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.