Sinkholes - USGS water science for schools

USGS: Sinkhole at Winter Park Florida

USGS

Urban sinkholes are devastating to a community's structures and infrastructures. But experts are equally concerned about the threat sinkholes pose to Florida's underground drinking water supply.

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A land made for sinkholes

I’m Kevin Pierce with the Florida Environment.

Imagine having a house overlooking a lake, and one day, the lake disappears. Or that a huge hole suddenly appears in place of the road you had been driving on. It's the reality of a sinkhole -- as many as 150 are reported each year in Florida. Ann Tihansky studies sinkholes for the US Geological Survey..

"Florida is basically all limestone, and then there's a thin covering, usually very loose sand... and in part of the state, there's a lot of thick clay. And those sediments kind of mantle the limestone."

But that mantle covering the porous limestone below is often only temporary ...

"Limestone is very soluble. And as water moves through it, small holes develop into larger holes. And the overburdened sediments can cover the hole for a certain amount of time. But once the hole gets larger than their ability to bridge across it, the sediments collapse into it."

Sinkholes are rare in the southern part of the state, with central Florida and the Big Bend seeing the most...

"What we think is a solid surface because it's all sand and clay to our eyes, it seems like it's not limestone. But it's not a very thick covering and there are usually sinkholes common wherever you have limestone terrain."

For more information, visit floridaenvironment.com. With help from its Environmental Studies Program we're produced at the Whitaker Center at Florida Gulf Coast University and funded by the Southwest Florida Council for Environment Education.

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The perils of urban sinkholes

I'm Kevin Pierce with the Florida Environment.

While more sinkholes are reported in Florida each year, experts attribute that in part to a greater population being around to spot the openings to the earth below. And it is those sinkholes near urban areas that cause much of the concern. Ann Tihansky studies sinkholes for the U.S. Geological Survey...

"In a field where you have cattle or something, aside from their being a risk of a cow getting into it, they don't pose a risk. The risk is in roadways, and certainly structural problems with house foundations or larger buildings. That's when property is damaged and that's when accidents occur related to sinkholes."

Because of Florida's geology, these sinkholes are just a fact of life...

"In the natural environment, it's rarely catastrophic or harmful. It may cause several trees to be uprooted or something, but it's not that kind of dangerous or expensive catastrophe that you get in an urban area when whole houses or vehicles... you can disconnect water mains..."

While studies try to forecast sinkholes and study their causes, a warning remains...

"There's not a lot you can do about it... large forces of nature that happen... and probably will continue to happen."

For more information, visit floridaenvironment.com. With help from its Environmental Studies Program we're produced at the Whitaker Center at Florida Gulf Coast University and funded by the Southwest Florida Council for Environment Education.

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An unseen threat from sinkholes

I'm Kevin Pierce with the Florida Environment.

While sinkholes are capable of swallowing buildings and destroying roadways, there's another danger, perhaps even greater. Hydrologist Ann Tihansky says it comes from a sinkhole's direct link between above ground water and underground water...

"The structural problems are pretty obvious. But the more insidious problem -- and what I consider a major threat -- is the risk to our ground water supply."

Sinkholes, though not causes of pollutants, can be the carrier for pollution...

"Once things get into the ground water, it's very difficult to remove them and to clean them. And when you have a direct connection with surface water, all kinds of wastes can get into the ground water system through sinkholes. And that to me is probably the most important risk besides obvious structural problems."

And when a sinkhole is formed, there's little limit to the type of pollution it can spread...

"When we have land use that allows nutrients, or chemicals or other kinds of contaminants to get into the ground water system, sinkholes are sometimes the most direct pathways for those wastes."

For more information, visit floridaenvironment.com. With help from its Environmental Studies Program we're produced at the Whitaker Center at Florida Gulf Coast University and funded by the Southwest Florida Council for Environment Education.

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The right weather for sinkholes

I'm Kevin Pierce with the Florida Environment.

The period of unusually dry weather that Florida has just come through will make conditions right for more sinkholes this year. Ann Tihansky studies sinkholes for the U.S. Geological Survey...

"Drought years, when we don't have as much rain and water levels decrease and become lower, are usually correlated to the highest incidences of sinkhole occurrence."

In addition to dry weather promoting sinkhole formation, dry weather followed by unusual rainfall creates an even more favorable condition...

"When we're coming out of the dry season and we've got an extreme deficit rainfall. And then let's say we went right into an extremely rainy time... that would probably be linked to the highest number of sinkholes."

Historically, years with that dry-weather followed by wet-weather situation have had some of the greatest sinkhole reports...

"When we have an awful lot of surface loading and rain... If we had a major rain event -- I'm talking several inches -- and we had it occur repeatedly so that surface water areas were flooded, that weighs on the land surface and that can trigger sinkholes."

For more information, visit floridaenvironment.com. With help from its Environmental Studies Program we're produced at the Whitaker Center at Florida Gulf Coast University and funded by the Southwest Florida Council for Environment Education.

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Human causes for sinkholes

I'm Kevin Pierce with the Florida Environment.

Sinkholes are a natural occurrence in Florida -- as many as 150 are reported each year, with countless others going unreported -- but human activity might be increasing their number and severity. Ann Tihansky is a hydrologist for the U.S. Geological Survey...

"We've collected quite a few stories about sinkholes forming. And all the ones that we've collected are related directly to some sort of (human) activity."

Most often, it's human effect on the underground water table that can contribute to sinkholes forming...

"Ground water pumping in specific areas when water levels are already low and you take them even lower, that can trigger a more sudden collapse of overburdened sediments that may not have happened if water levels weren't drawn down past some level."

Many human activities can contribute to sinkhole occurrence...

"...Increases in ground water pumpage, loading at land surface, retention pond building, altering a landscape where you're changing the overburdened thickness and the overburdened materials. Those are all activities that can induce sinkholes."

For more information, visit floridaenvironment.com. With help from its Environmental Studies Program we're produced at the Whitaker Center at Florida Gulf Coast University and funded by the Southwest Florida Council for Environment Education.