It’s happening around the world—the over-extraction of groundwater is causing cities to sink. But new research shows this sinking problem can be reversed if groundwater resources are given time to replenish.
“Cities have learned the hard way that over-extracting groundwater contributes to land subsidence, increasing the risk of flooding and exposure to sea-level rise. In Aotearoa New Zealand, we know about 80 percent of urban coastlines are sinking and groundwater extraction could be among the contributing factors in some areas,” said Dr Jesse Kearse, an Earth scientist at Te Herenga Waka—Victoria University of Wellington.
Research by Dr Kearse and colleagues from Japan’s Kyoto University and the University of Southern California in the US has now shown this sinking isn’t necessarily permanent.
“Think of a gigantic water balloon beneath the city. When groundwater is pumped out, the ‘balloon’ deflates and the ground sinks. This can make coastal cities more exposed to sea-level rise as the climate warms.
“But when groundwater is replenished, the balloon re-inflates and the ground above rises. So, if we allow aquifers to replenish, we could help protect coastal areas from future sea-level rise,” he said.
The city of Osaka, Japan, provided an “outdoor laboratory” for the researchers to study this effect.
Osaka had high rates of water-take between the 1920s and 1960s, with groundwater levels dropping by 30 metres and land subsidence of more than 2 metres. Strict rules were subsequently put in place, limiting groundwater take.
In the decades since these rules were introduced, Osaka has seen groundwater levels recover, something that happens naturally when surface water seeps down into the ground, said Dr Kearse.
“We were able to use Osaka’s long-term water monitoring data, combined with satellite observations, to show there was a clear relationship between groundwater recovery and areas where the city is uplifting at rates of up to 12 mm a year. Our findings show that if underground water resources are replenished, cities can rise again.”
The researchers also discovered that areas of land uplift differed around the city, influenced by faultlines in the Earth’s crust.
“These faultlines can act like a dam, effectively stopping the flow of water and creating pockets where water builds up. This means water levels, and rates of land uplift, are comparatively higher in these areas.”
Importantly, different rates of uplift could signal the presence of undiscovered faultlines beneath cities, he said.
“In New Zealand, we’ve seen major earthquakes happen in areas—such as Christchurch—where we didn’t previously know there was a faultline. Where aquifers and faults coexist, as they do in Christchurch, land uplift could be a proxy to tell us there’s a faultline in the area, providing vital information for future land-use planning.”
The research is published in the journal Nature Communications Earth & Environment.