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A year’s worth of Minnesota road salt, visualized [1]

['Christopher Ingraham', 'More From Author', '- January']

Date: 2024-01-11

With a return to more seasonable weather conditions the Minnesota Pollution Control Agency reminded residents on Tuesday to go easy on the road salt this winter.

The stuff is really bad for lakes and streams, as it turns out, because once certain salt compounds get into the water supply it’s virtually impossible to get them back out.

“It only takes one teaspoon of salt to permanently pollute five gallons of water,” the agency writes. “That’s right: Once the chloride is in our water, it’s there for good, and continues to build up year after year.”

Fifty lakes and streams are on the state’s impaired waters list on account of chloride, and 75 more are getting close to meeting the listing requirement, according to the MPCA. The chief driver is de-icing salt, with the problem especially bad in the southern and western parts of the state.

Overall, the MPCA estimates Minnesotans apply about 445,000 tons of salt to paved surfaces every single year. It’s a staggering amount that’s difficult to even visualize. What does one ton of road salt look like, or 100, or half a million?

We decided to find out.

Road salt is extremely heavy, as anyone who’s handled a bag of it knows. A cubic foot weighs 80 pounds, according to a pamphlet once published by the Salt Institute, an industry trade group that abruptly shut its doors several years ago. Other sources, like Minnesota’s own Cargill, post similar estimates, which means that a cubic yard of it weighs about a ton.

But large amounts of salt tend to come in big conical piles, not tidy square cubes. If you pour a given quantity of salt on the ground it will form a wide, flat cone with an angle of about 32 degrees, according to that Salt Institute pamphlet.

We’ll start by visualizing a pile of road salt as tall as a 6-foot Minnesotan, which would end up being 19.3 feet in diameter and contain 24 tons of salt.

Bear in mind that that 2D triangle represents a 3D cone viewed from the side, which will also be the case in all the illustrations that follow.

The Salt Institute’s pamphlet contains a handy table of calculations for salt piles of differing sizes, but that only gets us so far. One thousand tons of the stuff, for instance, would be a cone about 67 feet in diameter and 21 feet tall.

But that’s only 1/450th of where we need to be. We’ll need something much bigger than a human to compare it to. Something massive, instantly recognizable, and – most importantly – easy for the Reformer art department (me) to draw.

Fortunately, Minnesota is home to an iconic structure made exclusively out of simple polygons: the U.S. Bank Stadium in Minneapolis.

For this illustration, imagine that we’re standing on Chicago Avenue looking east toward the stadium, as in this Google Street View shot or this photo from the stadium architects Thornton Tomasetti. The peak of the stadium on the left stands at about 270 feet. We’re looking at one of the shorter sides of a building that’s longer than it is wide, so the width visible to us comes in at close to 500 feet.

If you look carefully at the lower left you can see our little guy next to his original 6-foot, 24-ton pile. Now we can finally start pouring some serious salt.

Taking the data from the Salt Institute’s tables we can reverse-engineer a formula to give us the height and diameter of a salt pile of any given weight. Plugging 450,000 tons into that formula gives us a cone with a height of about 160 feet and a width of roughly 500: similar in size to the stadium as viewed from this angle.

That’s how much salt we dump on the state’s roads and sidewalks every single year. Much of that salt ends up in the state’s waterways, fouling our lakes and rivers at a rate of one teaspoon to five gallons. It’s little wonder the MPCA is trying to get those numbers down.

Now, there are all sorts of caveats associated with an exercise like this. The 450,000 ton figure is, itself, an estimate, on top of which we’re extrapolating numbers from other numbers and converting them into screen pixels. It all makes perfect sense mathematically, but the real world tends to be a lot messier and more complicated.

No salt pile is perfectly conical, for instance, and pile densities may change as you add on more and more of the stuff and things start to deform. Maybe the MPCA’s assumptions are wrong, or maybe the Salt Institute’s formula is off. Consider the numbers above an approximation with a potentially large margin for error.

You might call it a ballpark estimate.

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[1] Url: https://minnesotareformer.com/2024/01/11/a-years-worth-of-minnesota-road-salt-visualized/

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