Tracing monsoon precipitation, surface–groundwater interaction and seawater intrusion using stable isotopes in a subtropical river system

This study identifies dominant moisture sources, develops an isotope-based framework for characterizing precipitation across a transboundary basin, and evaluates hydrological and anthropogenic influences using long-term isotope records from the lower Red River, Vietnam. Bi-monthly to monthly δ¹⁸O and δ²H measurements of rainfall and river-water were collected from February 2018 to June 2024. Strong rainfall–isotope relationships were observed, with d-excess values of 8–12‰ indicating dominant moisture contributions from the western equatorial Pacific. Isotope-altitude–continental–amount relationships were used to generate isotope time series for unmonitored sub-basins. River-water isotope seasonality agreed with GNIR data at similar latitudes but differed from the Mekong River due to contrasting latitudinal extent and moisture sources. Three river-water sources groups were identified: delta-origin, upstream inflow, and seawater-mixed. Enhanced evaporation, seawater intrusion, and increasing groundwater discharge due to anthropogenic activities are evident. d-excess is a robust indicator of seasonal evaporation in regulated and wetland-rich basins.