Evolution of the groundwater chemical composition in the Poyang Lake catchment, China

In this paper the water–rock interactions and anthropogenic effects in the Poyang Lake catchment (China) were distinguished using the chemical composition of the shallow groundwater. The analysis focuses on those changes which result from water–rock interactions. It is found that the shallow groundwater in the Poyang Lake catchment is HCO₃–Ca–Na or HCO₃–Na–Ca with an average TDS value of 183 mg/L. The pH value increases with groundwater salinity and changes from acidic to neutral and rarely to slightly alkaline. The content of δ¹⁸O and δD suggests that the shallow groundwater is of meteoric origin. A detailed analysis reveals that the groundwater can be confined to two different landscapes: (1) to red earth soils (natural landscapes) and (2) to paddy soils (landscapes influenced by intensive agricultural activities). The main distinction between two types of the groundwater is that the groundwater confined to red earth soils is in equilibrium with kaolinite and hydroxides, while the groundwater confined to agricultural landscapes is in equilibrium with montmorillonites and less often with illite and kaolinite. The differences between mineral equilibrium character of two main groundwater types result from changing in water exchange rate due to long-term agricultural activity with regular flooding and slope terracing. It is found that in some areas the shallow groundwater is rich in NO₃ ⁻, Cl⁻, SO₄ ²⁻, K⁺, NH₄ ⁺, NO₂ ⁻, PO₄ ³⁻, F⁻ due to overuse of fertilizers, sewage water leakage and other pollutants.