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It’s Sinkhole Season! 3 Things to Know

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I’ve heard of hurricane season and wildfire season, but this is the first I’ve ever heard of a sinkhole season. NBC News reported

Across Florida this time of year, it's the start of what's unofficially considered the "sinkhole season," State Geologist Jonathan Arthur said. It coincides with the beginning of the state's rainy season and usually lasts until the end of summer. ... Acidic rain can, over time, eat away the limestone and natural caverns that lie under much of the state, causing sinkholes. Both extremely dry weather and very wet weather can trigger sinkholes, he said. 

Sinkholes have of course become a hot news topic since a 37-year-old man is presumed dead in Florida after a sink hole that developed underneath the bedroom of his house swallowed him while sleeping. 

While the death was shocking, sinkholes in Florida are nothing new. (In one of the most well-known events in Winter Park, Florida in 1981 a 320-foot wide sink hole swallowed a home and a Porsche dealership.) Business Week reports:

The sinkhole that killed Jeff Bush wasn’t even one of the state’s 15,000 verified sinkholes, which are located mainly in central Florida and around Tampa. Plenty are unverified, according to research from CoreLogic. Springhill, on the state’s west coast, has the greatest number of  verified sinkholes, with 3,145—roughly one for every 31 residents. 

If your interest is piqued the United States Geologic Survey has more info.

1. So, what are they exactly?

The USGS reports

A sinkhole is an area of ground that has no natural external surface drainage. Basically this means that when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface.

Sinkholes are most common in what geologists call, “karst terrain.” What’s that? These are regions where the type of rock below the land surface can naturally be dissolved by groundwater circulating through them. Soluble rocks include salt beds and domes, gypsum, and limestone and other carbonate rock. Florida, for instance, is an area largely underlain by limestone and is highly susceptible to sinkholes.

When water from rainfall moves down through the soil, these types of rock begin to dissolve and spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a period of time until the underground spaces just get too big. If there is not enough support for the land above the spaces, then a sudden collapse of the land surface can occur.

Keep in mind though that while collapses are more frequent after intense rainstorms, there is some evidence that droughts play a role as well. Areas where water levels have lowered suddenly are more prone to collapse formation.

2. Where are they? 

They’re not just in Florida, according to the USGS:

About 20% of our country is underlain by “karst terrain” and is susceptible to a sinkhole event. The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania.

3. What causes them? 

While many are caused naturally, others are induced by us.

Many sinkholes form from human activity. Collapses can occur above old mines, from leaky faucets, when sewers give way, or due to groundwater pumping and construction.

Think about all the changes that occur when water-drainage patterns are altered and new systems are developed. And when industrial and runoff-storage ponds are created, the resulting substantial weight of the new material can trigger an underground collapse of supporting material.

Aquifer systems are another example. The sediment above the system is delicately balanced by ground-water fluid pressure, meaning that the water below ground is actually helping to keep the surface soil in place. Groundwater pumping for urban water supply and for irrigation can produce new sinkholes. If pumping results in a lowering of groundwater levels, then underground structures could fail and thus sinkholes can occur.

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