Many cities around the world are sinking because too much water is being extracted from the groundwater reservoirs beneath them.
This subsidence makes these coastal cities more exposed to sea level rise as the climate warms.
But as our new research shows, this process can work in reverse; When groundwater is replenished, cities can grow.
Our findings also reveal an unexpected clue: Faults play a key role in shaping where the most rapid groundwater recovery and land uplift occur.
The global problem of sinking cities
Land subsidence, the gradual sinking of the ground surface, affects many of the world’s major cities.
Parts of Jakarta, Indonesia’s capital, sink more than four inches each year, prompting the government to plan a relocation of the city.
The coastal city of Tianjin, China, is home to 15 million people. It is also a critical sinking point. If the sinking continues, 15% of the city’s population will be underwater by 2120.
Similar stories are unfolding from San Diego to major cities in Iran, wherever groundwater has been pumped out faster than it can be refilled.
In Aotearoa, New Zealand, recent studies show that around 80% of the urban coastline is sinking and that groundwater may be a contributing factor in some areas, including Christchurch and Wellington.
How urban groundwater leads to subsidence
Many cities are built on natural reservoirs of groundwater (aquifers) and are literally supported, in part, by the water beneath them.
Think of a gigantic water balloon beneath a city. The globe represents an aquifer filled with water. When groundwater is pumped, the “balloon” deflates and the soil above sinks.
This is more important on the coast. If the land sinks while the sea rises, the two effects combine, meaning coastal cities experience relatively higher sea levels and the impacts of climate change arrive sooner.
This effect of groundwater pumping on land subsidence is well understood and better groundwater management is often proposed as a key strategy to slow or stop subsidence.
What’s not clear is how the ground beneath a city responds when water is replenished from the aquifer over the long term. Does the ground simply rise again, uniformly, like a re-inflated balloon? Or is the answer more complicated?
To answer this, we need two key things: long-term records of where water accumulates in underground aquifers and precise millimeter-scale measurements of ground surface elevation.
Osaka, a natural laboratory
Osaka provided the perfect setting to explore this. Between the 1920s and 1960s, intense groundwater pumping reduced water levels beneath the city by up to 30 metres, with the land sinking by more than two meters in some places.
This led the Japanese government to introduce strict groundwater regulations in the early 1960s. In the decades since, groundwater levels have been steadily recovering.
To track groundwater levels, we analyzed data from 44 monitoring wells, some of which reach depths of up to 500 meters below the surface and date back to 1985.
To measure small changes in ground elevation, we use a technique known as interferometric synthetic aperture radar (InSAR). This involves the repeated acquisition of satellite radar images of the Earth’s surface, linked to very precise measurements from the ground stations of the global navigation satellite system.
The results are surprising. Across greater Osaka, home to some 15 million people, the terrain is improving, and quickly.
The rate of rise averaged about four millimeters per year and reached 12 millimeters per year in some places. Groundwater levels have also been rising steadily, at a rate of up to one meter per year. Where water levels rose fastest, so did the land.
But the improvement map was also very irregular.
A city block could rise quickly, while less than 100 meters from the road could be much slower. These abrupt changes are not random. They form narrow but continuous corridors that cross the city.
We soon realized something unexpected: these corridors corresponded almost perfectly to known tectonic fault lines.
Faults act like dams
Faults are fractures in the Earth’s crust and are generally considered in terms of earthquakes. But we discovered that they play another role here.
Imagine a fault as a curtain hanging vertically underground and cutting across the aquifer. Instead of retaining light, it retains water, blocking its lateral flow and acting as an underground dam.
On one side of Osaka’s Uemachi fault, groundwater levels have risen three times faster than on the other side.
We found that the same pattern is repeated in other faults in the city. Water dams on the “upstream” side of a fault, where it cannot flow easily, driving faster groundwater recovery and faster uplift there, while the “downstream” side lags behind.
Our research highlights a potentially important opportunity for coastal cities to adapt to sea level rise.
Where groundwater depletion has contributed to land subsidence, reducing abstraction and allowing aquifers to recover could reduce (and in some places potentially reverse) downward land movement.
As sea levels continue to rise, preventing the ground beneath coastal cities from sinking could be an increasingly important part of adapting to a warming world.
By Jesse Kearse, Earth Sciences researcher; Te Herenga Waka – Victoria University of Wellington
This article is republished from The Conversation under a Creative Commons license. Read the original article.
