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.