Characteristics of Water Resources in the Mekong Delta as Determined by Stable Isotopes Signatures

This study investigates the spatio-temporal variation of stable isotopes (δ¹⁸O, δ²H), deuterium excess (d-excess) in rainwater and surface water in the Mekong Delta, Vietnam to identify hydrological processes and freshwater–seawater interaction. The δ¹⁸O and δ²H of the rainwater samples from five sites experienced seasonal change, with an average δ¹⁸O and δ²H of -5.35‰ and -35.74‰, respectively. D-excess ranged from -13.25‰ to 26.09‰ with an average of 7.07‰, suggesting mixing of moisture sources and post-depositional evaporation. Surface waters were generally enriched (-4.23‰ for δ¹⁸O; -30.16‰ for δ²H), with inland river waters the most depleted (-6.17‰ for δ¹⁸O) and coastal waters the most enriched (-1.60‰ to -1.87‰ for δ¹⁸O), due to effects of tides and evaporation. Isotopic concentrations became more positive closer to the coast, and significant and positive relationships were observed between chloride and δ²H (r = 0.74) and δ¹⁸O (r = 0.60; p < 0.001). Rainwater presented the highest d-excess values, indicating a greater evaporative influence on surface waters. These observations suggest that the isotope-salinity relation may support to understand the influences of the mixing of water sources, seasonal recharge, and saltwater intrusion. The results also indicate that stable isotopes could be recommended as reliable hydrological indicators in some deltaic regions subjected to clearly distinct climatic and anthropogenic influences for analyzing hydrological processes and providing insights into water management.