4.3 Stable isotopes of groundwater around Qinghai Lake
The δ18O values of the groundwater around Qinghai Lake
ranged from -10.41 to -5.86 ‰ (mean -7.95 ‰), while the
δ2H values ranged from -66.64 to -41.99 ‰ (mean -51.37
‰) (Fig. 4). The values fell within the isotope ranges of precipitation
in the Qinghai Lake Basin (Fig. 3, Fig. 4). Comparing the groundwater
samples with the local meteoric water line (LMWL) was useful for
determining the water source in the investigation of the regional
hydrology (Clark & Fritz, 1997). Most of the isotope data points lay
close to the LMWL, and the slope of
local
evaporation line of groundwater (LEL: δ2H = 6.08
δ18O-3.01) was lower than the slope of LMWL (7.80;
Fig. 4). These all indicated that the groundwater around Qinghai Lake
mainly came from the precipitation in the basin, which had undergone
variable degrees of evaporation before infiltration (Friedman et al.,
1962; Gremillion & Wanielista, 2000; Cui & Li., 2014).
According to the Fig. 5, the δ18O values of the
groundwater lying on the east, south, west and north of Qinghai Lake
ranged from -10.41 to -7.78‰, from -9.03 to -6.57‰, from -8.98 to -5.86‰
and from -7.20 to -6.68‰, respectively, with average
δ18O values of -8.62‰, -7.95‰, -7.75‰, and -6.90‰,
respectively, indicating that the groundwater around the lake were
recharged by precipitation at different altitude, and the precipitation
had undergone variable degree of evaporation before infiltration
(Friedman et al., 1962; Gremillion & Wanielista, 2000; Cui & Li.,
2014). The average δ18O of the groundwater was in the
order:
δ18ONorth>δ18OWest>δ18OSouth>δ18OEast.
There
were two possible scenarios: One possibility was that the groundwater
lying on the east of Qinghai Lake could have a relative higher recharge
altitude than other regions around Qinghai Lake, the second possibility
was that the groundwater lying on the north of Qinghai Lake could have
undergone a stronger evaporation than other regions
around
the lake before infiltration. The LEL slope of groundwater lying on the
east, south, west and north of Qinghai Lake was 7.25, 4.48, 4.22 and
3.94, respectively (Table 3; Fig. 4), indicating that evaporation degree
of the groundwater before infiltration was in the order: North
> West >South > East (Weyhenmeyer
et al., 2002; Dogramaci et al., 2012), the result was agreed upon that
of the second possible scenario. Comparing with the east and south
regions, the north and west regions overlain by alluvial and lacustrine
sediments had a relatively flat terrain with low hydraulic gradient
(Fig. 1, Fig. 2, Table 2), leading the surface water flowed slowly and
had long time to infiltrate (Gibson et al. 2005; Buda, 2013). The longer
time surface water infiltrated into groundwater, the higher evaporation
degree the groundwater was undergone before infiltration (Weyhenmeyer et
al., 2002). These all suggested that evaporation degree of the
groundwater in west and north regions was higher than that in south and
east of Qinghai Lake.
In order to eliminate the influence of evaporation on the groundwater,
the intersection between the LEL of groundwater in each region and the
LMWL of precipitation was calculated (Table 3). The values of
δ2H and δ18O on the intersection
were the recharge (initial) isotope values from precipitation to the
groundwater (Clark & Frjtz, 1997; Cui & Li, 2014). The initial isotope
value of the groundwater was in the order:
δNorth>δWest>δSouth>δEast,
with value of -7.30‰, -8.09‰, -8.11‰, and -11.14‰, respectively (Table
3). Due to altitude effect of δ18O in precipitation,
the average recharge altitude of the groundwater in four regions was in
the order: East > South > West >
North (Cui & Li, 2014).
According to the Fig. 5, the δ18O value of G29
(-5.86‰) was higher than that of other groundwater and lower than that
of lake water (average δ18O: 1.61‰; Cui et al., 2016),
and the water level of G29 (relative altitude of -1.27m, hydraulic
gradient of -6.20‰) was lower than the water level of Qinghai Lake
(Table 2), suggesting that the groundwater in location G29 would be
recharged by Qinghai Lake water. The water level of G6 (relative
altitude of -0.12m) was also lower than the water level of Qinghai Lake
(Table 2), but the δ18O value of G6 (-9.03‰) was
relative low (Fig. 5). There could be two reasons for the low water
level and low δ18O value in G6. One possibility was
that the hydraulic connection between the G6 and lake water was not
closely due to the slightly negative hydraulic gradient (-0.24‰); the
second possibility was that the G6 was recharged by fissure water with
relative depleted isotopes, because G6 was located on the fault zone of
the southern margin of Zhongqilian Massif (Fig. 2). Overall, the
groundwater around the lake was mainly recharged by precipitation, and
the δ18O values of groundwater in different locations
suggested that the altitude of the recharge areas varied.