Earth Science

Is the Secret to Kentucky Bourbon Limestone Water?

We investigate this popular claim to see if the state’s water is really the key to its whiskey.

Photo Illustration by The Daily Beast

It’s hard to have a conversation in Kentucky without somebody bringing up the state’s famous limestone water. I’ve heard about it time after time on tours of bourbon distilleries, talking to distillers, marketers, sales reps, and even chefs. The special H2O is supposedly what makes Kentucky the perfect place to make bourbon, raise racehorses, and grow corn. (No word on any effect it has on the state’s reportedly huge illegal marijuana crop).

But what is limestone water? Does it exist outside of Kentucky? And does it really make a difference?

My daughter, Nora, recently graduated college with a geology degree. So, I figured I might as well get some benefit from all that tuition money and ask her for background on limestone water. “No one calls it limestone water but bourbon people. ‘Hard water’ is the more common term,” she patiently explained to me.

It’s pretty simple. “Hard water is water with a high mineral content, formed when water percolates through deposits of limestone or chalk, which are largely made up of calcium and magnesium carbonates.”

That explanation dovetails with what Buffalo Trace master distiller Harlen Wheatley told me. “The primary reason is that the limestone adds nutrients like magnesium and calcium that yeast needs to thrive.”

And to make this already complicated question more complex, there are a number of different water sources that distilleries can use to make whiskey. Buffalo Trace gets some of its water from the Kentucky river (which flows right beside the distillery) as well as nearby springs and the Frankfort municipal water system.

At Maker’s Mark, which is deep in the country and far from a municipality, the water used by master distiller Greg Davis comes from a spring-fed lake, which brings its own issues. For instance, the brand has to stock the lake with algae-eating fish to help prevent the growth of geosmin, which is a non-toxic but organic compound with a musty taste and odor. “We don’t change the mineral content or filter,” says Davis. “We bring [the lake water] straight into a tank, preheat it with steam and then it’s on to the cooker.”

Denny Potter, Heaven Hill’s master distiller, has a different challenge. The company’s Louisville distillery, only uses municipal water. While it’s limestone-based, it has been treated. That’s something Potter’s familiar with; earlier in his career, he was a certified drinking water operator and did all the hands-on testing of community water. He really knows about water.

“One thing I can tell you,” he told me, “more can go wrong than right with pulling water direct from a lake or well. Municipal water treatment takes minerals from the water, but they also clean out microbes. You have deionizing units, reverse osmosis units; we want a water source that’s as pure as possible. The water is clean, but we’ll run it through carbon filtration. Carbon will neutralize any other tastes or odors, and it’s not adding anything.”

Potter, like the other distillers, noted the chemical benefits of limestone water: elevated pH levels, beneficial mineral content, low or nonexistent iron content. But Kentucky’s distilling history goes back centuries; did people stop here because of the water? “Back in the day, people were migrating east to west, and distillers landed in Kentucky,” said Potter. “They didn’t know all that about the minerals and pH, but they realized that the water didn’t taste metallic, and it didn’t discolor the whiskey.”

That abundance of good distilling water allowed farm distillers to ultimately evolve into large whiskey producers. Maker’s Mark’s Davis confirmed my daughter’s research, indicating that limestone underlies most of the areas in and around the Appalachians.

“Geographically speaking, the Karst structure that is part of Missouri, all of Kentucky, Tennessee, southern Illinois, southern Indiana, and West Virginia is limestone,” he said. So distillers in those states should also have excellent limestone-rich water. But Davis allowed that other factors helped make Kentucky’s distillers preeminent. “I am just a firm believer that it takes the sum of all parts to make a great bourbon, and there is no better place than Kentucky!”

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Maybe so... or maybe it’s just an accident of history that the bourbon industry settled there. I grew up in eastern Pennsylvania, and limestone bedrock literally poked up through the grass in my neighbor’s yard. Our well pumped water that was so hard you could taste it. They produced whiskey not far away, at a distillery that’s since closed down: the original Michter’s distillery, where they made the excellent whiskey that has become a legend under the A.H. Hirsch name.

Though the distillery has been closed for almost 30 years, the master distiller, Dick Stoll, still lives nearby. He’s more than 80 years old and in startlingly good health, and is helping a new distillery, Stoll & Wolfe, get started there. I asked him what made the water at Michter’s good for making whiskey.

“The alkalinity in it, as opposed to acid,” he told me. “Everyone said that was better for fermentation. Much more than that, I can’t tell you, but it worked very well.” The new Stoll & Wolfe distillery will be using that same water, provided by a local spring water company.

While water doesn’t have to come from a well, or a lake, or even from Kentucky, the right water and mineral content definitely makes a difference in your bourbon.