Smack-dab in the middle of Florida is a farming community called Groveland. It is hard to get more central than this central Florida town. Groveland rides high on Florida’s limestone spine, a slight rise that puts the center of the state mildly above sea level. The limestone backbone, hidden under clay and sand, is riddled with caves and caverns. These aren’t usually visible from the surface, but they are vulnerable to collapse. On Sunday, three days ago, the lower than usual water table weakened the isostastic pressure holding up the roof of one such cave. The ceiling fell and a 20-metre-deep hole suddenly opened. Luckily, homes were spared, though part of a driveway was swallowed by the earth below.
Sinkholes mar the landscape around Florida like chicken pox on an unvaccinated third-grader. This aerial view shows about 25 circular sinkholes that are over 100 metres in diameter. The largest approaches 500 metres. There are many more sinkholes which are smaller than 100 metres. (The one being celebrated at the corner of East Waldo Street and Iowa Avenue in Groveland is just 15 metres in diameter). Smaller sinkholes have largely been filled in by natural erosion – even in the aerial image below you can see circular swamps and depressions which were likely once small lakes. By the way, this image covers about 12 square kilometres, of which ten percent is presently water-filled sinkholes.
I lived in this part of Florida for ten years. Groveland was my postal address, though my turf was seven kilometres south of town. It was the 1970s and early 80s. I knew about sinkholes, realized their danger, but spent almost no time thinking about them. Having a sinkhole suck up your home (and maybe your life) seemed less likely than being struck by a truck, a lightning bolt, or a water moccasin.
After a big one swallowed an Orlando house when I lived nearby, I asked a grove owner about the chances of his trees disappearing into a sinkhole one night. He looked at me as if I were speaking in tongues. “No, no chance,” he said. And I suspect people who are still living in the area are thinking about this Sunday’s sinkhole the same way. A grand nuisance, but no one was hurt, little damage was done, and school/work began pretty much as usual on Monday morning.
The University of South Florida (USF) has a list of 101 sinkholes which opened in Lake County during the 30 years of 1976-2006. There were likely more, but the list gives a sense for the frequency of these events. The USF has similar lists for sinkholes across other parts of Florida, but on Monday they found they didn’t need to travel far to find a new one to examine. Florida’s latest sinkhole – a tiny metre across but 6 metres deep – appeared right on the Tampa campus.
The Tampa area is a hotspot for sink- holes. In February 2013, one formed under the bedroom of Jeff Bush in Tampa’s Seffner suburb. Bush had just gone to bed. In a few minutes, he screamed for help. His brother Jeremy ran to see that Jeff Bush and most of his bedroom had disappeared. The young man’s remains were never recovered. Last week, on Wednesday, a 6-metre-wide sinkhole opened again at the same spot. After the initial sinkhole formed two years ago, the county bought the property and a neighbouring home to prevent another disaster. It reopened, but the county does not expect it to grow larger.
To prevent deaths and dissuade developments atop potential sinkholes, predictive tools would be useful. You would be correct to place money on a sinkhole reopening where one might have existed in the past. As we’ve just seen, the killer in Tampa opened again last week. Even the Groveland sinkhole that led this story is a revival of a smaller one that struck the same spot 40 years ago.
Florida has more sinkholes than any other state and they can strike almost anywhere in the state. It just takes the right combination of near-surface carbonate rocks (which are fairly ubiquitous) and disturbances in those limestone and dolostones. Florida’s shallow ground water is usually slightly acidic. As it seeps below the surface, it eats away at the carbonates, forming water-filled caves. The resulting karst topography includes unexpected fresh-water springs, disappearing streams, caves, and sudden sinkholes. The underground erosion can be exacerbated by droughts followed by heavy rainfall or by excavation for buildings and pumping of irrigation water.
Pumping water can be especially problematic. Without Florida’s groundwater, a lot of farming wouldn’t see its next harvest. On the other hand, the shallow underground pores, vugs, and caves are sometimes depleted with unexpected consequences. For example, in the winter of 2011, a young lady living near Plant City left her back door and fell through a freshly opened sinkhole, a deep cavity less than a meter in diameter. She had her cell phone in her hand, called the emergency line and was rescued. The sinkhole developed because local farmers were pumping water to protect winter strawberries from frost. The water table fell fifteen metres and the aquifer couldn’t recover quickly enough – hundreds of private wells ran dry. Because the pumping suddenly made the subsurface cavities hollow, one hundred forty sinkholes appeared within days. In addition to farm irrigation and emergency anti-frost spraying, Florida’s growing population and domestic water consumption add to the problem of drained aquifers and resulting sinkholes.
The hope of predicting the next sinkhole now rests on NASA technology. Satellites equipped with Interferometric Synthetic Aperture Radar (InSAR) may detect changes in ground elevations over time. Such changes may indicate an area’s vulnerability for sinkholes – sometimes a few centimetres of subsidence occurs before the ground collapses. InSAR can measure subtle deformations. This is done by comparing sequential radar surveys, searching for changes. At this moment, the technology is so new that these surveys are still in the planning stage. It might not even work as a predictor. Unfortunately, not every sinkhole is preceded by subsidence. Some simply pop open, swallowing the surface.
Not only are they sudden and (so far) unpredictable, you can’t be totally safe anywhere in the state, though some areas are riskier than others. If your retirement dream includes Florida, you can’t pick a safely non-sinkhole homestead. However, if we consider that about ten percent of the more active areas has been sunk sometime in the past 10,000 years, then you might assume that there is about a one percent chance that your property will experience a sinkhole in the next thousand years. So, you are reasonably safe – but you should probably avoid any real estate that has had a recently patched sinkhole.