Sinkholes

from Sinkholes, West-Central Florida USGS

Sinkholes can vary from small shallow depressions in the earth to holes that are hundreds of feet deep and cover hundreds of acres. Some sinkholes even hold water and form natural ponds and lakes. Typically, sinkholes form so slowly that little change is seen in one's life- time, but they can form suddenly when a collapse occurs. Such a collapse can have a dramatic and devastating effect if it occurs in an urban or suburban setting as recently happened in Hillsborough County near Tampa, Florida when a sinkhole opened beneath a house swallowing a man and his bedroom. The body was never recovered and the house was demolished. Western central Florida has a long history of sinkholes and because of geology and groundwater pumping is  particularly susceptible to sinkholes. In the water well fields for St. Petersburg located in Hillsborough County and surrounding counties sinkholes have occurred in conjunction with development of each of the well fields as well a throughout the region. Sinkhole formation is highest during dry months of the year and during drought, but overall appear to be increasing in frequency according to the U.S. Geological Survey, USGS, though there might be some reporting bias to the data.   

A landscape that forms sinkholes, sinking streams, caves, and springs is called a karst landscape. A karst landscape most commonly develops on limestone, but can develop on several other types of rocks, such as dolostone (magnesium carbonate or the mineral dolomite), gypsum, and salt, which are types of evaporates rocks. Rain is naturally mildly acidic, and slowly over time the weakly acids rainwater dissolves these deposits creating fissures. The deposits are highly permeable, and surface water passes through them quickly to underlying aquifers, eating away at the limestone and evaporates bedrock. Overtime this creates the voids that become sinkholes. There are three general types of sinkholes: dissolution sinkholes—depressions in the limestone surface caused by the erosion of limestone by rain; cover-subsidence sinkholes—formed as overburden materials gradually fill below surface fissures formed by the infiltration of rain; and cover collapse sinkholes—which occur in limestone terrain with a thick overburden or mantle after the fissures forms large cavities and the cover materials collapse into the subsurface voids. This third type of sinkhole is what occurred last week near Tampa.

Hundreds of collapse sinkholes of various sizes occur throughout the country each year and start unnoticed when infiltrating water or groundwater flowing in the subsurface creates a void where soil is washed away. Eventually the void or hole grows large enough that the soil above it can no longer bridge it. The soil bridge then suddenly collapses into the void below and a sinkhole forms. Often this happens when the water level that has been exerting an upward pressure- helping to hold up the soil bridge falls. This process usually takes many years to occur in nature, but it can be aggravated by human activities. Any activity that increases the amount of water flowing into the subsurface can speed up this process. Parking lots, streets, altered drainage from construction, irrigation, leaking swimming pools and roof guttering are some things that can increase runoff; even severe weather can cause sinkholes.

from the Florida USGS office

The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania thought large areas of the United States are underlain by evaporates rocks (salt, gypsum, and anhydrite) and carbonates (limestone and dolomite), the rock types that are most susceptible to being dissolved away by water. Even when evaporite rocks are buried at great depths, in so called Mantled Karst terrain sinkholes can form. These sinkholes are the most sudden when the mantle give way. The western central portion of Florida is an area of Mantled Karst terrain, but most of Florida is prone to sinkhole formation because it is underlain by thick carbonate deposits and is so rich in groundwater. Development and overuse of the groundwater resources for municipal, industrial and agricultural water supplies has resulted in falling groundwater levels that play a role in sinkhole formation as well as development.

According to Ann B. Tihansky of the U.S.G.S. in Tampa and author of Sinkholes, West-Central Florida,“Induced sinkholes are generally cover-collapse type sinkholes and tend to occur abruptly. They have been forming at increasing rates during the past several decades and pose potential hazards in developed and developing areas of west-central Florida. The increasing incidence of induced sinkholes is expected to continue as our demand for groundwater and land resources increases. Regional declines of ground-water levels increase sinkhole occurrence in sinkhole-prone regions.” The sinkhole prone regions of the country can be seen below.All of Florida is karst terraine, but as can be seen below karst terrain also covers much of the Valley and Ridge Province of Virginia in the western third of the state. Small karst areas occur in the Cumberland Plateau, Piedmont and even the Coastal Plain provinces.

From USGS

If you have questions or worries about sinkholes, settling or earth movement in your yard, Florida has an excellent question and answer web site. 

Protecting Drinking Water in Karst Terrain

Most of us are familiar with the caves and the more striking karst features that occur in karst terrain though visits to the National Park System and caverns. My own introduction to karst features was Howes’ Caverns in New York where the stalactites, stalagmites and flowstone created from the calcite dissolved from overlying rock was a wondrous site for a child. My husband, a native son of Virginia, went to Luray Caverns as a child. In Virginia karst terrain covers much of the Valley and Ridge Province in the western third of the state. Smaller karst areas occur in the Cumberland Plateau, Piedmont and Coastal Plain Provinces, too. Cave systems are just some of the features that occur in karst terrain. Karst terrain may including sinkholes, fractured bedrock, sinking streams, and sinkhole ponds that are all direct routes of groundwater recharge that provides little if any containment or removal of contaminants of surface waters that recharge karst aquifers. Ground water flows rapidly through karst aquifers, through the enlarged solution channels, discharging from springs and supplying base flow to surface streams and rivers.

Karst terrain occurs in areas where the underlying rocks are carbonate rock such as limestone (CaCO3), dolomite (CaMg(CO3)2) and gypsum (CaSO4.2H2O). These rocks are soluble in dilute acids and typically have only a thin soil overlay with areas of rock outcroppings. Rain water becomes slightly acidic when passing through decaying organic debris in the surface soils. The decaying organic material is a ready source of carbon dioxide, CO2. The CO2 and H2O chemically react to form a weak acid called carbonic acid. The slightly acidic water percolates down through the soil into fractures in the carbonate bedrock. These types of rocks are the very type of rocks used in the filters to neutralize acidic or corrosive well water because they easily react with the slightly acidic rain water. The carbonic acid in the moving ground water slowly dissolves the bedrock forming passageways and caves. This geological process results in unusual surface and subsurface features ranging from sinkholes, disappearing streams and springs to complex cave systems and caverns that are the characteristic Karst features.

These Karst features are very important to understand because approximately 20% of the land in the United States is classified as karst topography, but these areas produce 40% of the groundwater used for drinking water in the United States. World wide approximately 10% of the earth's surface is classified as karst; with an estimate 25% of the world's population living in karst areas. The hollow nature of karst terrain results in a very high pollution potential. The thin soils over fractured limestone allow precipitation to enter the subsurface with minimal natural filtration. Streams and surface runoff enter sinkholes, fissures and caves, carrying surface contaminants and without the natural filtration provided by soil and sediment cover the contaminants quickly flow to depth. Groundwater can travel quite rapidly through these underground networks. In tests by the U.S. Geological Survey and the U.S Environmental Protection Agency, groundwater was documented to travel thousands of feet, even miles, per day transmitting tracer dyes and potentially contaminants to wells and springs throughout the vicinity. In karst terrain groundwater can flow like an underground river.

Karst aquifers are among the most highly vulnerable to contamination, particularly where the overlying soil is thin. This vulnerability results from: sinkholes, widened flow paths, and rapid velocities of ground water and contaminants. Contaminants can be transmitted quickly from entry in a sinkhole to wells and springs in the vicinity. A sinkhole is generally a funnel-shaped or steep-sided depression that is caused by the underlying carbonate rocks dissolving away and the subsidence of the land surface into a subterranean passage, cavity, or cave. Sinkholes proved a direct path for surface contaminants to enter the groundwater. Sinkhole creation, sinkhole flooding, and groundwater contamination are the major hazards associated with karst terrain, and unlike other natural hazards they are chronic in nature. Rapid infiltration of surface water allows bacteria to reach groundwater depth while still alive. Also, rapid infiltration does not consume a lot of the oxygen in the water and nitrate does not denitrify making karst aquifer highly susceptible to bacterial and nitrate contamination (major contaminants in human and animal waste).

Sinkholes are easily formed in karst terrain. Alterations to surface runoff during development can cause sinkholes. Groundwater pumping can quickly lower the water level and result in a subsidence or sinkhole formation. Failing septic systems are a significant source of groundwater contamination in karst terrain. Also, there are many cases of septic tanks simply sinking into the underlying cave system in karst areas. Rather than devastating natural disasters, karst terrain unwisely developed has resulted in long-term economic burdens on individual property owners and communities for sewage and water treatment in sparsely developed areas. The residents of karst areas need to be aware of how day-to-day activities affect the groundwater and fragile ecosystems in their karst regions. In addition, surface water can directly influence groundwater carrying with it all the surface bacteria and contaminants that source groundwater is not typically treated for.