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Valley News – Making road salt a science


CLAREMONT — The Claremont Public Works Department has technology on its four newest trucks that calibrates salt delivery to road temperatures and air temperature. Drivers prescribe just enough salt to keep the roads safe, and no more.

“We were using 350 pounds per track mile; now it’s just under 200 per mile of lane,” said Ted Wadleigh, the city’s assistant director of public works. “We cut it in half.”

Reducing de-icing salt is an important environmental goal. The salinity of freshwater ecosystems is increasing across the world as salt from fertilizers, mines and de-icing highways pours into ponds, lakes and rivers. According to a new international study to which Dartmouth contributed, salt concentrations well below the Environmental Protection Agency’s threshold are still causing significant environmental damage in lakes and ponds.

The researchers concluded that there is an “immediate need to reassess current government thresholds to protect lakes from salinization.”

The EPA’s threshold for salt contamination is 230 milligrams of chloride per liter, while Canada’s is only 120 milligrams of chloride per liter. (Chloride is one of the two chemical components of rock salt). Even at the lower edge of Canada, researchers have found massive declines in zooplankton.

Dartmouth researchers collected water samples from Storrs Pond, Mascoma Lake, Boston Lot Lake and Goose Pond to recreate lake ecosystems in buried dumpsters at the Dartmouth Organic Farm. They were increasing the salt concentrations in the “mesocosms” every week. It only took zooplankton a few days to suffer from high salt levels, said Jennifer Brentup, a former postdoctoral fellow at Dartmouth who conducted research for the study.

Zooplankton are crustaceans so small that they are difficult to see with the naked eye. But they are an essential hub in the circulation network of lake ecosystems, said Kathryn Cottingham, professor of biological sciences who led the Dartmouth research team. Fish eat zooplankton and zooplankton eat phytoplankton, also known as microalgae. Zooplankton are at the center of the food chain, transmitting the energy phytoplankton captures from the sun up the food chain. Without them, the most recognizable inhabitants of our lakes and rivers, such as trout and salmon, cannot survive.

“Basically, if you break the circulation network, whatever is above will collapse,” Cottingham said.

As salt levels increase, negative impacts ripple through the ecosystem. Phytoplankton populations increased at nearly half of the study sites. In some sites, they formed a carpet on the surface of the water, blocking out light. Fish hunt by sight, and therefore they are hungry in the dark. Rapidly reproducing phytoplankton consume oxygen, smothering fish.

The Dartmouth researchers said lakes and ponds near roads are most at risk. In 2021, the US Geological Survey estimates that the United States consumed 54,000,000 metric tons of salt, and about 42% was used for highway de-icing. De-icing salt damages more than the natural environment. It also seeps into groundwater, contaminating drinking water with a salty flavor and harming people with high blood pressure. Salt corrodes cars and infrastructure. The EPA estimates that the United States spends $5 billion on annual repairs because of rock salt.

Climate change is putting upward pressure on the amount of de-icing salt New England needs to keep its highways safe. In New England, Brentup said climate change is driving more extreme winter rainfall and temperature swings. Rapid freeze-thaw cycles mean more ice on the roads and more salt.

The area’s lakes and ponds are suffering and the problem is getting worse every year. In 2008, New Hampshire listed 19 chloride-impaired water bodies that exceeded the EPA threshold. In 2020, this number has increased to 50.

“Chloride concentrations in New Hampshire lakes have been steadily increasing since the late 1980s and early 1990s,” Cottingham said. “Especially with interactions with global warming, we could hit thresholds that could cause big changes quickly. How do we slow these things down?”

Cottingham pointed out that what we do on land affects our water. The New Hampshire Department of Environmental Services warns that neither evaporation nor chemical degradation nor factories remove a significant amount of chloride from the environment. The salt we spread on our roads and sidewalks will almost entirely end up in our water. The ministry stressed that the only way to avoid further contamination is to limit the amount of salt we deposit, without compromising safety.

New Hampshire has conducted a road salt reduction program that trains snow and ice management professionals to use as little de-icing salt as possible. As of November 2020, the EPA credits the program with reducing road salt use by 20%.

Brentup has listed many ways to reduce our dependence on de-icing salt. The porous pavement soaks up water that would otherwise freeze into a treacherous layer of ice. Brining roads with a salt water solution before a snowfall event requires significantly less salt than spreading rock salt. And some road crews use biodegradable substances such as beet and pickle juice to slow the formation of ice and make the saline solution stick to the roads rather than washing them away.

Last year, the Claremont Public Works Department tried magnesium chloride mixed with molasses. The molasses limits rust on vehicles and sticks to roads, reducing the amount of magnesium chloride the department’s trucks spill, Wadleigh explained. But magnesium chloride only works when temperatures stay well below freezing. During a warmer winter, it becomes slippery, reproducing the dangers of the ice that it is supposed to limit. Over time, Wadleigh said, the department will update all of its vehicles with new technology that distributes salt more efficiently.

On average, over the past two winters, Claremont has spent $168,426 to purchase 2,700 tons of salt. This year, salt prices are on the rise, but the city needs far less than before.

Claire Potter is a member of the Report for America body. She can be reached at cpotter@vnews.com or 603-727-3242.