Numerical modeling for determination of the dominant factor inducing saltwater intrusion into shallow aquifer in the Mekong River Estuary within the Mekong Delta, Vietnam

Abstract

Due to climate change and ever-increasing groundwater exploitation under the background of population growth and economic development since the 1990s, saltwater intrusion (SWI) into coastal aquifers has been recognized as a significant geo-environmental issue in the Mekong Delta (MKD) in southern Vietnam. Previous research indicated that groundwater over-exploitation mainly causes SWI into deep aquifer and seawater is the dominant source of SWI into deep aquifer, however, whether seawater or saline river water is the dominant source of SWI into shallow aquifer remains unknown. In this study, a 3D groundwater flow and salt transport SEAWAT model was developed and calibrated/validated to simulate the processes of shallow groundwater and surface water interactions and saltwater and freshwater interactions in the Mekong River Estuary within the MKD during 2012–2021. Simulation results indicated that: (1) the saltwater/freshwater wedge within the qh aquifer migrates 4.3 km landward and saltwater (TDS 20–35 kg/m³) storage increases 101.5 % from 2012 to 2021; (2) the dominant source of SWI is seawater from annual perspective; (3) the dominant source of SWI is seawater from November to August (10 months) while is the Bassac River from September to October (2 months) from monthly perspective; and (4) contribution ratios of ocean, the Mekong River, and the Bassac River to SWI during one year were 80.29 %, 3.42 %, and 16.29 %, respectively. The outcome of this study can provide a useful reference for implementation of efficient and effective adaptation strategies of coastal water resources management and eco-environment protection and restoration against SWI in a changing climate in the MKD.