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Approximate location of maximum subsidence in the United States identified by research efforts of Dr. Joseph F. Poland (pictured). Signs on pole show approximate altitude of land surface in 1925, 1955, and 1977. The site is in the San Joaquin Valley southwest of Mendota, California.
Land subsidence occurs when large amounts of groundwater have been withdrawn from certain types of rocks, such as fine-grained sediments.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 When the water is withdrawn, the rocks falls in on itself.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 groundwater for irrigation has caused the land to drop. The top sign shows where the land surface was back in 1925! Compare that to where Dr. Poland is standing (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 of buildings in Mexico City, do you find yourself asking if these buildings in Mexico City seem to be following a wave pattern instead of a straight line from left to right? In fact, that is just what is happening not only to these buildings, but throughout Mexico City, where long-term extraction of groundwater has caused significant land subsidence and associated aquifer-system compaction, which has damaged colonial-era buildings, buckled highways, and disrupted water supply and waste-water drainage. Some buildings have been deemed unsafe and have been closed and many others have needed repair to keep them intact.
Land subsidence in California, due to groundwater withdrawal. Fissures near Lucerne Lake (dry) in San Bernardino County, Mojave Desert, California. The 5-gallon bucket is for scale.
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 groundwater levels. Here are some other things that can cause land subsidence:
The principal causes are aquifer-system compaction, drainage of organic soils, underground mining, hydrocompaction, natural compaction, sinkholes, and thawing permafrost.
Groundwater pumping and land subsidence
Compaction of soils in some aquifer systems can accompany excessive groundwater 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.
Map of the U.S. showing some of the areas where subsidence has been attributed to the compaction of aquifer systems caused by groundwater pumpage. From "Land Subsidence in the United States", USGS Fact Sheet-165-00, December 2000.