Multitemporal synthetic aperture radar (SAR) interferometry (InSAR) is a widely used technique to measure the ground subsidence and has already shown its ability to map such phenomena on a large spatial scale with millimetric accuracy from space. In Vietnam, to have independent SAR data for surface risk applications, a new X-band SAR mission (JV-LOTUSat) has been scheduled for launch for the 2019–2020 timeframe. However, Vietnam is located in tropical regions where their conditions are impacted by strong atmosphere. The aim of this article is to provide a better understanding of the capabilities of the X-band for estimating the ground subsidence under tropical atmospheric conditions. Analysis is carried out on two stacks, TerraSAR-X and Cosmos SkyMed X-band, from 2011 to 2014 in Ha Noi. We show that the results on the ground subsidence from InSAR processing can describe consistently the subsidence area based on ground measurements. This article demonstrates that the InSAR technique can be effective at detecting and estimating the subsidence phenomena even with the X-band and under conditions typical of tropical regions. The displacement results from TerraSAR-X and Cosmos SkyMed datasets are consistent, with a correlation coefficient ( $R^{2}$ ) of 0.91 for the period during which their coverage overlaps. Groundwater overexploitation is one of the main causes of the ground subsidence in Ha Noi. This study provides strong support for the scientific potential of the X-band SAR space-borne mission in Vietnam and other tropical countries because it demonstrates the feasibility of the ground subsidence estimates by the X-band SAR, even in conditions impacted by strong atmosphere.

In the investigation of optimal groundwater extraction in coastal regions, conventional assumptions typically revolve around unconfined aquifers with specified boundary conditions. In such cases, intricate solutions for groundwater management have been documented. However, within extensive delta plains, the extraction wells are frequently drilled in confined aquifers with not much variable-density flow. This circumstance, characterized by paleo-saltwater intrusion, is further complicated by the placement of wells at a considerable distance from the coastal line. As a result, the design and implementation of groundwater supply systems in these areas necessitate strategic groundwater management to optimize groundwater utilization while mitigating the potential risk of saltwater intrusion. Analytical solutions and an optimization problem approach have been applied to address this challenge and solve the differential equations governing confined aquifers with salt–freshwater interfaces. These methodologies provide simplified yet dependable conditions tailored to the study area. A case study conducted in Vinh Hung district, Long An province, is focused on determining the optimal pumping rate for production wells to forestall saltwater intrusion during groundwater extraction. Here, the focus is on the migration of older saltwater towards inland pumped wells, rather than the influence of recent seawater encroachment. The findings contribute valuable insights into achieving an equilibrium between maximizing groundwater utilization and preventing saltwater intrusion in the aquifer systems by a simplified approach.

Riverbank filtration (RBF) is a viable alternative to direct river water abstraction and groundwater abstraction, offering the potential to reduce post-treatment costs and mitigate aquifer overexploitation. Therefore, it should be considered in any large-scale water resources management plan for water supply. This study applies a recent quantitative multicriteria methodology, proposed by Hoang et al. (Hydrogeol J 30:1701–1716, 2022), using GIS-based spatial analysis to assess RBF potential along six major rivers in the Red River Delta (RRD), Vietnam. The RRD is a densely populated region, experiencing rapid urbanization, groundwater stress, and increasing demand for drinking water. The methodology consists of three sequential steps, that evaluate: (1) an essential criterion—the hydraulic connection between the river and the aquifer, (2) the quantity criteria—the quantity of water that can be potentially abstracted, and (3) the quality criteria—river and groundwater quality. Additionally, saline groundwater aquifers, and saline surface water boundaries are introduced as a key factor for evaluating RBF feasibility in coastal settings of the RRD. The approach requires the collection of various data types, their transformation into spatial layers, analysis at riverbank level, and strategies to address data gaps, each of which is demonstrated in this work. Results show that out of 797 km of rivers analyzed 10.8% (86 km) of them are considered suitable for large scheme RBF. The limited extent of suitable zones reflects key hydrogeological constraints, including widespread shallow saline groundwater and weak river aquifer interaction along approximately half of the river network due to a confined Holocene clay layer, which collectively reduces the overall feasibility of RBF in the region. The resulting Site Suitability Index map however serves as a pre-screening tool for decision-makers to identify potential locations for RBF application in the delta. Lessons learned from the application of this methodology can be applied to other regions. Notwithstanding limitations in data constraints at certain areas for certain factors, this work offers valuable preliminary results that can significantly reduce the time and cost required for identifying potential sites for detailed investigation, an essential step in hydrogeological assessment and the design of large-scale RBF systems.

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

The Vietnamese Mekong Delta (VMD) is one of the largest economic centers in southern Vietnam, home to approximately 18 million inhabitants. The geological composition of the region encompasses seven aquifers namely Holocene (qh), Middle-Pleistocene (qp3, qp2-3, qp1), Pliocene (n22, n21) and Upper-Miocene (n13) characterized by porous formations and fractures, facilitating the percolation and retention of groundwater. In the region, groundwater is a vital role as a fundamental water resource, serving essential functions in domestic, agricultural, and industrial domains. However, the intensive extraction of groundwater leads to several negative impacts, including reduction of groundwater level, saline water intrusion into the groundwater, and the potential threat of land subsidence in several locations. This study aims to quantitatively assess the future change in groundwater level in VMD through the utilization of a FEFLOW forecasting model consideing several climate change groundwater exploitation scenarios. The results reveals a substantial decline in groundwater levels by the year 2100, ranging from 15 m to 25 m in the aquifers qp2-3, qp3, n22, n21, n13 and a deeper more than 35 m in the aquifer qp1. Due to the influence of the groundwater extraction process in the main aquifer, the groundwater level will be lowered, forming lower funnel-shaped depressions and a subsequent shift of saline boundary in the aquifer system. The findings also underscore the occurrence of varying degrees of saline intrusion across all six aquifers, observed in both the Pleistocene (qp) and Neogene (n) aquifers. To mitigate the risks associated with saltwater intrusion, it is recommended to strategically plan the placement of water extraction wells at a considerable distance from the saline boundary. Additionally, it is advisable to implement measures focused on artificial replenishment, the development of water sources, and the promotion of surface water utilization, especially in regions experiencing water scarcity.