Origin and extent of fresh groundwater, salty paleowaters and recent saltwater intrusions in Red River flood plain aquifers, VietnamOrigine et extension des eaux douces souterraines, eaux anciennes salées et intrusion saline récente dans la plaine d’inondation de la Rivière Rouge au VietnamOrigen y extensión del agua subterránea dulce, paleoaguas salinas e intrusiones recientes de agua salada en los acuíferos de la planicie de inundación del Red River, Vietnam越南红河洪积平原含水层的地下淡水,古咸水和现代海水入侵的起源和规模Origem e extensão das águas doces subterrâneas, das paleoáguas salgadas e das intrusões marinhas recentes nos aquíferos da planície inundável do Rio Vermelho, Vietname

A model has been established on the origin and extent of fresh groundwater, salty paleowaters and saltwater from recent seawater intrusions in the Red River flood plain in Vietnam. This was done with geological observations, geophysical borehole logging and transient electromagnetic methods. Salt paleowater is present up to 50–75 km from the coastline, with occurrence controlled by the Holocene transgression. A density-driven leaching of salty porewater has occurred from high-permeability Holocene sediments into underlying Pleistocene deposits, whereas diffusion has dominated in low-permeability layers. In the Pleistocene aquifer, the highest content of dissolved solids is found below two intrinsic valleys with Holocene marine sediments and along the coastline. Recent intrusion of saltwater from the South China Sea is observed in shallow groundwater 35 km inland, probably a result of transport of salty water inland in rivers or leaching of paleowaters from very young near-coast marine sediments. The observed inverted salinity profile, with high saline water overlying fresher groundwater, has been formed due to the global eustatic sea-level changes during the last 8,000–9,000 years. The proposed model may therefore be applicable to other coastal aquifers, with a proper incorporation of the local geological environments.