Forget “The Sky Is Falling” – The Ground Is Sinking!

We were treated to yet another sinkhole story on May 20, when construction crews at Austin Peay State University's Governors Stadium in Tennessee had to excavate a 3-by-5-foot sinkhole to a size of 40-by-40 feet in order to locate stable bedrock. This latest news prompted us to ask: are there more sinkholes opening up nowadays, and if so, why? Or is there simply more media coverage, which makes it seem that sinkholes are increasing? The answer is: a little of both. Nevertheless, there are several factors that have contributed to a rise in sinkhole frequency around the world. 

Usually sinkholes occur in regions with large subsurface limestone deposits. Over hundreds of years, acidic groundwater gradually eats away the limestone bedrock, creating an underground cave that is visible only to geographical surveys. Thus we surface dwellers have no idea what’s happening down there until one day the cave collapses and our homes and buildings are swallowed up. Perhaps the most recent sinkhole tragedy involved a man in Seffner, Florida who fell through his bedroom floor last March when a hole opened—to the horror of his brother and the rest of America. The damage doesn’t end with loss of life and property, however. After the “cover collapse,” as it is known, soluble toxic contaminants such as gasoline, heavy metals, and organic compounds can leak through the cracks in the fractured limestone and reach the aquifer, poisoning our drinking water. 

That brings us to the delicate relationship between sinkholes and aquifers. By definition, an aquifer is a layer of water-bearing permeable rock. Yet the very same quality that makes such rock ideal for water storage—porosity—also means that it is less structurally sound than other kinds of rock. In other words, the pores that hold recharge water, usually from stormwater runoff, tend to become larger over time through erosion, which compromises the integrity of the whole layer, causing sinkholes to develop. Sinkholes aren’t always bad. In less populated settings, they can collect large volumes of recharge water in a short amount of time and transmit this water directly to the aquifer. Of course the volume will vary per season. Recharge can also occur through fractured limestone in a streambed. 

About 10% of the world’s surface consists of karst landscapes, or depressions that will eventually form sinkholes. Besides Florida and Tennessee, the states of Alabama, Kentucky, Missouri, Pennsylvania, and Texas have the most sinkholes due to their soluble bedrock. But sinkholes are becoming more prevalent in other states and around the globe because of climate change and human activity. Warmer temperatures result in heavy rains, which bring flooding and mud slides. The upper layers of clay and sand break down, seeping into the fissures in the bedrock; soon these layers deteriorate enough to actually fall into the cave below. Salt, gypsum, limestone, chalk, and sandstone are all soluble and will give way to excessive water. If the water-bearing rock lies under an impermeable layer, pressure will build up until the water bursts out, sometimes as a natural spring but occasionally as a fracture which then collapses.  

Humans can influence sinkhole development as well. Mining creates openings in rock, weakening the ground structure. If mining cavities are then abandoned and left to erode, entire buildings can be lost. In periods of drought, people draw more water from the aquifer. Regardless of the season, factories and farms pump out a large amount of water. When the water, which once supported the underground caves, drops without being recharged, the caves begin to crumble. While sinkhole occurrence sometimes scares off developers and prospective buyers, hurting the building industry, the risk of building on unstable bedrock is too great to ignore. Builders would do well to consult geologists before breaking ground. Homeowners too can help themselves by looking for signs of sinkholes, such as rainwater collecting in unlikely places, doors or windows that no longer close properly, and sagging trees or fence posts.  

With more frequent drought and tropical storms, we’re entering a new age that requires us to be alert to the stability of the Earth. We can’t always prevent sinkholes, but we can be prepared for them by better understanding how they form. While we’re striving to build “greener” houses, offices, and schools, we should pause for a moment to remember the temporary, fragile nature of all manmade structures in the face of climate change. We should also realize that what we sow in mining and water usage, we will reap in a planet crumbling beneath our feet. 

By Jessica Foster

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