6. Conclusion
This study has presented a comprehensive understanding of the primary geochemical processes that occur in an inland catchment of the Loess Plateau, based on an integration of information derived from the chemical and isotopic signatures of water and soil. Therefore, we conclude that: 1) Affected by the carbonate formation, the Mg-Ca-Na-HCO3 type groundwater is formed in the headwater. Groundwater in the headwater is also characterized by a lower87Sr/86Sr and depleted δ11B value. The dissolution of non-marine evaporites, such as halite and gypsum, is the main source of salinity in brackish groundwater of upstream, and these processes also contribute the strontium ions with high 87Sr/86Sr ratios and the boron ions with high δ11B value into water. Additionally, the cation exchange reaction occurs in upstream groundwater has a significant influence on the geochemical composition of water and resulting in a much heavier boron isotopic composition of water. After recharged by brackish groundwater, the river water was affected by evaporation and various soil erosion events during runoff. Then the river water showed further salinization, decease of87Sr/86Sr ratios, and enrichment in heavy boron isotopes.
2) The magnesium sulfate minerals in loess provide an initial source for magnesium enrichment in upstream groundwater. In addition, the incongruent dissolution of dolomite coupled with gypsum dissolution and calcite precipitation also contributed to the enrichment of magnesium in groundwater. In addition to the impact of brackish groundwater, the river water is further affected by evaporation, which enhance the level of ”bitterness”.
These results indicate that water salinization in the Zuli River catchment primarily originates from the dissolution of soluble minerals in the upstream area, large-scale treatment for the brackish water resources in the downstream area should be avoided, instead of strengthening the water treatment in front of the mountain pass or exploration of the high-quality groundwater in the headwater should be encouraged.