FLORIDA HAS MORE SINKHOLES than any other state in the nation. They are an obvious feature of Florida's natural karst topography. Sinkholes provide a primary pathway for rainwater to replenish subsurface groundwater; they are an important part of the aquifer system that supplies 95% of Florida's drinking water.

Appreciation and conservation of sinkholes is essential to ensure the future of Florida's freshwater supply.

If left unprotected, polluted surface water can drain into sinkholes and easily contaminate the aquifers.

Unfortunately, their bad reputation for property destruction frequently overshadows the vital role sinkholes play in Florida's natural environment.

One day in 1999 "outstanding Florida water body" Lake Jackson began to disappear when a sinkhole opened. Within a few days, this large well-known fishing lake in Tallahassee lost its water, its fish and its alligators into the aquifer beneath. Lake Jackson is one of Florida's "disappearing lakes": it has done so at least four times since 1907. For more information about this event, go to this page on our main web site.

Source: USGS

SINKHOLES and the AQUIFER
Sinkholes originate beneath the surface when groundwater moves through the limestone and erodes large voids, or cavities, in the bedrock. When water fills a cavity, it supports the walls and ceiling, but if the water-table drops, the limestone cavity is exposed to further erosional processes that eventually result in the collapse of the cavity, causing a surface indenture, or sinkhole. The sinkhole becomes a primary site of recharge, where surface water can enter the aquifer and replenish the groundwater supply.

SURFACE CHARACTERISTICS
On the surface, sinkholes may develop progressively as subtle, bowl-shaped depressions, or they may collapse suddenly into steeply sided, water-filled craters. The shape of the sinkhole, and the speed that it forms, depend on the size of the subsurface cavity and the thickness of the overburden (sediments or organic matter that rest on the limestone bedrock).

Source: USGS

COLLAPSE SINKHOLES
Collapse sinkholes are the most common type in Florida. They happen suddenly where the overburden is thick with soils and heavy clay. Collapse sinkholes are deep, steeply-sided holes in the ground. They are frequently triggered by fluctuations in the water-table. As water levels fluctuate, the roof of the cavity is stressed and weakened. When the water-table drops too far, the cavity walls are unsupported and the ceiling becomes too weak to hold the heavy overburden. Eventually, the ceiling collapses and a sinkhole is formed. A conical debris mound left on the sinkhole floor is all that remains of the cavity ceiling.

If the water-table rises, the collapse sinkhole can fill with water, and overflow like a spring. An off-set sinkhole will have an upstream and downstream conduit as water flows into the sink and siphons underground. If the water-table drops below the sinkhole, it will remain dry and accumulate sediments and vegetation.

SOLUTION SINKHOLES
If the overburden is thin or absent, the surface of the limestone bedrock is broken down by erosion from wind and surface water. A bowl-shaped depression, or solution sinkhole, naturally forms slowly and continuously as chemical and physical processes erode the rock.

Source: USGS

SUBSIDENCE SINKHOLES
Subsidence sinkholes form gradually where the overburden is thin. The dissolving limestone is replaced by sand granules that fall into the depression and fill the holes. They appear as a concave depression in the ground. Subsidence sinkholes are usually only a few feet in diameter and depth because the development of the cavities in the limestone is retarded since they are filled with clay and sand. As the sediments fill the depression, they restrict the flow of water through the bottom and the hole begins to retain water. As water accumulates, a lake is formed.

SINKHOLES and LAKE FORMATION
Many ponds and lakes in Florida are a result of sinkhole formation. The characteristics of a sinkhole lake can give clues as to how it was formed. A circular lake indicates that the lake evolved from a collapse sinkhole. A shallow circular lake results from impermeable sediments washing into a subsidence sinkhole. If a lake rests above groundwater level, it is above a confining bed.

SINKHOLES and URBAN DEVELOPMENT
Sinkhole formation is aggravated and accelerated by urbanization. Development increases water usage, alters drainage pathways, overloads the ground surface, and redistributes soil. According to the Federal Emergency Management Agency, the number of human-induced sinkholes have doubled since 1930, insurance claims for damages as a result of sinkholes has increased 1200% from 1987 to 1991, costing nearly $100 million.

To avoid the destruction of property and the contamination of groundwater, it is important to monitor potential sinkhole formation. Although a sinkhole can form without warning, specific signs can signal potential development:

• Slumping or falling fenceposts, trees, foundations, etc.
• Sudden formation of small ponds
• Wilting vegetation
• Discolored wellwater
• Structural cracks in walls, floors, etc.

WHAT TO DO if a SINKHOLE DEVELOPS on your PROPERTY

• Notify your local Water Management District
• Fence or rope the hole off
• Keep children away!
• Protect the area from garbage and waste
• Contact your homeowners insurance company

SINKHOLES and RECREATION
In addition to providing essential pathways for water to replenish the aquifer, a variety of sinkholes are protected and maintained as Florida state parks where they provide frost-free habitats for cold sensitive species and offer many opportunities for sports and recreation.