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.