Wisconsin Public Television University Place  Sinkholes David Hart  10/2/13 UP930  cc >> Welcome, everyone, to Wednesday Nite at the Lab. I'm Tom Zinnen. I work here at the UW Madison Biotechnology Center. I also work for UW Extension Cooperative Extension, and behalf of those folks and our other co-organizers, Wisconsin Public Television, the Wisconsin Alumni Association, and the UW Madison Science Alliance, thanks again for coming to Wednesday Nite at the Lab. We do this every Wednesday night, 50 times a year, and it's one of your opportunities to share in the discovery here at your public land-grant research university. Tonight, I'm delighted to be able to introduce to you one of my UW Extension Cooperative Extension colleagues. Dr. David Hart is a professor in the Department of Environmental Sciences at UW Extension, and he's a hydrogeologist and a geophysicist for the Wisconsin Geological and Natural History Survey. His current research interests include measurement of the hydraulic properties of mid-continent rift sediments to determine their suitability for carbon sequestration, thermal properties of Wisconsin rocks, and also he's studying the methods to measure flow in fractured rocks. He is also interested in developing web applications to allow citizen input of Wisconsin karst features and provide information on sites of geologic interest in Wisconsin. He received his BA from Luther College in Iowa and his PhD from the University of Wisconsin-Madison. 

Tonight, the really cool thing he gets to talk to us about is sinkholes. I feel the Earth move under my feet. [LAUGHTER] In 1978, I lived with Art and Marilyn -- and family on Townsvalley Road south of Hudson, Wisconsin. Later that year, a sinkhole swallowed a good part of their machine shed. I had never seen anything like it, and they thought the only thing that stopped it from swallowing the whole machine shed was one of the things in the machine shed was a camper. And that fell down in the hole and basically formed a cork in the bottleneck. If you've ever stood at the edge or the lip of a sinkhole, you get yourself a workout there because it's a scary thing. I think that's why many of us are here today because it's fickle. And so I'm looking forward to hearing what Dave has to say as he talks to us about sinkholes here in Wisconsin. Please join me in welcoming Dave Hart to Wednesday Nite at the Lab. 

[APPLAUSE]  >> Tonight I'm going to talk about sinkholes in Wisconsin. That's going to be my primary focus, but with all of the news about sinkholes that we've had internationally and nationally, I think it's worth asking the question: are we standing on solid ground? How concerned ought we to be about sinkholes? So I hope to kind of give you a sense of that and maybe a sense of what are the major issues with sinkholes in Wisconsin as well. Okay, some issues with sinkholes that we would have is, well, they create property damage. Here's in Winter Park, Florida, that's kind of a famous one. A whole city block disappeared in a hole in Florida. We have hazards. This poor car went into a parking lot hole. They indicate karst, and we'll talk about what karst is and some of the issues that that might present for us in Wisconsin. A large part of that is that it provides pathways for contaminants to get to our groundwater. Where there's sinkholes we need to be concerned about what we do in the land surface. 

I'll hit on that again later. >> [INAUDIBLE] >> Excuse me? Which one? Lower right? That one I believe is going to be up in the Pierce or St. Croix County. I forget which. It's from a Minnesota DNR person, Jeff Green, but they have the similar geology across the river. Sinkholes in the news. Of course, I think this is the death of the man while he was sleeping, that tragedy, I think it set off a lot of the concern and issues that we see. More recently we have the sinking of the resort. Basically one story went directly downward, and you can see here we have that. And, amazingly enough, I think this is in part maybe to the training and how alert people and how sensitized they are to sinkholes in Florida where they are a big deal where people do need to be concerned. People understood what was going on and evacuated that entire building. 

No injuries, no loss of life. I think that's really quite amazing when you look at the damage that was done there. Everybody got out. Another sinkhole that's made the news lately was at Bayou Corne in Louisiana. Here's a sketch of what was going on there. We'll talk about that in a bit, but basically they had a large salt dome here that's kind of a remnant of the seas that you see down in Louisiana long ago. And the salt is an economic asset. Chemical plants use it, and very often they'll drill a well into the salt dome, inject fresh water, and pull the brine out and use the chemicals, very often the chloride from the brine. The issue here is one of the wells was close to the edge of the salt dome, and the sediment at the edge of the salt dome does not have the strength to hold itself up, and when they excavated this or when this all dissolved, we had collapsed into this feature here. This is not a small feature. 

The scale here which is 5,000 feet, so that's nearly a mile below the surface, the sinkhole at the top that looks like such a tiny little thing, it's a thousand feet across. And it currently is 40 acres in aerial extent. So what happened was this sinkhole or this well was misplaced toward the edge, the sediment slumped into this very large cavity, and then it's still slumping today. They think it slowed down. It seems like they have a decent model for what's going on there. So things are starting to settle down. But the issue that we have is you have freshwater from the surface is going to go down and continue to erode the brines. And so you have this tradeoff between sediment filling the cavern, which will slow the sinkhole formation because things are going to get filled up, and freshwater coming in. Eventually the sediment is going to win and things will stop forming there. I don't know how many of you have seen this movie. 

I thought it's a phenomenal movie. It's, I think, actually quite frightening. So the main sinkhole is off to the left, or your right, and that's the 40-acre lake. It basically looks like a lake now. People have been monitoring the sinkhole. We're going to look at a video where the camera, they're standing on a nice platform so I'm not sure I would feel comfortable there but apparently they felt pretty comfortable while this thing was going in. So they weren't screaming get away, get away. They were just talking about keeping an eye on it. So I want you to just kind of keep you eye on this section of trees here. And kind of keep watch what's going on there. 

Watch the water. You can see the water is getting pulled in as the trees are slumping down. Remember, they're slumping into that mile-deep cavity. And this isn't really probably so much a suction of water moving. This is the clump of sediment that the root ball is attached to that's dragging the tree straight down. And you can see the water is rushing down into that very large, very deep cavity. And now you can see it's boiling backward out toward the camera there as well, too. This process has been ongoing for, I think, for maybe nearly a year now. They've bought out most of the homeowners who are surrounding the bayou or now the sinkhole here as well, too. Another more closer to home. 

I heard an Illinois joke earlier, but we had a golfer in Illinois fall into an 18-foot deep sink hole. Straight down. He was golfing. He actually saw a little dip in the ground, and then wondered what is that little depression, got too close, and down he went. The sinkhole that he fell into is shaped like a bell, and that is actually typical for the sinkholes that we see in Wisconsin and Illinois. And that sinkhole was 10 feet wide at the bottom. You can see at the top it's only a couple feet wide. Here, this is after they rescued the guy. The golfer dislocated his shoulder when he fell that 18 feet. He was golfing with his friends. 

They went and got a ladder, and I would call him a very good friend climbed down into the sinkhole and helped him, tied a rope around him and they pulled him to safety. >> [INAUDIBLE] >> I believe it was southern Illinois. It was farther south. So, that's a little bit closer to home. Even closer to home is this gentleman in Manitowoc on eastern Wisconsin was out walking in his backyard. Again, a bell-shaped, 10 feet wide at the bottom, several feet wide on top. I believe this sinkhole was only like 15 feet deep. But this fellow had to stand in two feet of mud for about 20 minutes. Luckily his wife was in the house. I think she probably heard him shouting. 

I think my wife could have heard me shouting. [LAUGHTER] Even in a sinkhole. And you can see they got him out. Here, again, the ladder seems to be the basic tool of sinkhole extraction in the upper Midwest. They got him out in about 20 minutes. The fire department did. They called 911 and they came. It's nice to know these guys are probably trained for this sort of thing. Another sinkhole that made the news in Wisconsin is a Milwaukee car and driver. It was a heavy, heavy rain, I believe in 2010, and this Escalade drove into the sinkhole or the sinkhole collapsed beneath him. 

A passerby rescued him. The driver climbed out on the hood of his car, and then they pulled him to safety. You can see how much water was flowing through the storm sewer at that point right there. There's a lot of water moving through this system, and that's part of the sinkhole story that we'll get to. And Minnesota Public Radio, the car was still running 20 hours after it fell into the hole, just sitting there idling and no one was going to go shut it off, apparently. [LAUGHTER] We've been worried about sinkholes for a long time. I think there are certain references. Perhaps this is a reference to a sinkhole, but there is a report of the Earth opening and swallowing a house, including the -- of Korah in the Book of Numbers. There was a disagreement with Moses and the house of Korah did not survive that. Also, then you have this cenote, or sinkholes, in the Yucatan Peninsula. 

Interestingly, there's very little surface water in that part of Mexico, so these were places where they built the cities around because there is a convenient water supply. You have a built in well there. And then they also, archeologists have also linked the skeletons in these cenotes to ritual sacrifices as well. And I believe National Geographic had a story on that a couple of months ago. And this is the sacred cenote at Chichen Itza. Okay, now we're going to switch gears a little bit and maybe talk a bit more about how sinkholes form. Maybe a little bit more of the science, a little bit less of the newspaper articles. So sinkholes can form naturally or by human action. They're not really hard to understand. Really what you need are two things. 

You need a void beneath the ground surface. How do you get that void? There's lots of ways to do it. And you need water to move that sediment into the void. You put those two things together and you stand a good chance of getting a sinkhole. Some possible voids? Cave, conduits, cracks and voids in carbonate rock, and when I say carbonate rock, I mean the dolomites that we have here in Wisconsin or the limestones that you have in Florida. Our sinkholes are going to be different than the Floridian sinkholes because the dolomite isn't as easily dissolved as the limestone. There's a big difference in how well things are dissolved. So that's one of the reasons we don't have the issues they do. Solution caves and salt. Even though there's a lot of these, it's a fairly common thing to mine salt in the way that they did at that Bayou Corne issue, and more than once it's happened that you get these big solution caves and things fall into it. 

You have a large void. In Kansas this has happened. Elsewhere in Louisiana it's happened as well. And then very common sewer pipes cause a lot of issues. We have a lot of aging infrastructure in our country, and if your sewer pipes crack, you saw how big the pipe was in Milwaukee, that can move a lot of sediment and a lot of water. Where did that sediment go that that thing fell into? That was water swirling around and moving that sediment. Water is very effective, especially when it's moving fast, at moving sediment. And where do we get our water sources? Precipitation is the most natural thing. And then our water mains and also injection wells. And so you can notice where in municipalities, when we see sinkholes, we actually put water mains right over our sewer pipes right underneath our roads. 

So that's why it's very common to see a vehicle falling into a sinkhole because the car causes the ultimate collapse. The void forms, the car drives over it, there's no underlying soil, in goes the car. We put those three things together, we couldn't design a better system to wash cars into sinkholes. Natural sinkhole formation, how do we get this? We get this karst. It develops by dissolution in carbonate bedrock. And I should tell you that I'm taking some liberties with the term karst. I'm going to kind of mean it to talk about cracks and fractured rock and solutions. Strictly, we should probably just talk about it as landforms. But I'm going to take some liberties with it in this presentation. And so we dissolve carbonate bedrock. 

How does that happen? We put some carbon dioxide, which we have in our atmosphere, some water, which falls to the ground, we mix those two together. We get a very weak acid, carbonic acid, and when we mix carbonic acid with our dolomite, which is our calcium or magnesium carbonate or our limestone, which is that calcium carbonate, we get some ions which means there's a solution in water. We take a solid, we put it into solution, we leave a void is what happens there. And so that's how we get our voids, and, ultimately, of course, we start off with a crack, then you maybe get a conduit, and then you maybe get a cave, something that we can climb into ultimately. And so what happens after we dissolve all that? Well, we now have our void. Water washes overlying material into those conduits and fractures, and then we can get our sinkholes, our natural sinkholes. And so in Wisconsin we just need carbonate rock and water. There was a story told about a machine shed up in Hudson. Here you can see we've got a carbonate rock shown in blue and yellow here, and we've got a lot of precipitation, averaging 31 inches. We have both the things we need to get sinkholes in Wisconsin. 

There are different types of sinkholes. This subsidence sinkhole forms if, and it's going to depend on your overburden on the material, on the sediment overlying that limestone or dolomite rock. So if your overlying sediment doesn't have a lot of strength, if it can flow fairly easily, if it's sand for example, you can just kind of wash it into the sinkhole here, wash it into that void, and you just get a shallow depression. And you can put more sand and take your dump truck and dump another couple cubic yards here. Five years later you can dump another five cubic yards there, and you can just continue this process for as long as you want. We'll talk about maybe some ways to get rid of these sinkholes. But if you just keep putting sediment on the surface, very often the sinkhole will just keep taking the sediment away. So these can be kind of frustrating and mysterious, why do I always have that depression at that point in my yard question. And then there's also the cover-collapse sinkholes. And, again, if you have a more stiff material, clay perhaps, or in Wisconsin kind of the glacial tills, the glacial sediment, that may be more stiff, and so we start to wash, we have some water percolate down, and we start to form a void underneath our ground. 

In the case of the gentleman from Manitowoc, this was probably happening earlier, and he may have walked over it when it was in this form. No problems. But when he walked over it when it was in this form, bloop, in we go. So very often these are the kind that you wake up the next morning and there's a hole in your yard. And you can see that bell shape that was talked about, you can see that would come up here where that center is the first thing to collapse and then this may not collapse on the sides. That would kind of explain that. In Wisconsin, one of the main things that we worry about when we see sinkholes is we know it's going to indicate karst. It's going to indicate this landscape of really well connected fractures and conduits that can move groundwater wherever. So if we put something on the surface here, it could end up a couple miles away coming out of spring over there. The Minnesota Geological Survey has done a lot of tracer tests where they'll put some kind of dye, like a fluorescein dye, it's like a bright green or a bright red dye, into a sinkhole and then they'll watch where it comes out. 

So let's say your sinkhole is at the top of some ridge of dolomite in southeastern Minnesota. They put the dye in there. Some of the dye comes out two or three miles over here. Some of the dye comes out two or three miles away over here. So that's where these conduits, this plumbing is really complicated, really complex, and it's very, almost impossible to predict once you put something on the land surface, where's it going to end up. It's very difficult to predict. It's going to depend on all this plumbing and conduit and how things might plug up. One of the obvious questions is, are these things always open? What happens to the sediment that gets washed into a sinkhole. Well, it kind of moves its way through this fracture network, through this conduit network, and very often things are plugged up. Things aren't moving. 

And then that kind of gives us that episodic nature of sinkholes where nothing happens, nothing happens, and all the sudden the sinkhole forms. Well, the sediments had time to move its way through that conduit network, that fracture network and to open things up again. But the main message we are most concerned about, one of the things we're most concerned about when we see sinkholes is we know surface contaminants can be quickly transported. You can see, here's a farm field with some vegetative matter on top of the farm field. That's what's been left over after someone's done some land spreading. So that's kind of recycled vegetative matter. It's had one pass through the cow there. So those are the sorts of things that we want to be careful that we don't immediately introduce in the groundwater. If we're going to do manure spreading on our land surface, we want to know if there are sinkholes, and if there are, then we want to be careful that things don't get into our groundwater. All right, so if we're going to look at our Wisconsin bedrock, we have three main groups or units that are associated with karst. 

And they're mostly dolomite. The magnesium has come in later and substituted for the calcium, made things a little bit less easy to be dissolved. We have Devonian and Silurian dolomites over in the east. We have the -- group, the Galena/Platteville in the little bit farther to the west and south here, and then we have the Prairie du Chien shown in purple going up along here. So we have three different rock types, and these rock types have a little bit different characteristics. Geologists use, the ones on top are youngest if you're a geologist usually, and so that's why I started with the Silurian and ended with the Prairie du Chien. The Silurian is the youngest rock, the Prairie du Chien is the oldest. As we go, you'll even kind of notice you can see that the Silurian isn't quite as beat up as the Prairie du Chien if you look at that. But here we can see, here's in the Waukesha Stone and Lime Quarry in Waukesha, Wisconsin, this is just south of the interstate, 90 and 94, as you go through Waukesha. You can see a nice, big rock wall here. 

It doesn't look too bad. And then here's, in Calumet County, County Highway F, you can see a nice little fracture. It isn't going to be much of a sinkhole, but there might be a little hole. There might be a little settlement that goes down through there. Here's another closeup of that Silurian dolomite. Here we're now looking at the Sinnipee dolomite, and here's a quarry near Ripon, Wisconsin. You can see things maybe look a little bit more beat up there. And it also has the Galena, Decorah, and Platteville formations in there. Here's some core. The Wisconsin Geological Survey, we go out and we collect cores so we can understand these rocks a little bit better. 

And here's an example. We have a drill rig that we can collect core from 700 feet or so and get a sense of the fractures, how much of a void there is in those rocks, how strong those rocks are as well. And then come up with the Prairie du Chien. And there was a mention of the machine shed falling in, also near the Hudson area by Pierce and St. Croix County, the Prairie du Chien is also the place where we have the largest sinkholes. Maybe a hundred feet across or so. Again, in that part of Wisconsin in the north-northwest. But here's, along the Rush River in Pierce County. This is a four-foot long staff, and you can see that's quite a conduit. It's almost what we would call a cave. I call a cave anything that I can climb into, and I can usually fit in the four-foot thing. 

Here's a quarry near Highway 71. You can see the nature of the rocks looking a little bit more, has more fractures and conduits and karst. The other thing we often do at the Geological Survey is when we drill a well, we can lower a video camera down into the well and record the image. What do we see at depth? So I'll tell you, here at 190 feet, that nice cream, buff colored rock, the dolomite, and as we go down, at 200 feet, a little bit of a fracture and conduit there. And then look at this. There's maybe a cave 200 feet down in the Prairie du Chien there. Something we could climb into. And that's near Pine Lake, and we'll see Pine Lake again in just a second. So, one of the things we want to do is we want to look at what's the potential for sinkholes, what's the potential for these karst issues to show up. And so we're going to combine depth to bedrock. 

If you have a shallow depth to bedrock, it's more likely that the sediment can slump in. We combine that with the carbonate rock, and that's how we come up with this map. If you go to our website, you can see this map and see if your house is on there and how worried you need to be. You don't need to be worried in Wisconsin, generally. All right, now we will do a tour of some sinkholes, mostly ones that I've visited, and some of the issues that we see. So we'll look at kind of a range of sinkholes, and we'll look at a range of issues as well too. Both in the drinking water perspective and from a hazard perspective. This is one of my favorites. This is one of the first sinkholes I worked on. It's a stormwater retention pond on the west side of Madison by Black Hawk neighborhood there. 

This pond has drained several times since it was constructed. It never went up to its designed fill depth. The city went in there and plugged the sinkhole several times. It kept forming. Dante Fratta, a professor in civil engineering, and I did a study there. We recommended that we stop trying to have this be a nice recreational resource because it's too hard to. The water levels never went to what they were designed to be, and so it really wasn't a very nice water. It was very green, the algae. >> [INAUDIBLE] >> Off Old Sauk. >> Old Sauk. 

>> Yeah. But it does have some nice park land around it. But the community expectation and developer sold the community on a pond, and I always tell, when I talk to developers or I talk to people who approve development plans, I say this is always something that you should watch out for when you're looking at the plats, when you're looking at the roads. This is from this neighborhood where you have Hidden Cave Road, and a couple blocks later you have Waterside. [LAUGHTER] These two things generally don't like to coexist very well. So I would be cautious if you're going to buy waterfront property and the next road over is Hidden Cave. You may have an issue with that. [LAUGHTER] Here's some site photos. Here's after the sinkhole, or actually, here's a nice air photo of the retention pond here. You can see it's this long, long structure. 

There's a nice bridge going across it. And there's some small bluegill that live it now as well, too. But this point here is out in the middle of that retention pond. You can see that's kind of a rather large sinkhole. And anecdotally, talking with one of the homeowners, the last time this drained, I guess there were some youth out in a boat or canoe or raft or something and apparently there was also a whirlpool as the thing was draining. [LAUGHTER] That's anecdotally, but you look at the size of that hole and it certainly could be true. That was a definite hazard, and I'm very glad no one was hurt. But what the city did is they dug this out and then they filled it with clay. It seems to have worked. They've done that several times, and it seems now to have worked, and it seems to have kind of, I don't know if it's every gone up to its actual designed depth, but it's not as low as it had been in the past. 

So it seems to be doing okay. Over here on this north side of the shore, there are numerous sinkholes formed there. Here's a pen for scale. They're just these little ones like that. You can see that nice bell shape again too as we go here. And here's one right at the water's edge. They also occur right at the water's edge. The sediment doesn't have a lot of strength there, and there's also water to move things vertically down. >> [INAUDIBLE] >> Yeah. Yeah. 

Another stormwater retention pond. I think one thing people need to be cautious of when building a stormwater retention pond if you don't want these issues is you need to leave enough sediment beneath carbonate rock in the bottom of your pond. If you dig out too much sediment, it's too easy for things to slough in, and then you get the sinkhole. And so there was some poorly cemented sandstone mined above that dolomite. If they left the sandstone in place, they wouldn't have had this issue. The pond drained after filling. This was a rather large sinkhole. 103 feet long, 62 feet wide, and seven feet deep. It has a crack at the bottom that's another 10 feet deep. I'm going over into Minnesota but the rocks are the same so we won't claim them. 

So what they did was they remediated this one. They fixed it with grout. Here's the sinkhole. Here's, in the wintertime there's a drill rig injecting grout, and they are plugging it up. Here's, I think, Calvin Alexander from Minnesota Geological Survey thinking about what's going on with this crazy sinkhole here. This is one of my favorites, Pine Lake in St. Croix County. Here you can see the lake's drained. If you drive by it, it's a really pretty, really nice lake, but it drained in 1919. Let's do a little math. What is that? About 20, I'm not going to be able to do it. 

17 years later. 16-17 years. And then again, what do we have? 23 years later. And then what do we have here? 39. And it hasn't drained yet again. I think in '88 they fixed it. I've been waiting for this one to go and see if it drained. But I think the last time they fixed it, we've gotten better at engineering these things. We now have this thing called geotextiles which are basically a big plastic sheet that's kind of reinforced but also prevents water from flowing. So last time when it drained, they repaired it with geotextiles and clay, and in 1988 it was 155 feet across. 

That's that same place. Remember I showed you the video image? At 200 feet there was that big four foot plus void or cavern. It's pretty easy to imagine where that water might go. There are other large sinkholes over a hundred feet across in that part of Wisconsin. Those are where the biggest, I don't know of any sinkholes that large in southern or eastern Wisconsin. It seems to only be up in that part in the St. Croix/Pierce area. And then we also have some sewage treatment lagoons. Three municipal waste sewage lagoons in Minnesota's karst region, which is almost identical to our karst region in southwest Wisconsin and Pierce and St. Croix failed since '74. 3.7, 2.3, and 7.7 million gallons of partially treated wastewater drained from these lagoons. They don't know where the water went. 

[LAUGHTER] Again, it's really that unpredictable conductivity of those fractures that make it really difficult. We don't know where the water went from that Black Hawk lagoon either. There's a City of Madison well located about a half of mile from there. That water may have gone toward that well. We're not certain. It just kind of disappeared. Same with this water. Again, this is kind of an oddball sinkhole. It's a farm sinkhole in southwest Waukesha County here. Right over here. 

There's pick up. There I am for scale. You can see there's the sinkhole right there. It formed in the fall of 2003 just after the harvest. The farmer drove his combine across, harvested his soybeans. The next morning, he woke up and he said it was like someone, a space alien or whoever... [LAUGHTER] Took a cookie cutter or something and just punched the Earth straight down. You can see the rows of crop here. Those are perfectly aligned, they're just 10 feet down. [LAUGHTER] Just flat and straight. 

Just went straight down. Very, very strange sinkhole. I went out and investigated this with a field tech at the time at the Wisconsin Geological Survey, and he wanted to jump down into it, and I said only if you tie off on a rope because I don't want to make that phone call. [LAUGHTER] Even with the rope it's not something I ever would have done. Some people are more driven to see geology up close than others, I guess. [LAUGHTER] Here's kind of the geology of this site. That's why I say it's an oddball sinkhole as well, too. Here's sand to 45 feet, clay 46-90, gravel 90-105, and then we have our limestone all the way down at 100 feet. And so here we would have had, there's probably a fairly large cave system underneath there, and so we had things collapse in. And I think the clay is part of the reason why the walls were so vertical. 

That clay just would have gone straight down and we would have lost that. It's really kind of puzzling. For a long time we thought perhaps that's where the Model-T was buried, but we took our magnetometer out which would have detected any metal. There's no metal under there. So it's almost certainly a natural sinkhole. Another formed sinkhole, this one is just a little bit west of Madison. It formed in '06. The farmer was pulling his manure spreader, and it dropped into the sinkhole while he was spreading the manure. The bottom of the sinkhole is 20 feet below the ground surface. So you can see this thing is only about three to four feet across, but it's 20 feet deep. 

I found a really long stick and stuck it down in there. Here you can see the long stick. You can see it's wet starting there and going down. This one is scary because you fall in that thing and you're in maybe some unpleasant water and you can't climb out. Sometimes these things can be dangerous. A highway sinkhole. This one formed in a highway in Monroe County. A driver was driving along, felt a slight thump, a little bit of a depression, had the wherewithal to check it out, and then called it in. Nobody was hurt. The Department of Transportation came out, and if you drive across this highway today you can still see the patch. 

The hole was 10 feet deep. It went straight down. The area has a lot of karst features. This beat up, rubbly zone here is at the same elevation as that sinkhole in the road. It does not correspond with this more competent rock but this rubblier rock. So the geology kind of tells us something about where these things can happen. This little, tiny sinkhole, they call swallets, that was in a ditch right beside this one here. Here's Professor Fratta by a conduit at the same elevation as this sinkhole here, just in another outcrop. No one was hurt. And here's another Madison sinkhole. 

This one is what I call typical. We get called for one of these maybe once every two years, once a year or so. Again, it's five to 10 feet across, five to 10 feet deep. It happens when there's lots of water. It opened in the springtime during the thaw and it's unexpected. So here I am. You can see I'm standing outside of the tape getting ready to leap away at a moment's notice. [LAUGHTER] If it caves in. And I've got ice. It's still wintertime there but it's warm. 

You can see there's been a big thaw. We had a lot of sinkholes last spring. More than I've seen for a very long time. It was a wet spring That gives us more sinkholes. What do sinkholes need? They need water and a void. Last spring we had a ton of water. >> [INAUDIBLE] >> In the east side of Madison. >> East side. >> Yeah. There's a lot of erosion, topography in Madison so you go through different rock formations depending where you live. 

And you can see, here's the bottom of the sinkhole, you can see a little bit of the bedrock and how it's kind of beaten up there. This is bedrock and this is sediment. There's the end of the ice, snow. One of the last examples is looking at a sinkhole from inside. This is at, I believe, in Mother's Cave at the Ledge View Nature Center. This cave was filled with sediment, which is the case for a lot of these conduit systems. They're plugged up most of the time, and when they come unplugged that's when we get our sinkholes. What they did in this cave is they found some sinkholes, they saw that the cave is a very large opening, and so they removed the sediment five-gallon pail by five-gallon pail. So now if you go there, it's really kind of a neat cave to go through and see. They also get people from the nearby prison who want some sunshine and fresh air, maybe not sunshine but who want some fresh air, they come out and help dig as well, too, and clear out the cave. 

One of the things, inside the cave is a nice, here's an example of a pretty good size conduit. This is almost, for me, not a cave because start to have trouble fitting through that. But if you're young, you can. >> The prison, you mean Waupun? >> I don't know where they come from. I don't know if it's a county or where, but sometimes they're out there working. A lot of times you'll see lots of small sinkholes. I can name within a mile of here more than probably or so that I've looked at over the years. And they're just little, tiny things like this. There's a little pocketknife at the bottom of this one here. Or they look like that swallet that was beside the highway. 

And what they do for a hydrogeologist, for someone thinking about groundwater, is they indicate that there are pathways and conduits that we need to worry about so we should be concerned about what we're doing at the land surface. They also, and this one is from, I believe, St. Croix County. That's where a stream disappears when there's water. It just goes away, falls in that sinkhole. Most sinkholes, Brown County. Here's some fractures, part of the conduit system. The alfalfa, there's more sediment in there, better at holding water so often you can see this in air photos. The alfalfa grows better along the fractures. Here's another little swallet. And here I wanted to give an example of a human caused one. 

I got a call, is this a sinkhole? I'm like, I don't know, maybe it's just bad, they didn't put down the subgrade very well. But then I started to look, and here you can see here's a patch, here's a patch, and here's manhole. So everything is kind of lined up. There's a void, probably a sewer that has a crack in it. So we have rain, you have a void, I think that's what's causing the issues there. So I think they were going to video the sewer to see if it had cracks, and if it did, then fix it and they wouldn't have to keep replacing. They dumped a lot of gravel there. This is gravel that was dumped and mounded a little bit. You can see it's already starting to disappear. And it looks like a sinkhole but doesn't. 

Just because you have a depression doesn't mean you have a sinkhole. You have shaft entrances and a rock pile in Iowa County. There's all sorts of mine openings and shafts in the southwest from the lead-zinc mining that was going on there, and glacial kettles, places where you had blocks of ice that melted out and left depressions. Some tools that you want to maybe think about for managing karst. Geologic maps are one way that help us understand if we need to have concern about karst. Is there carbonate rock beneath our feet? Some places in Madison have carbonate rock, some don't. If you're really risk averse, you can maybe find a geologic map, or if sinkholes just really frighten you, you could find a geologic map, build your house, buy your house over sandstone. You probably will be fine. Look at depth of bedrock. The thicker depth of bedrock, less chance of a sinkhole. 

We always love it when people contact us and help us understand sinkholes, help us understand karst issues. So there's some karst databases. You can go to our web page, fill something out, or there's also more in depth reporting form that you can do as well. I'm very careful to not share information. Don't worry about your property values. Unless you want me to say your address, I will not. But I'm still interested in seeing the sinkholes. If you notice I was a little cagey about where in Madison that example was. I can tell everybody in that neighborhood has the same problem, potential problem. So it's not really fair for one person. 

There's also some karst databases. St. Croix County has a really nice karst database. It would be great if we could do this for additional counties. And now we come to solutions. I've been kind of showing you all these things that can go wrong. Worrying about groundwater, worrying about potential damage or hazards. So when we hear about a sinkhole and are asked to reflect on it, we always ask: what's the land use, is there a human health hazard, falls or drowning, is groundwater going to get contaminated, who's going to be drinking that groundwater perhaps? And then we also have some environmental hazards too. Is it going to discharge to surface water? Is that sinkhole going to perhaps contaminate a spring or a trout stream or something. >> [INAUDIBLE] >> I wish it was. I wasn't that clever. 

[LAUGHTER] I'm going to claim that from now on. [LAUGHTER] And then we also know cost as well, too, is the other issue. Other possible solutions. Nothing at all. It's in the forest. It's cover-subsidence. It's kind of nice. This is what the Madison School Forest does. They've got a series of sinkholes in their forest. They use them for education. 

I did kind of urge them to no longer let the kids go down and tromp around on top of the sinkhole, but they're probably fine. It's a cover-subsidence sinkhole. But I said, you know, I'm not sure I'd be comfortable with doing that. That's usually how I think of it. Remediate it, fill it up, isolate it with a berm, a shallow ridge, or fence it off. Limit land use, that's one of the most powerful things we can do, and then also educate. So how do we remediate sinkholes? We can inject some grout, as they were doing here. Here's what I always tell homeowners. If any of you get a sinkhole in your house, what you can do is what we call reverse grade. Start with large rocks at the bottom. 

Here they say cabbage sized. You could use bowling ball sized. Then use fist sized and then gravel. Then they say geotextile or you could use clay. And someone asked me, where do I get clay? And I thought about it, I really don't know where you get clay. You could go buy non-scented kitty litter. But the clumping clay kitty litter and that would work as well, too, to make a nice seal. And then you just put soil. And you see they've got it nicely mounded here. Why is that? To keep water off of it. 

The water is not going to flow into that void anymore and keep things going. Even if you go through all of this work, repair is unlikely to be permanent, especially if it's not excavated down to the bedrock. If you just do this down to the soil, just kind of keep in mind that it may happen again. Don't put the edge of your house there. [LAUGHTER] But I should say, that's also a very common question. People worry about, there's a beautiful house, I believe it was designed by Frank Lloyd Wright kind of in the neighborhood of our office, and there was a sinkhole formed in the yard of that home. It was kind of a longer, thinner sinkhole, and it was kind of pointing right at the house. [LAUGHTER] Do we have to worry about that sinkhole getting bigger, wider, longer, and eventually engulfing that house? And the answer very, very, very unlikely. I know of no instances where that has happened in Wisconsin, and especially not southern Wisconsin. Houses kind of take care of themselves, especially if you do what we all, nobody likes a wet basement, so what do you do with that water? You route it away from your house. 

If you're routing the water away from your house, you're not putting anything into the voids. You don't wash in the soil, so it's unlikely then that the home is going to be impacted. It's only when the sinkhole gets larger than the footprint of the house that you have issues. We don't get that big a sinkhole in general in Wisconsin. Not southern Wisconsin. They do get that large of a sinkhole in Florida. That's very often where you'll see the large sinkholes. One thing to keep in mind with sinkholes, repair is unlikely to be permanent, as I said before. These things can come back very quickly and easily. The other thing you can do is limit land use. 

Put in a berm around the sinkhole. Prevent water infiltration. Fence it off. Limit human and livestock access. Limit land use, perhaps do some sort of set back. This is especially important for rural application. Decide when you're going to do that waste application. This is not the best time to do land spreading of manure because when things melt, it all rushes away. Nobody wants manure in surface water. You want it in the soil. 

Everybody wants it in the soil where the plants can use it. And then adjust expectations. They indicate karst, that means filling it may not improve water quality because there's all sorts of other rocks and conduits and cracks around. Often, the small features that you can't see are more important than you do see. It makes groundwater management very difficult if you have karst, and we need to really adjust our land use. No remediation is permanent. They're often episodic. Those conduits plug and unplug, I think, in chaotic, unpredictable ways. And, as I just said, they're unpredictable. You may not even find them when looking in the right spot. 

They sometimes have their own mind. For instance, that one that happened in the hundred foot thick sediment in southwest Wisconsin. In conclusion, the sinkholes occur when overburdened slumps in the carbonate bedrock or some other void driven by water flow. In Wisconsin, they indicate karst and issues surrounding karst, which very often means groundwater issues. And they're unpredictable but we can kind of plan for them somewhat. We generally know what the rock is underneath out feet. Remediation may be necessary, but don't count on it lasting forever. And you kind of need to look at each sinkhole individually. Where is the sinkhole in the landscape? What are the issues surrounding that sinkhole? So, thank you for your time. [APPLAUSE]