5.1.2 Formation of brackish groundwater in the upstream
The upstream groundwater shows an obvious salinization than groundwater in headwater. The TDS in upstream groundwater varies from 1177 to 8216 mg/l (Table 1), which is classified as brackish water (Freeze & Cherry, 1979). According to the previous work, the upstream groundwater is recharged by groundwater in headwater and precipitation. And the tritium data shows a rapid circulation of groundwater (less than 40 years) in the upstream area (Liu et al., 2019). Groundwater still evolves to brackish water from headwater to the upstream area under such a rapid replenishing, which is rare in most similar inland areas around the world.
The δ11B values of upstream groundwater are significantly lower than those of seawater (Barth, 1993), which indicates the geochemical composition in upstream groundwater is of non-marine origins (Fig. 7). The variation range of δ11B value in these water load samples (3.39‰-5.42‰; NO.61-63) and soil samples (acid-extracted) (0.6‰-7.42‰; NO.58-60) are in the range of δ11B value for the non-marine evaporates (Vengosh et al., 1994) (Table 1) (Fig. 7). The boron isotopic characteristic indicates that the dissolution of non-marine evaporates is the possible source of solutes in brackish groundwater.
In the quaternary loess layers of the study area, there are reserves a large amount of soluble evaporates minerals (Tsunekawa et al., 2014; Xiao et al., 2016), such as halite, gypsum, etc. The increase of strontium concentration and87Sr/86Sr ratio observed in upstream groundwater relative to the source groundwater (Fig. 5a) implies that there exists a strontium input with higher87Sr/86Sr ratio during the evolution of groundwater. However, there is no correlation between Sr/Cl ratio and Sr isotopic composition in upstream groundwater (Fig. 5b), as would be suggested weathering of carbonates rocks is no longer the major geochemical process in water. Strontium and calcium belong to one family and strontium usually reserve in Ca-containing minerals (Yokoo, Nakanob, Nishikawac, & Quan, 2004). Strontium shows a significant correlation with calcium (R2 = 0.9037; Fig. 5c) in upstream groundwater, which means strontium origin from the dissolution of Ca-containing mineral in the evolution of groundwater. The87Sr/86Sr ratios of upstream groundwater are close to those reported for aquifers where evaporates dissolution occurs (0.711; Palmer & Edmond, 1992). Therefore, the dissolution of gypsum mineral is the main source of strontium, which also is an important source of solutes in upstream water.
When there is a dissolution of non-marine evaporates mineral, the Cl/ Br ratio will significantly increase with the increase of chloride concentration in water (Brenot et al., 2015). The Cl/Br vs. Cl diagram displays a well-defined relationship between these two conservative indicators in the upstream groundwater and Cl/Br ratio of upstream groundwater shows obvious high relative to that of source groundwater (Fig. 4a), indicating progressive reaction derived from the dissolution of non-marine evaporates with high Cl/Br ratio during the evolution of groundwater. With the increase of Cl/Br ratio, the Na/Cl ratio of groundwater samples gradually approaches 1 (Fig. 4b), which clearly argues for the dissolution of halite mineral as an important source of salinity in groundwater.
Nevertheless, the δ11B values of upstream groundwater samples (11.48‰-22.58‰; NO.5-7,11,16-17) are significantly higher than those of water affecting by non-marine evaporates dissolution (-32‰-8‰) (Vengosh et al., 1994) (Fig. 7), meaning that the boron isotopic signature is altered by additional B inputs from other sources beyond the dissolution of non-marine evaporates. Indeed, the loess layer is rich in clay minerals which have a high cation exchange capacity (Cartwright, Weaver, & Petrides, 2007; Tsunekawa et al., 2014), such as kaolinite and illite. This characteristic usually promotes cationic exchange reaction to be active in groundwater (Ghassemi et al, 1995). The preferential10B adsorption occurs on exchange phases (clay minerals), while the 11B will remain in the liquid phase (groundwater), which will lead to the relative enrichment of11B in water (Palmer et al., 1987). Since chloride behaves as a conservative element, the low B/Cl ratio usually reflects a loss of boron related to the preferential adsorption of10B in cation exchange (Cary et al., 2015). The B/Cl ratio gradually decreased with the increase of TDS in upstream groundwater also reflect the existence of the cation exchange reaction (Fig. 6b).
As suggested above, the dissolution of non-marine evaporates is the main source of solutes in the upstream groundwater. These soluble minerals can dissolute into water rapidly during the flow path of groundwater and lead to the salinization of water. In addition, the cation exchange reaction occurs in the upstream groundwater significantly modify the boron isotopic signature of water, which also influence the geochemical composition of water.