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

This study identifies dominant moisture sources, develop an isotope-based framework for characterizing precipitation across a transboundary basin, and evaluate hydrological processes and anthropogenic influences using long-term water isotope records from the lower Red River system of Vietnam. Bi-monthly to monthly δ18O and δ2H measurements of rain and river water were collected from Feb 2018 to Jun 2024. Strong rainfall–isotope relationships were observed, with d-excess values of 8–12 ‰ and dominant moisture contributions from the western equatorial Pacific. Isotope-altitude–continental–amount relationships were used to generate 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.