This article presents the results of determining the age of groundwater in the Pleistocene aquifer in the south of Hanoi to clarify the hydraulic relationship among rainwater, Red River water, and groundwater in the study area. At the same time, the average age of water in the Pleistocene aquifer in some boreholes in the south of Hanoi was also determined by the carbon-14 method to determine the flow direction of groundwater in the Pleistocene aquifer at current time.

The result of isotopic composition of three types of water, including rainwater, river water, and groundwater showed that the abstraction wells at distances up to 1 km along the Red River received more than 90% river water recharge. The contribution of river water decreased when going deeper to the center of the aquifer. Data on the stable isotope ratios and age from 20 thousand to over 45 thousand years of water in drill holes P44a, P25a, P28a, and P61a showed that the Red River water did not contribute to water reserves in the above holes. Water in the Pleistocene here is taken from the storage and recharge from distance, from the western edge of the city. The results of estimating the recharge of Red River water to the Pleistocene under the isotope composition were quite consistent with the results calculated using other experimental methods.