Sample Collection

The study area is located in Laochang Town in the southeastern part of Fuyuan County, adjacent to Huangnihe Town in the east, with the geographical location ranging from 104°30′ to 104°35′E and 25°11′ to 25°16′N (Fig. 1). The area of Laochang Town is about 236 km², and the terrain is mainly mountainous, with elevation of 1480~2460 m, of which the karst area is about 159 km², located in the subtropical plateau monsoon climate area, with obvious three-dimensional climate, annual average temperature of 11-14ºC, rainfall mostly concentrated in May-October, annual rainfall of 1100-1500 mm, and average evaporation of 1885.5 mm. The regional geological work area is located in the old plant tectonic zone, which is oriented at 40°-45°N.E., with a length of about 20 km and a width of about 6 km.

The tectonic zone is mainly composed of the Old Plant backslope, the foot fractures parallel to the backslope and the Sulphur Plant fracture, which are all compression-torsional in nature. The exposed strata in the study area consist of the Lower Permian Mao Kou Formation tuffs, Upper Permian Longtan Formation tuffs, mudstones, siltstones, sandstones and coal beds, and the Lower Triassic Feixianguan Formation chip sandstones, rock chip siltstones and muddy siltstones. Groundwater mainly includes pore water of the loose layer of the Fourth Series, fracture water of coal clastic rock and fracture water of carbonate karst, etc. In this experiment, water samples of river water, groundwater, mine pit water and coal gangue drainage water were collected from the mine catchment area of Rongjia River Basin in Laochang in September 2020 (abundant water period) and March 2021 (dry water period), respectively. A total of 14 groups of isotope water samples were collected, including 12 groups of groundwater samples, 2 groups of river water samples; 14 groups of groundwater full analysis samples, 7 groups of mine water analysis samples and 16 groups of surface water samples (Fig. 2). The groundwater was divided into groundwater from the clastic rocks of the Triassic Feixianguan group, groundwater from the coal bed of the Permian Longtan group and groundwater from the chert of the Permian Maokou group, and the groundwater samples collected in this paper were mainly springs and deep groundwater from boreholes.

Testing Method

Sampling apparatus in accordance with the prescribed standards using polyethylene plastic bottles and brown hard glass bottles, sample bottles cleaned in advance with distilled water, and then rinsed with water samples 3 times before sampling [18]. The sampling bottle into the water filled with no air bubbles, the scene to add a good protective agent after sealing, low temperature storage and avoid direct sunlight. Groundwater sampling before letting it flow for 3-5 minutes, borehole sampling before pumping 10-15 minutes, surface water sampling sample bottle to the central river below the liquid level sampling. On-site use of portable multi-parameter water quality meter (DZB-718, Leici Co., Ltd, China) to determine the pH, temperature (T), electrical conductivity (EC) and mineralization (TDS) and other physicochemical parameters of water samples at the sampling site. The physical and chemical parameters such as K⁺, Na⁺, Ca²⁺, Mg²⁺ and metal content in the water are determined according to the relevant national standards of China [19-21]. Hydrogen (δD) and oxygen (δ18O) isotopes were determined using stable gas isotope mass spectrometer (253Plus, ThermoFisher Scientific, USA). All samples are additionally tested for two parallel samples to control the quality of analysis and testing. The isotopes were tested by testing the standard samples at the same time to ensure the quality, which ensured the real credibility of the analysis results.

Fig. 1.

Locations of in Laochang in the map of China.

Fig. 2.

Locations of sampling sites in Laochang.

Type of Water Chemistry

The results of water chemistry element testing at the water sample sites in the study area are shown in Table 1. Water chemistry type analysis was performed using Aquachem software to obtain the piper trilinear diagram (Fig. 3) [22]. The anions in the groundwater samples of the study area were dominated by HCO^3- and SO4^2-, and the cations were dominated by Ca²⁺ and Mg²⁺. The water chemistry types can be divided into HCO₃-Ca, SO₄·HCO₃-Ca·Mg, SO₄·HCO₃-Ca, HCO₃·SO₄-Ca, and HCO₃·SO₄-Ca·Mg, among which the HCO₃-Ca type was dominant. The anions in the pit mine water samples are mainly SO₄^2- and HCO₃^-, and the cations are mainly Na⁺, Ca²⁺ and Mg²⁺. The water chemistry types can be divided into SO₄-HCO₃-Na, SO₄-HCO₃-Ca-Mg, SO₄-Ca-Mg, etc., among which SO₄-HCO₃-Na and SO₄-Ca-Mg are mainly. The anions in surface water samples are mainly SO₄^2- and HCO₃^-, and the cations are mainly Na⁺, Ca²⁺ and Mg²⁺, and the water chemistry types can be divided into SO₄·HCO₃-Ca·Mg, SO₄·HCO₃-Ca, SO₄-Ca·Mg, SO₄-Ca, SO₄·HCO₃-Na·Ca, HCO₃·SO₄-Ca, HCO₃·SO₄-Na·Ca, etc., with SO₄·HCO₃-Ca·Mg, SO₄·HCO₃-Ca, and SO₄-Ca·Mg dominating (Fig. 3).

The study area is located in Yunnan, which is a typical karst landscape area [23], where the dissolution and precipitation of soluble rocks by groundwater and surface water are more obvious [24]. The reaction between the infiltrated recharged atmospheric precipitation and the overlying carbonate and saltstone minerals of the Maokou Formation and the pyrite and chalcopyrite minerals within the coal seam of the Longtan Formation, resulting in the generally high contents of HCO₃^-, SO₄^2-, Ca²⁺ and Mg²⁺ in the water samples of the study area [25, 26]. Similarly higher HCO₃^-, SO₄^2-, Ca²⁺ and Mg²⁺ were also observed in other study areas in Yunnan [25, 27, 28].

Recharge Characteristics of Groundwater

As can be seen from Table 1, the main ions of water samples in the study area are Ca²⁺, Mg²⁺, Na⁺, HCO₃^-, and SO₄^2-, and their sources are closely related to the lithology of the aquifer [29]. The area is mainly carbonate rocks as well as clastic rocks, and the dissolution filtration or precipitation of carbonate as well as silicate minerals affects the ionic composition in the groundwater of the study area [28, 30]. The analysis of the ion scale factor (Fig. 4) can effectively determine the main water-rock action in the study area [31]. Fig. 4a) shows the ratio of Ca²⁺ to SO₄^2-. The value of Ca²⁺/SO₄^2- in groundwater is basically greater than 1:1, and there may be dissolution of carbonate minerals such as calcite (CaCO₃) and dolomite (CaMg(CO₃)₂)), and the dissolution equation is shown in Equation (1) and Equation (2). The Ca²⁺/SO₄^2- value of surface water is basically close to 1:1, which shows that Ca²⁺ and SO₄^2- in water basically originate from the dissolution of gypsum (CaSO₄·2H₂O), and the dissolution equation is shown in Equation (3). The ratio of pit mine water and some surface water samples is less than 1:1, and the SO₄^2- content is too much, so there may be a cation exchange process (Equation (4)), which makes some Ca²⁺ replaced by Na⁺ [32].

Fig. 4b) shows the ratio of Na⁺ to Cl^-. The points of Na⁺/Cl^- values in groundwater, pit mine water and surface water are all located above the rock salt dissolution line with a ratio of 1:1, indicating that feldspar weathering and silicate mineral reactions may have occurred, or cation exchange, where Ca²⁺ is adsorbed onto clay minerals, allowing Na⁺ ions to be replaced and their content to increase [33]. As shown in Fig. 4c), the values of (Ca²⁺+Mg²⁺)/(HCO₃^-+SO₄^2-) in groundwater and most of surface water are on the 1:1 line, indicating that Ca²⁺, Mg²⁺, HCO₃^-, and SO₄^2- in water originate from the dissolution of carbonate minerals and sulfate minerals mainly, which verifies each other with the conclusion in Fig. 3a) [34]. Fig. 4d) shows the values of [(Ca²⁺+Mg²⁺) -(HCO₃^-+SO₄^2-)]/(Na⁺ -Cl^-), and all points are very close to the ideal value of -1, indicating that silicate dissolution and ion exchange interactions jointly influence the type of hydro-chemical interactions in the study area [35].

The ion exchange can be analyzed by Chloro-Alkaline Indices (CAI) [36], as shown in Equation (5). If the CAI is negative, it means that Ca²⁺ and Mg²⁺ in groundwater exchange ions with Na⁺ in the aqueous medium; if its value is positive, the Na+ in groundwater is replaced by Ca²⁺ and Mg²⁺. As shown in Table 1, the CAI -I values of the water samples are all negative, indicating that the replacement of Ca²⁺ and Mg²⁺ in water by Na⁺ in the water-bearing medium minerals is the main ion exchange process in the study area [37].

Fig. 3.

Piper diagram of water chemistry of water samples in the study area.

Fig. 4.

Proportion of major ions in water samples in the study area.

Sources of Main Groundwater Components

Hydrogen-oxygen isotope and water chemistry methods of water bodies are effective ways to study the water cycle, and stable hydrogen-oxygen isotope composition is an effective tracer to reveal the physicochemical processes of the water cycle, which plays an important role in studying the source and mode of groundwater recharge in the basin and the mutual transformation process of surface water and groundwater [14, 38-41]. A total of 14 sets of water samples were collected for hydrogen and oxygen isotope testing (Table 2), including 10 sets of groundwater samples, 2 sets of pit mine water samples, and 2 sets of surface water samples. The δD values of groundwater and pit mine water in the study area varied from -82.9 to -75.3 with a mean value of -78.82, and the δ18O values varied from -14.16 to -11.01 with a mean value of -12.13.

Craig (1961) proposed that the hydrogen-oxygen isotopic composition of atmospheric precipitation is linearly correlated [42]: δD = 8δ¹⁸O + 10, which is the accepted global meteoric water line (GMWL) [43]. The atmospheric precipitation line in the study area was selected from the east Yunnan-Qianxi atmospheric precipitation line with the relationship: δD = 7.848δ¹⁸O + 11. As shown in Fig. 5, the surface water sample points in the study area were distributed on the local atmospheric precipitation line, and most of the groundwater and surface water samples were distributed near the local atmospheric precipitation line, indicating that the groundwater mainly originated from atmospheric precipitation; however, the pit mine water and three groundwater samples, TW02, TW03 and TW05, were shifted, and the δ¹⁸O values decreased and were located to the left of the atmospheric precipitation line. According to Pang et al. (2016) the relationship between hydrogen and oxygen isotopes is divided into 13 lines [44], the points in the study area that are offset from the local atmospheric precipitation line are close to line 10 (Eq. (6)). For example, there is condensation of water vapor, resulting in low values of both hydrogen and oxygen isotopes with small slopes [45].

Gibbs plot can be effectively analyzed by Na⁺/ (Na⁺+Ca²⁺) and Cl^-/(Cl^-+HCO₃^-) compared with TDS, respectively (Fig. 7). The types of water chemistry can be classified into: “evaporation-controlled”, “rock-weathering” and “precipitation-controlled” [46]. Fig. 7 shows the Gibbs plot of the sampling points in the study area. The TDS ranges of groundwater and surface water are 56~370 mg/L and 158~876 mg/L, respectively, which are freshwater; the pit mine water has the highest TDS, ranging from 962~1656 mg/L, which are brackish water except for point KKS11. All water sample points fall in the rock weathering control area, which shows that the groundwater in the study area is mainly controlled by rock weathering [47].

Water-Rock Action Analysis

The saturation index (SI) is a parameter to study the saturation state of minerals in groundwater, which has an important indication in the study of water-rock interaction and is defined as [48]:

In Equation (7), LAP is the activity product of the relevant ions in the dissolution reaction of minerals; K is the equilibrium constant of the dissolution reaction of minerals at a specific temperature. When SI<0, it indicates that the mineral is in an unsaturated state in solution; when SI = 0, the mineral is in dissolution equilibrium in solution; when SI>0, the mineral is supersaturated in solution.

According to the water chemistry results, the main cations at the water sample sites are Na⁺, Ca²⁺, and Mg²⁺, and the main anions are SO₄^2- and HCO₃^-. Therefore, the possible mineral phases in the study area are calcite, dolomite, gypsum, and anhydrous gypsum. The saturation index of each mineral was calculated using PHREEQC geochemical software [49], and the results are shown in Fig. 8. The SI values of calcite at each water sample point are close to 0, that is, calcite is basically in dissolution equilibrium, and there may be mineral precipitation if it continues to dissolve [50]; the SI values of the remaining minerals (dolomite, gypsum, anhydrous gypsum) are basically less than 0, which are in unsaturated state, indicating that the water samples in the study area have the ability to continue to dissolve the surrounding rocks [48]. Furthermore, the ratio of pit mine water to some surface water is on the 1 :1 line (Fig. 3c), suggesting that other water-rock interactions are involved [51].

Water Risk Assessment in Mining Areas

It is worth noting that the study area contains relatively extensive semi-metallic deposits, so the risk of elemental exceedances in this area requires special attention [52, 53]. We can see from the data in Table 1 that groundwater is a little more polluted than surface water. Compared with surface water pollution, groundwater contamination is much more challenging to address once it has occurred. The most direct route of contamination of aquifers in the present study area is through open pit mining or underground mining below the water table [54]. According to the groundwater quality standards in the geological and mining industry standards of the People’s Republic of China (DZ/T 0290-2015). The pH values of groundwater sampling sites Q3, Q9 and Q12 reached Class IV and V category, respectively. In terms of SO₄^‑ content, Q2, Q6, Q7, Q9 and Q13 sampling points are category II, and Q12 is category III. Fe content of Q13 reached IV category; Q6, Q8, and Q12 have Mn levels up to Category IV. Class IV and V waters are not suitable for direct use as drinking water because of their high chemical content. Excessive levels of iron and manganese not only affect the color, odor and taste of drinking water, but also have a negative impact on human health [55]. For example, excessive iron intake may cause bacterial infections, acute iron toxicity, hemochromatosis and other diseases in patients [56]; excessive manganese may cause negative reactions in the gastrointestinal tract, lesions in the oral mucosa, and neurological damage [57]. Therefore, groundwater from the above exceedance sites requires some treatment before drinking, and future research should emphasize the migration and transformation of contaminants in groundwater.

Fig. 5.

Stable isotope distribution of groundwater, pit mine water and surface water.

Fig. 6.

Isotope geological indicators and water-rock interaction processes: (1) Global atmospheric precipitation line GMWL (2) Moisture cycle (3) H₂S exchange (4) CO₂ exchange (5) Water-rock action in underground hot water (6) Water mixing in andesite medium (7) Evaporation operation (8) Seawater mixing (9) Silicate hydrolysis (10) Water vapor condensation (11) Paleo-atmospheric water (12) Hydrocarbon exchange (13) Clay mineral exchange.