5.1.3 Further salinization of river water
After being recharged by brackish groundwater, the river water shows further salinization, with a higher strontium concentration, lower87Sr/86Sr ratios (Fig. 5a) and a much heavier boron isotopic composition relative to groundwater. Few river samples (NO.44-45) even evolve to saline water (TDS>10000 mg/l).
Because of chloride and bromide are conservative elements with similar geochemical behavior, evaporation would not change the Cl/Br ratio of water (Brenot et al., 2015). The river water shows a higher TDS compared to groundwater (Table 1), but the variation of the Cl/Br ratio of river water still keeps steadily (Fig. 4a), which implies that evaporation imparts a significant influence on the salinization of the river water. Under the condition of airflow, evaporation will cause the light10B in the water to enter the vapor phase preferentially, while 11B will remain in the liquid phase (Barth,1993), thus resulting in an enrichment of11B in water (Fig. 6a). Therefore, the heavier boron isotopic composition of river water can also attribute to evaporation.
According to the above discussion, the further increase of strontium concentration in river water compared to groundwater can be attributed to the concentrating by evaporation. However, the87Sr/86Sr ratio in river water shows a decreasing trend, which indicates there is additional strontium input in river water (Fig. 5a). These soil samples collected from the hillside is contained strontium with a lower87Sr/86Sr ratio (0.710912-0.711110, Table 1; Fig. 5a). And these soil profile samples (water-extracted) collected from one site where adjoin to the riverbanks also exhibit a depleted boron isotope (-2.13 ‰ to -1.72 ‰) in the upper soil (NO.55-56) relative to the deeper soil (NO.57) (Table 1). These characteristics suggest the occurrence of soil erosion events in the runoff of the river, which includes the leaching process along the riverbanks, and the infiltration process occurring on the hillside in precipitation events. These processes provide additional salts and strontium with a lower87Sr/86Sr ratio for the river, which also leached a large amount of 11B in the upper soil layer into the river water.