Hydrogeochemical and isotopic study of the origins of groundwater salinization in the deep confined aquifer of northern Yangtze River

The deep confined water of coastal plain of northern Yangtze River suffers salinization. That results from overexploitation. This work aims to investigate the geochemical processes that lead to the salinization for this aquifer. Multiple environmental tracers of major ions, minor ions and isotopes (O-18, Sr-87, C-13) were used to yield reasonable conclusions. The TDS of the aquifer ranges from 387 to 2600 mg/L. The aquifer is mainly composed of fresh water. Brackish water is distributed in the eastern and southern coastal areas, and is scattered in some inland areas. The water chemical type evolves from HCO3-Na to Cl-Na as TDS increases. Groundwater salinization is caused by mixing with saline water of marine origin. The aquifer is also affected by other hydrochemical processes. Silicate weathering and carbonate dissolution add Na+, K+, Ca2+, Mg2+ and HCO3- to groundwater. Sulfate reduction makes sulfate be deficient and HCO3- be enriched. Carbonate minerals equilibrium limits the increase in the concentrations of Ca2+, Mg2+ and HCO3-. Cation exchange reduces the concentration of Ca2++Mg2+ and increases the concentration of Na++K+. The aforementioned processes have a combined influence on the formation of fresh water with HCO3-Na type.