Italy’s fastest-sinking cities, including Ravenna and Venice, could benefit from a scientific solution that has helped reverse land subsidence in Japan: replenishing depleted underground aquifers.
Land subsidence is an increasing threat to urban areas worldwide, particularly in coastal regions where sinking ground compounds the risks posed by rising sea levels. In Italy, Ravenna is sinking by 7.8 millimetres a year, followed by Porto Tolle at 7.2 millimetres, Comacchio at 6.9 millimetres and Argenta at 6.5 millimetres. Venice ranks tenth, with an annual subsidence rate of 5.5 millimetres.
Scientists say the process can be slowed and, under the right conditions, potentially reversed by restoring groundwater reserves depleted by excessive extraction.
Replenishing underground water reserves
Pietro Teatini, a professor of hydrology and hydraulic engineering at the University of Padua, says excess river water could be diverted underground instead of being allowed to flow out to sea.
The process involves recharging aquifers through existing wells or purpose-built infiltration basins. In Italy’s Friuli Venezia Giulia region, researchers are working to convert disused quarries into recharge basins that could store winter water from the Tagliamento River underground.
The approach aims to restore groundwater levels and help counter the ground settlement caused by decades of water extraction.
Japan provides evidence that subsidence can be reversed
Japan offers a striking example of the potential benefits. Parts of Osaka sank by as much as two metres between the 1920s and 1960s as groundwater was heavily extracted.
A recent study published in Communications Earth & Environment found that the metropolitan area, home to around 15 million people, is now rising at an average rate of four millimetres a year, with some areas recording increases of up to 12 millimetres.
According to the researchers, the findings demonstrate that subsidence can be reversed.
Japan introduced restrictions on groundwater extraction in the 1970s. Italy later adopted similar measures in Ravenna and Venice. In Venice, excessive pumping had contributed to ground sinking of around 1.5 metres. Authorities subsequently banned new wells and shifted water supplies to surface aqueducts fed by rivers and mountain reservoirs.
These measures helped slow subsidence and halt it in some areas, but did not fully reverse the process.
Why Italy faces a different challenge
The Po Valley’s geology makes the problem more complex. Its young basin and soft sediments are naturally prone to compaction under the weight of buildings and infrastructure, Teatini says.
Hydrocarbon extraction has also contributed to subsidence around Ravenna, where underground gas reservoirs have been heavily exploited. However, Teatini says groundwater withdrawal has had a greater impact because aquifers can cover much larger areas, while gas reservoirs lie deeper in older, denser and less compressible geological layers.
Climate change could further complicate efforts to restore groundwater reserves. Although the extent to which changing rainfall patterns are reducing natural aquifer replenishment has not yet been quantified, severe droughts can push people to extract more groundwater when rivers run dry.
Restoring the natural balance
Teatini says Italy needs a combination of measures to protect its sinking cities. Buildings and infrastructure should be designed to minimise their weight, while rivers should be allowed to follow their natural courses wherever possible.
Before extensive human intervention, seasonal flooding deposited fresh sediment across river plains, helping to offset natural ground subsidence. Hydraulic engineering projects disrupted this process.
Artificially recharging aquifers beneath vulnerable urban areas could help restore part of that lost balance, offering a potential new tool in the fight against subsidence and rising sea levels.
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