Enhancing Predictions of Land Subsidence Induced by the Groundwater Withdrawal in the Mekong Delta, Vietnam

Globally, land subsidence caused by groundwater pumping is a common phenomenon. Numerous subsurface processes, both natural and anthropogenic, contribute to its occurrence. In coastal regions, severe land subsidence exacerbated by groundwater pumping is particularly detrimental. On top of that, coastal areas affected by natural compaction, river delta consolidation processes, and additional exposure to drainage-induced aquifer-system compaction are especially susceptible to flooding and salinization due to the steadily rising sea level triggered by climate change.

The Mekong delta, one of the world's largest deltas, is densely populated and crucial for agricultural production. The delta is low-lying and has a high rate of natural compaction, whereas human activities accelerate land subsidence. A numerical model of groundwater-extraction-induced aquifer-system compaction was developed in 2017 to demonstrate the effects of 25 years of groundwater extraction on land subsidence in the delta. The model encompassed the time range from 1991 to 2016 using geological, hydrogeological, geomechanical, and remote sensing data. In 2020, the model was updated to include a surface water network. Six scenarios were developed to simulate potential future pathways of hydraulic head evolution and aquifer-system compaction in the Mekong delta from 2019 to 2100.

Our research aims to enhance the reliability of the existing numerical model of groundwater extraction-induced aquifer-system compaction in the Mekong delta, given the significance of such scenarios in the development of policies to mitigate the negative effects of groundwater pumping.  Our research focuses on four steps.

First, a novel subsurface model representation.

The 3D subsurface model of the Mekong delta was developed using ten hydrogeological cross-sections derived from 96 geological borehole logs interpolated linearly. This resulted in a subsurface model consisting of 15 layers, including seven aquifers, seven aquitards, and a phreatic top layer. The goal of the current study is to develop a new schematisation of the aquifer system within the Mekong delta based on 522 borehole logs and to investigate the spatial variability of the aquifer system using advanced geostatistical tools.

Second, a hydrogeological schematization enhancement.

In the current schematisation, aquitards are discretized as a single layer, resulting in the inability to simulate delayed groundwater pressure propagation within the aquitard. Several additional models with refined aquitard discretization are constructed and compared to evaluate the effect.

Third, the quantification of the influence of deterministic modelling on compaction.

The hydrogeological model is deterministically parameterized and calibrated using hydraulic head time series. Utilizing stochastic modelling of hydrogeological parameters, the impact of this deterministic modelling approach on simulated compaction is determined.

Fourth, a hydrogeological and geomechanical parameters consistency improvement.

The previous hydrogeological and geomechanical model parameterizations are inconsistent since the groundwater model and the geomechanical module were initially parameterized and calibrated independently. To address this issue, an iterative procedure is used to calibrate storage and compression indexes consistently for each individual model layer. This is accomplished by utilising groundwater head datasets recorded by 358 piezometers and land subsidence datasets retrieved by InSAR from 2006 to 2010 and 2016 to 2019.