Cities across the world are sinking because of groundwater depletion, but the process can be reversed

Many cities around the world are sinking due to excessive groundwater extraction, exacerbating coastal flooding as sea levels rise. However, our research reveals that this process can be reversed when groundwater levels are replenished. Osaka, Japan, served as a natural laboratory for our study, as the city experienced significant land subsidence from 1920 to 1960 due to heavy groundwater pumping. By 1960, the government implemented strict regulations, and groundwater levels have been recovering since then.

To determine how the land responds to aquifer recovery, we analyzed 44 monitoring wells stretching back to 1985 and employed interferometric synthetic aperture radar (InSAR) for precise measurements. The findings were striking: across greater Osaka, the land is indeed rising rapidly, with an average uplift of four millimeters per year, reaching up to 12 millimeters in certain areas.

Groundwater levels have also been steadily increasing, with some regions experiencing up to a meter per year. However, the pattern of uplift was not uniform; it formed distinct corridors aligned with known tectonic fault lines.

Our research suggests that these fault lines act as barriers to groundwater flow, effectively acting like dams. On one side of a fault, groundwater levels can rise three times faster than on the other side. This leads to faster land uplift on the "upstream" side of the fault, while the "downstream" side lags behind. Therefore, managing groundwater extraction and allowing aquifers to recover could potentially mitigate land subsidence in coastal cities, ultimately reducing the impacts of rising sea levels in the face of a warming world.