There are 220 CBM extraction wells in Linfen mining area. The main CBM wells are developed in the No. Five coal seam of Shanxi Formation and No. Eight coal seam of Taiyuan Formation. The well types are straight wells and directional wells. According to the location of CBM wells and single layer drainage, a total of 14 representative coalbed-produced water samples were collected to cover the whole area as much as possible, including 10 samples from No. Five coal seam and four samples from No. Eight coal seam. Sample numbers were J5-1, J5-2, J5-3, J5-4, J5-5, J5-6, J5-7, J5-8, J5-9, J5-10, J8-1, J8-2, J8-3, J8-4, respectively. Before sample collection, polyethylene sampling bottles were sterilized in the laboratory under UV light. The water samples were collected using 2.5 L and 500 ml polyethylene sampling bottles at the drainage of CBM wells to collect coalbed-produced water. The 500 ml polyethylene sampling bottles were acidified to pH <3 with dilute hydrochloric acid on site for subsequent cation detection. The 2.5 L polyethylene sampling bottles were used to collect raw water samples for pH, anion, TDS, trace elements, hydroxide isotopes and dissolved inorganic carbon isotope composition. The water samples were collected in the field by rinsing the polyethylene sampling bottle three times with the coalbed-produced water. Then filtered microorganisms, coal dust and suspended substances with 0.5 μm filter paper placed at the mouth of the bottle, and filled the bottle to remove the air from the bottle. Next sealed the bottle with the cap and checked whether it leaked. Finally marked the sampling time and place and sent it to the Guizhou Institute of Geochemistry, Chinese Academy of Sciences for testing.

The water samples were tested for anions and cations, hydrogen and oxygen isotopes of water, dissolved inorganic carbon isotopes and trace elements. Among them, the instrument used for cation test is Vista MPX inductively coupled plasma - emission spectrometer of Varian, United States. The anion test used ion chromatography (Thermo Fisher ICS-90). The HCO₃ ⁻ and CO₃ ²⁻ ion test used acid titration method to detect. Anion and cation detection standards referred to GB/T 5750.6-2006 and DZ/T 0064.51-1993, respectively. The instrument used for hydrogen and oxygen isotope testing was the liquid isotope analyzer 912-0026, with a standard deviation of 0.6‰ for δD and 0.1‰ for δ ¹⁸O. The isotopes of dissolved inorganic carbon were measured by gas isotope mass spectrometer MAT252 with an accuracy of ≤0.01‰. The isotopes of hydrogen, oxygen and inorganic carbon were measured according to DZ/T 0184.19-1997, DZ/T 0184.21-1997 and GB 13193-91, respectively.

The pH was measured by PP-50-p11 m and TDS was measured by DDSJ-308A conductivity meter. The testing standards were executed according to GB/T 6920-1986 and DZ/T 0064.9-1993, respectively.

The results of water quality testing and analysis of coalbed-produced water in the Linfen mining area were shown in Table 1. The pH of the produced water of No. Five coal seam ranged from 6.91 to 7.85 with an average value of 7.38. The produced water of No. Five coal seam was weakly alkaline, which was favorable to the survival of methanogenic bacteria in the coalbed-produced water. The pH of the produced water of No. Eight coal seam ranged from 6.78 to 9.14 with an average value of 7.62. But the pH of No. Eight coal seam had a larger span and a higher average pH than that of No. Five coal seam. The cations of coalbed-produced water in the study area are mainly Na⁺. The concentration of Na⁺ ranges from 2134.10 to 8506.80 mg/L. In addition, they also contain certain concentrations of Ca²⁺, Mg²⁺ and K⁺. The anions were dominated by Cl⁻. The concentration of Cl⁻ ranges from 2170.09 to 13,785.20 mg/L. In addition, they also contained CO₃ ²⁻, HCO₃ ⁻ and SO₄ ²⁻. Among them, CO₃ ²⁻ was only detected in J8-4 with a concentration of 249.73 mg/L. The concentration of HCO₃ ⁻ ranges from 671.24 to 1509.02 mg/L, and higher levels of HCO₃ ⁻ were detected in 10 CBM wells in No. Five coal seam and four CBM wells in No. Eight coal seam. The higher content of HCO₃ ⁻ suggested a higher degree of confinement in the groundwater environment. The SO₄ ²⁻ concentration ranges from <0.10 to 4.23 mg/L. The concentration of TDS in the study area ranges from 5011.45 mg/L to 23,405.39 mg/L. The TDS of J5-7, J5-8, J5-9, J5-10, J8-1 and J8-2 were all higher than 13,000 mg/L. The higher mineralization may be related to groundwater rock action.

The data of 14 sets of water samples from the Linfen mining area were plotted on a Piper trilinear graph (Figure 1A), showing that the water chemistry type in the study area is Cl-Na type. According to the box plot of ion concentrations in coalbed-produced water in the study area (Figure 1B), the concentrations of Na⁺, Cl⁻ and HCO₃ ⁻ are much larger than other ions. The data are more concentrated, with the median line located almost half of the length of the square box, indicating that the concentrations of Na⁺, Cl⁻ and HCO₃ ⁻ ions do not vary much. The concentration of SO₄ ²⁻ is lower and the data are scattered. The lower quantile line of Ca²⁺ and SO₄ ²⁻ box plots with the lower quantile line far from the square box imply that the Ca²⁺ and SO₄ ^2- concentration content of each individual well is low and varies more than most CBM wells. The anomalous data were marked by boxes in the graph. The presence of anomalous values of K⁺, Ca²⁺, Mg²⁺, and HCO₃ ⁻ concentrations in the coalbed-produced water indicate that they are related to the groundwater environment and that groundwater-rock interactions need to be identified.

Table 2 shows the hydrogen, oxygen and carbon isotope values of the coalbed-produced water in the Linfen mining area. As shown in, the hydrogen and oxygen isotopes of the produced water from No. Five coal seam and No. Eight coal seam do not differ significantly. δD ranges from −85.84‰ to ⁻64.31‰, with an average value of −75.22‰. δ ¹⁸O ranges from −12.14‰ to −9.07‰, with an average value of −10.74‰. However, the inorganic carbon isotopes of the produced water from No. Five coal seam and No. Eight coal seam differed significantly. The dissolved inorganic carbon isotopes δ ¹³C_DIC of the produced water from No. Eight coal seam ranges from 7.98‰ to 12.68‰ with a mean value of 10.36‰. The dissolved inorganic carbon isotopes δ ¹³C_DIC of the produced water from No. Five coal seam ranges from 25.79‰ to 36.50‰ with a mean value of 30.59‰. All of the δ ¹³C_DIC in the study area exhibited positive values, indicating that dissolved CO₂ in groundwater is enriched in ¹³C (Li et al., 2022).

In this article, a total of 22 trace elements were detected in the coalbed-produced water of the Linfen mining area. The average content of Cd, Cu, Hg, Pb, Sb, Sn, Ti, Tl, U, and Zr elements below 0.10 μg/L were not analyzed, and the statistical results of other elements with stable distribution and high content were shown in Table 3. The trace elements in the produced water of No. Five coal seam and No. Eight coal seam do not vary much. The content of Li in this area ranges from 287.88 to 1923.32 μg/L, with an average value of 721.11 μg/L. The content of Al ranges from 1.60 to 61.57 μg/L, with an average value of 14.77 μg/L. The content of As ranges from 1.25 to 9.48 μg/L, with an average value of 4.24 μg/L. The content of Ba ranges from 4323.18 to 69,528.23 μg/L, with an average value of 35,064.76 μg/L. The content of Co ranges from 0.10 to 5.02 μg/L, with an average value of 1.31 μg/L. The contents of Ni ranges from 0.52 to 39.43 μg/L, with an average value of 15.28 μg/L; The contents of Cr ranges from 0.65 to 1.13 μg/L, with an average value of 0.85 μg/L; The contents of Zn ranges from 0.75 to 16.89 μg/L, with an average value of 7.23 μg/L; The contents of Sr ranges from 3996.26 to 107,240.41 μg/L, with an average value of 28,282.97 μg/L; The contents of Rb ranges from 13.25 to 79.23 μg/L, with an average value of 37.30 μg/L. The content of Mo ranged from 1.52 to 143.37 μg/L, with an average value of 20.00 μg/L. The content of Mn ranges from 21.47 to 1116.92 μg/L, with an average value of 434.81 μg/L. The concentrations of Li, Ba, Sr, Rb, Mo and Mn in the coalbed-produced water are relatively high, and the average values are all greater than 20 μg/L. Ba is the trace element with the highest content in the coalbed-produced water in this area. The average concentrations of trace elements in the coalbed-produced water in this area are Ba>Sr>Li>Mn>Rb>Mo>Zn>As>Co>Cr.

The hydrogen-oxygen isotopic composition of coalbed-produced water can reflect the source of produced water. Craig (1961) obtained the global meteoric water line (GMWL) as: δD = 8δ ¹⁸O+10. The Chinese meteoric water line (CMWL) first proposed by Zheng et al. (1983), expressed as: δD = 7.9δ ¹⁸O+8.2. The hydrogen-oxygen isotope data in Table 2 were projected onto the δ ¹⁸O-δD relationship diagram (Figure 2), it was found that the hydrogen-oxygen isotope projections of the produced water from 14 CBM wells in the study area were all located near the GMWL and CMWL. It showed that the coalbed-produced water in the study area originated from atmospheric precipitation and received recharge from atmospheric precipitation, providing a way to carry microorganisms into the coal seam for microbial degradation to occur.

Dissolved inorganic carbon in coalbed-produced water is generally considered to originate from dissolution of carbonate minerals, dissolution of CO₂ in CBM, and microbial action (Lemay and Konhauser, 2006). Figure 3 showed the relationship between HCO₃ ⁻ and δ ¹³C_DIC in the coalbed-produced water in the study area. As shown in, both were positive and showed a significant negative correlation. This suggested that strong microbial action had occurred in situ in the coal seam in the study area. The positive δ ¹³C values indicate the occurrence of methanogenesis in groundwater environments because methanogens preferentially utilize ¹²CH₄, resulting in the remaining ¹³C enriched in CO₂ and DIC (Meng et al., 2014; Li et al., 2022). The comparison shows that No. Five coal seam δ ¹³C_DIC is heavier than that of No. Eight coal seam, implying that microbial degradation occurs more strongly in No. Five coal seam than in No. Eight coal seam.

Figure 4 shows the radar map of trace element content distribution in coalbed-produced water. The content of Ba, Sr, Mn, Li and other elements are relatively high. On the one hand, these trace elements have their own active chemical properties and easily loss or gain electrons with existing in ion form in produced water. On the other hand, Ba, Sr, and Mn elements have high affinity for silica-aluminates. Li element has carbonate affinity (Dai et al., 2005), and these properties result in different trace elements. During the contact between the coal seam and the surrounding rocks, a certain degree of enrichment of these trace elements will occur in the water through dissolution-precipitation, adsorption of substances such as iron-manganese oxides and hydroxides, carryover of organic matter and ion exchange of clay minerals, etc. In addition, the stronger the groundwater dynamic conditions, the stronger the water-rock interaction will be, and the higher the element dissolution will be. As a result, the coalbed-produced water contains higher concentrations of trace elements such as Ba, Sr, Mn and Li.

The source of ions and the process of water-rock action can be determined from the relationship between the content of major ions in water (Lakshmanan et al., 2003). The occurrence of cation exchange is usually reflected by the ratio relationship between γ(Na⁺-Cl⁻) and γ[(Ca²⁺+Mg²⁺)-(HCO₃ ⁻+SO₄ ²⁻)] (Tang et al., 2013)^. The relationship between the chloride base index (CAI-Ⅰ=[Cl⁻-(Na⁺+K⁺⁾/Cl⁻]) and (CAI-Ⅱ=[Cl⁻-(Na⁺+K⁺)]/(SO₄ ²⁻+HCO₃ ⁻+CO₃ ²⁻)) relationships can characterize the direction and intensity of ion exchange (Li et al., 2013). The ratio relationship between γ(Na⁺+K⁺) and γ(Cl⁻) can reflect the source of Na⁺ and K⁺ (Li et al., 2015). The relationship between the ratio of γ(Ca²⁺+Mg²⁺) and γ(HCO₃ ⁻+SO₄ ²⁻) can determine the source of Ca²⁺ and Mg²⁺ in the produced water (Tang et al., 2013). The relationship between the ratio of γ(Ca²⁺+Mg²⁺-HCO₃ ⁻) and γ[SO₄ ^2--(Na⁺-Cl⁻)] can reflect the source of SO₄ ²⁻ in the produced water (Singh et al., 2015).

The relationship between γ(Na⁺-Cl⁻) and γ[(Ca²⁺+Mg²⁺)-(HCO₃ ⁻+SO₄ ²⁻)] of the coalbed-produced water in the study area is shown in (Figure 5A), which show a significant negative correlation (R ²=0.77), indicating that cation exchange is occurring in this mine. The chloride and alkali index of the coalbed-produced in the Linfen mining area is almost less than 0 (Figure 5B), implying that positive cation exchange occurs mainly in the study area. Exchange Ca²⁺ and Mg²⁺ in the water with Na⁺ in the surrounding rocks, resulting in an increase in the Na⁺ content in the coalbed-produced water. The reaction mechanism is shown in Eq. 1 (Houatmia et al., 2016). N a + R o c k + C a 2 + + M g 2 + W a t e r = C a 2 + + M g 2 + R o c k + N a + W a t e r (1)

The sample points of the coalbed-produced water in the study area basically fall above the γ(Na⁺+K⁺)/γ(Cl⁻)=1:1 line (Figure 5C), indicating that the main source of Na⁺ and K⁺ in the study area is the dissolution of silicate minerals such as potassium feldspar and sodium feldspar. The reaction mechanism is shown in Eqs 2, 3. 2 N a A l S i 3 O 8 S o d i u m   f e l d s p a r + 2 C O 2 + 11 H 2 O → A l 2 S i 2 O 5 O H 4 + 2 N a + + 4 H 4 S i O 4 + 2 H C O 3 − (2) 2 K A l S i 3 O 8 p o t a s s i u m   f e l d s p a r + 2 C O 2 + 11 H 2 O → A l 2 S i 2 O 5 O H 4 + + 2 K + + 4 H 4 S i O 4 + 2 H C O 3 − (3)

The ratio of γ(Ca²⁺+Mg²⁺) to γ(HCO₃ ⁻+SO₄ ²⁻) in the coalbed-produced water in Linfen mining area is almost all less than 1 (Figure 5D), suggesting that the dissolution of silicate minerals and evaporite is the main source of Ca²⁺ and Mg²⁺ in the coalbed-produced water (Li et al., 2015). Figure 5E shows the scatter plot of γ(SO₄ ²⁻+Cl⁻) to γ(HCO₃ ⁻) milligram equivalent ratio of coalbed-produced water, which can reflect the dissolved evaporite and carbonate rocks in the water column (Tang et al., 2013). The sample points of the coal seam produced water in the study area were mainly distributed above the 1:1 line, exhibiting that the dissolution of evaporite in the produced water mainly occurred in the study area. γ(Ca²⁺+Mg²⁺-HCO₃ ⁻) can characterize the Ca²⁺ concentration and γ[SO₄ ²⁻-(Na⁺-Cl⁻)] can characterize the SO₄ ²⁻ concentration of gypsum dissolution in the produced water. The sample points of coal seam output water in the study area were all distributed above the γ(Ca²⁺+Mg²⁺-HCO₃ ⁻)/γ[SO₄ ²⁻-(Na⁺-Cl⁻)] = 1:1 line (Figure 5F), displaying that SO₄ ²⁻ in coalbed-produced water mainly originated from the dissolution of gypsum and mannite.