5 December 2013
On Friday, March 2, 2013, most of the residents of a suburban Tampa neighborhood were getting ready to go to bed when Jeremy Bush heard something that sounded like a car crashing into his home. He said, later, that he thought he heard his brother, Jeff, calling for help as the concrete floor began to give way under Jeremy’s feet. As he moved toward his brother’s voice, he fell into a hole where, a moment before, the home’s floor had been. Sliding down, Jeremy couldn’t see his brother, but only “the cable wire running from the TV going down into the hole” and, also, one corner of the box springs from his brother’s bed.
He would never see or hear his brother again. When Jeremy tried to get out of the hole himself, he was unable to do so. Eventually, firefighters arrived the extracted the surviving brother from what would turn out to be a sinkhole. Rescue efforts were hampered by uncertainty as to whether the ground around the home was stable enough to serve as a base of operations for rescue workers and their equipment. Jeff Bush’s body has never been recovered.
Although the tragic death of Jeff Bush is unusual, sudden collapses of the ground under the feet of Floridians are not. Sinkholes in Florida are frequent and have swallowed, at one time or another, almost everything including homes, swimming pools and even a whole shopping center. Although the motto would never be chosen for a license plate, Florida might be called “The Sinkhole State.”
Florida comes by its sinkholes naturally. Sinkholes are formed when a limestone bed deep beneath the surface layers of soil is gradually eroded away by groundwater. The sole above, eventually, but suddenly, falls into the gap in the limestone bed. Areas with this type of subsurface strata are often known for caves, which are formed in limestone, directly, by the action of water passing through the porous rock.
Florida’s subsurface layer of limestone is so full of cavities and pockets that it has been compared to a giant piece of Swiss cheese. Beneath the surface of the entire state is soil and stone “riddled” with holes that might cause a sinkhole at any time. “How riddled is riddled?” asks one writer. “Anywhere you drill you’ll find them!”
In fact, sinkholes or, at least, the results of sinkholes have created some of Florida’s most prominent features. However, most of these are now known by different names. If the ground sinks slowly, Florida’s sandy soil and water continually fill the sinking area of land. Instead of calling these sinkholes, they’re called bog swamps and estuaries. In time these features spread out to form caves and wetlands, which, in turn, were responsible for what is now known as Florida’s Everglades. Robert Brinkmann, a geologist at the University of South Florida in Tampa, says, “Every little wetland is a little sinkhole.”
When the New York Times reported the tragic death of Jeff Bush, it began by explaining that it was “sinkhole season” in Florida. That’s a new one. Sinkhole season? Since when do unpredictable collapses of the earth’s surface happen seasonally? They do in Florida. And that takes some explaining.
An aquifer describes an area of ground water. It’s a kind of earth-filled lake. There’s an aquifer beneath most of our feet right now. Early settlers would establish a homestead by digging a well. In many places, when you dig down far enough, you reach water. If you dig down just a little deeper than the water level, you have a well. An open well is a hole, a shaft, going directly down into the ground to a point a bit beneath the water table. Because the shaft extends a bit beneath the level of the ground water, a small pool of water forms at bottom of the shaft. Then, all you have to do is lower a bucket and bring up some water.
Later, pumps displaced the simple wells. Nowadays, water is piped into our homes directly from reservoirs in which the water is treated and purified. Where does the water in theses reservoirs come from? Well, it depends on where you live. If, like me, you live at the confluence of the Mississippi and Missouri Rivers, large quantities of available water are no problem. As a matter of fact, seasonally, we often have more water than we need or want. When that happens, it’s called a flood.
Florida does not have an abundance of natural water sources. Although the state is surrounded by water, it’s the wrong kind of water. Oceans and seas can only provide salt water. And, regularly consumed, salt water is fatal to human beings. Why not remove the salt? Salt removal, or desalinization, is too costly. Use of the water in inland swamps and bogs not only presents conservation issues but, also, purification issues that are too costly to resolve.
Florida does have one substantial source of water for human and agricultural use: groundwater from its aquifers. Most of the fresh water used by Florida’s population is pumped out of the ground — right out of the state’s aquifers. But what does all this have to do with sinkholes?
Well, eroding limestone deep beneath the ground isn’t the only thing that causes sinkholes. The water in saturated soil provides support for that same soil. Just as large bodies of water create a tremendous pressure near their bottoms, so water in the soil exerts a pressure that supports the ground around and above it. Groundwater fills the tiny spaces between particles of earth and gives the ground, at the surface, a strength to bear more weight than it could without the water’s pressure.
So, what happens if a normally moist region has a dry spell or pumps a good part of the groundwater out of the ground? The ground dries out and, as it does, becomes filled with tiny air pockets. Air provides little, if any, support at all. So, the sudden drying of the normally moist ground can cause something, very much like sinkholes, called “subsidence”. However, the differences in terminology seem less important in Florida, because the two, dry earth subsidence and the collapse of subsurface pockets in the deep limestone, work together. Many sinkholes are a combination of the both: the sudden loss of support in the dry soil near the surface weakens the very support preventing the collapse of an empty pocket in the deeper limestone below.
So, does pumping groundwater for residential and agricultural uses cause sinkholes? Well, a little. Remember that Florida has always experienced an exceptional number of sinkholes — long before human beings arrived. And groundwater depletion was always part of the problem.
On the good side, Florida’s aquifers are not depleting in the way that these are in other states. Kansas is trying to implement conservation measures to preserve its groundwater and hope to extend its aquifer’s life for another century. In Texas, the chronic depletion of groundwater is severe. But Florida isn’t experiencing chronic groundwater depletion. The average yearly level of groundwater is quite stable. But within each year, Florida’s ground water levels go up and down like a rollercoaster.
In a sense, Florida’s aquifers are substantially depleted every year. Then, they are completely rehabilitated a few months later with a period of abundant rainfall — only to be depleted, again, later that same year. And the cycle continues. In other words, in Florida, your open well will be full of water for part of the year and bone-dry during the rest of the year.
The long-term strength of Florida’s aquifers is good news, but the extreme seasonal variation is bad news if you want to avoid sinkholes. The reason why other regions of the country don’t have sinkholes with every draught is that these areas don’t have such moist soil to begin with. Sinkholes, from loss of groundwater, happen when the water leaves the soil suddenly. And that’s just what Florida’s climate assures. Each and every year, a season of heavy rainfall is followed by a dry season. The ground is filled with water and, then, dries out completely. In fact, Florida’s climate and geology seem designed to produce the a continuous stream of sinkholes.
So, what did the Times mean by Florida’s sinkhole season? Late spring and summer tends to be quite wet. But Florida winters are dry. And, after the winter dry season, comes early spring — sinkhole season.
However, climate and the pumping of ground water aren’t the only contributors to the Florida’s sinkhole problem. Human development of the land as well as the use of groundwater aggravates the already substantial risk of sinkhole development. How does “development” contribute to the formation sinkholes? It does so in two ways.
First, housing and commercial developments as well as roadways are being built to cover a greater proportion of Florida’s total available land. Even if the frequency and distribution of sinkholes remains the same, with more of the land’s surface covered with structures and pavement, we can expect more frequent “collisions” between human-built structures and sinkhole occurrences.
Second, human-built surface structures actually increase the incidence of sinkholes by (1) increasing the weight load on the earth’s surface and (2) blocking the flow of rainwater to the soil beneath the structures. In other words, the structures, themselves, aggravate the groundwater depletion immediately beneath their foundations and increase the likelihood of the formation of a sinkhole immediately beneath the structure itself – just where we don’t need a sinkhole.
Another aspect of the problem is determination of where sinkholes are most likely to form. There really is no way to know. Ground penetrating radar equipment can sometimes detect large underground cavities. But subsidence from dry earth is extremely difficult to detect . . . until it happens. And when it happens, the inability to know the extent of the undermined area substantially affects rescue and other related remedial efforts. In other words, when a sinkhole forms, the last thing anyone should do is approach it. It may suddenly grow larger and swallow you up (or, rather, down). Or another adjacent sinkhole may open beneath your feet.
If a sinkhole causes injury or creates a dangerous condition, there is a real and immediate danger of the failure of adjacent ground support. This assures that emergency crews really cannot safely approach the area and render immediate assistance. And that may be one of the biggest problem with sinkholes.
Florida used to require that all homeowners’ insurance policies cover sinkhole damage. However, a number of years ago, enterprising lobbyists convinced the Florida State Legislature to repeal that requirement. This measure was supposed to provide lower premiums, which would make insurance policies more attractive to purchasers. Unfortunately, these same purchasers will be financially devastated if their homes are ever damaged or destroyed by the formation of a sinkhole.
The state has a variety of measures in place, and in development, to deal with the sinkhole problem. Unfortunately, even the best available measures are limited. Ground-penetrating radar equipment is regularly used throughout the state to attempt to locate subsurface cavities that might lead to sinkhole formation. Special drainage equipment is being developed and installed to direct rainwater to areas beneath existing construction in an effort to avoid dry soil conditions and reduce the danger of future sinkhole formation.
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