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Tuesday, January 11, 2005

Expert: Most sinkhole fears groundless

By DINAH VOYLES PULVER | News-Journal Environment Writer

Most people never stop to think about the world beneath their feet — until it suddenly opens and swallows a house. Then it gets a lot of attention.

But while people are washing dishes, showering and buying homes near tidy storm water retention ponds, all that water is moving around below.

And just as ocean waves wash away the beach and rain cuts gullies in dirt roads, when a lot of water suddenly goes underground, it can erode rock and dirt, allowing the ground to collapse and form sinkholes like the one that opened Sunday morning on Springbank Avenue near Orange City, swallowing one home and part of another.

The hole stabilized Monday and workers dumped about 10 loads of dirt into it. The job should take most of the week, said Gerald Brinton, Volusia County engineer.

Fortunately for most area residents, sinkholes aren’t going to be a problem. The chances are really remote for anyone who lives east of Interstate 95, for example. State officials have a record of only three sinkholes on the east side of the county, and none is listed for Flagler County.

But the chances are greater in West Volusia and far greater in other Central Florida locations.

Sinkhole reporting is voluntary, but according to a state database, at least 70 sinkholes have been reported in Volusia County in the past 32 years, most in Pierson, Orange City and DeLand. But that’s far below the 332 sinkholes found in Hillsborough County or the 309 in Citrus.

There’s no way to know when or where the next sinkhole might form.

“I wouldn’t lose a lot of sleep over it,” said Frank Rupert, a geologist with the Florida Geological Survey in Tallahassee. “The chance of a sinkhole opening under your house is slim, even though we’ve seen some dramatic examples recently.”

Homeowners and prospective homeowners can hire professional engineers to test the ground to get an idea of the risk, but even those tests can’t tell you when.

Sinkholes show up most often during long droughts and after heavy rainfall, Rupert said. Half the Volusia sinkholes have occurred in the months of December and January, possibly, experts said, because of heavy water pumping to keep farm crops from freezing.

The new Orange City sinkhole and the one that opened in December in Deltona are being attributed to heavy rain.

It’s happening all over the state, just on a smaller scale. Some 600 sinkholes have been reported in Florida since the hurricanes last summer, geologist Anthony Randazzo said Monday.

An area’s tendency toward sinkholes can be identified if you have enough time and resources, using such techniques as ground-penetrating radar, said Randazzo, a retired University of Florida professor who co-owns a company that does sinkhole research for homes and businesses.

A basic geophysical survey costs roughly $1,500, he said. In some cases, additional tests might cost another $1,000.

The foundation for subterranean activity was laid millions of years ago, when layers of rock and shell created a bed on which modern-day Florida was formed by layers of sand building up over time, Rupert said.

In Central Florida, much of the rock is fractured limestone with cracks and crevices filled with water Floridians drink.

When there is a drought, the caverns in that limestone can collapse when the water level drops in the caves, creating sinkholes.

Layers of soil, dirt and rock above the limestone also hold water, which naturally seeps or is drawn down by gravity into the layers below. After heavy rain, the force of the water and its high acid content can erode the limestone, also creating sinkholes. The water gets its acidic qualities naturally from decaying vegetation and soil, as well as contaminants from storm water.

The weight from volumes of groundwater also can create sinkholes by collapsing the ceiling of limestone caverns.

Filmmaker Wes Skiles of Karst Productions in High Springs has seen the underground caverns up close. A cave diving expert, he studies the water’s flow and the erosion. His company released a PBS film in 2003 that took viewers into the caverns to see the water’s movement.

The water shifts and moves at different speeds depending on where it is, Skiles said. In some places, it may move as much as a mile a day, he said, while in other places it may move much slower.

Area residents need look no further than Blue Spring to see how the water moves, Skiles said.

The spring is “flowing like crazy” and has been since shortly after the hurricanes, said Tom Carey, groundwater expert for the Volusia County Environmental Management Department.

Staff Writer Christine Girardin contributed to this report.

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