Résumé
Study region: The studied region is the Saigon River basin, located along the eastern border of the Mekong Delta in southern Vietnam. Study focus: In this rapidly growing economic region facing with land subsidence, aquifers are a key resource for agricultural and urban uses. This study focuses on the vulnerability of shallower aquifers (0–150 m deep) in elevated zones (+15 m asl), strategically considered as “refuge zones” compared with the inundated lowlands. Clayey aquitard layers were analyzed for their capacity to protect, or not, the aquifer from infiltration of polluted waters. 260 Time Domain Electromagnetic (TDEM) soundings were used to detect clayey layers and create resistivity maps. New hydrological insights for the region: TDEM results revealed an unknown geometry of the aquifer-aquitard down to 150 m depth. On the right bank of the river, there were no superficial clayey layers to protect the aquifer from infiltrating pollutants. In comparison, on the left bank, the flood plain is clay-rich, thus favoring runoff rather than infiltration of polluted waters. Observations of the deeper clayey layers highlighted discontinuities, which follow ancient meanders and floodplain shapes that originated from quaternary transgression sequences. These results provide clues for understanding past regional sedimentation processes, and links with current aquifer vulnerability. Such information is critical for the strategic planning of elevated areas of coastal megacities such as Ho Chi Minh City