Groundwater-related subsidence

Groundwater-related subsidence is the subsidence (or the sinking) of land resulting from groundwater extraction. One estimate has 80% of serious land subsidence problems associated with the excessive extraction of groundwater [http://water.usgs.gov/ogw/pubs/fs00165/] , making it a growing problem throughout the world.

Groundwater is considered to be one of the last 'free' resources. Anybody who can afford to drill, can draw up merely according to the their ability to pump. However, as seen in the figure, the act of pumping draws down the free surface of the groundwater table, and can affect a large region. Thus, the extraction of groundwater becomes a Tragedy of the commons, with high economic externalities.

Perhaps as a consequence of global warming, the desert areas of the world are requiring more and more water for growing populations, and agriculture. In the San Joaquin Valley of the United States, groundwater pumping for crops has gone on for generations. This has resulted in the entire valley sinking an extraordinary amount, as shown in the figure. Fortunately, there is little consequence in a wide, flat, agricultural basin, since the settlement is uniform. Total settlement can only be determined by surveys and GPS measurements.

Unfortunately, not all groundwater-related subsidence is benign. In Mexico City the buildings interact with the settlement, and cause cracking, tilting, and other major damage [http://www.geotimes.org/july01/sinking_titanic_city.html] . In many places, large sinkholes open up, as well as surface cavities. Damage from Hurricane Katrina was exacerbated due to coastal sinking, associated with groundwater withdrawal.

The cause of the long-term surface changes associated with this phenomenon, are fairly well known. [http://geochange.er.usgs.gov/sw/changes/anthropogenic/subside/] . As shown in the USGS figure, aquifers are frequently associated with compressible layers of silt or clay.

As the groundwater is pumped out, the effective stress changes, precipitating consolidation, which is non-reversible. Thus, the total volume of the silts and clays is reduced, resulting in the lowering of the surface. The damage at the surface is much greater if there is differential settlement, or large-scale features, such as sinkholes.

The only known method to prevent this condition, is by pumping less groundwater, which is extremely difficult to enforce, when many people own water wells. Attempts are being made to directly recharge aquifers, but this still a preliminary effort.