5.1.1 Formation of freshwater in headwater
The strata of the mountain area in the headwater of the Zuli River are mainly composed of a set of marine carbonate rocks (Zhan et al., 2010). In our previous study, groundwater was considered as recharged by precipitation in the headwaters based on the analysis of stable isotopes data (δ2H, δ18O) (Liu et al., 2019). The groundwater in the headwater region is characterized by higher strontium concentration and87Sr/86Sr ratios relative to those determined for precipitation (strontium concentrations: (0.002-0.3mg/l),87Sr/86Sr ratio:0.7078-0.7092 (Bullen et al., 1996))(Table 1). This suggests that there is dissolution of Sr-containing minerals in the source groundwater, with this process increasing the strontium concentration and affecting the strontium isotopic composition of water. The higher Sr/Cl ratio in source groundwater probably reflects rock weathering as an important control of the chemical composition of groundwater with low TDS (Monjerezi et al., 2011) (Fig. 5b). The Sr/Cl ratio and the87Sr/86Sr ratio show an inverse correlation in the source groundwater (Fig.5b). The inverse correlation between the Sr/Cl ratio and87Sr/86Sr ratio is generally related to the significant dissolution of carbonate minerals (Harrington & Herczeg, 2003). This process is also supported by the87Sr/86Sr ratio of source groundwater (0.710903-0.711041, NO.2,4) compatible with the signature of limestone (87Sr/86Sr: 0.709-0.713) (Cartwright et al., 2010) (Table 1).
Studies focusing on boron isotopic composition in precipitation in the Loess Plateau are sparse. In this work, the source groundwater (Sample NO. 1-2) shows lighter boron isotopic composition than precipitation (Sample NO. 46-50) (Table 1), which may indicate that there is an input of boron with low δ11B value in source groundwater. In the mountain area of headwater, there are many exposed carbonatite rocks (Zhan et al., 2010). The narrow range of δ11B variation in the source groundwater (-8.35‰~-5.77‰, NO.1-2) can be attributed to carbonate rock weathering in which δ11B values usually range from -15‰ to -5‰ (Palmer & Swihart, 1996; Romer, Meixner, & Förster, 2014). Due to the conservative property of chloride and bromide, the Cl/Br ratio (weight) was used as an indicator to judge the source of salinity in groundwater, especially in distinguishing between marine and non-marine (Cartwright, 2004; Edmunds, Ma, Aeschbach-Hertig, Kipfer, & Darbyshire, 2006). In this study, the Cl/Br ratio of groundwater in headwater is similar or slightly higher than that of seawater (Alcalá & Custodio, 2008) (Fig. 4a), suggesting these ions are of marine origin (Skrzypek et al., 2013). These above characteristics indicate that the dissolution of carbonate minerals is the main process that influences the geochemical and isotopic composition of groundwater in headwater.