Cities sinking due to too much groundwater extraction may eventually lift again - study

Cities that are sinking because too much groundwater is being extracted may eventually lift again if the water is allowed to replenish, new research shows.The study found the Japanese city of Osaka is rising again after decades of strict groundwater regulations, introduced after over-extraction caused some areas to sink by nearly three metres.New Zealand earth scientist and co-author Jesse Kearse said the research had important implications for coastal cities in particular, because of accelerating sea-level rise and coastal flooding.That included New Zealand, where nearly 80 percent of the coastline was already sinking, and pressure on freshwater - which was currently abundant - would grow as the climate warmed."We're not quite at that same stage as other countries around the world - I can't really point to a particular city in New Zealand and say, this city's sinking because we're using way too much groundwater," Kearse said."But the key message is, as we move into a world where we're getting more and more reliance on groundwater resources, we just need to be aware that that's one of the implications."The link between over-extraction and land subsidence had long been known, but researchers had not been sure if allowing groundwater to replenish would reverse the subsidence over time.The study used Osaka as a natural laboratory, because of its long history with groundwater and vertical land motion, Kearse said."The city realised in the early part of the 20th century that they were taking far too much water from underneath the ground, running out of water, and the city dropped by close to three metres in some places - which is really extreme subsidence - as a response to groundwater extraction."Osaka put strict regulations in place in the 1960s and groundwater had gradually replenished over the last 50 or 60 years, he said.The researchers combined long-term groundwater monitoring data from wells as deep as 500m with repeated satellite observations to measure the effect on land movement."The city has bounced back up again... Osaka is actually rising."Kearse described the effect as "a gigantic water balloon" beneath the city."When we take up water from underneath the ground, that balloon deflates and the city sinks. But as the groundwater is replenished, that balloon reinflates and the city pops back up again - so it works in both directions," he said.The patchiness of the subsidence meant it could cause great damage, Kearse said."If you've got pipes under the ground and some bits are moving differently to others, that can cause stress on the infrastructure. Roads are cracking, building are tilting, it's not a good thing."The research showed it was possible to predict "quite precisely" where the damage would occur."The sharp barrier between uplift and subsidence tends to occur along the known mapped faultlines - they are acting as underground dams," he said.That created corridors of damage - "not because of earthquakes, but because of how they're disrupting the groundwater flow."The same subsidence could occur with any sub-surface fluid, such as oil and gas extraction, he said."It's essentially the same geomechanical process... The physical relationship there is really well-understood."New Zealand's coastal subsidence had been mapped in detail as part of the NZ SeaRise project, which projected sea-level rise out to 2300.What was not well-understood was why that subsidence was happening, Kearse said.