Land subsidence occurs when large amounts of ground water have been withdrawn from certain types of rocks, such as fine-grained sediments. The rock compacts because the water is partly responsible for holding the ground up. When the water is withdrawn, the rocks falls in on itself. You may not notice land subsidence too much because it can occur over large areas rather than in a small spot, like a sinkhole. That doesn't mean that subsidence is not a big event -- states like California, Texas, and Florida have suffered damage to the tune of hundreds of millions of dollars over the years.

This is a picture of the San Joaquin Valley southwest of Mendota in the agricultural area of California. Years and years of pumping ground water for irrigation has caused the land to drop. The top sign shows where the land surface was back in 1925! Compare that to where the man is standing (about 1977).

Subsidence is a problem everywhere

Subsidence is a global problem and, in the United States, more than 17,000 square miles in 45 States, an area roughly the size of New Hampshire and Vermont combined, have been directly affected by subsidence. More than 80 percent of the identified subsidence in the Nation has occurred because of exploitation of underground water, and the increasing development of land and water resources threatens to exacerbate existing land-subsidence problems and initiate new ones. In many areas of the arid Southwest, and in more humid areas underlain by soluble rocks such as limestone, gypsum, or salt, land subsidence is an often-overlooked environmental consequence of our land- and water-use practices.

When you look at this picture, do you feel yourself tilting your head a bit to the left? If you do, it is because the building appears to be leaning to the left, which it really does because it is sitting on soil that has subsided. In Mexico City, rapid land subsidence caused by groundwater withdrawal and associated aquifer-system compaction has damaged colonial-era buildings, buckled highways, and disrupted water supply and waste-water drainage.

Causes of land subsidence

Land subsidence is most often caused by human activities, mainly from the removal of subsurface water. This pictures shows a fissure near Lucerne Lake in San Bernardino County, Mojave Desert, California (photograph by Loren Metzger). The probable cause was declining ground-water levels. Here are some other things that can cause land subsidence:

Ground-water pumping and land subsidence

Compaction of soils in some aquifer systems can accompany excessive ground-water pumping and it is by far the single largest cause of subsidence. Excessive pumping of such aquifer systems has resulted in permanent subsidence and related ground failures. In some systems, when large amounts of water are pumped, the subsoil compacts, thus reducing in size and number the open pore spaces in the soil the previously held water. This can result in a permanent reduction in the total storage capacity of the aquifer system.

Ground-water true/false quiz

Information on this page is from Waller, Roger M., Ground Water and the Rural Homeowner, Pamphlet, U.S. Geological Survey, 1982