Origin of groundwater in Hanoi, Vietnam, revealed by environmental isotopes

In 2015 and 2016, groundwater samples were collected in Hanoi to analyse the isotopic composition (δ²H, δ¹⁸O and ³H) and elucidate the relationship between groundwater and surface water, as well as the origin of the groundwater. The values for δ¹⁸O and δ²H indicate that the groundwater originated from evaporated meteoric water and the isotope enrichment is due to the evaporation of shallow groundwater. Evaporation is the primary process affecting stable isotope signatures. Water samples collected from both Holocene and Pleistocene aquifers are more depleted in the heavy isotopes ¹⁸O and ²H than the rainfall in the area. This indicates that part of the groundwater is paleo-groundwater or may be caused by the altitude effect due to recharge at a higher elevation. The results also show the close interaction between two granular aquifers and the Red River. Furthermore, the contribution of modern groundwater could be observed by the appearance of tritium in both aquifers. The presence of tritium indicates that originally tritium-free groundwater from the margins of the basin has been diluted by young water. The results of this study might help managers to evaluate the origin and reserves of groundwater more accurately.