Formation and Freshening of Paleo‐Saline Groundwater in a Typical Hilly Coastal Zone of Southeastern China

Groundwater systems in coastal zones are strongly influenced by sea‐level fluctuations, paleo‐seawater intrusion, and anthropogenic activities and commonly exhibit complex hydrogeological architectures characterized by saline–freshwater coexistence and mixing. However, the origin and freshening mechanisms of paleo‐saline groundwater in hilly coastal settings composed of porous‐fractured media remain insufficiently understood. In this study, a typical hilly coastal zone in southeastern China was investigated through hydrogeological drilling and stratified sampling of aquifers and aquitards. Hydrogeochemical and isotopic tracers, including stable hydrogen and oxygen isotopes, Cl/Br and Sr/Cl ratios, and radiocarbon dating, were integrated to constrain the origin and freshening evolution of subsurface paleo‐saline groundwater. Groundwater salinity exhibits pronounced spatial variability, generally increasing toward the coastline. Vertically, borehole profiles show an asymmetric fresh‐saline‐fresh distribution. Stable isotope compositions indicate that the high‐salinity groundwater in the intermediate layer represents relict paleo‐seawater emplaced during the Early Holocene transgression. The combined Sr/Cl and Cl/Br evidence further demonstrates that the paleo‐saline groundwater originated during the Early Holocene marine transgression and has since undergone progressive freshening. This evolution is primarily controlled by the interbedded aquifer–aquitard framework and is characterized by a composite freshening mechanism involving both lateral recharge and upward inter‐aquifer leakage of deep paleo‐freshwater. The upward migration of deep paleo‐freshwater exerts a persistent dilution effect on the intermediate paleo‐saline groundwater, a process closely associated with regional geological structures and past sea‐level fluctuations. This study not only deepens the understanding of saline‐freshwater evolution in hilly coastal groundwater systems but also provides important theoretical support and practical guidance for the scientific assessment and sustainable management of groundwater resources under the background of rapid urbanization in southeastern coastal China.