Groundwater recharge mechanisms in the lower Tri An hydropower reservoir region, Dong Nai Province, Vietnam, as studied using isotopic technique.

In recent years, climate change-induced drought has increasingly led to water shortages for domestic and industrial use in Dong Nai Province, Vietnam. Within this province, the Tri An hydropower reservoir plays a crucial role in the sustainability of regional water resources. This study aims to assess groundwater recharge mechanisms in the lower Tri An hydropower reservoir (LTAHR) region using isotopic technique based on the water stable isotope ratios (δD and δ¹⁸O) in groundwater, surface water and local precipitation. The results obtained indicate that the stable isotope values of precipitation in the region exhibit significant seasonal variations, whereas that values of surface water in the reservoir maintains more stability. On the other hand, stable isotopic compositions in groundwater are significantly influenced by monsoonal activity and temperature fluctuations. Applying the end-member mixing analysis (EMMA) method, the study revealed that surface water is the primary contributor to groundwater recharge, accounting for 60-85 %, while precipitation contributes 15-40 %, depending upon the season. Thus, the change of water storage level in the Tri An hydropower reservoir, along with the impacts of El Niño and climate variability, may disrupt seasonal water balance and significantly reduce annual groundwater recharge.