Abstract: In order to find out the hydrochemical characteristics and salinization causes of shallow groundwater in Caofeidian area, we collected different types of water samples such as river water, underground fresh water, brackish water, salt water, brine, rainwater and seawater in the study area. We analyzed the chemical composition and ions ratio, Piper diagram, Gibbs diagram, hydrogen and oxygen isotope composition and 14C dating results of water samples. The results showed that the shallow groundwater included the phreatic water in the Holocene sedimentary layer and the faint confined water in the Late Pleistocene sedimentary layer, which were not originally enclosed in the strata but formed in the middle and late Holocene. The results also showed that there were freshwater, brackish water and salt water quality types for the phreatic water from alluvial fan to coastal plain and estuary. The salt water and brine are the main faint confined water quality types. Groundwater hydrochemistry in the frontal alluvial fan, reclamation area, and estuary and coastal plain are mainly controlled by rock weathering, seawater mixing, and evaporation/crystallization respectively. The salinization process of shallow groundwater in Caofeidian is mainly caused by repeated dissolution, by atmospheric precipitation and river water for many periods, of sea salt crystallized during the transgression and retreat period since the late Pleistocene. The salt in phreatic water of the estuary and reclamation area is mainly derived from modern seawater intrusion.

Key words: Caofeidian, groundwater, hydrochemistry, hydrogen and oxygen stable isotope, salinization