Hydrogeochemical and isotopic insights into groundwater evolution in the agricultural area of the Luanhe Plain

The scarcity of freshwater resources in coastal plains has raised global concerns. Understanding the chemical characteristics and evolution of groundwater in coastal plains is crucial for ensuring sustainable water supply for residents and irrigation. This study focuses on a representative profile of the Luanhe plain, systematically analyzing hydrochemistry and stable isotopes of seawater, river water, and groundwater samples to elucidate groundwater chemistry evolution and investigate the influence of evaporation and mixing processes. Results showed that shallow groundwater was predominantly of the Ca-HCO₃ type, formed through the dissolution of halite, carbonate, and silicate minerals in the aquifer. Under the influence of evaporation and mixing processes, there was a transition in the chemical type towards Ca-Na-HCO₃ type. Especially in areas influenced by seawater intrusion at estuaries, a rapid shift towards Na-Cl type occurred. In addition, deep aquifers evolved predominantly into Ca-HCO₃ type groundwater through the leaching of silicate and carbonate minerals within the aquifer matrix. Along the flow path, influenced by cation exchange and mixing processes, the water chemistry underwent a shift from the Ca-HCO₃ type to the Na-HCO₃ type. The river water chemistry was primarily influenced by evaporation and the erosion of silicate and sulfate minerals from the riverbed. At estuarine zones, intense seawater intrusion significantly increased salinity and shifted the water chemistry from a Ca-Cl type to a Na-Cl type. This study elucidated the evolution mechanisms and influencing factors of coastal groundwater chemistry, providing scientific insights for local groundwater resource management.