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).