Sinkholes

Sinkholes are natural, circular depressions that form when water erodes easily dissolved or soluble rock located beneath the ground surface. Water moves along joints, or fractures, enlarging them to form a channel that drains sediment and water into the subsurface. As the rock erodes, materials above subside into the openings. Sinkholes range from a few feet (m) to several hundred ft (m) in width and depths up to 150 ft (50 m). The subsidence associated with sinkholes can be a rapid or gradual process and poses considerable risks associated with damage to surface structures. Though many of these depressions are filled with water, sinkholes have no external drainage and are potential sites for the pollution of groundwater.

Sinkholes occur worldwide, and in the United States are common in southern Indiana, southwestern Illinois, Missouri, Kentucky, Tennessee, and Florida. Abundant sinkholes as well as caves, disappearing streams, and springs, characterize a type of landscape known as karst topography. Sinkholes are the most characteristic feature of karst topography. Karst topography forms where groundwater erodes subsurface carbonate rock, such as limestone and dolomite, or evaporite rock, such as gypsum and salt. Carbon dioxide (CO₂), when combined with the water in air and soil, forms carbonic acid, acidifying the water slightly. The slight acidity intensifies the corrosive ability of the water percolating into the soil and moving through fractured rock.

Geologists classify sinkholes mainly by their means of development. Collapse sinkholes are initiated by solution of rock beneath the surface. As the corrosion of subsurface rock proceeds, a cavity is produced within the rock. The sinkhole itself is formed when the roof of the resulting cavern collapses. These sinkholes are often funnel shaped, with characteristically steep sides and a debris-covered bottom. They form when soil or rock material collapses into a cave. Collapse may be sudden and damage is often significant; cars, roads, and homes may be swallowed by these sinkholes.

In contrast to collapse sinkholes, solution sinkholes are formed from the surface downward. Solution sinkholes form in rock with multiple vertical joints. Water passing along these joints expands them allowing cover material to move into the openings. Solution sinkholes usually form slowly and minor damage occurs, such as cracking of building foundations.

Alluvial sinkholes form where carbonate rocks are overlain by unconsolidated debris. Solution of the underlying rock causes the overlying sediment to sag and form the surface depression. This phenomenon can also occur where the overlying material is solid, insoluble rock. They can be hard to recognize and some are relatively stable. Rejuvenated sinkholes are previously stable alluvial sinkholes in which the cover material once again begins to subside, producing a growing depression.

Vanishing Lake, a large sinkhole. Photograph by W.K. Fletcher. Photo Researchers, Inc. Reproduced by permission.

Uvalas are large sinkholes formed by the joining of several smaller sinkholes. These features often have a scalloped, irregular outline that is the result of their mode of origin.

Sinkholes occur naturally, but are also induced by human activities. Pumping water from a well can trigger sinkhole collapse by lowering the water table and removing support for a cave's roof. Construction over sinkholes can also cause collapse. Sinkhole development damages buildings, pipelines, and roadways, costing millions of dollars each year in the United States alone. In May 1981, a large sinkhole began to develop in Winter Park, Florida. After three days, the sinkhole had swallowed a house, several cars, parts of two businesses, part of a community pool, and a section of road. Damage from the Winter Park sinkhole is estimated at greater than $2 million. As is often the case, this sinkhole formed during a drought period, as a result of lower groundwater levels.

In areas where evaporite rock is common, human activities play an especially significant role in the formation of sinkholes. Evaporites dissolve in water more easily than do carbonate rocks. Salt mining and drilling into evaporite deposits allows water that is not already saturated with salt to easily dissolve the rock. These activities have caused the formation of several large sinkholes.

Sinkholes may also serve as routes for the spread of contamination to groundwater when people use them as refuse dumps. These natural depressions have long been attractive sites for dumping all types of waste. In rural areas, it is common for household trash, dead livestock and game carcasses, and other trash to be carelessly disposed of in these pits. Furthermore, agricultural chemicals, including fertilizers and pesticides, road salt, sewage, and leaking and waste petroleum products often flow into the features. Because sinkholes are intimately tied to the groundwater that helps to form them, they carry any toxins present within them directly to that groundwater system. The rapid flow characteristic of karst systems does not allow for the normal filtering and purification of groundwater. Instead toxins are carried rapidly to any down-gradient groundwater users.

Research into the formation and characteristics of sinkholes continues. One area of concern is the hydrologic effect of wastes and toxins on groundwater systems near sinkholes. Dye tracing methods have been used to delineate the areas affected by such wastes and the rate at which such material is carried downstream. In this method, a dye is placed in the groundwater and collected downstream at wells or other sinkholes. In areas with known karst topography, subsurface drilling or geophysical remote sensing may be used to pinpoint the location of hidden sinkholes. This is of particular importance for the protection of existing or planned structures. Detailed elevation mapping, ground penetrating radar, and the electrical resistance of the earth can be used for the identification of incipient sinkhole locations.