We need sand—and not just any sand. As rivers carry sediment, grains collide, creating craggy surfaces on the tiny rocks. Those rough surfaces translate into stability when river sand, gravel, and cement are combined to make concrete, the physical foundation of our modern world.
Though it’s the second-most sought-after resource next to water, sand is understudied and overused, in part because it seems like a limitless commodity, said Mette Bendixen, an assistant professor and physical geographer at McGill University in Montreal, Que. “We don’t have the full overview of where sand is being mined,” she said, “or how much is being taken.”
To that end, Chris Hackney, a Fellow and fluvial geomorphologist at Newcastle University in the United Kingdom, and his colleagues used satellite imagery to track sand mining in a short stretch of the Mekong River in Cambodia. He and his colleagues found that in this reach, the amount of sand that was removed increased annually, from about 24 million to 59 million metric tons per year between 2016 and 2020. This new estimate far exceeds previous ones collected before a recent uptick in urbanization, and was published in Earth Surface Dynamics in September.
Mining and Incising
The Mekong River begins its southeastward journey high on the Tibetan Plateau. It then flows on uneven terrain as it mostly follows the borders of Laos, Myanmar, and Thailand.
After crossing into Cambodia, the Mekong eventually flattens, depositing its sediment load onto the floodplain, said Hackney. The triangular apron of sediment at the Mekong’s mouth forms much of southwestern Vietnam. The Mekong Delta nourishes the rice that feeds the region’s nearly 20 million denizens.
Throughout the Mekong’s course, anchored sand mining vessels suction sand (or dig with excavators) for 2–3 hours per day, said Hackney. With full holds, they chug elsewhere to unload. After noticing these ships in satellite imagery, Hackney and his coauthors systematically combed monthly satellite composites that combined cloud-free snapshots taken from the sky every 2–3 days, and they counted the mining ships.
“We wanted to understand exactly how much material these miners were extracting,” said Hackney, “and how that…varies year on year.”
Satellite imagery from 2016 onward showed an increase in the sheer number of ships. To convert ship counts to sand volumes, Hackney and his colleagues calculated carrying capacity of the ships’ holds using Google Earth imagery and drone footage. They found that the amount of sand removed from this reach in 2020 was greater than the previous estimates for the entire lower Mekong basin.
Worryingly, Hackney’s estimate is about 9–10 times more sand than the river can naturally supply, in part because of hundreds of dams that bar sediment from flowing downstream. The resulting sand deficit causes the river to incise downward, lowering river levels. Fisheries and other delicate ecosystems can be destroyed. The combination of lower river levels and less sand allows the salty ocean to encroach upstream, affecting agriculture, he said. Along with climate change, limited sand means “you’re not getting the material delivered to the delta, to help it build out and fight rising sea levels.”
Regulation and Automation
Many Western countries have outlawed exploitation of river sand for commercial gain, said Hackney. Those countries often have enough money to import sand or mine ancient river deposits outside active channels.
Developing countries like Cambodia—with large, sediment-laden rivers funneling sand into their backyards—cannot afford to look elsewhere as their populations urbanize and grow, said Bendixen, who was not involved in this study.
In 2017, Cambodia banned the export of sand. However, the government still issues licenses to mine certain stretches that supply sand for local projects, requiring reports of extracted volume, Hackney said. But standing on a riverbank is not sufficient to assess the full scale. “You wouldn’t see all of the activity that was going on,” he said. Satellites solve that problem.
Hackney and his colleagues have since automated their method. Instead of monthly counts, he said, they can track Mekong sand mining about every 2 days. This method is also transferable to other parts of the world in which sand mining continues, he said, so long as satellites snap images with regularity.
However, in some places, manual operations involving canoes, or even people scooping sand into buckets by hand, could be difficult to spot with satellites, said Arpita Bisht, a postdoctoral researcher with the International Institute of Social Studies in the Netherlands and the Graduate Institute in Geneva. In those cases, drone monitoring could help, she said, “but for now, it’s extremely limited.”
“Sand is so ubiquitous around us, but it’s so invisible,” said Bisht. Many of sand mining’s impacts, she noted, are also “invisibilized,” especially in populous regions where people depend on local ecosystems—like the Mekong Delta.
Yet sand mining employs millions of people, said Bendixen. “In many countries, because of climate change, people might not be able to sustain their livelihoods as they’ve always done by fishing or agriculture, so they turn to sand mining.” The building boom, fueled by population growth, needs so much more sand, she said. “This is, in my opinion, one of the biggest overlooked global challenges.”
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer