Understanding the Science of Sinkholes

Have you ever landed at Orlando International 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 the result of sinkholes that have occurred both recently and over millenia. In fact, there is 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 sinkholes, 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. Over time, cover-subsidence sinkholes slowly form as the porous limestone rock dissolves and the land sinks or settles. 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 and with little warning. Interestingly, sinkholes are round because when the weakest spot underground fails, everything falls in toward this central axis, forming a circular-shaped hole.

Some sinkholes occur naturally, but human activity can increase the risk of their formation. Because water flow and fragile limestone are the ingredients of sinkhole formation, development in our rapidly growing state should 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 layer of clay and soil is too thin, and overuse of water or drought has caused a void, a collapse can occur.

Similarly, when construction is underway and substantial weight is placed on areas that are fragile, buckling and sinking also occur. Water movement within the aquifer weakens the limestone from which it is made. Some lakes and retention ponds have been engineered to overflow 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 critical, too. This strategy keeps the slow, natural balance of the water-rock ratio in place.

The ecology of Florida is intricate and fragile. Understanding where we live and how we utilize water is as important as the ground we stand on.

Patricia Wilson moved to Orlando in 1990 and since then has been involved in understanding the science of Florida’s unique environmental issues. She has volunteered with NOAA’s Marine Microplastics Program and has worked with the American Association of University Women to develop a free educational program about how simple choices that we make can help improve water quality.