is high. A large amount of coal mining has brought a series of ecological and environmental problems (Yang et al. 2018), among which ground subsidence is one of the most serious. In coal mining areas, the mining of underground coal mines causes the overlying strata of coal seams to move downward and form a subsidence basin, commonly known as a subsidence area (Zhang et al. 2020). The large-scale collapse caused by mining activities had created a large-scale subsidence lake. The surface subsidence lake is a natural water storage area, which is the main source for local residents, mining production, aquaculture and agricultural irrigation. Besides atmospheric precipitation and surface runoff, the main supply source of the subsidence lake is shallow groundwater. During the flood season, a subsidence lake can recharge groundwater, while during drought and dry periods groundwater can act as an important source to feed the subsidence lake. Consequently, groundwater and subsidence lakes are closely linked components of the hydrologic system. Therefore, the connection between subsidence lake and groundwater and the environmental quality of both are highly concerned.

It is generally believed that hydrochemical composition records the formation process and runoff path, and environmental isotopes tracking has been utilized to assess the source and transformation during the hydrologic cycle (Najafi Saleh et al. 2020), which are also effective tracers to explain the connection between surface water and groundwater. Numerous studies have been conducted to understand this connection, including the ratio of hydrochemical parameters (such as cations and anions), the environmental isotopic tracers, and exchange characteristics of water quantity and quality (Kshetrimayum 2015; Yang et al. 2016). In the northern Pakistan, Rashid et al. (2018) studied the fate, source distribution and hydrogeochemistry of the groundwater and surface water hydrogeochemical profile, which was based on chemical parameters such as pH, TDS, Na⁺ and SO₄²⁻, and so on Gu et al. (2017) analyzed the differences in hydrochemical and isotopic composition between surface water and groundwater to clarify the transformation relationship in Liu-jiang Basin, Hebei province, China. Zhao et al. (2018) evaluated the groundwater discharged and associated chemical inputs through the analysis of stable isotopes, ²²²Rn measurements, and corresponding models. In addition, water quality assessment is of great significance for environment protection and sustainable utilization of water resources. Misaghi et al. (2017) used an improved water quality index to evaluate the irrigation water quality of Ghezel Ozan river. However, for the subsidence lake around the coal mine, the hydrogeochemical connection between local, shallow surface water and quality remain unclear, and needs to be addressed urgently.

Hence, the objectives of this paper are as follows: (1) Investigate the connection between surface water from subsidence lake and shallow groundwater based on hydrogeochemical characteristics, formation mechanism and source analysis. (2) Assess the suitability for irrigation purposes. The outcomes of this research provide a better understanding of subsidence lakes and groundwater and it will help environmental protection and rational utilization of water resources.

Luling coal mine is located southeast of Suzhou City (Figure 1(a) and 1(b)) in the Huaibei coalfield (117°06′30″E, 33°35′59″N), with an area of 33.877 km². It was built and put into operation in 1970, with actual annual production capacity of 2.3 million tons per year. In the mining area, except for the subsidence lake formed by mining, the minefield is farmland with flat terrain, which tends to be higher in the West and lower in the East, with an elevation of +22–25 m. The climate in this area is mild, belonging to the north temperate monsoon area marine continental climate. The climate change is obvious, with four distinct seasons. The average annual rainfall is 766 mm, most of which are concentrated in July and August. The area is suitable for the comprehensive development of agriculture. The planting industry is dominated by wheat, soybeans, corn and lettuces.

The surface water and shallow groundwater systems of Luling coal mine are relatively abundant. The surface water system in the mine field is mainly a subsidence lake and several artificial rest ditches. The Luling coal-mining subsidence lake is formed by coal-mining collapse, which was formed about 50 years ago, and its shape is irregular with an area of nearly 1,000 hm², the accumulation water is mainly derived from atmospheric precipitation and shallow groundwater. The average water depth is about 6 m, and the maximum water depth is 8.33 m. There is a lot of coal gangue buried along the coast (Figure 2). The shallow groundwater system is composed of a thick loose layer with a thickness of about 200–300 meters. It is divided into four aquifers and three aquicludes (Gui et al. 2018). Among them, the first aquifer is located in the top layer, which is in direct contact with the bottom of the subsidence lake. It belongs to phreatic water type, the thickness of aquifer is about 30 m, and the buried depth of water level is 3–5 m. The lithology of the first aquifer is mainly composed of sandy soil and sandy clay with a thin layer of fine silt, containing abundant calcareous nodules and humic acid.

On the basis of hydrogeological survey, in order to reveal the transformation relationship between the coal mine subsidence lake and shallow groundwater, a total of 37 water samples were collected in May 2018, including 31 samples from 3 subsidence lakes and 6 samples from shallow groundwater. The location of the sampling points is shown in Figure 1(c). According to the technical specification for Surface Water and Sewage Monitoring (HJ-T91-2002, China), the water samples 0.5 m below the surface of the subsidence lake were taken from four subsidence lakes in Luling coal mine. The groundwater is taken from the first aquifer with a depth of about 35 m. The samples were collected in pre-cleaned HDPE bottles. Before sampling, the samples were moistened with water samples 3 times and were sealed with sealing film on site. The electrical conductivity (EC), pH, and total soluble solids (TDS) of all samples were measured by portable devices from OHAUS corporation (Shanghai, China). Portable instruments to test EC, pH and TDS were ST20R, ST20 and ST20T-B, respectively. The measurement accuracy of ST20R, ST20 and ST20T-B reached 1 μs/cm, 0.01 mg/L, 0.01. Then, the collection samples were immediately filtered with 0.45 μm filter paper within 24 h after being collected, and the water samples were kept under low temperature conditions.

The contents of main cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) and main anions (SO₄²⁻, Cl⁻) in samples were measured by Ion chromatograph (ICS-600-900, USA). The contents of CO₃²⁻ and HCO₃⁻ were determined by acid-base titration (analysis error of anion and cation should be controlled within 5%). The stable isotopes of hydrogen and oxygen (δD, δ¹⁸O) were measured by isotope analyzer (LGR-LWIA-45EP, USA) of the LGR company. The measurement accuracy of δD and δ¹⁸O was 1.0 ‰ and 0.2 ‰ respectively. Before all samples were tested, the stability of the test instrument was tested with standard samples, parallel samples were set, and the relative deviation of the parallel samples was less than 5%.

The Pearson's correlation analysis was carried out to display the relation of each hydrochemical parameter in subsurface water and shallow groundwater separately. All samples were analyzed with IBM SPSS Statistics (version 26, IBM, USA) and graphs were generated by Origin 9.0 (version 9.1, Originlab, USA).

The Chadha diagram can be used to determine the hydrochemical type of water and explain its source (Chadha 1999). The Gibbs diagram can effectively determine the control mechanism of chemical components (Gibbs 1970). The relationship between the ion proportion coefficient and TDS is generally used to decide the origin of different water samples, the sources of chemical components and the formation process (Jin et al. 2018). The variation of hydrogen and oxygen isotopic composition of water during evaporation follows the principle of Rayleigh fractionation, which can be used as a natural tracer to study the runoff transformation process of surface water and groundwater (Bu et al. 2018).

Due to high salinity or alkalinity affecting the growth of crops and soil quality, it is necessary to evaluate water quality for irrigation. At present, the common evaluation criteria of agricultural irrigation water quality mainly include Sodium Adsorption Ratio (SAR), sodium percentage (% Na), residual sodium carbonate (RSC) and permeability index (PI).

The unit of all parameters in formula (5)–(8) is meq/L, and the classification of the evaluation index (Asghari et al. 2018) is shown in Table 1.

Through the statistical analysis of the Na⁺, Mg²⁺, Ca²⁺, K⁺, Cl⁻, SO₄²⁻, HCO₃⁻, CO₃²⁻, pH, Ec, TDS of the samples of the subsidence lake and shallow groundwater, the relevant parameters are shown in Table 2. ANOVA analysis was used to compare characteristics of samples, which can be seen in Figure 3.

Table 2 and Figure 3(a) show that the range values of pH, EC, TDS of the subsidence lake are 8.59–8.78, 734–784 μs/cm and 453.82–470.64 mg/L, while the range values in shallow groundwater are 7.20–8.23, 812–1,176 μs/cm and 534–682.53 mg/L, respectively. The result of pH value shown in this area was alkali, and the values of pH in the subsidence lake was found to be slightly higher than that of shallow groundwater due to evaporation and human activities, which belong to low variance (CV < 15%). The value of TDS in the subsidence lake was lower than the shallow groundwater, which indicates that the water in this area was low salinity fresh water and shallow groundwater possessed fine solution filtration conditions. The value of EC from surface water to shallow groundwater showed an increasing trend, and the spatial variability was low, from which it can be seen that there was a transformation relationship between them, which was mainly due to the slow runoff of shallow groundwater and the dissolution of more minerals.

The range values of cations of Na⁺, Mg²⁺, Ca²⁺ in the subsidence lake is 119.25–124.55 mg/L, 23.96–24.69 mg/L and 16.98–19.79 mg/L, while in shallow groundwater it is 126.64–197.57 mg/L, 24.13–53.55 mg/L and 18.22–32.12 mg/L (Table 2, Figure 3(b)). The mean concentrations of Na⁺, Mg²⁺, Ca²⁺ in the subsidence lake are all lower than those in shallow groundwater, and Na⁺, Mg²⁺, Ca²⁺ in shallow groundwater (16%, 30.69%, 18.45%) belonged to the medium variation (15% < CV < 36%). The range values of anions of Cl⁻, SO₄²⁻ and HCO₃⁻ in the subsidence lake is 49.16–55.75 mg/L, 65.39–70.62 mg/L and 299.39–340.07 mg/L, while in shallow groundwater is 11.71–42.22 mg/L, 25.53–87.50 mg/L, 549.74–707.19 mg/L (Table 2, Figure 3(c)). The mean concentrations of Cl⁻ and SO₄²⁻ in the subsidence lake were larger than in the shallow groundwater; however, the mean concentration of HCO₃⁻ in the subsidence lake were lower than in the shallow groundwater. The CV of Cl⁻ (49.53%) and SO₄²⁻ (47.68%) in shallow groundwater was attributed to high variance (CV > 36%). The result showed that both the subsidence lake and shallow groundwater were weakly alkaline with low salinity, Na⁺ and Mg²⁺ are the two most abundant cations, followed by Ca²⁺, while the concentrations of anions are in the following order: HCO₃⁻ > SO₄²⁻ > Cl⁻.

The Chadha diagram is the modified form of the Piper diagram. It reveals an association between cations and anions of surface water and groundwater. According to the Chadha diagram, four different water types were formed, including NaHCO₃, CaHCO₃, NaCl and Ca-Mg-Cl, and the water data were plotted as the difference between (HCO₃⁻)-(SO₄²⁻ + Cl⁻) against (Ca²⁺ + Mg²⁺)-(Na⁺ + K⁺), expressed as a percentile (Chadha 1999).

The cations and anions of the subsidence lake and shallow groundwater were calculated simultaneously, as shown in Figure 4. Almost all water samples (n = 36) located in Field-4 revealed 99.3%, there was only one shallow groundwater sample in Field-1 and it revealed 0.7%, there were no samples located in Field-2 and Field-3. The result showed that all subsidence lake and most shallow groundwater samples were of the type of NaHCO₃. The formation of this water type mainly resulted from ion exchange processes. It showed a higher concentration of Na⁺ and HCO₃⁻ in the subsidence lake and shallow groundwater. The halite, albite, carbonate and calcite minerals were the dominating minerals to form NaHCO₃, CaHCO₃ and NaCl water type. There was a large amount of coal gangue buried along the subsidence lake (Figure 2), whose main components were silicate and carbonate minerals. The water type revealed that the coal gangue around the subsidence lake was mostly dissolved within the lake and aquifer. Also, HCO₃⁻ was derived from the dissolution of CO₂ in the atmosphere and the dissolution and leaching of various carbonates and weathering materials. In addition, the variation in the formation of subsidence lake was minor, which was relatively obvious in shallow groundwater. The phenomenon was mainly due to the fact that the runoff velocity of the subsidence lake was faster than that of shallow groundwater, and the hydrological cycle period was shorter than that of the shallow groundwater. It indicated that the chemical composition of the shallow groundwater was affected by human factors to a certain extent, showing obvious spatial differences.

The Gibbs diagram is widely used to distinguish the main factor controlling the formation mechanisms of chemical components. The relationships between the Gibbs ratios, Na⁺/(Na⁺ + Ca²⁺) for cations and Cl⁻/(Cl⁻ + HCO₃⁻) for anions are usually used to analyze the dominance factors that influence water composition, including evaporation, rock and precipitation effect (Gibbs 1970). In the Gibbs diagram, the evaporation factor is located at the upper right corner with the TDS value above 10,000 mg/L and Cl⁻/(Cl⁻ + HCO₃⁻) value above 0.5. The precipitation controlling field, located at the lower right, has low value of TDS and Cl⁻/(Cl⁻ + HCO₃⁻) value above 0.5. The rock weathering dominance field is located in the middle of the diagram (Figure 5). The data points of both subsidence lake and shallow groundwater fell into the rock dominance field, with high values of Na⁺/(Na⁺ + Ca²⁺) more than 0.8 and low values of Cl⁻/(Cl⁻ + HCO₃⁻) less than 0.3, implying formation mechanisms of chemical components in the study are mainly influenced by rock dominance. Meanwhile, the data points fell outside the solid line, which indicated that there were other factors, such as ion-exchange interactions and human factors.

The sources of Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻ can be estimated by the ratios of (Mg²⁺/Na⁺)/(Ca²⁺/Na⁺) and (HCO₃⁻/Na⁺ vs Ca²⁺/Na⁺). It is determined that high values of the ratio indicate the domain of bicarbonate dissolution, while low values of this ratio mean the dissolution of silicates (Simsek et al. 2007). The processes controlling the two waters are discussed in detail in Figure 6. Through the use of the bivariate diagrams of Figure 6, it is indicated that all the subsidence lake and shallow groundwater samples were mainly affected by silicate weathering, with ratios of (Mg²⁺/Na⁺)/(Ca²⁺/Na⁺) approximately equal to 1 and (HCO₃⁻/Na⁺)/(Ca²⁺/Na⁺) slightly more than 1. The samples of subsidence lake were concentrated and mainly affected by silicate weathering, there was a partial influence of evaporate dissolution of the cations. The cations of shallow groundwater were dispersed, showing the influence of both evaporate dissolution and silicate weathering. The subsidence lake is a perennial lake located in the discharge zones of the shallow groundwater flow system. The hydrochemical formation mechanism and source analysis of the subsidence lake was similar to that of shallow groundwater. These similarities were clear evidence of local subsidence lake and shallow groundwater interaction, followed by subsequent evaporation and silicate weathering affecting both the groundwater and subsidence lake.

The negative CAI-I and CAI-II values suggest Ca²⁺ in samples has been exchanged with Na⁺ in surrounding material. Besides, the larger the absolute values of CAI-I and CAI-II are, the stronger the ion-exchange interaction is. In this study, the CAI-I and CAI-II of subsidence lake ranged from −1.54 to 1.22 and −0.19 to −0.16. In shallow groundwater, their CAI-I and CAI-II were ranged from −12.55 to −2.68 and −0.31 to −0.14 (Table 3). The subsidence lake and shallow groundwater all show negative CAI-I and CAI-II value, implying Ca²⁺ in this study have been exchanged by Na⁺ in surrounding rock. Moreover, the shallow groundwater has higher absolute values of CAI-I and CAI-II than those of the subsidence lake, suggesting ion-exchange interactions were more intense in the subsidence lake.

Studies show that there is a significant correlation between the contents of chemical components, which can reflect that each component has the same source or geochemical process. If there is a significant and extremely significant correlation between the elements, it indicates that the elements generally have a certain homologous relationship or belong to the compound formation environment.

The correlation of hydrochemical parameters has been given in Figure 7. The correlation of cations and anions in the subsidence lake and groundwater was similar, and the highly significant correlation for the subsidence lake and shallow groundwater were to be found as follows: Cl⁻ and SO₄²⁻, Cl⁻ and Na⁺, Na⁺ and Mg²⁺, Mg²⁺ and Ca²⁺, indicating that there was a hydraulic connection between the subsidence lake and the surrounding shallow groundwater. The Cl⁻ and SO₄²⁻, Cl⁻ and Na⁺ showed highly significant correlation, which indicated the weathering of calcite and dolomite minerals. The highly significant occurs between the cations of Na⁺, Ca²⁺, Mg²⁺, which may be attributed to cation exchange. However, in the subsidence lake, a highly significant relationship existed between Cl⁻ and Mg²⁺, HCO₃⁻ and Ca²⁺, a slightly better relationship existed between SO₄²⁻ and Mg²⁺, SO₄²⁻ and Ca²⁺, while there was no correlation between them in the shallow groundwater. There is a large area of agricultural activity around the Luling coal mine area, and the subsidence lake is significantly affected by human activites. It is therefore inferred that Cl⁻, HCO₃⁻, Mg²⁺, Ca²⁺ and SO₄²⁻ mostly originated from the human activities and fertilizer (Leone et al. 2009).

Generally, the chemical composition is restricted by the sources of replenishment, evaporation and mixing, which reflect different characteristics of isotopes. The hydrogen and oxygen isotopic compositions of surface water and groundwater can indicate the difference of recharge source and water circulation process, and partly reflect the interaction between them. The δD and δ¹⁸O are used as an ideal indicator to trace water recharge and discharge within different flow systems. During the water circulation process, due to the equilibrium fractionation and thermal fractionation of isotopic composition, there is a linear relationship between stable isotopes of δD and δ¹⁸O in global precipitation, which is expressed as δD = 8δ¹⁸O + 10 and defined as the Global Meteoric Water Line (GMWL) by Craig (1961) D-excess (d = δD − 8δ¹⁸O) can reveal the degree of imbalance of evaporation and condensation process of regional atmospheric precipitation, as well as the characteristics of the water environment. With strong evaporation of water, isotope fractionation occurred, which leads to the decrease of D-excess, indicating stronger evaporation.

The hydrogen-oxygen stable isotope δ water samples of the subsidence lake (n = 5) and the shallow groundwater (n = 6) in Luling coal mining area is chosen. The numerical results were calculated and are shown in Table 4. In general, the isotopic values of subsidence lake samples ranged from −41.02‰ to −25.00‰ for δD, and from −4.69‰ to −2.15‰ for δ¹⁸O in the subsidence lake. It ranged from −56.01‰ to −47.10‰ for δD, and from −7.54‰ to −6.16‰ of for δ¹⁸O in the shallow groundwater. The δD and δ¹⁸O mean values of the shallow groundwater are relatively poorer than those of the subsidence lake. The results also show that the D-excess value of the subsidence lake (−2.93‰) is smaller than that of the shallow groundwater (4.03‰), indicating that the evaporation of the subsidence lake is stronger.

The values of δD and δ¹⁸O described in the δD-δ¹⁸O coordinate system are composed of the Global Meteoric Water line (GMWL), the Local Meteoric Water line of the China (LMWL) and the regional Local Evaporation line (LEL), as shown in Figure 8.

All samples were located below the Global Meteoric Water line (GMWL), the Local Meteoric Water line of the China (LMWL), indicating that the subsidence lake and shallow groundwater were mainly from atmospheric precipitation. The values of δD and δ¹⁸O were fitted by linear regression, and the equations were as follows: ⁠, ⁠. The d values of the subsidence lake and shallow groundwater were significantly smaller than that of the Global Meteoric Water Line (10‰), revealing different degrees of evaporation.

The two slope values of the fitting equation were both less than 8 (GMWL) and 7.83 (LMWL), indicating δD and δ¹⁸O enrichment occurred during evaporation due to dynamic fractionation. The subsidence lake samples were distributed approximately along LEL, and the content of δD and δ¹⁸O was higher than that of the shallow groundwater, indicating that secondary evaporation of the lake water is stronger than that of the shallow groundwater, which led to the excessive fractionation of δD and δ¹⁸O in the lake water. In addition, a large amount of coal gangue was accumulated around the subsidence lake shore, and the subsidence lake water was in full contact with silicate minerals of the coal gangue for a long time, resulting in oxygen isotope exchange equilibrium reaction, which was called silicate exchange and the reaction equation was (3). Due to the existence of various hydrocarbon groups and other hydrogen-bearing materials in coal-bearing strata, a hydrogen isotope exchange equilibrium reaction is produced on the basis of oxygen isotope exchange equilibrium, which is referred to as hydrocarbon exchange reaction and the equation was (4). The sufficient exchange of silicate and hydrocarbon groups in the subsidence lake resulted in the enrichment of δD and δ¹⁸O in the lake compared with that in the shallow groundwater.

There are many kinds of utilization forms of subsidence lake, such as industry and agriculture, breeding, landscape and so on. In this study area, agricultural activities are more intensive around the subsidence lake, and the shallow groundwater is also the main body of agricultural irrigation. Therefore, it is necessary to study the water quality of the shallow lake for irrigation.

The SAR, %Na, RSC and PI were used to evaluate the single index irrigation water of the subsided pond and shallow groundwater. The statistical summary of results are shown in Table 5 calculated by formulas (1), (2), (3) and (4).

According to the evaluation results of SAR and PI, the SAR index of all subsidence lake and shallow groundwater samples was at the very suitable grade and PI was at suitable grade. However, RSC and %Na have different evaluation results as follows: the RSC index of all samples was unsuitable grade; the %Na of all subsidence lake samples were in unsuitable state, and 50% of shallow groundwater was unsuitable and 50% was basically suitable. The results of evaluation indexes are obviously different, which is mainly due to the different emphasis of evaluation. The RSC was characterized by CO₃²⁻, HCO₃⁻, Ca²⁺ and Mg²⁺ to define alkali damage, and SAR, PI and %NA emphasize the characterization of Na⁺ to alkali damage. To find whether the water quality is suitable for irrigation, it is necessary for it to be further analyzed by applied comprehensive indexes.

The U.S. Department of Agriculture utilized the U.S. Salinity Laboratory's diagram (USSL diagram) to evaluate the quality of irrigation water, which combined the effects of SAR and EC values on soil (Richards 1954). According to the degree of salinization damage, irrigation water is divided into C1 low salinization (EC < 250 μS/cm), C2 medium salinization (250–750 μS/cm), C3 high salinization (750–2,250 μS/cm), C4 high salinization (>2,250 μS/cm). Then, according to the degree of alkalinity damage, irrigation water can also be divided into four types: S1 low degree alkali damage (SAR < 10 (meq/L)^1/2), S2 medium degree alkali damage (10(meq/L)^1/2–18(meq/L)^1/2), S3 high degree alkali damage (18(meq/L)^1/2–26(meq/L)^1/2), S4 very high alkali damage (>26(meq/L)^1/2). Therefore, the irrigation water can be divided into 16 categories by USSL.

The samples collected from the subsidence lake and shallow groundwater in the study area were plotted in the USSL diagram. As shown in Figure 9, the distribution of EC in the subsidence lake is concentrated with an average value of 764.74 μS/cm. The EC of the shallow groundwater varied significantly, ranging from 812 to 1,176 μS/cm, with an average of 996.83 μS/cm. All water samples are located in C3 area. Combined with the SAR value on the ordinate, most of the water samples are distributed in C3S1 and C3S2 areas, indicating the risk of high salt and low-moderate alkali damage. If the soil is excellent at leaching and drainage, irrigation can be carried out. Otherwise, the water should be treated and then irrigated, or plants with good salt tolerance should be selected to reduce the risk of salinity damage.

The Wilcox diagram was characterized by %Na and EC, divided into five regions as follows: Excellent to good, Go to permission, Permission to Doubtful, Doubtful to unsuitable and Unsuitable (Wilcox 1955). Assuming that the sample plotted in Excellent to good and Go to permissive region, such water for agricultural irrigation will not bring salt or alkali damage. While the sample plotted in Permissible to Doubtful region, the water may lead to a minor risk of alkali damage, which can be prevented by adopting appropriate measures. Also, there would be risks of salt and alkali damage plotted in the Doubtful to Unsuitable region. The water plotted in the Unsuitable region is not suitable for irrigation, and will bring serious salt and alkali damage.

The samples collected from the subsidence lake and groundwater in the study area were plotted in the Wilcox diagram. As shown in Figure 10, almost all the samples were distributed in the Permissible to Doubtful area, and only one sample was plotted in the Go to permission area, which indicating subsidence lake and groundwater for irrigation may lead to minor risk of alkali damage that can be prevented by adopting appropriate measures.

In this study, the analysis on the connection between the subsidence lake caused by coal mining and the shallow groundwater was firstly carried out. Moreover, the formation mechanisms and source were discussed based on Gibbs, correlation and stable isotope. In addition, water quality for irrigation purpose of two types of water were assessed. The outcomes of this research provide a better understanding of subsidence lake and groundwater and it will help in securing safe irrigation water sources for local agriculture. However, the various factors such as test data, reference values of atmospheric line, variability of samples and seasonal changes caused uncertain results, the subsidence lake and shallow groundwater need to be further studied.

Based on comparative analysis on the hydrogeochemical characteristics, the formation mechanism of chemical composition and sources of the subsidence lake and shallow groundwater were studied, and the suitability as irrigation water was evaluated. The results are as follows:

• (1)

  In both the subsidence lake and shallow groundwater, Na⁺ and Mg²⁺ are the two most abundant cations and Ca²⁺ is the second most abundant cation. The contents of anions in two types of water follow the same order: HCO₃⁻ > SO₄²⁻ > Cl⁻. The hydrochemical facies types of two types of water samples are controlled by NaHCO₃ (99.3%). Compared with the shallow groundwater, the surface water from the subsidence lake has a higher content of Na⁺ and HCO₃⁻, due to the dissolution of silicate minerals from coal gangue buried along the subsidence lake.

• (2)

  The chemical composition of surface water in the subsidence lake and shallow groundwater are similar, and are all mainly influenced by silicate minerals weathering and ion-exchange interactions. In addition, ion-exchange in the subsidence lake is more intense than in the shallow groundwater.

• (3)

  The values of δD and δ¹⁸O of samples follow the line: and ⁠, implying two kinds of water were mainly from atmospheric precipitation. Besides, the two slope values of the fitting equation were both less than 8 (GMWL) and 7.83 (LMWL), indicating the enrichment of δD and δ¹⁸O is related to dynamic fractionation. These similarities were clear evidence of connection between the local subsidence lake and shallow groundwater. The evaluation for irrigation purpose suggested that there is high salt and low-moderate alkali damage in the study area, which can be prevented by adopting appropriate measures.

• (4)

  In this research, the accuracy of test data, reference values of atmospheric line, variability of samples and seasonal changes lead to different degrees of uncertainty to results, which need to be further studied in the future.

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 the Key natural science research projects of Suzhou University (2020yzd03, 2020yzd07, 2019yzd01), National Natural Science Foundation of China (41773100) and Funding projects for research activities of academic and technological leaders of Anhui Province (2020D239).

The authors declare no conflict of interest.

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