1. Introduction
Groundwater is a strategic and life-sustaining resource that is utilised
for drinking water by 1.5 billion people around the world (Alley et al.,
2002). Meanwhile, it plays significant roles in agriculture, the health
of ecosystems, and the sustainable development of human societies and
economies, especially in cold and semi-arid regions where surface water
resources are relatively deficient (Giordano et al., 2009; Siebert et
al., 2010; Qiu, 2010; Gleeson et al., 2012; Gleeson et al., 2016; Van
Loon et al., 2016). In recent years, groundwater quality has been
affected by the intensification of human activities as well as global
climate change, which has attracted extensive attention worldwide (Li et
al., 2013). Previous studies have found that groundwater levels have
fallen at a phenomenal rate on both regional and global scales (Postel
et al., 1996; Vorosmarty et al., 2000; Oki et al., 2006; Giordano et
al., 2009; Rodell et al., 2009; Gleeson et al., 2012; Wada et al.,
2012). Unsustainable depletion of groundwater would lead to a series of
environmental problems, such as ground subsidence, aquifer unwatering,
saltwater intrusion, and serious ecological degeneration (Matter et al.,
2006; Edmunds, 2009; Liu et al., 2015; Goldin, 2016). Therefore,
comprehensive studies of the spatial distribution, water quality,
recharge source and hydrochemical evolution of regional groundwater are
the foundation for the rational use of groundwater resources, which are
also essential and contribute to the scientific management and
sustainable development of regional water resources (Adams et al., 2001;
Edmunds et al., 2006; Chang & Wang, 2010). The integrated use of
isotopic and hydrochemical technology is an effective approach for
investigating the complex hydrological processes of groundwater over a
range of spatial and temporal scales (Clark & Frjtz, 1997; Gibson et
al., 2005; Raghavendra et al., 2015; Wassenaar et al., 2011; Cui & Li,
2014).
The
recharge area and recharge elevation of groundwater could be determined
based on the elevation effect of isotope in precipitation and the Local
Meteoric Water Line, due to the isotope composition of groundwater
mainly depends on the recharge source (Clark & Frjtz, 1997; Gu et al.,
2011; Wang et al., 2015). Therefore, isotope and hydrochemical
techniques are widely used to explore the recharge source and cyclic
evolution of groundwater. For examples, Bicalho et al. (2019) developed
a conceptual model for groundwater circulation by using isotopes and
geochemical tracers. Younas et al. (2019) evaluated the recharge sources
and geochemical identification in groundwater of the semi-arid alluvial
aquifers of Pakistan based on stable isotopes and other major elemental
data. Li et al. (2019) investigated the formation mechanism and mixing
behaviour of Nanyang thermal spring based on isotopic and hydrochemistry
techniques.
Qinghai Lake, the largest saltwater lake in China with an area of 4264
km2, lies in the cold and semi-arid region of the
north-eastern Tibetan Plateau. It is an important water body as well as
an international wetland and China’s national nature reserve for
maintaining the ecological security of the north-eastern Tibetan Plateau
(Tang et al., 1992; Cui & Li, 2015a). Meanwhile, it is a key area for
social and economic development in Qinghai Province, such as ecological
tourism and animal husbandry. Furthermore, the Qinghai Lake Basin is a
closed drainage basin, it has become an ideal area for studying global
climate change, environmental evolution and the uplift process of the
Qinghai Tibetan Plateau, as well as the water cycle and eco-hydrological
processes, because of its sensitivity to global climate change. In
recent decades, a series of ecological and environmental problems have
already arisen under the influence of climate change and human
activities, such as grassland degradation, wetland reduction, and
biodiversity decline, which have attracted attention from the local
governments as well as the international community ( Li et al., 2007;
Xin, 2008; Cui et al., 2016). Previous studies, referring to
geochemistry and water cycle of the Qinghai Lake Basin, were focus
mainly on paleoclimate and environment change, lake evolution and its
response to climate change, sources of precipitation, characteristics of
river runoff, and so on (Chang et al., 2009; Cui & Li., 2015a;
Cui
& Li., 2015b; Fu et al., 2016; Tang et al., 2018). However, there are
few studies having used the stable isotope and hydrochemical techniques
to investigate the sources and water quality of the groundwater around
Qinghai Lake.
Therefore, this study investigated water level, stable isotope and
hydrochemistry of the groundwater around Qinghai Lake. The objectives
are (1) to obtain the water level, stable isotope and hydrochemical
characteristics of groundwater around Qinghai Lake; (2) to reveal and
assess the water quality for drinking by analysing the ion
concentrations (TDS, TH, Na+, Cl-,
SO42-, and
NO3-); (3) to explore the possible
causes of groundwater evolution and water quality. The results would
contribute to knowledge about the hydrological and geochemical
evolutions of groundwater around saltwater lakes in the Qinghai-Tibet
Plateau, and inform water resource management in the Qinghai Lake Basin
and Qinghai-Tibet Plateau.