25 Jan 2016

How America got addicted to road salt — and why it's become a problem

The US economy doesn't just grind to a halt every time there's a major blizzard. And for that, we can thank the 15 million tons of salt we dump on our roads and sidewalks each winter to melt away the snow and ice.
There are huge upsides to salting the streets. One 1992 study found that spreading salt can reduce car accidents by 87 percent during and after a snowstorm. (The salt works by lowering the freezing temperature of water, preventing ice from forming.) De-icing allows traffic to keep moving, a benefit worth many billions of dollars
But road salt also comes with major drawbacks: Salt is corrosive, chewing through cars, trucks, concrete, and steel bridges. Worse, when all that salt dissolves and washes away, it steadily accumulates in rivers and streams. In some areas, that makes the drinking water saltier, bad news for people trying to cut their sodium intake. The salt in those waterways also kills off fish, plants, and amphibians. In some areas, moose and elk get attracted to the salt and wander onto roads, raising the risk of crashes.
"We've become salt-addicted over the last 50 years, and we're now discovering that there are all these hidden costs," says Xianming Shi, an associate professor in civil and environmental engineering at Washington State University. He estimates the US now spends $2.3 billion each year to remove snow and ice from highways. It then costsanother $5 billion to pay for the resulting damage caused by salt. And that's not even counting the cost of salting cities or rural roads.
So, in recent years, some states and localities have been looking for ways to reduce their reliance on road salt. There are common tricks like pre-salting roads before storms hit, which prevents ice from sticking in the first place. There are exotic remedies like adding beet juice to the de-icing mix, which can help the salt stick in place and lessen the amount needed.
And that's just today. Engineers like Shi have been working on more futuristic technologies, like "smart" snowplows that are thriftier with salt, or ice-free pavement.
Unfortunately, no one's yet figured out a perfect alternative to salt, which is still the cheapest and easiest way to unfreeze roads. But the hunt is on — especially since America has seen major salt shortages these past few winters, and other options are starting to look more enticing.

How America got hooked on road salt 

Before World War II, few US cities used salt in the winter. When snow fell, local governments would plow the roads and then spread sand and cinders around to improve traction. Cars would don snow chains. And people generally accepted that the roads weren't always passable in icy conditions. 
But as America's highways expanded and became ever more crucial to the economy,that changed. Increasingly, truckers and commuters needed to be able to drive in all conditions. New Hampshire's state government became the first to use salt on the roads in 1941-42 and the practice spread as the interstate highway system grew.
By 2013, 26 states were sprinkling roughly 17 million ton of salt on their roads each winter. (Usage can vary wildly by state: An old National Research Council survey found that Massachusetts used about 19.5 tons per lane mile, whereas Idaho used 0.5 just tons.)
To put it another way, while consumption varies each year, the US now puts approximately 10 times as much salt on its roads as it does in processed food:
Salt consumption in the US 
Road salt is basically sodium chloride — much like table salt — and comes from depositsleft over after prehistoric oceans evaporated, with huge mines in Ohio, Michigan, New York, Kansas, and Louisiana. Often times, extra chemicals will be mixed in: For instance, road salt is less effective at melting ice when temperatures drop below 20°F, so when it gets extremely cold, other chemicals like magnesium chloride or calcium chloride are mixed in.
In recent years, however, there's been a salt shortage. State and local officials have sometimes struggled to get enough salt for their roads, after the particularly brutal winter in 2013 depleted stockpiles. In some areas, salt prices have risen as much as 30 percent. Cities like Milwaukee are trying to ration what salt they have. And that's led to a search for alternatives.

The downsides of salting the roads 

Salt, after all, has plenty of drawbacks. It can corrode the steel in cars, trucks, bridges, as well as reinforcing rods in concrete — weakening valuable infrastructure. Transportation departments can add chemicals to the salt to inhibit corrosion or add coating to steel, but this gets pricey. One study in Utah estimated that salt corrosion now costs the US$16 to $19 billion per year
Just as alarmingly, when that salt dissolves and splits into sodium and chloride, it washes away into rivers and streams. Chloride, in particular, doesn't get filtered out naturally by soil and accumulates in waterways. In December 2014, a study by the US Geological Survey found that chloride levels were on the rise in 84 percent of urban streams studied — with 29 percent exceeding federal safety limits of 230 milligrams per liter for at least part of the year.
To some extent, that's a concern for humans. The average American already has too much salt in his or her diet, and having saltier drinking water isn't all that healthy. (Sodium chloride is essential for life, but too much of it has been linked with high blood pressure and even cardiovascular disease). In 2009, the USGS said that about 2 percentof US drinking-water wells it studied had chloride levels higher than the EPA's recommended threshold.
But it's an even bigger deal for all the other freshwater organisms in those lakes and streams. As Nina Rastogi reported for Slate in 2010, high chloride levels interfere with the ability of amphibians to regulate how fluids pass through their permeable skins. Extra salinity can also affect oxygen levels and create dead zones in lakes. The extra chemicals added to road salt can cause fish die-offs. And the salty soil near roadways can kill trees and other plants.
Perhaps the most unexpected effect comes with land animals. Moose, elk, and other mammals often visit natural salt licks to fill up on sodium. But during the winter, they often wander up to salted roads instead — raising the chances of crashes and roadkill.

Why it's hard to find alternatives to road salt 

For all those reasons, many state and local officials have been looking for ways to cut back on road salt use. In 2013, the US Department of Transportation established theCenter for Environmentally Sustainable Transportation in Cold Climates, where Xianming Shi is an assistant director. He notes that there are a handful of different ideas out there:
1) Pre-salting the road before a storm: If officials have advance warning of a storm, they can off spread salt on the roads beforehand — which prevents ice from sticking to the pavement and lessens the need for salting after the fact. The EPA says this can reduce salt use 41 to 75 percent and is best done two hours before the storm. Diluting the salt with a bit of water to allow it to spread can help, too.
The downside? This can cost a bit more upfront. But it helps. One 2010 study from the University of Waterloo found that a handful of "best practices" can reduce local chloride levels in half.
2) Smart snowplows to use salt more precisely. Shi has worked on research for newer "smart" snowplows that not only measure pavement temperature, but also detect residual salt that's already been laid down, as well as the presence of ice on the road. This can help prevent salt overuse, and they're already being rolled out in some cities. In a similar vein, newer Maintenance Decision Support System software gives cities more precise weather forecasts in order to help them use salt more precisely.
3) Using different chemicals: Sodium chloride isn't the only chemical that can lower the freezing point of water. It just happens to be the easiest and cheapest to get ahold of. But, for instance, New England governments often use calcium chloride in areas where sodium levels in the water are high — this doesn't kill off vegetation but it can be more corrosive to concrete and metal.
4) Beet and tomato juice de-icers: Many cities now use beet juice or pickle brine to help salt and sand stick to roadways and minimize runoff. (Wisconsin has even been using cheese brine for this purpose.) The upside? Beet juice or cheese brine is biodegradable and less harmful to wildlife. Still, these only reduce the need for salt somewhat. They don't solve the problem entirely. Shi has been experimenting with other mixtures, including one with leftover barley residue from vodka distilleries, that might help even further.
5) Pavement that doesn't freeze or corrode: The real dream is that one day we might have pavement that's resistant to freezing, or roads that can heat up to melt ice (solar-powered roads, perhaps). Shi says these are still further off, though, and likely to be pricey — it's unlikely, for instance, that we'll get self-heating roads in remote mountain passes, where ice is really a problem. "You'll probably see these ideas first tried in airports, or with the military," he says.
The big drawback for many of these solutions, Shi notes, is that they tend to cost more. Even with the recent shortage, salt is extraordinarily cheap, and cities have a limited budget for de-icing their roads. Plus, of course, they don't necessarily pay for all the indirect costs, like the corrosion on trucks or the environmental damage. "If you take those all into account, then salt is really expensive," he says. "But if you don't, then salt is still the cheapest option, and unless that changes, I don't see it going away for the next 20 to 30 years."


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