In the course of Snowzilla, the Baltimore Department of Transportation laid down more than 11,000 tons of sodium chloride—aka road salt—to prevent ice from forming. Sodium chloride is incredibly effective at keeping ice from forming, making it more difficult for water to freeze. The problem is that the salt doesn't stay put—it moves as the snow and ice melt and ends up in the water supply.
Salt is a compound of two elements, sodium and an ion of chlorine, known as chloride. Once salt becomes wet, it dissolves, breaking down into sodium and chloride. Neither sodium nor chloride are considered water contaminants by the Environmental Protection Agency (Canada considers sodium a contaminant, however), but there are "recommendations" for the concentration of each in drinking water—chloride largely for taste and sodium for potential health effects.
In recent years, the average levels of sodium in water that's leaving treatment plants in Baltimore to go to taps has been approaching—and in one case, surpassing—the recommended level.
Levels in Baltimore
The EPA recommends that sodium in drinking water not exceed 20 ppm (parts per million) for consumption by infants and those with kidney or heart problems. In the 2014 water quality report put out by the Department of Public Works, the average level at one plant was 21.8 ppm and, at another, 19.6 ppm. To compare, a can of Coke has 45 mg of sodium. The average liter of water coming out of the treatment plants in Baltimore in 2014 was about half as salty as a can of Coke.
Data for 2015 have not been released yet.
Notably, in the years 2007-2013, the DPW water quality reports said that the levels of sodium were "considered low." This designation was absent from the 2014 report.
The water quality reports do not include any data on chloride in the same way that they do sodium. Chlorine is added to drinking water to prevent algae growth and kill bacteria, and is not the same, chemically, as chloride. (Quick chemistry lesson: Elemental chlorine is two atoms of chlorine bonded together. Chloride is just a singular chlorine atom that is missing an electron. Small differences on the atomic scale have huge payoffs—chlorine was used as a weaponized gas; chloride is essential to life.)
However, as pointed out in a presentation to the Maryland Department of the Environment by Joel Moore, an assistant professor in the department of physics, astronomy, and geosciences at Towson University, chloride levels in the Baltimore drinking water supply have increased as the sale of road salt in the United States has increased—demonstrating a correlation between the use of road salt and the increased levels of sodium and chloride in the water.
Moore observed a threefold increase of chloride in the supply since 1960. A study published by Sujay Kaushal, an associate professor at University of Maryland, College Park, showed sodium levels in the Baltimore drinking water supply increase from less than 5 ppm in the 1970s to hovering right below 15 ppm in recent years.
Health and other effects
Sodium chloride is essential for life, but too much of it in humans can be dangerous. Too much salt in someone's diet can lead to high blood pressure and cardiovascular disease. For infants and individuals with kidney disease or heart disease, the EPA recommends that sodium in drinking water not exceed 20 ppm (which is the same as 20 milligrams per liter).
While, no, drinking water with a high salt content isn't the same as downing a large order of french fries every day, it's definitely not helping. And, in recent years, some of Baltimore's drinking water supply has been saltier than recommended, as seen in the recent averages released by the DPW’s water quality report.
Road salt also speeds corrosion in the metal in bridges, automobiles, and reinforcing rods in concrete. Reports have shown that corrosion costs from road salt total between $16 and $19 billion a year.
When road salt leaves the streets, it can enter waterways and soil, harming plant, aquatic, and amphibian life. Because of the way that water filters through the landscape, salt levels in streams and waterways stay elevated throughout the year—they are highest in the winter, but do not return to "baseline" levels. Salt in the groundwater eventually finds its way to surface water, where it can reach plants, animals, and soil.
How road salt works/Why we use it over alternatives
Road salt works by altering the freezing point of water. The more salinity water has, the colder it has to be before it freezes to ice.
And it's cheap. Of all the methods out there for de-icing roads, chunks of salt are the easiest to get ahold of—and the cheapest. There's a reason that 26 states, in 2013, used more than 17 million tons of salt on their roads, according to Vox.
There are alternatives to just dropping salt rocks on the road but, as Moore, the Towson professor, said, "there are no silver bullets." As with just about any environmental issue, any technique used to de-ice roads comes with costs and benefits, he said.
Roads can be pre-salted before a storm hits, for example, which typically uses less salt—by up to 75 percent, according to one EPA report. Pre-salting, however, is typically more expensive upfront. "Smart" snowplows, which can read pavement temperatures and detect how much salt is already on the road to prevent over-salting, are being developed, but they cost more than regular plows. Beet juice can be used to prevent runoff of salt, but salt is still required to keep ice from forming.
There are other chemicals—calcium chloride, for example—that are less harmful in the water supply and less harmful to plant life, but calcium chloride is more corrosive to metals than sodium chloride.
Alternative products and alternative techniques exist—it’s just a matter of municipalities and states deciding and prioritizing them. As it is with any environmental issue, there will be costs up front. However, those up front costs would be much cheaper than repairing bridges and dealing with the consequences of fishless streams and rivers.