Road Salt Pollution Levels Deemed Safe in U.S. and Canada May Not Protect Freshwater Ecosystems Enough

At current thresholds, salinization can kill off zooplankton, a crucial microorganism at the center of many food webs

An image of blue road salt covering an icy brick road.
Even at thresholds of sodium chloride that were considered safe at 230 milligrams of chloride per liter of water in the U.S. to 120 milligrams of chloride per liter in Canada, researchers found a significant loss of zooplankton populations an increase in algae.
  Douglas Sacha via Getty Images

Using salt to clear icy roads may be an effective winter safety measure, but excess salty meltwater can wreak havoc on freshwater ecosystems and drinking water resources. 

In a new study, researchers show current water quality guidelines in North America and Europe are not enough to prevent dangerous levels of salinization. The findings were published last month in the Proceedings of the National Academy of Sciences

"Salt concentrations are rising in lakes and rivers across North America and Europe over recent decades due to road deicing," says University of California San Diego ecologist Jonathan Shurin, who was not involved in the study, to Science Alert's David Nield. "The study suggests that the levels considered safe need to be revised downward."

Previous research has shown salt leaching from agricultural and mining operations cause freshwater organisms, like zooplankton, to die off in alarming numbers. The cascading effect from the massive die-off can generate an enormous shift in the freshwater system's food web, reports Tara Yarlagadda for Inverse

An image of Lakeshore State Park's bridge overlooking the skyline of Milwaukee, WI
Between 1800 and 2020, sodium chloride levels in Lake Michigan rose from less than 2 milligrams per meter to 15 milligrams per meter. (Pictured: Lakeshore State Park bridge in Milwaukee, Wisconsin.) Joe Daniel Price via Getty Images

In the new study, researchers conducted experiments at several different sites in the United States, Canada, and Europe. To observe the isolated effects of salination on zooplankton in a controlled environment, the team altered sodium chloride levels in 16 large tanks of lake water called mesocosms, which allow scientists to focus on specific factors more easily than they could at an actual lake, according to Inverse.

“With an experimental mesocosm approach, we can be more confident that we are indeed observing the effect of salt and not some other factor in the environment,” study author William Hintz, a freshwater ecologist at the University of Toledo, tells Inverse.

Even at thresholds of sodium chloride that were considered safe, researchers found a significant loss of zooplankton populations and an increase in algae, per Science Alert. The safety threshold for sodium chloride is 230 milligrams per liter of water in the U.S. and 120 milligrams per liter in Canada. At 73 percent of the test sites, half of the zooplankton populations died off in salinity conditions considered safe by both countries.

Zooplankton occupy almost all water bodies except rivers and streams. From oceans, lakes, and ponds, the tiny microorganisms are abundant. Most lakes will hold over 40 species of zooplankton and play a crucial and central role in freshwater food webs. Because the microorganisms are hold a central position in lake food webs, they can affect algal densities, water quality, fish production, and nutrient cycling. When salinization kills off zooplankton populations, it can affect fish growth rates and populations.

"Almost all fish species consume zooplankton when they are young and need zooplankton to grow and eventually become bigger fish," Hintz tells Inverse.

With fewer zooplankton, freshwater ecosystems are more prone to harsh algae blooms that affect water quality. Saltier freshwater also means less available drinking water. The team calls for policymakers to craft legislation that will lower approved sodium chloride concentrations to protect freshwater sources.

"We — as a society — need to recognize that salt pollution is a major ecological issue affecting our freshwater ecosystems," Hintz tells Inverse. "We all need fresh water."

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