Characteristics and genesis of groundwater salinization in coastal areas of the Lower Reaches of Oujiang Basin

The coastal areas of the lower reaches of Oujiang River Basin are rich in groundwater resources. However, the unsustainable exploitation and utilization of groundwater have led to significant changes in the groundwater environment. Understanding the characteristics and genesis of groundwater salinization is crucial for preventing its deterioration and ensuring sustainable utilization. In this study, a comprehensive approach combining the ion ratio method, mineral saturation index method and multivariate statistical analysis was employed to investigate the hydrochemical characteristics and main controlling factors in the study area. The findings reveal that: (1) Groundwater samples in study area exhibit a neutral to slightly alkaline pH. The predominant chemical types of unconfined water are HCO3-Ca·Na, HCO3·Cl-Na·Ca and HCO3·SO4-Ca·Na, while confined water mainly exhibits Cl·HCO3-Na and Cl-Na types. (2) Salinity coefficients indicate an increase in salinity from unconfined to confined water. TDS, Na+ and Cl– concentrations show an increasing trend from mountainous to coastal areas in unconfined water, while confined water displays variability in TDS, Na+ and Cl– concentrations. (3) Groundwater salinity is mainly influenced by water-rock interactions, including the dissolution of halite and gypsum, cation exchange, and seawater intrusion etc. Additionally, human activities and carbonate dissolution contribute to salinity in unconfined water. Seawater intrusion is identified as the primary factor leading to higher salinity in confined water compared to unconfined water, with increasing cation exchange and seawater interaction observed from unconfined to confined water.