The Mekong Delta in southern Vietnam is subsiding rapidly. At the current rates large parts of the delta will have sunk below sea level by 2050. This constitutes a threat to the many millions of people who live there. Scientists of Utrecht University, Deltares and TNO are working closely together with their Vietnamese colleagues to identify the causes of subsidence and develop sustainable solutions to mitigate the potential catastrophic consequences of subsidence.
Floating market of Cần Thơ in the Mekong Delta. Photo: Internet
The outflow area of the Mekong River near the capital of Ho Chi Minh City in the southwest of Vietnam stretches out over thousands of square kilometers. Home to twenty million inhabitants spread over large cities and densely populated rural areas, the area is bustling with nature along with farming and fishing activity. Recently scientists discovered that the Mekong delta is subsiding by one to four centimeters each year, with groundwater extraction as a possible major cause. At this rate, large parts of the delta will have disappeared below sea level halfway this century. Subsidence drastically increases the risk of flooding and saline sea water is replacing fresh groundwater and surface water posing a threat to flora, fauna and people.
2. Collaborating within and across borders
The Rise and Fall project is part of the research focus area Future Deltas initiated and led by Utrecht University. Within this project scientists from Utrecht work closely together with Vietnamese researchers to identify the causes and consequences of subsidence, and to develop solutions. Philip Minderhoud, earth scientist at Utrecht University and Deltares, is one of the researchers: “This close collaboration with our Vietnamese partners gives the project an extra dimension, which is crucial to successfully conduct the investigation. Our Vietnamese colleagues have a solid understanding of the local environmental situation and knowledge on groundwater extraction and quality and associated regulation. We add our expertise and technology from the Netherlands and together we come to a better understanding of subsidence in the Mekong delta. Furthermore, our close collaboration ensures the results of our project to be supported locally.”
Within the Netherlands there is close collaboration between different knowledge institutes as well. Minderhoud continues: “The partnership between the Utrecht University, Deltares and TNO means we have access to many experts and knowledge in a vast array of areas. This enables us to efficiently and effectively conduct research to achieve better results and solutions.”
3. Divided Opinions
The exact causes of the subsidence are still unclear. Many scientists believe that the extraction of groundwater is the major cause. Earth scientist Esther Stouthamer, who supervises Minderhoud, agrees that the withdrawal of groundwater plays a major role, however, she believes that there are also other drivers at play.
“The total subsidence observed at the surface of a delta is caused by various natural and human-induced drivers,” says Stouthamer. “Tectonics is a natural driver, but there are also human-induced drivers, such as the extraction of oil, gas and groundwater, loading by buildings and other infrastructure, and the lowering of groundwater level, which causes oxidation of organic material above groundwater level. In order to take effective measures to prevent subsidence, it is essential to know to what extent drivers contribute to total subsidence in a particular area, and how they vary from area to area, even within a single delta.”
4. Living below sea level
In the article, entitled ‘Alarm over sinking deltas’, that was recently published in Science, renowned delta scientist James Syvitsky of the University of Colorado, suggests that pumping water back into the subsurface aquifers (recharge) may reduce the amount of subsidence by restoring pressure in the aquifers. Both Stouthamer and Minderhoud confirm that this is one of the possible solutions. If population growth and urbanization and associated land-use and current groundwater management is maintained, the delta will sink below sea level, just as is the case in the Netherlands.
Regarding the possible measures to deal with subsidence Stouthamer paints a nuanced picture: “There are mega-cities in deltas located close to the coast, like Jakarta in Indonesia and the Randstad in the Netherlands, that are already up to a few meters below sea level due to subsidence. These cities can only be protected against natural hazards, like flooding, by drastic measures, such as the construction of heavy dikes and measures, like limiting the extraction of groundwater, preventing these cities from subsiding further. This situation is highly undesirable for the Mekong delta and Ho Chi Minh city.”
Stouthamer continues: “However, by taking the right ‘tailor made‘ measures, the amounts and rates of human-induced subsidence can be reduced. For example, by adapting the locations and quantity of groundwater extracted, recharge of clean water into groundwater aquifers, limitation of groundwater level lowering for aqua- and agricultural purposes, or by naturally or artificially raising land. Furthermore, the ultimate consequence of subsidence could be that people need to move away from low-lying areas that are threatened and become uninhabitable, to settle on higher ground.”
5. Future Deltas: Together towards sustainable future deltas
Both Syvitsky and Stouthamer will be presenting on July 1 at the symposium “Future Deltas: Together towards sustainable future deltas”, at Utrecht University. To attend the symposium, please register on the website.
Future Deltas: focus area of Utrecht University