A multiple isotope (H, O, N, C and S) approach to elucidate the hydrochemical evolution of shallow groundwater in a rapidly urbanized area of the Pearl River Delta, China.

A comprehensive understanding of the impacts of natural and human activities on groundwater evolution is critical for sustainable groundwater resource management, as groundwater quality degradation from urbanization has raised widespread concerns. However, conclusions based only on basic hydrochemical data would be fragmentary because complex processes occur with high concentrations of pollutants in rapidly urbanized areas. Thus, the hydrogeochemical and multi-isotope approaches were combined to elucidate the groundwater hydrogeochemical evolution in such an area. The results demonstrated that the major hydrochemical types of groundwater were ClNa and HCO3-Ca in 2018 and that the hydrochemical patterns had changed since 1980. The predominant controlling factors for groundwater hydrochemistry were rock weathering due to carbonic, sulfuric and nitric acids, while the cation exchange and evaporation processes acted as natural factors; redox reactions, including denitrification, sulfate reduction, and methanogenesis, also affected groundwater hydrochemistry. The impacts of anthropogenic activities on groundwater hydrochemistry consisted of direct impacts that referred to the infiltration of manure and septic waste responsible for the occurrence of high NO3- content and part of the SO42- content in groundwater and indirect impacts that included the following issues: (1) acid rain accelerated water-rock interactions and resulted in the accumulation of SO42-; (2) sulfate reduction and methanogenesis increased the HCO3- content and expanded the distribution of HCO3-type water; (3) organic matter associated with manure and septic waste accelerated the development of a reducing environment in groundwater; and (4) the occurrence of a strong reducing environment promoted the release of Mn, aggravated heavy metal pollution and imposed adverse effects on the ecological system.