The restoration of groundwater levels can allow urban areas that subsided due to over-exploitation of aquifers to rise again, reducing coastal cities’ exposure to flooding and to sea-level rise. This is shown by an international study that analyzed decades of groundwater data and satellite observations of the Japanese city of Osaka.
The investigation, published in a peer-reviewed scientific journal, involved researchers from Te Herenga Waka – Victoria University of Wellington, Kyoto University in Japan, and the University of Southern California in the United States. Osaka served as an ‘open-air laboratory’ to study the relationship between groundwater recovery and land uplift.
Between the 1920s and 1960s, the city underwent intense groundwater extraction. The aquifer levels dropped by about 30 meters and the ground subsided by more than two meters. Subsequently, more restrictive rules on groundwater extraction were introduced.
In the following decades, groundwater levels recovered as surface water naturally infiltrated into the soil. By cross-referencing groundwater monitoring records with satellite observations, the researchers found a clear relationship between groundwater recovery and land uplift, which reached up to 12 millimeters per year in some areas of Osaka.
“When groundwater resources are replenished, the ground can recover and cities can rise with it,” explains Jesse Kearse, a geoscience researcher at Victoria University of Wellington and co-author of the study.
The phenomenon is related to the pressure exerted by water in underground materials. When water is withdrawn in large quantities, the reduction in pressure can contribute to sediment compaction and surface subsidence. When aquifers recover, part of this deformation can be reversed.
However, the researchers found that the uplift does not occur uniformly across the city. The distribution of geological faults influences groundwater circulation, with some acting as a kind of underground barrier and creating zones where water accumulates in greater amounts.
This difference in uplift rates could also help identify geological structures still unknown beneath cities. For Kearse, this possibility could be particularly relevant in New Zealand, where urban areas lie close to aquifers and geological faults.
In Christchurch, for example, the researcher points to areas where the land is currently subsiding at about 10 millimeters per year, a rate that, according to the data presented by the authors, is roughly three times the rate of sea-level rise. The subsidence increases the vulnerability of homes, roads and other infrastructure to flooding.
The problem is not exclusive to New Zealand. The over-exploitation of groundwater has caused subsidence in several cities around the world, worsening risks associated with rising mean sea level and extreme precipitation events.
The authors therefore argue that groundwater management should also be considered from an urban resilience perspective. The study shows that controlling water extraction and allowing the recovery of groundwater resources can have effects that go beyond water availability, potentially helping to reduce the physical vulnerability of coastal areas.
However, the results do not mean that all sinking cities can simply regain the same amount of uplift. The answer depends on local geological characteristics, the scale of water extraction, and the conditions of the aquifers. The study demonstrates the recovery observed in Osaka and suggests that groundwater management can be an additional adaptation tool for coastal cities subject to subsidence.
