Analysis of coastal groundwater hydrochemistry evolution based on groundwater flow system division

Groundwater hydrochemistry evolution in carbonated coastal aquifers is impacted by seawater intrusion. A well-documented groundwater flow system division is the premise and the key of hydrochemistry evolution or seawater intrusion research under complex geological and topographical conditions. In this research, the regional groundwater system in the study area is divided into three groundwater flow systems, according to topographic, geological, and hydrodynamic conditions. The hydrogeochemistry evolution in the Daweijia coastal aquifer is characterized on the basis of the groundwater flow system division. The results show that the groundwater hydrochemistry evolution varies in different groundwater flow systems in the Daweijia area: (1) Hydrochemistry evolution in groundwater flow system I (SI) is controlled by lateral recharge, dissolution of evaporates, and seawater intrusion. The hydrochemistry presents characteristic HCO3-Ca·Mg, Cl·HCO3-Ca, and Cl-Na·Ca along the runoff path, and seawater intrusion is found 1.3 km inland. (2) Hydrochemistry composition in groundwater flow system II (SII) is influenced by water–rock interaction, freshwater-seawater mixing, evaporation, and reverse cation exchange. Additionally, human activities are potentially a primary factor in the hydrochemical evolution of SII and cause NO3 type groundwater. Seawater intrusion is found 2.8 km inland, with a mixing proportion of < 5%. (3) Hydrochemistry evolution in groundwater flow system III (SIII) is influenced by seawater intrusion, local precipitation, and a short runoff path. A classic seawater intrusion model is present in SIII. The hydrochemical distribution presents a regular changing pattern following the order: HCO3-Ca and HCO3·Cl-Ca (freshwater) and Cl-Mg·Ca/Na·Ca (brackish water). The degree of seawater intrusion in SIII is more serious than that in the SI and SII. Isotopic results generally reveal that the groundwater is primarily derived from the combination of local precipitation and lateral recharge from the system boundary. The study proposes an effective method to analyze groundwater hydrochemical evolution under complex geological conditions in coastal areas.