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New rash of sinkholes tells the same old story

Add soaking showers to a dry winter and you have a recipe for sinkholes familiar to those in Hernando and Pasco.

By MATTHEW WAITE, Times Staff Writer
© St. Petersburg Times
published June 23, 2002

After a winter's worth of drought, Tony Gilboy knew when he woke up last weekend that soaking rains pouring on the Tampa Bay area meant sinkhole reports were coming.

"I got my raincoat and knew I was going to be busy," said Gilboy, a sinkhole expert at the Southwest Florida Water Management District.

If the recent reports of dozens of sinkholes opening up in southwest Hernando County sound familiar, they are. The same thing happened last year about the same time, under the same conditions.

But Spring Hill's vanishing real estate goes back further than last year.

Since the 1970s, more tightly clustered sinkholes have been reported in Spring Hill and just south of there, in Hudson and Bayonet Point in western Pasco County, than anywhere else in west central Florida, according to data from the Southwest Florida Water Management District, commonly known as Swiftmud, and the Florida Sinkhole Institute.

The Swiftmud database contains information on sinkholes reported from the early 1970s to 2001 from local and county governments, the Florida Sinkhole Institute and the water management district. Swiftmud authorities warn, though, that they have no way of knowing how many of the sinkholes ended up in their database.

Some of the sinkhole locations were plotted using sophisticated Global Positioning Systems; others were plotted through newspaper reports. And sinkholes in populated areas are more likely to be reported than ones in rural areas, where they could go unnoticed.

The district warns that the data should be used for regional trend analysis and not to target specific holes. Gilboy said insurance companies have far more accurate and complete maps of sinkholes.

But Gilboy said patterns emerge from the data they have. Big sinkhole outbreaks are almost always reported in the late spring, he said.

Why? It has to do with geology, he said. Spring Hill and northwest Pasco have relatively little top cover -- sand and clay -- over the limestone bedrock. Under that limestone, the Floridan Aquifer is usually applying upward pressure, keeping the limestone in place.

But during times of drought, aquifer levels go down, reducing the pressure keeping the limestone in place. And on top, water and other factors cause fissures or cracks in the limestone.

Add a foot of rain falling before the aquifer has been recharged, and the extra water makes the top heavy while less holds it up.

"When we get that heavy saturating rain ... at the beginning of the rainy season, you can just pretty much guarantee that you're going to get sinkholes," Gilboy said.

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