Geochemical characterization of shallow groundwater using multivariate statistical analysis and geochemical modeling in an irrigated region along the upper Yellow River, Northwestern China

The geochemical characterization of groundwater in irrigated regions along the upper Yellow River is a pivotal issue to be addressed. In this study, the integration of multivariate statistical techniques and geochemical modeling was used to decipher the geochemical processes and agricultural practices affecting chemical evolution of phreatic water in a typical irrigated area along the upper Yellow River, Northwestern China. Seventy-eight water samples were collected in September 2018 for chemical analysis. Results showed that groundwater samples were classified as the low mineralized Group 1 with mixed cations-HCO3-SO4 water, and Group 2 with highly saline Na-Cl-SO4 water. Both groups were mainly distributed on both banks of the Yellow River, respectively, suggesting different recharge sources with distinct chemical compositions. Water-rock interaction in alkaline environment and nitrate contamination were mainly responsible for changes in groundwater chemistry. Rock weathering and evaporation-crystallization predominate the geochemical evolution of Group 1 and Group 2, respectively. The geochemical processes occurring in the aquifer mainly include the dissolution of halite, gypsum, Glauber's salt, fluorite, feldspars and dolomite, calcite precipitation, along with cation exchange. Nitrate contamination in the study area may be attributed to the combined effects of high-dose application of nitrogen fertilizers and the localized fine-grained zone with low permeability. Additionally, the inverse geochemical modeling showed that relative to the west bank of the Yellow River, the groundwater flow path on the east side of the Yellow River experienced larger amounts of evaporites' dissolution and cation exchange, and was more affected by the dedolomitization processes. The findings of this research not only contribute to the geochemical characterization of shallow groundwater in the irrigated area, but also provide valuable information for the authorities to formulate the strategies of groundwater sustainable management in such irrigated regions along the upper Yellow River.