Fresh groundwater was found in the Pleistocene aquifer in the southern part of the Red River Delta in 70’s of last century. It is located mainly in the south of Nam Dinh province and small part in southeast of Ninh Binh province. The fresh-saline boundary seems to migrate southward recently and downsize the area of fresh groundwater. | Salt groundwater intrusion in the Pleistocene aquifer in the southern part of the Red River Delta, Vietnam Hoang Van Hoan1*, Flemming Larsen2, Nguyen Van Lam1, Dang Duc Nhan3, Tran Thi Luu4, Pham Quy Nhan5 1 Hanoi University of Mining and Geology, Duc Thang, North Tu Liem district, Hanoi, Vietnam 2 Geological Survey of Denmark and Greenland, 10, Øster Voldgade, DK-1350 Copenhagen K, Denmark 3 Vietnam Association of Hydrogeology, Nghia Do, Cau Giay district, Hanoi, Vietnam 4 Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan district. Hanoi, Vietnam 5 Hanoi University of Natural Resources and Environment, Phu Dien, North Tu Liem district, Hanoi, Vietnam Abstract Fresh groundwater was found in the Pleistocene aquifer in the southern part of the Red River Delta in 70’s of last century. It is located mainly in the south of Nam Dinh province and small part in southeast of Ninh Binh province. The fresh-saline boundary seems to migrate southward recently and downsize the area of fresh groundwater. It is necessary to find out the mechanisms of salt intrusion into the zone of freshwater in order to mitigate the negative impacts to the quality of water supply. Based on the survey data of groundwater chemistry, transient electromagnetic sounding, borehole logging, drilling, and chemical analysis of pore water squeezed from the low permeable sediment in the study region, it was determined the current spatial distribution of saline water in the Pleistocene aquifer and in marine clay layers. By combining these data with the results from previous studies, this study has determined the mechanisms of the salt intrusion into the freshwater zone. It was found that salinity in the Pleistocene aquifer is generated from two main sources (1) Vertical salt intrusion from the upper marine clay layer which is controlled by the diffusion and density flow and (2) Horizontal saltwater incursion due to the high hydraulic gradient which is controlled by the convection and .
