Many cities around the world are sinking because too much water is extracted from the groundwater reservoirs beneath them.
Author
- Jesse Kearse
Research Fellow in Earth Science, Te Herenga Waka — Victoria University of Wellington
This sinking makes these coastal cities more exposed to rising seas as the climate warms.
But as our new research shows, this process can work in reverse; when groundwater is replenished, cities can rise.
Our findings also reveal an unexpected clue: faultlines play a key role in shaping where the fastest 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, the capital of Indonesia, are sinking more than ten centimetres 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, too, is a subsidence hotspot . If sinking there continues unabated, 15% of the city's population will be underwater by 2120.
Similar stories are playing out from San Diego to Iran's major cities , wherever groundwater has been pumped faster than it can refill.
In Aotearoa New Zealand, recent studies show 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 sinking
Many cities are built above natural groundwater reservoirs (aquifers), and are literally held up, in part, by the water beneath them.
Think of a gigantic water balloon beneath a city. The balloon represents an aquifer full of water. When groundwater is pumped out, the "balloon" deflates, and the ground above sinks.
This matters most at the coast. If the land is sinking while the sea is rising, 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 improved groundwater management is often put forward as the key strategy for slowing or halting the sinking.
What is not clear is how the ground beneath a city responds when long-term water aquifer recovery happens. Does the ground simply rise back up, uniformly, like a balloon reinflating? Or is the response more complicated?
To answer this, we need two key things: long-term records of where the water is accumulating in the underground aquifers, and precise millimetre-scale measurements of elevation of the ground surface above.
Osaka, a natural laboratory
Osaka provided the perfect setting to explore this. Between the 1920s and 1960s, heavy groundwater pumping lowered water levels beneath the city by up to 30 metres, and the land sank by more than two metres in places.
This led the Japanese government to introduce strict groundwater regulations in the early 1960s. In the decades since, groundwater levels have been recovering steadily.
To track the groundwater levels, we looked at data from 44 monitoring wells, some reaching as deep as 500 metres below the surface, and stretching back in time to 1985.
To measure the tiny changes in ground elevation, we used a technique known as interferometric synthetic aperture radar ( InSAR ). This involves the repeat acquisition of satellite radar images of the Earth's surface, tied to very accurate global navigation satellite system measurements of ground stations.
The results are striking. Across greater Osaka, home to about 15 million people, the ground is rising, and fast.
The speed of uplift averaged about four millimetres a year, and was as high as 12 millimetres a year in some places. Groundwater levels have also been steadily rising, at rates of up to a metre a year. Where water levels climbed fastest, so did the land.
But the map of uplift was also very patchy.
One city block could be uplifting fast, while less than 100 metres down the road it could be much slower. These abrupt changes aren't random. They form narrow but continuous corridors that ran across the city.
We soon realised something unexpected: these corridors mapped almost perfectly onto known tectonic fault lines.
Faults act like dams
Faults are fractures in the Earth's crust, and are usually thought of in terms of earthquakes. But we found they play another role here.
Picture a fault as a curtain hanging vertically underground, cutting through the aquifer. Instead of holding back light, it holds back water, blocking its sideways flow and acting like an underground dam.
On one side of Osaka's Uemachi Fault, groundwater levels have been rising three times faster than on the other side.
We found the same pattern repeats at other faults across the city. Water dams on the "upstream" side of a fault, where it can't easily flow through, driving faster groundwater recovery and faster uplift there, while the "downstream" side lags behind.
Our research highlights a potentially important opportunity for coastal cities adapting to sea-level rise.
Where groundwater depletion has contributed to land subsidence, reducing extraction and allowing aquifers to recover could reduce - and in some places potentially reverse - the downward movement of the land.
As seas continue to rise, keeping the ground beneath coastal cities from sinking could be an increasingly important part of adapting to a warming world.
Jesse Kearse receives funding from Te Apārangi Royal Society of New Zealand.
