Factors controlling groundwater chemical evolution with the impact of reduced exploitation

The ongoing groundwater exploitation reduction (GWER) project in the North China Plain (NCP) is causing a series of aquifer changes in the both hydraulic and chemical signals. However, how groundwater chemistry will evolve with the impact of GWER is still unclear. In this context, this study aims to identify the impacts of natural factors and anthropogenic activities on groundwater chemistry in the Heilonggang region, a pilot area of GWER in the NCP. A total of 150 groundwater samples were collected and major ions were measured during four sampling campaigns (June 2019, December 2019, December 2020, May 2021). Results show that the shallow aquifers are dominated by Na+ and mixed anions, while the deep aquifers are mainly characterized by Na-Cl and Na-SO4 types. Groundwater chemical evolution is dominated by water-rock interaction, evaporation, and cation exchange. The major ions in groundwater are mainly derived from various geochemical processes, including the dissolution/precipitation of gypsum, halite, Glauber's salt, feldspars, calcite and dolomite. Human interventions (i.e., groundwater pumping reduction, industrial, agricultural activities and domestic sewage) also regulate the chemical compositions in the shallow groundwater. GWER indirectly improves the overall quality of the shallow aquifers, lowering ion concentrations (Na+, Mg2+, and SO42-) and salinity. Saline water largely evolves into brackish water with an increase in freshwater. This may be attributed to the shallow aquifers partly diluted by upward leakage flow from the deep aquifers. However, insignificant variation in the salinity of the deep aquifers suggests that GWER has little effects on its chemical evolution. This study facilitates the understanding of groundwater chemical evolution in such a region where groundwater exploitation reduction is under way, with important implications for groundwater sustainable management strategies in other similar basins worldwide.