Salting icy roads affects water supplies

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Courtesy of Fluence Corporation

It’s basic chemistry: Saline water’s freezing point is much lower than fresh water’s, which can help keep ice from forming on roads.

To reduce winter accidents, municipal and state workers often apply salt and other compounds to icy roads, because if not accidents start to happen and people end up in need cash in emergency. Unfortunately, this may have unintended consequences on water supplies.

Salt is sodium chloride, which in high concentrations can be toxic to plants and aquatic life. Rain and runoff from melting snow and ice can move road salt into both groundwater and surface bodies of water. The Minnesota Pollution Control Agency says a single teaspoon of road salt can permanently pollute 5 gallons of water.

Excess chloride (from NaCl and other salts) also affects trees and other plants. The Chicago Botanic Garden lost its 50-tree dogwood collection in the 1970s due to heavy salt use on Interstate 88. This can adversely affect diversity in ecologically sensitive wetlands.

Despite this, major metropolitan cities keep road salt well stocked to prepare for snow and ice. Of all the salt produced in the United States in 2014, 43 percent was intended for highway de-icing, according to the U.S. Geological Survey (USGS). That same year, salt production and imports increased significantly due to harsh winter weather.

Chloride Pollution in Streams

In a 2014 USGS study, Midwestern streams that were tested for chloride levels showed increases, and not just during the winter. Concentrations increased throughout the year at its testing sites, including Chicago, which uses more than 350,000 tons of salt annually.

Increased chloride levels were found in urban streams throughout Wisconsin, Illinois, Colorado, Michigan, Ohio, Pennsylvania, Maryland, Texas, and the District of Columbia. Fifty-five percent of the streams sampled contained chloride levels in excess of federal standards; roughly 30 percent of these sites exceeding the EPA’s chloride standard for a total of about four months a year, twice the amount assessed a decade earlier.

In 2018, Illinois plans to institute a strict water quality standard for chloride due to higher-than-permissible chloride levels in Chicago-area rivers. Some rivers have more than double the permitted levels of chloride, with smaller Chicago River tributaries containing the highest concentrations. The EPA estimates that more than 600,000 tons of chloride is added to Lake Michigan each year. A new standard of 500 milligrams per liter takes effect July 1, 2018.

New Hampshire, which uses about 200,000 tons of road salt a year, has spent $3 million decommissioning contaminated drinking water wells. The state reported that the number of water bodies impaired by chloride from road salt doubled between 2008 and 2016.

Removing chloride from drinking water supplies raises the cost of water treatment, as well as the cost of repairing and replacing salt-corroded pipes and other infrastructure.

Because of the cumulative effects, governments are becoming more thoughtful in their use of road salt and other chloride-based de-icing agents. In one example, Canada’s national government has imposed strict guidelines for the use of road salt to decrease negative environmental effects.

Downside to Reducing Salt Use

There is a downside. After several consecutive snow events in early 2017, Oregon and Portland officials have been rethinking their minimalist approach to using road salt. The Oregon Department of Transportation initiated a pilot study in 2012 to test the use of both solid road salt and liquid magnesium chloride on 11 miles of Interstate 5 near the California border and 121 miles of U.S. 95 between Idaho and Nevada. It may expand its use of rock salt into eastern Oregon and from the California border into the cities of Grants Pass and Canyonville.

When compounds such as calcium chloride, magnesium chloride, and calcium magnesium acetate are used as de-icing agents, they present new challenges. For example, calcium chloride corrodes metal bridges, industrial pipes, and concrete. And, despite manufacturer claims, no scientific studies support manufacturer claims that magnesium chloride is safer for the environment.

Officials charged with keeping roads and highways safe say they are still trying to determine which compounds and approaches work best for local conditions. Some areas have even experimented with alternatives, including beets, molasses, and cheese brine.

The state of Minnesota offers its Winter Maintenance Assessment tool, which is designed to help governmental agencies both use less salt and save money on road maintenance.

There Isn’t Just One Correct Approach

Conditions vary considerably, which means there is no one right approach. Road surface conditions and temperatures factor into decisions, as well as the potential for runoff. This means a combination of de-icing approaches may ultimately prove the best option to protect watersheds and the environment.

Ron Wright, central lab manager with the Idaho Transportation Department, told The Oregonian:

They’re all chemicals. The best thing to do is use the least you can. […] They’re both effective tools.

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