In the Red River Delta, the concentrations of Arsenic in groundwater of alluvial dominated systems are very high, exceeding the WHO’s permissible. The correlation between the Arsenic concentrations in groundwater and the age of Holocene sediment as a key controlling groundwater Arsenic concentration in the Red River delta has been investigated. The evolution of sediments in the Holocene is closely related to paleo-riverbed migration in the past. A combination of methods is implemented including remote sensing, multi-electrode profiling (MEP), gamma-logging, drilling, soil sample and groundwater modeling. The resul has identified the shape, sediment compositions and location of the six paleo-riverbed periods. The age of the paleo-riverbed is determined by drilling, soil sampling and optically stimulated luminescence (OSL) in the laboratory. The oldest sediments is 5.9±0.4 ka BP in Phung Thuong near the mountain, the youngest one is from 0.4÷0.6 ka BP in H-transect near the Red River and the rest of the other is around 3.5 ka BP. The modeling results by using MODFLOW and MT3D show that the dynamics of paleo-riverbeds controlling Arsenic mobilization in groundwater in the Red River Delta. When the river moved to another position, the current river position at that time was filled with younger sediments and became paleo-riverbed formation with reducing conditions, Arsenic content which was adsorbed in the previous stage then released into groundwater. Therefore, Arsenic concentration in groundwater of young Holocene sediments is higher than in older ones which elucidates that paleo-riverbed migration controls on Arsenic mobilization in groundwater in the study area.

This article has established a set of empirical equations to describe the relationship between bulk resistivity and TDS of the Upper-Middle Pleistocene aquifer (qp2-3) and Lower Pleistocene aquifer (qp1) of the for field survey in 2017 in Ca Mau province. This article has determined the content of TDS based on EC (TDSEC) and established correlation regression equation between TDSLAB and TDS is based on the EC of the qp2-3 aquifer: Y = 0.549X – 0.081  with R2 = 0.975 and Standard Error (SE) = 0.1591. The qp1 aquifer: Y = 0.4669X + 0.0483 with R2 = 0.9869, Standard Error (SE) = 0.0949. Based on these regression correlation equations, we found a high correlation coefficient and small deviation between TDSLAB and TDSEC. Therefore, the determination of TDS content through the EC has reduced the cost of groundwater samples of the aquifers of laboratory analysis in Ca Mau province.

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In recent years, the situation of salinity intrusion is very complicated and becomes more serious in the coastal area of Soc Trang province. In this research, SEAWAT modified water density flow model was used to forecast the salinity intrusion for Holocene aquifer (qh) and upper Pleistocene aquifer (qp3) according to the water use planning scenario. The results show that the impact of groundwater extraction process increases the TDS content in the aquifer, the salt water/fresh water areas change over time, the trend of salinity change increases. In Holocene aquifer, salt water area is relatively large and the TDS concentration increases towards the sea, however, fresh water area changes inconsiderably: In 2015 accounting for 17.42% and by 2030 accounting for 17.77% of aquifer area. In upper Pleistocene aquifer, the fresh water area declines significantly: In 2015 accounting for 9.35% and by 2030 only accounting for 7.53% of aquifer area.