Researchers Look at Impact of Road Salt on Freshwater Lakes

  • Mascoma Lake in Enfield is seen on a rainy day, April 7, 2016. (ELIZABETH FRANTZ / Monitor staff)

Valley News Staff Writer
Published: 4/13/2017 12:37:07 AM
Modified: 4/13/2017 2:33:56 PM

Hanover — Road salt is an unavoidable fact of life in the Upper Valley, helping to ensure the safety of those who travel on winter roads. But this safety measure may pose long-term dangers to freshwater lakes and ponds, according to a study published this week in Proceedings of the National Academy of the Sciences.

The study of 371 North American freshwater lakes — most of them in New England, Michigan, Minnesota, New York, Ontario and Wisconsin — found that the presence of roads and other impervious surfaces within 500 meters of a shoreline were a “strong predictor” of elevated chloride concentrations.

At lakes where at least 1 percent of the buffer zone was impervious, that “increased the likelihood of long-term salinization,” the process of becoming saltier over time, the study found.

And if the use of road salt continues in current trends, many of the lakes would surpass EPA-recommended chloride levels within 50 years, and 14 of them would exceed 230 milligrams of salt per liter of lakewater, the EPA standard for protecting aquatic life.

“The picture is sobering. For lakes, small amounts of shoreline development translate into big salinization risks,” Hilary Dugan, a limnologist at the University of Wisconsin-Madison and the lead author of the study, said in a news release accompanying the study.

The lakes in the study were larger than 4 hectares and had to have at least 10 years of recorded data about chloride levels. Impervious surfaces include pavement, brick and compacted soil — any material that does not absorb water, creating runoff.

In the Twin States, the study found increasing salt concentrations in Lily Pond in Windham, Vt., .54 mg/l; Lake Ivanhoe in East Wakefield, N.H., at 15.27 mg/l; and Mirror Lake, located in the Hubbard Brook research area of the White Mountains, at 2.45 mg/l.

Griffith Lake in Vermont’s Green Mountain National Forest, Kettle Pond in Plainfield, Vt., and North Pond at Pillsbury State Park in Washington, N.H., showed no major changes in chloride concentration over time.

Mirror Lake, in Woodstock, N.H., was one of the first places in the Northeast where the salinization phenomenon was noticed, “because salt concentrations began to rise steeply” when Interstate 93 was built nearby, according to Flora Krivak-Tetley, a Dartmouth College researcher who co-authored the paper.

Since the 1940s, the use of road salt to keep roads navigable in winter has been escalating. Each year, some 23 million metric tons of sodium chloride-based deicer is applied to North America’s roads to melt away snow and ice. Much of this road salt washes into nearby water bodies, where it is recognized as a major source of chloride pollution to groundwater, streams, rivers, and lakes.

Though the 371 North American lakes that the study looked at did not include any major lakes in the Upper Valley, such as Mascoma Lake and Lake Sunapee, it is safe to assume that they may be at a higher risk of salinization than the more rural lakes in New Hampshire and Vermont that were included in the study, said Krivak-Tetley. This is largely due to the impervious surfaces that tend to surround lakes in more developed areas, she said.

Mascoma Lake has 5.3 percent impervious surface within 100 meters of the shoreline, and Lake Sunapee has 7.2 percent, according to calculations Krivak-Tetley did for the Valley News.

Although there was not enough longitudinal data on chloride levels to include these lakes in the PNAS study, their high percentage of impervious surfaces suggest that these lakes are likely at risk of salinization, Krivak-Tetley said.

“In general, as salt levels increase in lakes, it tends to stress the biological system of aquatic species more and more,” Krivak-Tetley said, adding that a saltier environment usually favors exotic and invasive species, which are able to out-compete native species for resources. “Even small increases of chloride concentration can result in a loss of native species 10 or 20 years down the line,” she said.

And in lakes that incur higher chloride concentration levels, with thousands of milligrams of chloride per liters of water, the environment can become toxic, and unable to support life.

Most road salt is sodium chloride, which dissolves easily in water, at which point it either leaches into the soil, or, in the case of impervious surfaces, runs downhill and eventually trickles into larger bodies of water.

One of the issues with road salt runoff is that it’s not likely to disappear anytime soon. Salting has been the primary method of de-icing roads since its inception in the 1940s, and remains the preferred material over more time-consuming chemicals and abrasives, Bill Boynton, the New Hampshire Department of Transportation spokesman, said.

“While anti-icing alternatives exist, sodium chloride remains the most economical, effective and safest ice control method that we have at our disposal to maintain over 4,600 miles of state-maintained highways,” Boynton said via email. “We continue to research any and all anti-icing techniques and chemicals as they become available.”

One of these initiatives involves using a salt brine — a mixture of salt, water and the anti-icing chemical Ice B’Gone — as a pre-storm treatment that works to attack snow from the ground up, Boynton said.

The NHDOT’s guidelines on snow and ice treatments recommend no more than 300 pounds of salt per lane mile, supplemented with chemicals or abrasives as needed, for sleet and freezing rain, and no more than 250 pounds of salt per line mile for snowy conditions.

Meanwhile, individual business and property owners account for up to half of all salt used in winter, according to the Washington Post.

“We all have to be aware of balancing our needs to clear those surfaces with the potential impacts on freshwater,” Krivak-Tetley said in an email to the Valley News.

Though the recovery trajectory of a given lake depends on many factors, including its shape and size and how quickly water travels through it, she said that efforts to reduce road salt are “likely to be very worthwhile.”

EmmaJean Holley can be reached at 603-727-3216 or eholley@vnews.com.




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