AP-FL--Florida Sinkhole, 1st Ld-Writethru,610
Geologists study massive Florida sinkhole
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The Associated Press
LAND O’LAKES, Fla. (AP) — Geologists are studying a massive Florida sinkhole that destroyed two homes and rendered five others uninhabitable.
Experts might ever know exactly what caused Pasco County’s largest sinkhole in decades, but a team of University of South Florida researchers have descended upon Lake Padgett Estates to learn as much as they can about what happened.
The Tampa Bay Times writes that geologists want to know if anything might have predicted the devastation, such as way the lakefront neighborhood was developed.
“If we can say where these things are likely to occur,” said Lori Collins, an archaeologist leading the university team, “smarter decisions and regulations may emerge from that.”
The sinkhole opened July 14 in a suburb north of Tampa called Land O’Lakes. It now stretches about 260 feet (79 meters) at its widest point, and it’s estimated to be 50 feet (15 meters) deep.
There have been 336 reports of sinkhole activity in Land O’Lakes since the county started keeping track in 2003.
Lake Padgett Estates sits in the heart of a so-called “sinkhole alley” in central Florida. Like much of the state, the land sits atop porous limestone that groundwater flows through and over time can erode.
“I don’t want to sound alarming, but a lot of people who move here from up north may not be aware of this type of terrain,” said Dave DeWitt, a chief geologist with the Southwest Florida Water Management District. “And in this part of the state, you just don’t know what’s underneath you.”
Collins’ team is studying the area’s history, from its decades as an expansive orange grove to its conversion into a neighborhood. It grew from just 20 homes in 1967 to 300 in 1974, according to the U.S. Geological Survey, and building continued there until the 1990s.
Three instances of sinkhole activity were recorded by the Florida Geological Survey in the 1980s along Lake Saxon’s northern shore, not far from the current sinkhole. Both the homes it destroyed last month previously had been fortified after experiencing settling.
Sinkholes occur naturally in Florida. But they also can be provoked by human activity, including intensive groundwater pumping for irrigation or residential taps, or by work above ground that destabilizes the limestone below.
Geologists are calling the Land O’Lakes crater a “cover-collapse” sinkhole, which occurs when soil above a void in the limestone gives way. They tend to occur abruptly.
Sinkholes caused by human actions are most often the cover-collapse kind, according to a 1999 report from the U.S. Geological Survey.
Lake Padgett Estates sits between two large groundwater well fields. The day the sinkhole developed, groundwater levels recorded at the closest well to the neighborhood were within a normal range, according to water management district data. The agency said it was unlikely that pumping spurred the sinkhole, at least in the immediate sense.
But that doesn’t mean water wasn’t a factor: Florida had just emerged from a drought, and then welcomed consistent rainfall at the end of June. When water levels drop and then heavy rains suddenly pound the dry ground, the crust above a sinkhole is more likely to collapse, said DeWitt.
With its history of sinkholes, Pasco County passed an ordinance in 2008 requiring engineers to test for potential geological hazards before building new neighborhoods. If they detect a sinkhole risk, they make recommendations to avoid a problem.
County officials plan to revisit the ordinance after the Lake Padgett Estates sinkhole is emptied and stabilized, said Kevin Guthrie, assistant county administrator for public safety.
For now, he said, the county must clean up the hole and figure out what to do with prime real estate marked by a yawning chasm.
Information from: Tampa Bay Times (St. Petersburg, Fla.), http://www.tampabay.com.