Abstract
The low-lying and populous Vietnamese Mekong delta is rapidly losing elevation due to accelerating subsidence rates, primarily caused by increasing groundwater extraction. This strongly increases the delta’s vulnerability to flooding, salinization, coastal erosion and, ultimately, threatens its nearly 18 million inhabitants with permanent inundation. We present projections of extraction-induced subsidence and consequent delta elevation loss for this century following six mitigation and non-mitigation extraction scenarios using a 3D hydrogeological model with a coupled geotechnical module. Our results reveal the long-term physically response of the aquifer system following different groundwater extraction pathways and show the potential of the hydrogeological system to recover. When groundwater extraction is allowed to increase continuously, as it did over the past decades, extraction-induced subsidence has the potential to drown the Mekong delta single-handedly before the end of the century. Our quantifications also disclose the mitigation potential to reduce subsidence by limiting groundwater exploitation and hereby limiting future elevation loss. However, the window to mitigate is rapidly closing as large parts of the lowly elevated delta plain may already fall below sea level in the coming decades. Failure to mitigate groundwater extraction-induced subsidence may result in mass displacement of millions of people and could severely affect regional food security as the food producing capacity of the delta may collapse.
Highlights
The world’s third largest delta, the populous and low-lying Mekong delta in Vietnam is facing increased river flooding (Kuenzer et al 2013), decreased sediment delivery (Kummu et al 2007, Xue et al 2011, Kondolf et al 2014, Darby et al 2016, Kondolf et al 2018), coastal erosion (Anthony et al 2015) and salinization (Renaud et al 2015, Smajgl et al 2015, Eslami et al 2019)
Land subsidence is the main source of relative sea-level rise in the Mekong delta and is caused by various driving processes: i.e. natural processes like tectonics and natural compaction of the Holocene sediments (Zoccarato et al 2018) and human-induced processes driven by amongst others groundwater extraction (Erban et al 2014, Minderhoud et al 2017), drainage of shallow sediments and loading by buildings and infrastructure (Minderhoud et al 2018)
We aim to study long-term, hydrogeological and geotechnical behavior of the Mekong delta’s aquifer system and to provide the first process-based quantification of possible future groundwater extraction on delta-wide subsidence to support informed decision-making in the delta
Summary
The world’s third largest delta, the populous and low-lying Mekong delta in Vietnam is facing increased river flooding (Kuenzer et al 2013), decreased sediment delivery (Kummu et al 2007, Xue et al 2011, Kondolf et al 2014, Darby et al 2016, Kondolf et al 2018), coastal erosion (Anthony et al 2015) and salinization (Renaud et al 2015, Smajgl et al 2015, Eslami et al 2019). Like many other deltas in the world (Syvitski et al 2009, Nicholls and Cazenave 2010), the Mekong delta experiences accelerating rates of relative sea-level rise, the combined effect of absolute sea-level rise and land subsidence. Land subsidence is the main source of relative sea-level rise in the Mekong delta and is caused by various driving processes: i.e. natural processes like tectonics and natural compaction of the Holocene sediments (Zoccarato et al 2018) and human-induced processes driven by amongst others groundwater extraction (Erban et al 2014, Minderhoud et al 2017), drainage of shallow sediments and loading by buildings and infrastructure (Minderhoud et al 2018). Extraction-induced subsidence has accelerated over the past decades to rates exceeding 25 mm yr−1 in certain areas, making it at present the main contributor of delta-wide subsidence in the Mekong delta (Minderhoud et al 2017)
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