Washington (AFP) - Salting of roads in winter helps drivers navigate snow and ice, but the runoff may be irreparably damaging freshwater lakes in the United States and Canada, researchers warned Monday.

Most of the 371 North American freshwater lakes in the US Northeast and Midwest and Ontario province are showing an increase in salinity from chloride runoff, according to a study published in the journal Proceedings of the National Academy of Sciences.

And if the trend continues it could doom aquatic life and reduce water quality, limiting the supply of drinking and irrigation water, the researchers said.

"The picture is sobering," said lead author Hilary Dugan, a freshwater specialist at the University of Wisconsin-Madison.

"We compiled long-term data, and compared chloride concentrations in North American lakes and reservoirs to climate and land use patterns, with the goal of revealing whether, how, and why salinization is changing across broad geographic scales," Dugan said in a statement.

"For lakes, small amounts of shoreline development translate into big salinization risks."

Each lake studied was larger than four hectares (0.02 square mile) and had at least 10 years of chloride data.

The majority (284) of the lakes were located in the North American lakes region that includes 10 US states -- Connecticut, Maine, Massachusetts, Michigan, Minnesota, New Hampshire, New York, Rhode Island, Vermont, and Wisconsin -- as well as Ontario province.

The use of road salt to keep winter roads navigable has been rising since the 1940s. The researchers determined that each year, some 23 million metric tons of sodium chloride-based de-icer are applied to North American roads.

- Underestimated problem -

Much of that road salt washes into nearby water bodies, becoming a major source of chloride pollution to groundwater, streams, rivers and lakes.

To measure the quantities of road salt applied to roadways and other impervious surfaces, such as parking lots and sidewalks, the researchers evaluated road density and land cover within a 100- to 1,500-meter (0.06-0.09 mile) buffer around each of the study lakes.

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Their findings were clear: roads and other impervious surfaces within 500 meters of a lake's shoreline were a strong predictor of elevated chloride concentrations in the water.

When the results of the study are extrapolated to all lakes in the North American lakes region, some 7,770 lakes may be at risk of rising salinity.

If the escalation in salinization continues, it said, many lakes will exceed in the next 50 years the aquatic life threshold criterion for chronic chloride exposure set by the US Environmental Protection Agency of 230 milligrams per liter.

According to the study, 14 lakes are expected to exceed the EPA standard by 2050, and 47 are on track to reach chloride concentrations of 100 milligrams/liter during the same time period.

"These results are likely an underestimation of the salinization problem, as a number of regions with heavy road-salt application, such as Quebec or the Maritime Provinces of Canada, had no long-term lake data available," said co-author Flora Krivak-Tetley, a graduate student at Dartmouth College.

High chloride levels in lakes have been shown to alter the composition of fish, invertebrates, and the plankton that form the base of the aquatic food web.

That can reduce aquatic species and, in extreme cases, salinization can cause low oxygen conditions that smother aquatic life and reduce water quality, the study noted.