The origin and chemistry of the groundwater in the middle Pleistocene (qp2 3 ), lower Pleistocene (qp1 ), upper Pliocene (n2 2 ), lower Pliocene (n2 1 ), and the Miocene (n1 3 ) in the Mekong river delta (MKRD) were investigated by using isotopic and geochemical techniques. The origin of the groundwater was evaluated based on the composition of the water stable isotopes (d2 H and d18O) in the local precipitation, in water from the rivers system, and in the groundwater samples. The hydraulic interaction between the surface water and the groundwater as well as between the aquifers was assessed by a statistical treatment for the mean and standard deviation of the d18O signature and based on the 14C-ages of the water samples taken from different aquifers. The salinization of groundwater in the deep aquifers was investigated using the d18O signature combined with the geochemical composition of the water samples. It was revealed that the groundwater in the deep aquifers in the MKRD could be divided into two groups. The first group is fresh and represents the regional precipitation with the long traveling time ranging from older than 100 years to older than 40 ka BP (kilo years Before Present). The second group is the regional precipitation that is recharged from the remote areas mixed with the seawater. Statistical treatment with the mean d18O using the Mann-Whitney test showed that the water from the Mekong river system did not or very weakly recharged the deep aquifers. | Formation and chemistry of the groundwater resource in the Mekong river delta, South Vietnam
