Issue papers
- Cooperative Funding
- Comprehensive Watershed Management
- Exotic Species Management
- Flood Protection
- Funding Water Resources Protection
- Minimum Flows and Levels
- Mitigation
- Partnership Agreement
- Permitting
- Protecting Lands and Providing Public Uses
- Responsibilities
- Sinkholes
- Springs — A Unique Resource
- A Sustainable Water Supply
Sinkholes are as common in Florida as lakes, rivers and warm weather. In fact, many lakes in central Florida were formed by sinkholes.
As the name suggests, sinkholes result from the ground “sinking” and creating a hole or depression. The cause of the sinkhole is underground in the porous limestone base.
Over millions of years, limestone was deposited creating the Florida peninsula. Subsequent rainfall filtered through the ground to the limestone, eroding and dissolving the soluble rock and creating cavities in the subsurface. These cavities serve as part of what is now the aquifers containing our underground freshwater supplies.
Layers of sand and clay rest between the land surface and the limestone cavities below. This “support” layer separates the land’s surface from the limestone cavities. The thinner the support layer, the more likely it is that sinkholes will occur. The “sinking” can be caused by added pressure from above ground, such as development or standing water from heavy rains. It can also be caused by the continued, natural erosion of the limestone base, enlarging the existing cavities by the flow of water.
The lowering of underground water levels, either by lack of rainfall or over-pumping for water supply, also can contribute to the development of the sinkholes. The water in the underground cavities helps support the layers holding up the land surface. As the water level drops, so does the added support from the underground water, allowing the land surface to collapse into the cavity below.
In the Southwest Florida Water Management District area, sinkholes develop more frequently north of Tampa Bay where the limestone base is closest to the land surface and the supporting sand and clay layers are thin. Geologists have a good idea where sinkholes are likely to form geographically, but it’s much more difficult to accurately predict specifically where sinkholes will occur. Special ground-penetrating radar equipment can be used to create a map of the underground area, but this information provides only a clue where the cavities are in the subsurface.
Sinkholes usually offer some warning signs before occurring:
- Previously buried parts of fence posts, foundations and trees become exposed because of sinking ground.
- Slumping, sagging or slanting fence posts, trees of other objects.
- Doors and windows that fail to close properly.
- Cracks in walls, floors pavement and ground surface.
- Small ponds of rainfall forming where water has not collected before.
- Wilting of small, circular areas of vegetation because the moisture that normally supports vegetation in the area is draining into the developing sinkhole.
- Muddy water in nearby wells during early stages of sinkhole development.
Repairs of sinkholes are the responsibility of the property owner. The District is the agency responsible for protecting the water and water-related resources. As part of its mission, the District ensures that permitted systems work and function properly. Stormwater ponds, which are issued an environmental resource permit, can fail as a result of sinkholes in the bottom of the pond. If a sinkhole opens in a stormwater pond it is the responsibility of the permittee to contact the District with a repair plan. If a sinkhole opens all the way to the aquifer, surface pollutants can contaminate the underground water supply. Sinkholes that form in lakes can drain portions of the lake and cause environmental impacts to wildlife.
If your home is threatened, contact your homeowner’s insurance company. Make sure the sinkhole area is fenced, roped or taped off to prevent injuries. And if it is large enough, contact your local law enforcement office or emergency management office.
May 2005
