Delta subsidence – An imminent threat to coastal populations

Several interconnected factors contribute to land subsidence in delta regions. Delta plains were built up from thousands of years’ worth of silt deposition, producing richly fertile lands. Populations are skyrocketing in these agricultural areas, with many serious repercussions for the land. Construction of levees, dams, and embankments blocks the natural deposition of fresh silt, depriving the land of replenishment and creating bowls where floodwaters pool with nowhere to go. Extraction of groundwater and the fossil fuels that often underlie deltas allows the land to deflate. The sheer weight of growing urban and industrial infrastructure further compresses the land, and impervious surfaces—roofs and pavements—prevent the replenishment of groundwater. Saltwater intrusion, which occurs naturally in most coastal areas, is exacerbated as the depletion of groundwater reduces water pressure. Growing populations and water-intensive industries such as shrimp farming place a heavy demand on groundwater resources. Wells must be drilled deeper, and the water coming up is saltier, as ancient seawater is pulled up by excessive pumping. © Daniel Gallant; adapted from materials provided by Deltares Research Institute

Sea-level rise from a warming climate threatens to inundate coastlines around the world. But some of the world’s most vulnerable coasts – those fringing flat delta plains, mainly in Southeast Asia – face the far more immediate threat of sinking land. Induced mainly by human activities on a local rather than global scale, this phenomenon, known as land subsidence, can outpace sea-level rise substantially.

Apart from the increased risk of floods and associated diseases, experts consulted for this article say subsidence threatens health in other ways. It accelerates the contamination of freshwater resources with saltwater, making them unsuitable for drinking and agriculture. Subsidence stresses gas lines, sewage pipes, and other infrastructure, which can crack as the land buckles and heaves, increasing the risk of explosions and contamination of surface and groundwater. Finally, the stress of the threat to drinking water supplies, homes, and livelihoods can adversely affect people’s sense of well-being.



Deltaic Processes

The world’s deltas were built up mostly by aggradation, or the deposition of fertile river sediments over thousands of years. As such, they comprise important food-producing areas that attract large populations. The Mekong Delta, for instance, which is now subsiding at an average rate of 1.6 cm per year, is one of the world’s major rice exporters and home to more than 20 million people. Unlike rocky continental coasts, delta plains tend to be soft and easily compressed. They’re often propped up by underlying oil, gas, or fresh groundwater that flows through the pores of sediment deposits. As those resources are extracted, the sediments compress, and the land shrinks like a dried sponge. Some sediment, especially those rich in organic matter, such as peat, also oxidize when they dry. Oxygen binds with carbon in the soils, creating carbon dioxide that is released to the atmosphere. Deprived of the carbon lost to this reaction, the soils lose mass and compact.

Gualbert Oude Essink, a hydrogeologist at Deltares Research Institute and associate professor at Utrecht University, says saltwater is penetrating farther into the Mekong Delta every year. Being heavier than freshwater, saltwater migrates down through sediments into shallow aquifers from above, he explains. That makes the groundwater increasingly nonpotable. Furthermore, he says, salt ions also react chemically with the sediments, making the ground more prone to oxidation, compaction, and therefore subsidence. Where there’s a lot of pumping, salt water can also contaminate fresh groundwater resources from below. Oude Essink explains that fresh groundwater typically resides over more ancient seawater that can be pulled upward by excessive pumping. That process usually takes several years. Yet it can take much longer—decades or more—for the salt levels in contaminated freshwater aquifers to decline once extraction has ceased. That’s because compared with the pumping pressure that draws saltwater up, the gravity that pulls it down is a much weaker force, Oude Essink explains. A newer concern is that excessive pumping also could introduce arsenic into deep groundwater aquifers that would otherwise be free of the contaminant. Erban and colleagues reviewed arsenic measurements from nearly 43,000 deep wells in the Mekong Delta and found that many of them had become contaminated over time. It appears that excessive pumping could force that arsenic into deep groundwater, threatening the health of those who drink it. Erban speculates that pumping-related subsidence effectively squeezes dissolved arsenic from the clay layers as they compact. These findings contradict earlier assumptions that intervening clay layers protect deep aquifers from shallow arsenic contamination.

In a study of subsidence in the Mekong Delta over the period 1995–2010, Laura Erban and colleagues used well-monitoring data to estimate annual average rates of aquifer drawdown (A) and associated compaction-based subsidence at the well locations (B). These estimates corresponded closely to subsidence rates estimated from satellite imagery (C). Overall, the Mekong Delta is estimated to be subsiding at a rate of 1.6 cm per year. Source: Erban et al. (2014)

Tackling the Problem

Officials in Vietnam remain hopeful that research will point to remedies other than limits on freely available groundwater, which is an engine for economic growth. At the Rise and Fall project’s kickoff meeting, some officials were skeptical that groundwater exploitation is what drives subsidence in the Mekong Delta. “People just say ‘groundwater is causing this,’ but we have no data to prove it,” says Bui Tran Vuong, deputy director general of the Division of Water Resources, Planning, and Investigation for South Vietnam. Stouthamer insists accumulated evidence from around the world points to groundwater overuse as the main culprit. But she agrees that other factors are likely involved, such as the compaction that results when urban infrastructure is built on poorly supported clay or peat sediments. Changing codes so that infrastructure is engineered for better support from below and built using lighter-weight materials could help with subsidence, she says. Another possible option is to pump water back underground to counteract subsidence. Known as managed aquifer recharge (MAR), this can be a controversial proposition. Syvitski warns it could have unpredictable consequences. “Even if you could do it, roads and buildings would buckle as the land rises.” Oude Essink disagrees, saying that MAR projects around the world show it to be a potentially worthy approach for reducing the groundwater declines, and therefore subsidence.

These examples illustrate the challenges of addressing a creeping problem that’s barely perceptible to the population in real time. It’s hard to notice a drop in land elevation of a few centimeters per year until its consequences materialize in a catastrophic event, such as a devastating flood. Yet over time, these declines become significant. Where sea level is rising by an estimated 32 cm per century, land subsiding by 10 cm per year will sink that far in just over three years. Although sea- level rise gets most of the attention, for vast numbers of people worldwide, subsidence is by far the more immediate problem. But because subsidence is a local problem, local solutions are needed to keep it bay.

Data from Deltares compare historical subsidence rates from coastal areas around the world with estimates of absolute global sea-level rise. These are average rates; subsidence can differ considerably within a given city, depending on groundwater levels and subsurface characteristics. In some cities subsidence is accelerating as a result of economic growth. Tokyo, however, has shown that locally based mitigation measures can help stem the trend. © Erkens et al. (2015)

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