4.1 Geochemical results
Geochemical composition of CBM co-produced water is useful to identify hydraulic zone as water and rock interact along the flow paths. CBM co-produced water usually contained several important ions, with Na+, K+, Ca2+, Fe3+, Mg2+, Cl-, HCO3-, CO32-, NO3- and SO42-accounting for most of the total solute in groundwater. Generally, shallow coal seam water in runoff area was characterized with lower KDS and higher Fe3+, NO3- and SO42-contents, whereas deep coal seam water in stagnant area was characterized with higher KDS contents and lower Fe3+, NO3- and SO42-contents.
The concentrations of Na+ (K+) in the water samples ranged from 241 to 1187 mg/L, whereas the concentrations of Cl- ranged from 35 to 1609 mg/L. The concentrations of Ca2+ in the CBM co-produced water ranged from 0.12 to 10.7 mg/L. The concentrations of Mg2+ ranged from 0.57 to 4.31 mg/L. Dissolution of halite (NaCl) or sylvite (KCl) is the main source of Na+ or K+ in coal reservoir water. Additionally, silicates weathering could release Na+or K+, respectively. Cation exchange between Ca2+ or Mg2+ and Na+ or K+ can increase content of Na+ and K+ by reducing Ca2+ or Mg2+. The water rock reaction in the stagnant area was more sufficient, which led to the increase of mineralization (KDS) in the stagnant area.
In contract, NO3-, SO42- and Fe3+ were more abundent in the runoff area, they are anaerobic electron acceptors closely realted to anaerobic respiration of microorganisms. The concentrations of NO3- ranged from 0.24 to 1.01 mg/L. The concentrations of SO42- ranged from 6.93 to 17.02 mg/L. The concentrations of Fe3+ ranged from 0.3 to 5.57 mg/L.
The hydrogen and oxygen isotope composition of water could be used to identify the groundwater source. The following equation of atmospheric precipitation line in China was adopted: δD = 7.9δ18O+8.2. According to Fig. 7a, the water samples taken were distributed near the atmospheric precipitation line, indicating that the source of coal seam water is predominantly atmospheric precipitation.
The value of δ13CCH4 ranged from -20 to -40‰ and δD ranged from -270 to -130‰, both the methane gas samples and soluble methane samples suggested methane in the study area is thermogenic origin (Fig. 7b).
The value of δ15NNO3ranged from -1 to -10‰ and δ18ONO3ranged from -6 to -7‰, suggesting the nitrate may come from biological nitrogen fixation or nitrification of NH4+ fertilizer on the ground (Fig. 7c).