The stable isotopes of oxygen and hydrogen can provide useful insights into water origin and hydrological processes. The present study aims to investigate the characteristics of stable H/O isotopes of groundwater and surface water in a coastal area of the Vietnamese Mekong Delta. Isotopes and chloride concentrations of surface water show a highly seasonal and linearly spatial variability, depending on the distance to the sea. The seasonal variation of upstream discharge and rainfall plays an important role in changes of the isotopic compositions and chloride concentrations. Tide also influences on chloride concentrations of surface water while it does not change the isotopic compositions. Evaporation plays a crucial role in changes of isotopic compositions, while the influence of freshwater/seawater mixing on isotopic variabilities is negligible. Groundwater has a spatial heterogeneity in isotopic compositions and chloride concentrations, reflecting different recharge sources and seawater intrusion processes. Groundwater in shallow aquifers originates from rainfall and surface water with small evaporative losses, and it experienced different magnitudes of mixing with seawater. Groundwater in deep aquifers might be recharged by open-surface water evaporation in the last glacial age with minor impacts of seawater intrusion on these aquifers.
Keywords: Coastal aquifers; Mekong Delta; Vietnam; deuterium excess; evaporation process; hydrogen-2; oxygen-18; seawater intrusion.