4.5 Evaluation of groundwater quality
In drinking water, high concentrations of some ions could cause great harm to human health, such as Na+, Cl-, NO3-, SO42-, and son on (Wasana et al., 2016; Nixdorf et al., 2017; Wasana et al., 2017). For example, high NO3- concentrations could result in birth defect, hypertension and high-Fe hemoglobin (Carpenter et al. 1998; Ho et al., 2011; Jones et al., 2016); high SO42- concentrations could cause diarrhea, dehydration and weight loss (World Health Organization, 2008). According to the Chinese State Standards for drinking water quality (Chinese Ministry of Health, 2006) and Chinese State Standards for groundwater water quality (Chinese General Administration of Quality Supervision, 2017), the highest acceptable limits of pH, TDS, total hardness (TH) and the concentrations of Na+, Cl-, SO42-, and NO3- were 8.5, 1000 mg/L, 450 mg/L, 200 mg/L, 250 mg/L, 250 mg/L and 20 mg/L, respectively. As shown in Table 1 and Fig. 8, most of the groundwater was slightly and moderately hard freshwater, which fell within the standards for drinking water in all indices. However, the TDS of groundwater at location G4 (1520.05 mg/L), G6 (1092.44 mg/L) and G25 (1497.42 mg/L) was higher than the highest acceptable limits set by the standard (1000 mg/L) (Table 1); the Na+ concentration of groundwater at G6 (371.34 mg/L) and G25 (308.21 mg/L) were higher than the standard of 200 mg/L; the Cl- concentration of groundwater at G4 (294.79 mg/L) was higher than the standard of 250 mg/L; and the NO3- concentration of groundwater at G4 (100.88 mg/L), G11 (53.82 mg/L), G16 (74.46 mg/L), G24 (26.71 mg/L) and G25 (117.64 mg/L) were higher than the standard of 20 mg/L (Fig. 8). These all indicated that the groundwater in location G4, G6, G11, G16, G24 and G25 were all currently exceed the drinking standards and not suitable for drinking (Carpenter et al. 1998; Ho et al., 2011; Wasana et al., 2016; Wasana et al., 2017). According to the results of water level, isotope and hydrochemistry of groundwater around Qinghai Lake, high TDS and concentrations of groundwater in location G4 and G25 were dominated by heavy evaporation or dissolved solids (Xiao et al. 2012; Cui & Li, 2014). And high TDS and concentrations of groundwater in location G6 were dominated by recharging source of fissure water (Xu et al., 2010; Cui & Li, 2014). These all reflected the influences of natural factors on groundwater quality, such as geomorphology, terrain, groundwater aquifers, fault, and so on (Jin et al., 2009; Xiao et al., 2012).
In generally, the concentration of nitrate nitrogen in the water was higher than 3.0 mg/L, indicating that the water body was influenced by human activities (Babiker et al., 2004). As shown in Table 1, the concentration of NO3- in the groundwater were all higher than 4.60 mg/L, indicating that the groundwater around Qinghai Lake was affected by human activities, especially in location G4, G11, G16, G24 and G25 with high NO3- over the highest acceptable limits of NO3- (20 mg/L). Qinghai lake basin, lying in the Northeast Qinghai-Tibet Plateau, was a sparsely populated region with few mineral exploitation and large-scale factories (Li et al., 2018). Meanwhile, the region around the lake was a center of ecological tourism and animal husbandry for the Qinghai province (Tang et al., 1992). Therefore, the main sources of nitrate (NO3-) in groundwater around Qinghai Lake were animal feces and sewage. In order to further identify the source of nitrate in groundwater around the lake, the relationship between NO3- and Cl-of groundwater was analysed (Fig. 9). Because the ratio analysis of major ions in groundwater, such as NO3-/Cl-, SO42-/Cl- and Cl-/Br-, could be used to determine the source of nitrate pollution and the migration process of pollutants (Liu et al., 2006; Chen et al., 2009; Murgulet et al., 2013). As shown in the Fig. 9, there was a positive correlation between NO3- and Cl- ions in groundwater (n=34, P<0.001, R=0.747), further suggesting that the NO3- in groundwater was mainly originated from animal feces and sewage, which mainly came from the livestock breeding around Qinghai Lake.
Overall, the groundwater around Qinghai Lake came primarily from the atmospheric precipitation in the basin, the isotope and hydrochemistry of groundwater were mainly depended on the initial precipitation and the dissolution of surrounding rocks in runoff process, which were controlled by groundwater aquifers, fault, terrain and geomorphology (Jin et al., 2009; Xiao et al., 2012; Cui & Li, 2014). But the impacts of livestock manure and waste water on groundwater quality could not be ignored around the lake, and even high concentrations of some ions in groundwater at east and west of Qinghai Lake had exceeded the highest acceptable limit values for drinking water. These suggested that the pollution of groundwater in animal husbandry areas on the Qinghai-Tibet Plateau should be paid more attention, although the industrial and urbanization rates were evenly relative low on the plateau (Wang et al, 2014). Hence, the scientific planning and engineering management of livestock manure and wastewater discharge in animal husbandry regions are very necessary to be carried out urgently, which could not only protect water resources for drinking, but also contribute to human health and sustainable development of the environment of the Qinghai-Tibet Plateau.