Investigation of GW-SW is crucial in hydrology and water resource management, employing diverse methodologies. This study integrates hydrogeological measurements and isotopic techniques to assess water sources, using δ2H and δ18O values. In the Thach Han River region of central Viet Nam, challenges like declining groundwater levels and salinization persist. During the dry season, groundwater recharge mainly comes from reservoir water for about 7 months. Base flow contributes 80-85% to streamflow in the rainy season, rising to 100% in the dry season, with an average travel time estimated at 120 ± 10 days.