Land subsidence occurs when large amounts of ground water have been withdrawn from certain types of rocks, such as fine-grained sediments.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 When the water is withdrawn, the rock falls in on itself.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).
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.
Causes of Land Subsidence
The basic cause of land subsidence is a loss of support below ground. In other words, sometimes when water is taken out of the soil, the soil collapses, compacts, and drops. This situation occurs throughout the United States, but has had more impact in California, Texas, and Arizona.
Land subsidence is most often caused by human activities, mainly from the removal of subsurface water. Here are some things that can cause land subsidence:
Loss of water in organic soils
Dissolving of subsurface limestone rock
First-time wetting of formerly dry, low-density soils
Natural compaction of soils
Underground mining
Withdrawal of ground water and petroleum
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.
The basic cause of land subsidence is a loss of support below ground. In other words, sometimes when water is taken out of the soil, the soil collapses, compacts, and drops. This situation occurs throughout the United States, but has had more impact in California, Texas, and Arizona.
Land subsidence is most often caused by human activities, mainly from the removal of subsurface water. Here are some things that can cause land subsidence:
Loss of water in organic soils
Dissolving of subsurface limestone rock
First-time wetting of formerly dry, low-density soils
Natural compaction of soils
Underground mining
Withdrawal of ground water and petroleum
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