4.2 Level variation of groundwater around Qinghai Lake
Due to some residents’ wells being sealed, groundwater level was investigated in only 14 sites among the 34 sampling sites around Qinghai Lake (Table 2). The buried depth of the groundwater level ranged from 0.85 to 10.87 m (mean 3.97 m). The depth range was similar as that of previous study (0.9 m-8.2 m, mean 3.80 m; Xiao et al., 2013). However, the buried depth of groundwater level couldn’t reflect the direction of groundwater due to the different elevation among the sampling sites. The relative altitude between the groundwater level and the water level of Qinghai Lake was calculated at each sampling site (Table 2). The relative altitude of groundwater level ranged from -1.27 to 122.91 m (mean 25.70 m). The relative altitudes of most of the groundwater level, excluding G6 and G29, were positive values (Table 2), indicating the groundwater level were higher than the water level of Qinghai Lake, and most of the groundwater flowed into the lake, while the groundwater in location G6 and G29 would be recharged by the lake water. Based on the data of the positive altitude of groundwater level (Table 2), the average relative altitude was calculated for each region around the lake and were arranged: south > north > west > east, with altitude of 46.70 m, 38.32 m, 38.04 m, and 7.58 m, respectively.
The hydraulic gradient of each groundwater was calculated based on the relative altitude of groundwater level and the distance between sample site and Qinghai Lake (Table 2). The hydraulic gradient ranged from -6.20 to 43.14‰, with the average of 5.80‰. The average hydraulic gradient of each region around the lake was in the order: south > east > north> west, with value of 19.70‰, 4.61‰, 4.33‰, and 1.73‰, respectively, indicating that the driving force of groundwater at the south of Qinghai Lake was stronger than other regions. According to the Fig. 2, fault escarpments and lacustrine sediment terraces had been developed extensively along the southern shore of Qinghai Lake, while the areas surrounding the lake and rivers were overlain by alluvial and lacustrine sediments whose age ranges from Quaternary to recent times(Fig. 2). The mountain runoff in the diluvial and alluvial plains recharged the groundwater via infiltration (Xiao et al. 2012). Therefore, the distribution and variation of groundwater level around Qinghai Lake would be mainly related to geomorphology, terrain, groundwater aquifers, fault, and so on (Jin et al., 2009; Xiao et al., 2012).