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.