After the snow from last week’s bomb cyclone melts, cities and towns across the northeast will find their roads and sidewalks still a ghostly white.
That fine white powder is salt, America’s favorite de-icing chemical. More than 15 million tons of the stuff is applied annually only to wash away with snowmelt or spring rains. But just because it’s gone from your block doesn’t mean it’s gone for good. In fact, it may end up in your faucet.
Road salt is an influential part of a decades-long chemistry experiment local governments have unwittingly conducted on soil, waterways, and infrastructure. Studying river data nationwide, a study published Monday explains that 37 percent of all U.S. river systems have greater salinity, and 90 percent have seen a decrease in acidity, compared with a century ago. The two trends are related, scientists conclude, as urbanization and widespread use of chemicals such as road salt damage America’s water and soil quality while corroding infrastructure.
Why is less acid bad? Here’s how nature usually works. Raindrops absorb atmospheric carbon dioxide, turning them slightly acidic. It’s the same reaction that makes drinking a lot of seltzer potentially bad for your teeth. When that rain hits the ground, specifically rock and soil, the acid frees up ions from mineral salts and washes things like magnesium and calcium into rivers and out to sea, a process that scientists call “weathering.”
Humanity has put this process on steroids. First, we’ve increased the amount and types of acid in rain. Industrial pollution turned precipitation so ecologically harmful by the middle of the last century that Congress had to amend the Clean Air Act to reduce emissions. The move worked, which means there’s less acid pounding the pavement. But there’s still plenty.
Second, man-made environments such as cities are often made of materials much more easily “weathered” than natural ones the man-made “rocks” of roads and buildings are washed away into rivers more easily. Infrastructure is an easier target for stronger acids to dislodge calcium, magnesium, sodium, manganese and other positively charged ions from otherwise stable salts.
Finally, winter storm managers take what’s already an easily weatherable, salt-rich urban and suburban environment and slather it in road salt. North American rivers take in this additional salt, wreaking slow-moving havoc on drinking water and infrastructure in the process.
Road salt, sewage, irrigation run-off, and briny water from fossil-fuel production and mining all change the chemical composition of soils, dislodging the calcium, potassium, and magnesium that are supposed to be there, and replacing them with sodium, which isn’t.
Put simply, a lot more salt is being pushed into our water, and that’s bad. Easier weathering and the resulting increase in the alkalinity of waters (the opposite of acidity) across the North American continent is a process that now has a name “fresh-water salinization syndrome” according to the authors of the new paper in Proceedings of the National Academy of Sciences.
Sujay Kaushal, lead author and associate professor at the University of Maryland, said that his research hit home, literally. A few years ago, a neighbor knocked on his door asking him why his tap water had turned black, hours after a local journalist had come to his house reporting a story on the very topic.
The culprit, he suspected, was manganese and iron released from aging Washington cast-iron pipes, and probably helped along by decades of periodic pulses of road salt. Manganese is a potential neurotoxin, and has been linked to Parkinson’s disease and impaired child neurological development, he said.
Kaushal said chemists talk of the process by which water becomes less acidic, or more “base,” as “mobilizing a strong base,” a bit of dark humor among beltway scientists. Only it’s probably certain there’s little political support for the wrong kind of minerals in your drinking water.