2:12pm

Thu March 21, 2013
Science

The Abnormally Normal Science Of Sinkholes

Originally published on Fri March 22, 2013 2:58 pm

When a Florida man vanished into a massive sinkhole that opened underneath his bedroom in February, the case garnered national attention. Every so often, tragedies like this put sinkholes in the spotlight.

Researchers say that minor sinkholes occur all the time around the world without much notice.

Lewis Land, a hydrologist at the New Mexico Bureau of Geology and Mineral Resources, is one of the organizers of the 13th Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst. Karst is a term for landscapes made of porous, soluble rock — areas where sinkholes are naturally prone to occur.

Since 1984, geologists, hydrologists, biologists and other researchers have gathered every few years to share knowledge and research about karst terrain.

Land talks with NPR's Celeste Headlee about what happens when Earth swallows things.


Interview Highlights

On where sinkholes occur

"If you see one sinkhole, then chances are that you are in an area that is prone to sinkhole formation. There are specific geological conditions that make an area more likely to have sinkholes. And so there are parts of the country — and north and central Florida is notorious — for sinkhole formation. Western Illinois is another area, western Kentucky, and ... they occur pretty frequently in those parts of the country.

"One of the things that causes sinkholes — one of a number of mechanisms — is excessive groundwater withdrawals. And if you're in an area that's experiencing a drought ... there might be a lot more pumping going on. And that's one thing that could trigger a sinkhole formation."

On possibilities for predicting sinkholes

"You might get an early warning. You might see some, say, concentric fractures forming in the ground. But in some cases, it just happens without much warning at all, unless your property ... has all sorts of geotechnical monitoring systems on it. In some cases, you just don't know until it happens. ...

"We could probably make a lot of money if we could predict exactly where and when a sinkhole is going to occur. But in general, we can't do that. We can say that one area is more prone to sinkhole formation than others with confidence in the same way that seismologists can say that one area is more prone to earthquakes than others.

"But no seismologist can predict exactly where and when an earthquake will occur within a few days or weeks. ... There's a variety of different geophysical techniques that you can use to survey an area and determine if there [are] cavities in the subsurface. They cost money. ... Most homeowners can't afford to have that kind of thing done."

On how human activity can lead to sinkholes

"Excessive groundwater withdrawal for irrigation purposes, for example, can sometimes trigger a sinkhole because it depressurizes the aquifer. Another mechanism that ... occurred here in New Mexico a couple of years ago was brine well pumping. There was a company here that was injecting freshwater into the salt beds in the subsurface and dissolving out the salt to form brine. ...

"They're using the brine for drilling fluid in the oil fields here. And it was legal. They had a permit from the state. But what you effectively are doing is creating a big cavity in the subsurface. And that big cavity collapsed in two different places, about 20 or 30 miles north of Carlsbad, and created very large sinkholes."

On what you do to fix a sinkhole

"A mistake that a lot of people make when they are doing what we would call sinkhole remediation. They think the simplest thing to do is to just fill it full of dirt. ... The problem is you've got a conduit into the subsurface that ultimately caused that sinkhole.

"It's connected to the aquifer — in most cases to some underlying aquifer. And if you don't take care of that, then all you're doing is exacerbating the problem, and you're likely to have another collapse. ... There are engineers whose specialty is sinkhole remediation, and they spend a lot of time and a lot of engineering skill to try to identify those subsurface passages so that they can fill them up with things like grout, for example."

On how the sinkhole convention came to be

"It's a pretty specialized field. ... It was initiated because of the sinkhole that formed in Winter Park, Fla,, and swallowed up a woman's house and part of an import car dealership and a swimming pool. ...

"Insurance companies didn't really know anything about sinkholes, and so ... for a while there, the Legislature was funding a sinkhole research institute. ... And the conference series has been going. It takes place every two or three years and has been since the 1980s."

Copyright 2014 NPR. To see more, visit http://www.npr.org/.

Transcript

CELESTE HEADLEE, HOST:

But speaking of the news headlines, it feels like every single day we get another one about a hole that's opened up in the ground somewhere. Some of these are tragically fatal incidents. Some are relatively minor three-foot crevices. But is there really an increase in the number of sinkholes, or just an increase in the headlines about them? And what causes these, anyway? We want to hear from you. What are your sinkhole stories? 1-800-989-8255. Our email address is talk@npr.org. Or you can join the conversation at our website. Just go to npr.org, and then click on TALK OF THE NATION.

Joining me now is Lewis Land. He's a hydrologist at the New Mexico Bureau of Geology and Mineral Resources at the National Cave and Karst Research Institute. In May, he'll be one of the organizers of a national sinkhole conference, and he joins us by phone from his office in Carlsbad, New Mexico. Welcome to the program.

LEWIS LAND: Thank you.

HEADLEE: Can't be too bad if you're in Carlsbad, which is beautiful. But first, let me ask you the question of - is there really an increase in sinkholes. Is this kind of our sinkhole spring?

LAND: Well, it may be that there's an increase in sinkholes in some parts of the country. One of the things that causes sinkholes, one of a number of mechanisms is excessive groundwater withdrawals. And if you're in an area that's experiencing a drought, that - there might be a lot more pumping going on. And that's one thing that could trigger a sinkhole formation. And I'm - you'd probably be better off talking to somebody with the Florida Geological Survey, but I think there has been a bit of an increase in the incidence of sinkholes there.

HEADLEE: Scientists are always referring me to other scientists.

(LAUGHTER)

HEADLEE: It happens all the time, Lewis. But you're stuck with me...

LAND: OK. Sure. OK.

HEADLEE: ...at least the next 15 minutes. But, first of all, let's talk about what causes sinkholes. There's more than one reason why a sinkhole forms, right?

LAND: That's right. A lot of them are naturally occurring. A lot of them are either directly or indirectly associated with human activity.

HEADLEE: Human activity - construction work?

LAND: Well, let's see, as I've mentioned - I mentioned just a minute ago that excessive groundwater withdrawal for irrigation purposes, for example, can sometimes trigger a sinkhole, because it depressurizes the aquifer. Another mechanism that, you know, occurred here in New Mexico a couple of years ago was - is brine well pumping. There was a company here that was injecting freshwater into the salt beds in the subsurface and dissolving out the salt to form brine.

HEADLEE: Oh.

LAND: They're using the brine for drilling fluid in the oil fields here. And it was legal. They had a permit from the state. But what you effectively are doing is creating a big cavity in the subsurface.

HEADLEE: Yeah.

LAND: And that big cavity collapsed in two different places, about 20 or 30 miles north of Carlsbad and created very large sinkholes.

HEADLEE: What about...

LAND: Now, that...

HEADLEE: What about this, Lewis? Bonnie emailed us with this story, and Bonnie says: Suddenly, upon replacing a brick on our walkway, there seemed a large space underneath. Upon further investigation, there it was, a sinkhole. We think it probably came from a root that rotted away. Would that be correct? And then...

LAND: Oh, well...

(LAUGHTER)

LAND: ...that would be a very tiny sinkhole, but I suppose that could be some mechanism that could cause a micro sinkhole. That's probably not the most common thing that causes them, though.

HEADLEE: Well, let's say that a sinkhole appears in my backyard, even if it's small, let's say...

LAND: Yeah.

HEADLEE: ...it's three-by-two. What do I do? Can I just pour dirt in it?

LAND: Not necessarily. I think that is a mistake that a lot of people make when they are doing what we would call sinkhole remediation. They think the simplest thing to do is to just fill it full of dirt.

HEADLEE: Right.

LAND: The problem is you've got a conduit into the subsurface that ultimately caused that sinkhole. It's connected to the aquifer - in most cases to some underlying aquifer. And if you don't take care of that, then all you're doing is exacerbating the problem, and you're likely to have another collapse. Engineers, there are engineers whose specialty is sinkhole remediation, and they spend a lot of time and a lot of engineering skill to try to identify those subsurface passages so that they can fill them up with things like grout, for example.

HEADLEE: Well, we have another sinkhole story here. This is Melanie(ph) in Quincy, Florida, if I can get this to work. Melanie, hi. How are you doing? What's your sinkhole story?

MELANIE: Well, I grew up in a little town in Marianna, Florida, that has a lot of caverns and caves underneath it. But we had a Walmart store right on our main drag, and a sinkhole opened up under the parking lot and kind of collapsed the parking lot.

LAND: Yeah.

MELANIE: It ended up making a lot of the stores move out of the area. And they just kind of fenced it off, and we've all looked at it for a few years.

(LAUGHTER)

HEADLEE: Well, what...

MELANIE: And I always thought that it was cave that maybe caught, you know, there might have been a cave under the parking lot, but they said, no, it was a sinkhole.

HEADLEE: All right. That's Melanie, and Melanie brings up a good point. Thank you so much for you call, Melanie. Lewis - we're speaking with Lewis Land. And I wonder, Lewis, is it - if you see one sinkhole, does that necessarily mean there are going to be more?

LAND: Well, if you see one sinkhole, then chances are that you are in an area that is prone to sinkhole formation. I mean, there are specific geological conditions that make an area more likely to have sinkholes. And so there are parts of the country, and north and central Florida is notorious for sinkhole formation. Western Illinois is another area, western Kentucky, and it's, you know, they occur pretty frequently in those parts of the country. And so if you see one, chances are - I mean, it's just telling you that you are in an area that's prone to sinkhole formation.

HEADLEE: OK. So if Florida is prone to sinkhole - and if we can judge from the headlines lately, I think we all assume that.

LAND: It's famous for sinkholes.

HEADLEE: Is it the geology that it's not on a hard rock? Is it that it's not, you know, is it that there's a lot of groundwater underneath Florida?

LAND: Well, there is a lot of groundwater underneath Florida. The entire state of Florida practically is underlain by an aquifer called the Floridan aquifer, and it's formed in limestone bedrock. Limestone is one of these rock types that is particularly susceptible to dissolution, to being dissolved by circulating groundwater, and that's what creates caves and that's what creates, you know, cavities in the subsurface that contribute to sinkhole formation. The - in most parts of Florida, that limestone bedrock is covered up by tens to maybe a couple of hundred feet of non-soluble rock, like sand and gravel and clay. So the dissolution will occur in that limestone bedrock of the subsurface, and then in most cases, that overlying sediment just collapses into it.

HEADLEE: I wanted - this is - David Logan brings up a very good point. We've got an email that says: It sounds like many of your callers and emails are confusing sinkholes and erosion. Can you talk about the difference? So, Lewis Land, can you explain the difference?

LAND: Well, see, sinkhole - erosion is just kind of a broad term that covers all these different processes that weather and breakdown rock. Dissolution of limestone is just a type of what we call a chemical erosion. It's a more specific process. But erosion is just kind of a generic term that refers to a lot of different processes.

HEADLEE: We're speaking with Lewis Land. He's a hydrologist at the New Mexico Bureau of Geology and Mineral Resources. He's calling us from his office in Carlsbad, New Mexico. And if you have a question for him, you can call us at 800-989-8255. And it sounds like Gasper(ph) in Edenton, North Carolina, has a question. Gasper?

GASPER: Hello there. Thank you for having me on your show. Yes. I often wonder - I'm not a geologist nor a scientist, none of that, but I often wonder what happens with all the billions of oil barrels that we drill out of the ground and gas? What replaces all this empty, you know, space, if you will, under the ground? I mean, if - I often wonder if that has anything to do with the sinkholes, if it creates a vacuum.

HEADLEE: Well, let me ask that (unintelligible) - thank you very much for the call. It's Gasper in Edenton, North Carolina. What's the answer here, Lewis?

LAND: Well, in most cases, just the rock framework will keep sink - I mean, most oil and gas is extracted from reservoirs that are thousands of feet under ground. And so the rock overlying it, generally, is strong enough to prevent any catastrophic collapse. It's - and what takes the place of the oil and gas is water because at that depth, the rocks are saturated with one type of fluid or another. It's either oil, natural gas or saltwater.

There are places where there's been long-term pumping of oil from relatively shallow oil reservoirs where there's been subsidence. I think the Long Beach oil field is one example. That's kind of unusual because that reservoir was shallow and the pumping had been going on there for, I believe, decades. In most cases, oil and gas drilling is not unlikely to cause sinkholes except under very unusual circumstances.

HEADLEE: OK. We are asking for your sinkhole stories, and we have a real hydrologist with us, Lewis Land. This is Keith in Philadelphia, Pennsylvania. And, Keith, you have a sinkhole story?

KEITH: Hello.

HEADLEE: Hi.

KEITH: Yes, actually, I do. It involves my mother's house. My mother lives in Reading, Pennsylvania, which is a notorious - in the limestone belt in Pennsylvania. And she's had one that's been a persistent problem for her in front of her house for about the past three to five years. It opened up first when they were replacing gas and water lines on her street, and there was a lot of rain. And then all of a sudden, one opens up and dislodged and it's opened up a few other times. And they dumped truck load of fill into it. And the last time, they pumped a whole bunch of cement into it, but she still gets pretty nervous about it and even updated her homeowner's insurance policy to include sinkhole insurance.

LAND: That's a good idea.

(LAUGHTER)

HEADLEE: Yeah, probably is. Thank you very much. That call came from Keith in Philadelphia, Pennsylvania. And we also have a call here from Chris, also in Philadelphia. Chris, I guess, Pennsylvania has a lot of sinkholes. What's your story?

CHRIS: Well, this one goes way back to 1960. A sinkhole had opened up in front of a small neighborhood church that my father and my brother and I used to go to in North Philly, you know, what's now a pretty congested area. I guess, you could say it's an impoverish area. But anyway, it had swallowed a police car. It had opened up and the intersection of 6th - no, 5th and Indiana, had swallowed a police car. And, unfortunately, the officer died. They had to put a boat down there. They tried to shore it up.

HEADLEE: A boat?

CHRIS: An actual small boat with a light on the front of it. That's how big the underground river was. And they found that - they had tried to shore it up with, I guess, poles, like telephones and supposedly it swept them aside like matchsticks. It turned out that my father did some research on this before there was any sort of an Internet, so, you know, he did a lot of work on this and found out that Philadelphia - large portions of Philadelphia were built on quite a vast network of underground rivers.

HEADLEE: That doesn't make me feel good about (unintelligible).

CHRIS: Yeah. Well, it also accounts to some extent for sinking Logan and so forth.

HEADLEE: Yeah.

CHRIS: But...

HEADLEE: Well, that's interesting.

CHRIS: Yeah. And so this - the river is still there. And sometimes, if you go pass 6th or 5th or 3rd or 4th and Indiana on a rainy night, you can still hear them through the sewage system.

HEADLEE: Chris, thank you so much for your call from Philadelphia. You are listening to TALK OF THE NATION from NPR News. We are still here with hydrologist Lewis Land from New Mexico. And, you know, let me ask you, Lewis, you know, you say that a lot of this can be called by groundwater depletion. And I can't help but think that we're actually using up our freshwater in a pretty quick rate. That would imply that, perhaps, if we're pumping a lot of water out of the ground that we're going to see more and more of these sinkholes.

LAND: Well, that is a possibility, yes. I mean, in southeastern - in New Mexico in general, it's a pretty arid environment, and we're always concerned about our water resources, so...

HEADLEE: And is that the - I mean, you're holding a sinkhole conference, which implies that there are many questions that haven't been answered about sinkholes. Is this one of them?

LAND: Well, there's - no. It's - there's ongoing research on the topic. It's a pretty specialized field, but the - this is a conference series that's been going on since, I believe, the late 1980s. And it was initiated because of the sinkhole that formed in Winter Park, Florida, and swallowed up a woman's house and part of an import car dealership and a swimming pool. So after that, the Florida Legislature - I believe, this is what happens - said, well, insurance companies need to start providing homeowners with sinkhole insurance.

Insurance companies didn't really know anything about sinkholes, and so they - for a while there, the legislature was funding a sinkhole research institute. I think it was associated with the University of Florida. And the conference series has been going. It takes place every two or three years and has been since the 1980s.

HEADLEE: So what did we call them before they were called sinkholes?

LAND: That's what they call them. Well, that's a standard term for sinkholes in the United States. In Europe, they're more often referred to as dolines.

HEADLEE: Is there any way - I mean, is there any way to know that a sinkhole is coming? I mean, what do you look at on the ground, a telltale sign that the ground is being excavated out from underneath you?

LAND: Well, we have - there was an - I told you - I mentioned that brine well operation that caused a human-induced sinkhole...

HEADLEE: Yeah.

LAND: ...up in - near my town. There's another brine well that was located right in Carlsbad. And when those two other sinkholes formed up in northern Eddy County, they immediately shut that brine well down, and it's being continuously monitored with tiltmeters. So there is a big cavity in the subsurface, just maybe a couple of hundred feet below ground level.

HEADLEE: But what does that look like? If I'm going to look at my backyard, if I live in one of these places in Florida, can I see maybe wrinkling in the groundwater? Do I see a depression? What...

LAND: You might get an early warning. You might see some, say, concentric fractures forming in the ground. But in some cases, it just happens without much warning at all, unless your property is - has all sorts of geotechnical monitoring systems on it. In some cases, you just don't know until it happens. And, you know, we could probably make a lot of money if we could predict exactly where and when a sinkhole is going to occur. But in general, we can't do that. We can say that one area is more prone to sinkhole formation than others with confidence in the same way that seismologists can say that one area is more prone to earthquakes than others. But no seismologist can predict exactly where and when an earthquake will occur within a few days or weeks.

HEADLEE: We can't - we don't have instruments that allow you to sort of X-ray the ground and see beneath it to see...

LAND: Oh, yes, we do.

HEADLEE: Wouldn't that be able to tell you?

LAND: Yes, that's one thing that you can do. There's a variety of different geophysical techniques that you can use to survey an area and determine if there is cavities in the subsurface. They cost money. Most homeowners...

HEADLEE: Well, there's the problem.

LAND: Yeah. Most homeowners can't afford to have that kind of thing done.

HEADLEE: I would imagine. There's the rub. Let's take one more - unfortunately, Heather, it'll have to be short because we only have couple of minutes left. But you had - you were a cave diver in Pensacola, Florida?

HEATHER: Yes.

HEADLEE: And what are you seeing in reference to sinkholes?

HEATHER: We see an increase in sinkhole formation in the last 15 years or so. Florida has been under an extreme drought over the last several years and that, combined with water mining, specifically for bottled water, is contributing to a significant decrease in the amount of water coming out of the aquifer system. We're seeing not only sinkholes but springs literally dying, and so the water is not there anymore.

HEADLEE: That's Heather in Pensacola, Florida, making the argument to stop drinking bottled water. I mean, if we conserve water, are we going to kind of ease the situation, Lewis?

LAND: Yes, I think that that's definitely one strategy. I mean, because as Heather pointed out, you're - when you're depressurizing the aquifer by excessive groundwater pumping, that's one mechanism that can trigger sinkhole formation.

HEADLEE: All right. That's Lewis Land, a hydrologist at New Mexico Bureau of Geology and Mineral Resources. He joined us by phone from his office in Carlsbad, New Mexico. Thank you so much, Lewis.

LAND: Oh, no problem.

HEADLEE: Tomorrow, it is TALK OF THE NATION: SCIENCE FRIDAY. Ira Flatow will be here with a look at some of the innovations pioneer at New York's Grand Central terminal. In the meantime, this is TALK OF THE NATION from NPR News. I'm Celeste Headlee from Washington. Transcript provided by NPR, Copyright NPR.