Just a couple days before January’s “snowzilla” storm buried much of the region under 2 feet of snow, District of Columbia Mayor Muriel Bowser apologized for the city’s “inadequate response” to less than an inch of snow that left motorists variously sliding though icy streets or stranded in backups.

“We should have been out earlier with more resources,” she said. And the mayor assured residents that — with a blizzard officially named Jonas bearing down — that lapse wouldn’t occur again. She then outlined plans to deploy plows, front-end loaders, dump trucks — and 39,000 tons of salt. That’s 118 pounds for each district resident.

Since 1938, when New Hampshire began experimentally salting winter roads, the substance has increasingly become an effective — and cost-
effective — way to combat the slippery effects of ice and snow.

The lapse in preparedness that Bowser apologized for resulted in more than 1,000 accidents in the District. Nationwide, 1,300 people are killed on average and another 116,000 injured on icy pavement each year. Salt applications reduce crashes by 88 percent and injuries by 85 percent, according to the Salt Institute, an industry trade group. The costs of de-icing pay for themselves within 25 minutes, the group says, by reducing accidents and maintaining commerce.

Nationwide, 10 times as much salt goes on the road as is used to season all processed foods. But as with food, too much salt in freshwater is harmful. It’s a growing problem that threatens efforts to protect stream health in the Chesapeake watershed, and even in the Bay itself.

“Salt is an unfolding train wreck for streams across the mid-Atlantic and I think across the United States,” said Robert Hilderbrand, a stream scientist with the University of Maryland Center for Environmental Science’s Appalachian Laboratory, at a conference last winter.

Yet he noted the paradox that confronts elected officials and highway crews each winter: He said he was only able to attend the conference because the roads had been salted the day before because of an ice storm.

Salt works by lowering the temperature at which water freezes, thereby preventing ice from forming on the roads.

But when the snow and ice melt — and water runs off — it carries that salt along with it into freshwater streams where everything from insects and mussels to fish and salamanders evolved in very different conditions.

“The bottom line is that freshwater animals don’t tend to like saltwater — particularly the most sensitive of animals, like some amphibians and mussels,” said Scott Stranko, who oversees stream monitoring programs at the Maryland Department of Natural Resources.

Rock salt is sodium chloride, which is typically about 40 percent sodium ions and 60 percent chloride ions. Salt not only increases salinities in streams, but also concentrations of chloride in the water. In addition, it raises the conductivity of water — a measure of its ability to conduct electricity, which is enhanced by the presence of salt, chloride and other chemicals.

These measurements spike in runoff events after snowfalls. But studies show that chloride levels and measures of stream conductivity — which are commonly used surrogates for measuring salt inputs — are also increasing over time in many areas. That’s because chloride not only runs off directly into streams, but also sinks into groundwater, where it builds up over decades before it emerges in a stream or river.

“Once it reaches the groundwater, we are talking about decades or centuries before it is going to come out,” Hilderbrand said. “It is going to be a constant reservoir of elevated salt concentrations coming into our freshwaters for years to come. I do not see that decreasing anytime soon because we have so many people, and we use road salt for safety’s sake.”

The impact of that long-term trend is evident in New Hampshire, the first state to use road salt. By 2008, it had 19 waterbodies listed as impaired by high levels of chloride. Just two years later, that increased to 40.

Now, much of the nation is on that trajectory. A study by U.S. Geological Survey scientists published in 2014 found substantial chloride concentration increases in 84 percent of the urban streams it analyzed from the 1960s through 2011.

Many of the measured chloride concentrations were higher than the Environmental Protection Agency’s water quality criteria of 230 milligrams per liter, though those were seen mainly in snowy northern sites and in urban areas where a lot of roads get salted. But the study also found that chloride concentrations were increasing faster than the rate of urbanization. Road salt sales data
show a similar spike, the paper said, but it also suggested that the chloride that has been building up in groundwater over time is now making it into streams.

“The high concentrations that we found could negatively affect a significant number of species,” said Steve Corsi, a USGS scientist and lead author of the study. “If urban development and road salt use continue to increase, chloride concentrations and associated toxicity are also likely to increase.”

Studies around the country have found that as salt levels increase, streams begin to lose their most sensitive species. Caddisfly, stoneflies and mayflies are among the first things to disappear. Amphibians and fish follow shortly after.

In a 2013 review of salt impacts on streams, Maryland DNR biologists concluded that increased chloride levels in state waterways were associated with decreased diversity in stream-dwelling organisms. The Maryland Biological Stream Survey shows declines in amphibian populations as salinity levels increase. For instance, salamanders populations decline steadily as chloride levels rise, and when levels hit 440 mg/l, they are gone, the DNR review said.

Likewise, brook trout are only present in streams with chloride levels less than 280 mg/l, and the highest densities of the brilliantly colored fish are found where chloride concentrations are less than 100 mg/l.

In Maryland, water quality monitoring has shown a significant increase in water conductivity at 80 percent of the stream sites monitored over the years. Although other minerals can increase conductivity in certain areas, road salt is thought to be a major driver behind the overall pattern — conductivity routinely spikes after snow events.

Stream monitoring in Maryland also has shown reduced anadromous fish spawning as water conductivity increases, the DNR paper said.

Those trends could hinder a goal of the Bay Program’s 2014 Chesapeake Bay Watershed Agreement to improve the health and function of 10 percent of the stream miles in the Chesapeake watershed by 2025. The management strategy written to achieve that goal cites road salt as one of several stressors that could affect the region’s ability to improve stream health.

Environmental damage caused by road salt — and potential impacts to the Bay — actually start well before the mineral reaches a stream. Salt-laden runoff kills roadside plants and trees, which in turn increases erosion, resulting in more sediment being washed into waterways, the DNR paper said.

Scientists also worry that road salt in soils along roads can alter the bacterial community, limiting its ability to remove nitrate — a key Bay pollutant — and to break down other pollutants.

“The magnitude and extent of roadside soils containing salt could be an important factor to consider for meeting Maryland’s nutrient reduction goals and efforts to reduce nutrient runoff to Chesapeake Bay,” the DNR review said.

In Maryland, as with the USGS study, the highest chloride levels are found in urban areas with the most impervious surfaces — and therefore lots of salt-laced runoff. Surveys in recent decades show that average annual chloride levels in three streams entering Liberty Reservoir, a drinking water supply for Baltimore City, increased by as much as 300 percent.

Sujay Kaushal, an associate professor with the Earth System Science Interdisciplinary Center at the University of Maryland, reported in a 2005 paper that chloride levels in Baltimore area streams were “strongly related to the amount of impervious surface coverage” and that in areas with more than 40 percent pavement and buildings, the mean average annual concentration of chloride often exceeded EPA criteria for safe levels of chloride, and would harm sensitive freshwater species.

Although chloride levels peak during spring runoff — when salinity was sometimes a quarter that of seawater — they remained high much of the rest of the year, Kaushal reported. In spring, summer and fall, they were as much as 100 times greater than concentrations found in forest or agricultural watersheds without roads, where chloride concentrations typically range 3–8 mg/l.

The impact is less pronounced in rural areas, but a salt signal can still be seen around heavily salted roads. About a decade ago, Richard Raesly, a biology professor at Frostburg State University, and his students monitored salinity levels near Interstate 68 in Western Maryland. They consistently found elevated chloride levels downstream of the expressway, peaking as much as 16 times greater than upstream concentrations at one of the sites.

They did not find differences in stream diversity between the upstream and downstream locations. But, he noted, some of the highest concentrations were in the summer, which suggested that chloride was building up in groundwater and working into the stream.

Indeed, Kaushal’s study also found growing chloride concentrations in many rural streams, and warned that if trends continued, many rural waterways would have chloride levels that exceed EPA criteria in a century.

It’s not just an environmental issue, but an increasingly pressing health issue. Salt is expensive to remove from drinking water and builds up over time, which could pose a threat to people on salt-restricted diets, Kaushal said.

“If you would have asked 10 years ago, I would have said, ‘It’s important, but there’s other [stream] issues,’ ” Kaushal said. “But I think it’s one of the top issues now.”

In recent years, states have become more cognizant of the threat road salt poses to the environment, and have adopted protocols to optimize applications and explored a wide range of alternatives — even substituting beet juice for salt in some cases.

Yet as more areas are paved, the national trend has been toward increased salt use, though the exact amount varies from year to year based on the severity of the winter. In the winter of 2014–15, Maryland applied 337,492 tons of salt — or about 112 pounds for each resident. And those figures only count what’s reported by state highway agencies — not what’s used by local governments, businesses or homeowners.

Depending on where it comes from, Kaushal said, salt can contain a variety of other pollutants as well.

But, for officials trying to balance safe streets and clean streams, there are no easy solutions, he acknowledged. Non-salt alternatives such as grit, which in some areas includes coal fly ash, carry their own contaminants that can end up in streams.

“Anything that you apply to the roadway, any chemicals that you apply, they have effects,” Kaushal said. “There is nothing that’s just completely inert.”