Study finds dozens of major cities sinking due to groundwater extraction
Excessive ground subsidence could cause major infrastructure issues in the coming decades, a recent study found
New York City is sinking. Washington, D.C., is sinking, as well. So are Los Angeles, Detroit, Dallas, and more than two-dozen of the most populous cities in the United States.
Just over one year after researchers revealed that the U.S. East Coast is sinking at an alarming rate, the lead authors have announced that the ground is also sinking beneath dozens of cities across the U.S.—and the same likely goes for many more major cities around the world.
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Cities are pumping themselves into the ground
Ground subsidence—the ground sinking in elevation—is a major global issue. The phenomenon is a complex ordeal that involves land usage, local geography, and water management.
Image courtesy of Nature Cities.
The study, published this month in Nature Cities, utilized satellite imagery between 2015 and 2021 to analyze how much the land subsided beneath 28 of the most populous U.S. cities, accounting for more than 35,000,000 people.
“In every city, at least 20% of the area is sinking [...] and in 25 out of the 28 cities, at least 65% is sinking,” covering nearly 18,000 square kilometers of land, according to the publication.
Many of the cities included in the study saw an average subsidence rate of 2 mm per year. Some locations such as Houston saw areas where the ground sank at a rate of more than 4 mm per year.
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The rate of subsidence varies greatly across each city, and sometimes between one neighbourhood and the next. Some areas are even witnessing a slight increase in elevation, largely due to glacial isostatic adjustment—the ground bouncing back from the weight of glaciers that covered the land some 10,000 years ago.
It’s far more than just U.S. cities at risk.
The research team advised that “a broader spectrum of major cities worldwide, including inland metropolises such as Mexico City, Beijing and Tehran, are experiencing major subsidence at rates that necessitate immediate attention due to their potential impacts on infrastructure.” Given the global scope of the problem, this is likely an issue for Canadian cities, as well.
Major causes include groundwater use and cities themselves
Researchers noted that the vast majority of this ground subsidence occurs as a result of groundwater extraction. Aquifers compact as they’re depleted of the water that once filled them, causing the ground above to sink.
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Groundwater extraction has caused some dramatic sinking in a few locations around the world. One extreme example is 9 metres of ground subsidence in California’s San Joaquin Valley between 1925 and 1977.
Another factor behind this subsidence, the experts said, is “the loading effect of urban development”—quite literally the weight that cities exert on the earth beneath them.
Subsidence can add up to major issues
Relatively small amounts of ground subsidence can cause damage to buildings, roads, bridges, tunnels, as well as pipes carrying water, sewage, and natural gas. Small changes in elevation year after year eventually build up to big problems in the not-too-distant future.
Subsidence can also exacerbate the risk for flooding during severe weather, especially as climate change influences heavier and more frequent rainfall events. Cities such as Houston and New York are particularly vulnerable to this increased flooding risk.
Header image created using imagery and graphics from Canva.