Subsidence

Many forms of human activities can result in subsidence. One of the most widespread of these problems involves the removal of groundwater for agricultural, municipal, and other purposes. In large parts of the United States, for example, farmers and ranchers depend heavily on water removed from underground aquifers to irrigate their crops and water their livestock. Such activities have now been going on with increasing intensity for at least a century.

This practice will not lead to subsidence as long as enough rainfall filters downward to replace, or recharge, the groundwater removed by humans. However, when the rate of removal exceeds the rate of recharge, significant decreases in the volume of the aquifer begin to occur. The pore spaces between the grains of the aquifer, previously occupied by water, are emptied. The grains then begin to compact more tightly, and they collapse. Eventually, the aquifer begins to subside.

A dramatic example of subsidence as a result of groundwater removal has taken place in a region southeast of Phoenix, Arizona. There a section of land covering 120 sq mi (310 sq km) has sunk more than 7 ft (2 m). This phenomenon has occurred at many locations above the Ogallala Aquifer, which lies beneath the High Plains region, stretching from Kansas and Nebraska to Wyoming and from Texas and New Mexico to Colorado.

The removal of fossil fuels is also a major human cause of subsidence. A traditional method of removing coal, for example, is known as room-and-pillar because vertical columns of the coal (the "pillar") are left in position while the coal around it is removed. When such mines are abandoned, however, the pillars of coal left behind are often not strong enough to hold up the overlying ground. When the pillars break, the ceilings of the mined room collapse and the overlying ground does so also. With more than 90,000 abandoned mines in the United States, this source of subsidence is likely to be a problem into the foreseeable future.

The pumping of oil and natural gas from underground sources can have similar effects. Similar to removal of water from an aquifer, when these materials are removed from the reservoir, the reservoir's grains compact and the reservoir occupies a smaller volume than it did before the oil or gas was removed. As a result, overlying ground subsides as the reservoir slowly collapses.

One of the most famous of these instances occurred as far back as the late 1920s in Southern California. Oil removed from the Wilmington and Signal Hill oil fields caused unstable ground to subside by as much as 29 ft (9 m). Since this region lies along the coastline of the Pacific Ocean, drastic efforts were required to prevent ocean water from flowing into lands that were now lower in some places than sea level. By 1968, subsidence in the area had been stopped, but only after huge quantities of seawater had been injected into the empty oil wells in order to prop open the pores in the reservoir. This success in restoring stability to the area came at a high price, however, as docks, highways, sewer systems, and other municipal structures had, by that time, been completely rebuilt.