By Pat Wilson

Have you ever landed at the Orlando airport and marveled at the many round, mirror-like lakes that dot the surrounding area? According to the St. Johns River Water Management District, many lakes in central Florida are actually the result of sinkholes that have occurred both recently and over millenia. In fact, there is even an area to the west and south of Seminole County that is known as ‘sinkhole alley’. The western counties of Central Florida are at high risk for sink holes while our area is considered at moderate risk.

The limestone geology of central Florida makes it prone to these occurrences for several reasons. The mechanism of sinkhole formation involves water dissolving limestone. Abundant rainfall that is slightly acidic dissolves the limestone under the soil. Slowly, over time, cover-subsidence sinkholes form as the porous limestone rock dissolves and the land sinks, or subsides. Lakes emerge as the depression reaches the aquifer and fills with surface water. Sometimes, as the limestone dissolves, caves take shape. A more dramatic and catastrophic type of sinkhole called a cover-collapse sinkhole occurs when the clay or sand covering a thin area of limestone caves-in and a deep hole occurs suddenly, with little warning. Interestingly, sinkholes are round because when the weakest spot underground fails, everything falls in towards this central axis forming a circular hole.

Some sinkholes occur naturally but some factors of human activity can increase the risk of sinkhole formation. Since waterflow and fragile limestone are the ingredients of sinkhole formation, development pressures in our rapidly growing state need to be carefully considered. Pumping too much water out of the aquifer for city and agricultural use can lead to empty areas below the surface. When the overlying clay and soil is too thin, and overuse of water or drought has caused a void, a collapse can occur. Similarly when construction occurs and substantial weight is placed in areas that are fragile, buccaling and sinking also occurs. Water movement within the aquifer weakens the limestone that it is made from. Some lakes and retention ponds have actually been engineered to overflow directly down into the aquifer to keep surrounding developed areas from flooding. This causes too much hydrologic pressure and movement below leading to rock fractures.

Recently, however, large wetlands have been created to allow a more natural return of treated and run-off water to the aquifer. It’s important as more development occurs to allow water to slowly percolate down to the aquifer through engineered wetlands, but maintaining a network of open fields, forests and natural wetlands is fundamental. Thus keeping the slow, natural balance of the water-rock ratio.

The ecology of Florida is intricate and fragile, and so understanding where we live and how we utilize water is as important as the ground we stand on!

For more information: https://floridadep.gov/fgs

Florida’s Round Lakes; The Science of Sinkholes

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