If a disabled individual wishes to obtain the information contained in thisdocument in another form, please call Tony Gilboy at 1-800-423-1476,extension 4305; TDD only 1-800-231-6103; fax (352) 754-6882.

For more information call theSouthwest Florida Water Management District

2379 Broad Street • Brooksville, Florida 34609-6899Phone: 1-800-423-1476

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Sinkholes

Florida. Land of sunshine, beautiful beaches and... sinkholes!Sinkholes can be depressions or collapses in the land surface, or may

be hidden from view below the surface. Sometimes referred to as “sinks,”they are widely distributed in the northern and central counties of theSouthwest Florida Water Management District. They can be shallow ordeep, small or large, but all are a result of the same general geologicprocesses.

For more than 60 million years, ancient shallow seas covered whatis now the state of Florida. Factors relating to the chemistry of the waterand the chemistry of the plants and animals that lived here resulted inlimestone being deposited beneath these ancient seas. Eventually, theseas lowered to where they are today leaving a base of predominantlylimestone bedrock covered with sand and clay for us to live upon.

This limestone is thousands of feet thick and underlies the entirestate and portions of the Atlantic Ocean and the Gulf of Mexico. Whenthe limestone was originally deposited, it was laid down in layers orbeds. These layers have slight differences in chemical composition,hardness, and thickness. Earth processes are in constant motion andas a result, the limestone cracks, breaks, develops fractures and weaklayers, and slowly dissolves.

The essential factor of all sinkhole development is the dissolving ofthe underlying limestone by slightly acidic water. As rain falls through theatmosphere, it absorbs carbon dioxide and forms a weak carbonic acid. Asthis water moves through the soil zone, it reacts with living and decayingplant matter and becomes more acidic. The acidic water slowly dissolveslimestone, especially along the fractures and weak layers. This chemicalerosion eventually causes voids or cavities into which overlying sedimentsmay collapse or subside. The end result of chemical erosion of limestone,followed by physical collapse or subsidence, is a sinkhole.

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INTERMEDIATE AQUIFER SYSTEM

General Hydrogeologic Cross Section of the Region

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Possibilityof SinkholeDevelopment

Natural or Induced?

Much of Florida’s landscape is comprised of what is known as“karst.” A karst terrain is a land surface produced by water dissolvingthe bedrock and is characterized by sinkholes, cavern systems andsprings.

The buried karst of the southern half of the water managementdistrict receives much less infiltration of rain water and is much morestable than the northern half of the District. The reason is that theground water in the southern part of the District is well-confined due tosediment formations. These formations contain beds of sand, clay andthick limestone. The thickness and the type of material that overlies thelimestone determines the type of sinkhole that may form.

Sinkholes occur as a natural process of erosion of the limestone by water. Ancient cavities dissolved in the limestone need a

triggering mechanism to cause the collapse. In predevelopment times,sinkholes were usually triggered by heavy rains or a flood which madethe soil “roof” over the cavity very heavy, so that it eventually collapsed.Droughts can also lower the ground-water levels, reducing the buoyantsupport of a cavity roof and prompting a collapse. Natural sinkholes stilloccur in Florida.

Possibilityof SinkholeDevelopment

Natural or Induced?

Confined Aquifer and/or morethan 100 feet of overburden abovelimestone. Sinkholes rare.

Limestone at or near land surface.High sinkhole density, but moderateto small intensity of surface collapse.

Moderate overburden thickness overlayingcavernous limestones with significant water level differences between the surficial and Floridan aquifer systems. History of sudden steep-walled, wide sinkhole collapse.

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Cover-Collapse Sinkholes

Cover-Subsidence Sinkholes

Stages of a Cover-subsidence and Cypress Dome Sinkhole’s Development

Limestone Solution Sinkholes

Cover-Subsidence SinkholesIncreased numbers of sinkholes can generally be attributed tochanging or loading of the earth’s surface with development such asretention ponds, buildings, changes in drainage patterns, heavy traffic,drilling vibrations or declining ground-water levels. In urban areas, allthese impacts may occur at the same time, accelerating any sinkholetendencies. Urban construction, coupled with limestone depths of lessthan 200 feet, contributes to many of the modern sinkholes. Forexample, the lack of sinkholes in metropolitan Manatee and Sarasotacounties can be attributed to the depth to limestone which is generallygreater than 100 feet. The thick sequence of sand and clay above thelimestone, shown in the cross-section on page 1, seems to support thisobservation.

Whether natural or induced, there are several types of sinkholes.The majority of these occurring in southwest Florida are limestonesolution sinkholes, cover-subsidence sinkholes, and cover-collapsesinkholes.

In parts of Pasco, Hernando, Citrus and Levy counties, limestone is exposed at the surface or is covered by a thin layer of soil. Thisleaves the limestone subject to both physical and chemical processesthat break down the rock. When this breakdown occurs, it usuallyforms a saucer or bowl-shaped depression. Due to the naturaldissolving of limestone, these sinkholes develop continuously, butslowly.

Where the sand layer may be as thick as 50 to 100 feet, with very little clay below it, the dissolving limestone is replaced by

granules of sand that cascade down to fill the void. This type ofsinkhole is referred to as a cover-subsidence sinkhole. These sinkholesare only a few feet in diameter and depth. Their small size is due to thefact that the cavities in the limestone cannot develop to appreciablesize before they are filled with sand.

Generally, the deeper the soils, the more clayey the soils tend to become. This clay provides some cohesiveness to the soil materialabove it, allowing it to bridge any existing cavity in the limestone. If this“bridge” collapses, it results in what is called a cover-collapse sinkhole.

The size of the sinkhole depends upon the size of the cavity. Cover-collapse sinkholes form the same way as cover-subsidence sinkholesbut differ mainly in the bearing strength of the soil above the cavity andwhether the sinkhole subsides slowly or collapses abruptly.

Stages of a Cover-subsidence and Cypress Dome Sinkhole’s Development

Limestone Solution Sinkholes

Cover-Collapse Sinkholes

Thick Sand Clay Layer Limestone Water Table in Sand Layer Potentiometric Surface of Limestone Aquifer Flow to Discharge Area

Rainwater percolatesthrough incohesivedeposits to theunderlying limestone.Highly transmissivejoints dissolve fasterthan others.

Differential solutionof bedrock isexpressed by adepression at landsurface that funnelswater to the enlargedjoints.

Sinkhole intersects the water table and cypress trees begin to grow. Rate of dissolution is reduced because there is less head difference between the water table and potentiometric surface and, thus, less percolation.

Sinkhole spreadslaterally faster thanit subsides. A cypressdome forms with oldertrees in the center andyoung trees on theperimeter.

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Sinkhole Misconceptions

Sinkhole Warning Signs

Water and its Role in SinkholeDevelopment

Water not only contributes to the chemical dissolving of the limestone but it also contributes to the support or lack of support

it provides to the formation material when it is lowered or raised. Thewater provides a hydrostatic pressure which is exerted on the claylayers which separate the shallow surficial aquifer from the deeperFloridan aquifer (drinking water).

Fluctuations in this hydrostatic force have a weakening effect on alimestone roof or a clay layer spanning a cavity. Often times the waterlevel will reduce the hydrostatic support to the clay “bridge” andincrease the downward gradient of the overlying surficial aquifer,resulting in a sinkhole collapse.

Although the sinkhole may have occurred eventually, the loweringof the water level accelerated the sinkhole collapse, acting as a catalystor triggering mechanism.

When a sinkhole develops, everything on the land surface or in the soil below has the potential to end up in the aquifer. The

same fascinating system that allows dissolved rock to carry tremendousamounts of clean water for our use also allows surface water to carrycontaminants through the sinkhole into the drinking water.

Many people see sinkholes as drainpipes, calling them “go-awayholes.” They think that out-of-sight is out-of-mind when in actuality, thedrain may go directly into their drinking water!

The area around a sinkhole should be kept free of garbage andwastes. Don’t use it as a garbage dump or compost pit.

Make sure that fertilizers, chemicals, motor oils, animal wastes, orpesticides are not allowed to drain into a sinkhole from the surroundingarea.

• Fresh exposures on fence posts, foundations, and trees that result when the ground sinks;

• Slumping, sagging or slanting fence posts, trees or other objects; doors and windows that fail to close properly;

Sinkhole Warning Signs

Sinkhole Misconceptions

Water and its Role in SinkholeDevelopment

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• Ponding — small ponds of rainfall forming where water has not collected before;

• Wilting vegetation — wilting of small, circular areas of vegetation. This happens because the moisture that normally supports vegetation in the area is draining into the sinkhole that is developing below the surface;

• Turbidity in well water — muddy water in nearby wells during early stages of sinkhole development;

• Structural failure — cracks in walls, floors, and pavement; cracks in ground surface;

• Keep children away!

• If lake or river levels are affected or you think ground-water quality is endangered by a sinkhole, please report it to the Southwest Florida Water Management District at 1-800-423-1476.

• If your home is threatened, contact your homeowners insurance company.

• Make sure the sinkhole is fenced, roped, or taped very clearly. In some areas a local enforcement agency can help. Usually, the property owner will be liable if someone is hurt in the sinkhole.

• Check carefully for signs of the sinkhole enlarging, especially toward buildings, septic tanks, drain fields, and wells (flowing water into a sinkhole will continue or accelerate its growth). This can be done with a thin hard metal rod that can be pushed into the soil. Areas near the sink will offer less resistance to the rod than the unaffected soil.

• Do not throw any waste into the sinkhole. Fill the hole with clean sand.

• Do not use the sinkhole as a drainage system. Pesticides and other wastes seep easily through the sinkhole into the aquifer — your drinking water.

If a Sinkhole Developson Your Property:

If a Sinkhole Developson Your Property:

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