on sinkholes


Eats up stairs

Alone or in pairs

And sucks em right into the ground

A hole, a hole, good god what a hole

Everyone knows the sinkhole!

In my senior year of high school, I took the Ecology class, which more specifically was Floridian Native Ecology And Other Supercool Things That I Wound Up Being Totally Into Despite Failing The Class. I had a lot going on at home and I was really good at hiding it. Still don’t know how I graduated. But that class gave me a firm grounding – hah – on the subject of sinkholes, aquifers, karst (not loess), saltwater inclusion, drought, water treatment, and all the other things that make Florida such a crumbly crust of sand to build upon.

Today I feel like sharing my knowledge with you. You are about to learn Sinkholes from a Genuine Lifelong Florida Girl. A defective one, mind, as I got the Estonian pallor and couldn’t tan at gunpoint, but a Floridian nonetheless. I instinctively do the stingray shuffle and I can identify bug bites by the welts they leave. I’ve earned my cred.

NOW THE DISCLAIMER: everything I am about to relate comes from the initial base of knowledge I learned in that class, bolstered by things I learned on the internet because I am a nerd and I like to spend hours learning about local geology in my free time. If I get something wrong, and you are in a position to know that for a fact and tell me what is the correct bit of information, PLEASE DO. Then I will edit this post, credit you for the corrections, and be more useful.

To explain this I need to get to very basic things and ancient history.

In the beginning, Florida was a sandbar barely peeking out from older, warmer oceans. The shellfish and tiny crustaceans that collected on it, over time, were calcified and compressed into limestone, which is porous, relatively fragile, and has a curious chemical reaction to acids like vinegar. (It was a fun day when we did Vinegar Rock Tests.) Limestone is made mostly of calcium carbonate, which is easily demolished by acids; if you drop ordinary white household vinegar on the stone, it will fizz impressively. It is not strong stuff, as rocks go, but it is perfect for the Floridan Aquifer. (Not Floridian. I don’t know why, don’t ask.)

Image via Wikipedia; fair use etc.

Image via Wikipedia & USGS.

The aquifer is made of porous limestone and water. Think of it as a saturated sponge, except the sponge is made of stone. This construction, a soluble waterlogged bedrock, is known as karst. There are lots of karst areas in the world; another that immediately comes to mind is the Yucatan cenotes, and I believe there’s another substantial one under the midwestern US, which has for decades supplied water to all of the farming that goes on out there.

This karst aquifer is a magnificent system. It has been tested by relentless nature for longer than humans have existed. It regulates itself. It functions just fine on its own terms. The problem is that those are not human terms.

Here’s how it goes: rain leaches through the soil and clay and sand, losing impurities as it goes. It sinks until it reaches the limestone bedrock which, being porous, absorbs and contains it. It stays there, circulating in a thousand beautiful subterranean rivers, until it burbles back to the surface in springs which feed rivers and streams.

Note I do not say lakes; although there are some spring lakelets (I’ve swum in beautiful Lithia which feeds the Alafia) most lakes in the Floridan Aquifer system are the result of sinkholes.

A sinkhole is simple enough. Water is dense and solid. So, too, is rock — even fragile rock like limestone. This delicate-seeming combination is quite sturdy and normally can support the ground above it. Sometimes, usually due to drought, the aquifer’s water level goes down. The limestone alone cannot support whatever is over it, so eventually gravity does what it does best and brings things crashing down. That is a sinkhole.

Most Florida lakes are sinkhole lakes: they began as sinkholes. Since water is always going down to the aquifer, it brings debris with it; if this plugs the hole, the water collects in the hole and becomes a lake. The lake then seeps into the ground and feeds the aquifer again, and all is well. On rare occasions, the aquifer level may drop and the hole may reopen, and where there once was a lake there is suddenly a dry mudflat.

Take a look at Google Maps, here; you can see all the natural sinkhole lakes, which are round, and then the manmade reservoirs which probably were built onto lakes, and are not round.

This is how karst functions. There is nothing wrong with this system. It’s been doing this since before our ancestors were still hiding from giant reptiles. The problem, as I said, is that it’s not very good for humans to live on.

Or, more accurately, I might say the problem is that humans do not know how to live on the karst.

For at least the past thirteen years, we’ve been under significant drought conditions. Not enough rain coming in, despite what we’d have you believe when we cry havoc about all the storms. Florida is also a very attractive place for farming, since the winters are so mild, and we can grow lots of things here that we cannot grow in many other places. All of this farming requires water, and lots of it: when the agriculture was getting started, it was not a problem, because the aquifer was full and seemed a perfect endless reservoir.

We’ve since learned that it is not, but we haven’t learned to slow down.

There are other elements at work here that I do not know as much about. Saltwater intrusion is one: when the fresh aquifer water is low, and it is near the sea, the saltwater will be pulled into the limestone. This does two things: it salinizes the fresh water and it erodes the limestone further. Another is the use of fertilizers, which acidify the groundwater and, again, cause more limestone erosion. The mixture of water and rock is precise, and dictated by nature: when this is out of whack, it all comes crashing in.

Refer to what I wrote above; when there isn’t enough water to support the limestone, it collapses into sinkholes. This has been happening more and more frequently lately, in places where it hadn’t been before, and that is directly due to pumping more water out of the aquifer than it can physically support.

We don’t seem to realize that we stand on water as much as on rock. Without the water, the rock can’t hold us.


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