4.4 Hydrochemistry groundwater around Qinghai Lake
The
pH values of the groundwater around the lake ranged from 7.54 to 8.36
with a mean value of 8.04, indicating that the groundwater was slightly
alkaline. The pH values were similar as those of the river waters
(7.60-8.55) in the Qinghai Lake Basin (Jin et al., 2010). The electrical
conductivity (EC) and total dissolved solids (TDS) values of most of
groundwater samples, excluding G4, G6 and G29, ranged from 0.39 to 1.76
mS/cm and from 269 to 885 mg/L, respectively, with averages of 0.89
mS/cm and 523 mg/L, respectively. Compared with the river water (the
average EC and TDS were 0.17 mS/cm and 341.79 mg/L, respectively) (Cui
& Li, 2015b), the EC and TDS of the groundwater were relatively high,
indicating that the interaction between water and rocks was stronger in
the groundwater than that in the river water. The hydrochemical type of
most of groundwater samples, excluding G1, G4, G6, G7, G10, G11, G23,
G25, and G29, was Ca-Mg-HCO3 (Fig. 6). The
concentrations of Ca2+ and Mg2+ were
relatively high, accounting for more than 60% of the cations. This
finding indicated that the chemistry of groundwater would be mainly
controlled by carbonate dissolution around Qinghai Lake.
For each index according to the Table 1, The TDS of groundwater at G4,
G6 and G25 were more than 1000 mg/L; The Na+concentration of groundwater at G1, G4, G6, G25 and G29 were more than
170 mg/L; The Mg2+ concentration of groundwater at G4
were more than 110 mg/L; The Cl- concentration of
groundwater at G4 and G6 were more than 140 mg/L; The
SO42- concentration of groundwater at
G1, G4 and G25 were more than 150 mg/L. And the hydrochemical types of
G1, G4, G6, G25 and G29 were Na-Cl-SO4, Ca-Mg-Cl,
Na-Cl-CO3, Na-Ca-HCO3,
Na-Mg-HCO3, respectively (Fig. 6). There could be two
reasons for the high TDS and high concentrations of
Na+, Mg2+, Cl-,
and SO42- in the groundwater. One
possibility was that the longer flow path increased the interaction time
between water and rocks and raised the dissolved solids; the second
possibility was that the groundwater was recharged by the lake water.
The relative altitude of water level of G29 (-1.27 m) was lower than
water level of Qinghai Lake; the location of G29 is very near from
Qinghai Lake, with the distance of 0.20 km (Table 1, Fig. 1); and the
δ18O value of G29 (-5.86‰) was higher than other
groundwater, indicating that there would have a significant hydraulic
contact between G29 and the lake, and G29 would be recharged partly by
the Qinghai Lake water (Cui et al., 2016). The δ18O of
G1 (-8.29‰) and G6 (-9.03‰) were relatively lower than the
δ18O of other groundwater around Qinghai Lake (Table
1), and G1 and G6 were located on the fault zone of the southern margin
of Zhongqilian Massif (Fig. 2), indicating that the G1 and G6 were
recharged partly by fissure water with relative depleted isotope. The
relative altitudes of water level of G4 and G25 were higher than that of
Qinghai Lake, and the δ18O value of G4 (-7.85‰) and
G25 (-7.54‰) was slightly high (Table 2), indicating there were no
significant hydraulic contact between G4, G25 and the lake. High TDS and
high concentrations would be dominated by heavy evaporation or dissolved
solids (Xiao et al., 2012; Cui & Li, 2014). Meanwhile, G1, G4, G6, G25
and G29 were all located at east and west of Qinghai Lake (Fig. 1),
suggesting that the sources of groundwater at east and west of Qinghai
Lake were relative complex.
As shown in Fig. 7, all of the samples fell within the evolutionary path
from “Rock dominance” to “Ocean” in the Gibbs boomerang envelope
(Gibbs, 1970; Machender et al., 2014), suggesting that the chemical
composition of the groundwater around the lake was dominated by
rock-weathering. The results were supported by that rock weathering, ion
exchange and precipitation were the major geochemical processes
responsible for the solutes in the groundwater within the Qinghai Lake
Basin (Xiao et al. 2012; Cui & Li, 2014). Meanwhile, relative high
concentrations of Na+ and Cl- in
some groundwater were due to the evaporite dissolution in this area (Xu
et al., 2010) or to the long migration path of groundwater with strong
water-rock interaction (Cui & Li, 2014).