A Tampa Bay scientist on the nature of sinkholes

Times Correspondent

From time to time in West Central Florida, the ground opens up and swallows a home.

One of the biggest sinkholes in Florida in recent decades occurred in Winter Park in the 1980s, notes Philip van Beynen, associate professor of geosciences at the University of South Florida. It consumed a home, community swimming pool, road junction and part of a Porsche dealership. He says the biggest sinkhole he’s ever heard of happened in China.

“That’s probably two miles across and about half a mile deep.’'

Dr. van Beynen, 53, specializes in karst terrains like the honeycombed limestone rock that underlies much of this part of Florida. He talked with the Tampa Bay Times about the nature of sinkholes.

What causes a sinkhole?

Limestone is susceptible to being dissolved away by either water that’s coming down from the top, rainwater, or water that is mixing with salt water. What that does is it creates an acidic, corrosive solution, and limestone is very susceptible to being dissolved by this water. So, it creates these voids, or gaps, holes, under the ground. And therefore, if you have these voids or holes full of water, that water creates a buoyancy. It holds up the rock above that void in the ground. And then when we go through a dry period, that water starts to drop in that – I guess you could use the word cave because that’s really what it is.

And when the water starts dropping there’s nothing to hold up the roof or the ceiling of that cave, therefore it collapses into that void.

So that’s one way… Another way is if you put a lot of weight on top of the rock that’s, again, above one of these voids or caves. And then that ceiling can’t support the weight and therefore it caves in. Sometimes they build these big buildings and a sinkhole forms because of the weight above the ground. It just can’t support it anymore.

When the ground collapses because of weight, can it happen when the cave below is full of water?

Yeah, it can.

Is there a time of year when sinkholes are more likely to happen?

An ex-colleague of mine did a study on when sinkholes occur, and they found that it occurs both in dry season, which is actually the first process I spoke to you about, and the second one was in the wet season, which is starting finally to kick in at the moment. … When you have a lot of rainfall you also increase the weight that’s sitting above the caves. … It’s almost like putting a building on that weak ground. So it happens both times of the year, when you drop the water table and also when you increase the weight.

In a nightmarish incident that occurred in 2013, a Seffner man was presumably asleep in his bed when a sinkhole pulled him down; his body was never recovered. Had he been awake, would he have had some warning?

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I would imagine he probably could have felt something, heard something. I imagine as the ground is starting to give way beneath your house that you would have some indication. But really, how quick that was, I really don’t know. I would imagine it would have been at the most, probably, minutes. If it happened over an hour, I would have assumed that he would have woken up.

Our lack of soil above the limestone increases the chance of a sinkhole, you say.

One of the reasons that you see it here is because we’re on the coastal plain. So, quite flat, and there’s not a lot of sand and clay above the limestone. There’s not a lot of thick sediment to actually protect the limestone below it. The thicker the material you have above the rock, the more support there is to hold that sediment in place.

If you’ve got, like, three feet of sediment between, say, a road or a building and the actual bedrock, there’s not much support. There’s not much to hold it up. If you’ve got 60 feet of sediment, then there’s a lot more protection from the bedrock to stop a sinkhole from falling.

For people who are buying homes, is there any indication that the ground below is vulnerable?

If you’re buying in Pasco county, especially in west Pasco county, you’ll see there are a lot of sinkholes. So you know they occur. ... that would be the first thing that would jump out at you. …

If you were in an area where there’s sinkholes and you saw there were houses in the neighborhood where there’s cracks in the wall, or you can see maybe they’re doing some patching in places, or maybe they even had to destroy a house because it was too badly damaged, then that may give you some evidence to think, oh, wait a second, maybe there is a chance that there could be a sinkhole.

Do humans have an effect on the occurrence and frequency of sinkholes?

Oh, absolutely. Think back to… 2000, I think it was. … It was a cold winter and strawberry farmers had to pump a lot of water out of the ground to actually protect their crops. … and I think something like 160 sinkholes appeared in an 11-day period. And they dropped the water table down to a point where certain people’s wells that they used for drinking water just dried up. I think it created a sinkhole under I-4 as well. That’s probably the most dramatic… one I’ve heard of as far as having big impacts on the communities.

You’ve written about groundwater vulnerability. Don’t we get our drinking water from groundwater?

We tend to get our water from different sources. We usually get a fair amount from the surface, from the rivers, from our rain, from desalinization. ... However, we also get it from different aquifers. For example, if you go to the southern counties, they tend to get theirs from more of an upper aquifer called the surficial aquifer. … We get our drinking water in the Tampa Bay area, some of it anyway, from the upper Floridan aquifer. ... That’s protected by a layer of sandy clay that prevents the pollution from trickling down into it. The only way, often, that pollution can get down is if you have sinkholes. It just so happens that sinkholes can allow pollution to get down into the drinking water.