| dc.description.abstract | The Vietnamese Mekong Delta (VMD) is one of the largest economic centers in southern Vietnam, home to approximately 18 million inhabitants. Moreover, groundwater is a crucial water resource for domestic, agricultural, and industrial uses in the VMD. For decades, due to over-extraction and urbanization, groundwater levels have been depleting rapidly and being salted, restricting groundwater usability and negatively impacting human lives. Therefore, it is necessary to understand the hydrological mechanisms and forecast groundwater levels as well as groundwater salinity in the VMD, supporting groundwater resource management. Several studies have been conducted to understand the mechanism as well as forecast groundwater reserves and quality in the VMD. However, most of them consider particular areas rather than the entire VMD. Since groundwater is a basin-wide system, where activities in a local area affect the surrounding area, it is necessary to develop a complete basin groundwater model in order to forecast groundwater resources and relevant issues for the entire VMD basin. This study adopted the USGS-SEAWAT, which is the coupled version of MODFLOW and MT3D, to simulate groundwater flow and solute transport in variable-density conditions. The model was developed and calibrated by modifying boundary condition values and physical properties of materials such that it could reproduce groundwater responses. The model can accurately simulate groundwater levels after calibration (RMSE < 1 m, NSE > 0.95). Additionally, the model shows that it can simulate groundwater salinity from 2000 to 2021 with acceptable statistical parameters (RMSE < 3 g/l, NSE > 0.85). After demonstrating its ability to reproduce the past, the model was applied to forecast groundwater levels and salinity by using reasonable scenarios. The scenarios applied in this study include the following: the sea level would rise to around 0.77 m by the end of this century, and the groundwater recharge volume could reach a maximum of 0.56 million m3/day in 2030 before decreasing by 0.5% annually until 2100. The result shows that the groundwater levels for 7 aquifers will continue to decrease, with deeper aquifers having a more significant decline. By 2100, the lowest groundwater levels could drop to -17 m in the Holocene aquifer, roughly -30 m in Pleistocene aquifers, -38 m in Pliocene aquifers, and -47 m in the Miocene aquifer. In contrast to groundwater levels, groundwater salinity is predicted to increase significantly in shallow aquifers. Meanwhile, deep aquifers would only see a modest groundwater salinity increase. In detail, in the Holocene, Pleistocene, and Pliocene aquifers, it is anticipated that the saline area would grow by 939 km2, 5632 km2, and 992 km2, respectively. Noticeably, the Miocene aquifer is predicted to experience more desalination than salinization, resulting in 43 km2 of saline water potentially changing to freshwater by 2100. The forecasted groundwater levels and salinity results further emphasize the necessity of efficiently managing groundwater resources. Finally, this study successfully forecasts the groundwater levels and salinity of 7 aquifers for the Vietnamese Mekong Delta, which may support policymakers in developing effective plans and strategies for managing groundwater resources in the VMD. | en_US |