The Ca Mau Peninsula (CMP), the southernmost region of the Mekong Delta, is increasingly threatened by groundwater salinization, posing severe risks to both the freshwater supply and land sustainability. This study develops a three-dimensional, density-dependent groundwater flow and salinity transport model to investigate salinization dynamics across the CMP’s complex multi-aquifer system. Unlike previous studies that largely rely on model calibration, this research introduces a novel approach by systematically deriving the spatial distribution of longitudinal dispersivity based on sediment characteristics. Moreover, detailed land use mapping is integrated to assign spatially and temporally variable Total Dissolved Solids (TDS) values to the uppermost layers, thereby enhancing the model realism in areas where monitoring data are limited. The model was utilized not only to simulate the regional salinity evolution, but also to critically evaluate conceptual hypotheses related to the mechanisms driving groundwater salinization. Results reveal a strong influence of seasonal and land use factors on salinity variability in the upper aquifers, while deeper aquifers remain largely stable, affected primarily by paleosalinity and localized pumping. This integrated modeling approach contributes to a better understanding of regional-scale groundwater salinization and highlights both the potential and the limitations of numerical modeling under data-scarce conditions. The findings provide a valuable scientific basis for adaptive water resource management in vulnerable coastal zones.

The Vietnamese Mekong Delta (VMD) is a low-lying region and one of the largest economic centers in southern Vietnam, with a population of approximately 18 million people. Groundwater is a crucial resource in the region, serving essential functions in domestic, agricultural, and industrial domains.Due to increasing surface water pollution and salinization, its importance has been increasing. The geological composition of the region includes seven sedimentary aquifers: Holocene (qh), Middle-Pleistocene (qp3, qp2-3, qp1), Pliocene (n22, n21), and Upper-Miocene (n13). The lower confined aquifer system contains non-renewable groundwater that was replenished during the last sea level low stand. Unfortunately, the intensive extraction of groundwater has led to several negative impacts, including declining groundwater levels, saline intrusion and the potential threat of land subsidence in various locations. This is especially concerning due to the low elevation and low hydraulic gradient of the delta.This study quantitatively the future changes in groundwater level in the VMD using a FEFLOW variable density flow and transport model, considering various climate change and groundwater exploitation scenarios. The results reveal a significant decline in groundwater levels by the year 2100: 15-25 m in the aquifers qp2-3, qp3, n22, n21, n13 and more than 35 m in the aquifer qp1 under the . The model results also showed the varying degrees of ongoing saline intrusion observed in both the Pleistocene (qp) and Neogene (n) aquifers, resulting from increased hydraulic gradients near the cones of depression and draining of saline paleo-groundwater from aquitards. To mitigate the risks associated with saltwater intrusion, it is recommended to strategically plan the placement of groundwater extraction wells at a considerable distance from the local saline boundary.To decrease the demand for groundwater and slow down the decline in groundwater levels and subsequent land subsidence rates, it is advisable to implement measures such as (a) managed aquifer recharge, (b) the development of additional water sources like rainwater harvesting and desalination, and (c) the promotion of surface water treatment and utilization, especially in regions experiencing water scarcity. To assess the potential for managed aquifer recharge, an aquifer storage and recovery (ASR) pilot project was implemented, recharging highly-treated surface water by gravity into two different confined aquifers. The results showed a considerable absorption capacity for both aquifers. Based on the recovery efficiency, ASR seems to be a feasible option for slightly brackish aquifers, but is not recommended for saline aquifers. For the upscaling of MAR, site feasibility must be assessed based mainly on water source availability and quality, as well as cost-benefit considerations.Keywords: Mekong Delta, Groundwater Modelling, saltwater intrusion, Vietnam, FEFLOW, Managed Aquifer Recharge