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A Necessary Evil: is road salt a threat to aquatic life in the Mad River?
By: friendv3 (friendv3) 2008.05.01

While many wise Vermonters are expecting one last spring snowstorm, the tulips and daffodils are blooming and winter is for the most part behind us in the Mad River Valley.  Winters are especially harsh here in the Green Mountains of Vermont, with heavy snowfalls bringing joy to skiers and headaches to commuters, snow removal specialists and town road crews alike.

Expenditures on road maintenance, including snow removal and road salt, comprise a large part of town budgets in the Valley.  Road salt (a.k.a. sodium chloride), the least expensive and most effective material for de-icing roads, is essential in helping make roads safer for winter travel.  Since the 1940’s, the use of road salt in the US has increased steadily; the World Salt Institute estimates that more than 16 million tons was used on US roads in 2004.

Road salt is widely used in Vermont, and with an especially snowy start to winter this year, town road crews across the state feared that their salt stockpile would not last until the end of the season. In Warren, one of the five towns in the Mad River watershed  (along with Duxbury, Moretown, Fayston, Waitsfield) 300 tons of salt were used on the roads during the 2007-2008 winter season.  If the salt application rate stays constant, this will amount to more than 3,900 tons used in the town by 2020.

We have been using road salt as a de-icing agent for nearly 70 years, and scientists have been documenting related negative environmental impacts since the 1970's.  However, there is currently no water quality standard which addresses road salt in Vermont.  Unfortunately, increasing numbers of recent studies indicate that road salt applied at current rates is showing up in our waterways and is adversely affecting our land, water and wildlife.

Research Links Road Salt Use to Elevated Chloride in Waterways

When the snow banks thaw and the spring rains wash the accumulation of a winter’s worth of sediment, debris and salt into our rivers, it doesn’t disappear. The salt is showing up in our rivers and streams in the form of increased chloride levels. The more road salt that is used, the more that is found in waterways:  in 2003, a USGS report on water quality in New England Rivers during the 20th century found the pattern of chloride elevation in the Merrimack, Blackstone and Connecticut rivers during the last century follows the same pattern of increasing road salt usage in the United States.

In some cases increased chloride levels do not return to normal even after the salt laden spring run-off is long gone; instead elevated chloride levels have been shown to persist through the summer. A study conducted by Beverly Wemple at the University of Vermont looked at the effects of development on high elevation watersheds in the Waterbury/Stowe area.  The study compared a developed watershed including a large ski resort to a watershed which was largely forested and undeveloped. As one might expect, chloride concentrations in waters from the developed watershed were higher than in the waters from the undeveloped watershed.  More significantly, the research showed elevated chloride levels well into the summer months.  The authors concluded that road salt applications in the developed watershed resulted in elevated chloride concentrations year round.  This finding has implications for aquatic biota since chronic, long term exposures to chloride even at relatively low levels can affect stream life.

What is more, the problem is not limited to streams. Road salt is showing up in lakes and groundwater, and scientists are finding negative impacts.  A study done on Lake Ontario found that high salt concentrations in the water caused a chemocline in the lake, preventing mixing and creating low oxygen conditions. When better practices were implemented and road salt use reduced, mixing returned and conditions improved.  In Sweden, research has linked increased use of road salt to increased levels of chloride in groundwater.

How does road salt affect the environment?

Terrestrial environments

Road salt is retained in the soils along roadways over time, and can cause them to become compact and unstable. According to the Salt Institute, high concentrations of chloride can interfere with a plant’s absorption of moisture from the soil and cause browning or burning of leaves. It can cause severe root, shoot and leaf damage and affect reproduction.  The composition of plant communities is changed as salt intolerant species (such as the sugar maple) are edged out and replaced with salt tolerant ones. Car collisions can increase as moose, deer and other wildlife are attracted to roadsides turned into artificial “salt licks” by the presence of road salt. Birds may ingest salt, mistaking it for grit, and this can be lethal.

Aquatic environments

In freshwater aquatic environments, the effects of road salt are numerous.  In lakes, the presence of chloride changes the density of lake water, disrupting the stratification process and natural mixing cycle. In streams, salt has been found to be toxic to freshwater aquatic life.  After conducting an environmental assessment of road salt in 2001, Canada declared it to be “toxic” under the Canadian Environmental Protection Act (www.ec.gc.ca/substances/ese/eng/psap/final/roadsalts.cfm).

The effects of chloride on stream life are highly variable.  While many organisms are tolerant of chloride even to very high levels, some creatures are affected at low or moderate levels.  The timing and duration of exposure is key, but it is not well understood how differences in these variables may affect aquatic life.  It is known that chloride is toxic to organisms from short-term exposures at a level of 1,000 mg/L (natural levels are about 8-10 mg/L in streams), and that chronic exposures at lower levels also affects organisms.

As part of their assessment, Environment Canada developed predictive models of the average levels at which species would be affected by chronic exposure to chloride.  Nearly half of species studied were affected at chronic exposures of 550 mg/L chloride. The study found concentrations in the study area far exceeding this level: up to 5,000 mg/L in urban lakes and close to 4,000 mg/L in urban streams.  A study of selected streams in New Hampshire, New York and Maryland documented extremely high chloride concentrations, also topping out at approximately 5,000 mg/L.  This salinity level is 25% that of saltwater, and the elevated concentrations persisted through the summer.  The authors warned that if chloride is not controlled, northeastern surface waters will become unpotable and toxic to freshwater organisms.

Road Salt in Vermont and the Mad River Valley

Historically, chloride has not been considered a pollutant in Vermont.  It is unclear whether new information about the impacts of road salt.  There is evidence that road salt affects chloride concentrations in Vermont waters.  Angela Shambaugh from the Vermont Department of Environmental Conservation states that the data show that chloride levels have been increasing in Lake Champlain, though concentrations have not yet reached levels of biological concern. 

Shambaugh also relates that 13 of the 18 major tributaries to Lake Champlain are carrying more chloride now than they have in the past.  However, because the tributary monitoring program doesn’t target periods when chloride levels are likely to be high, we only have a partial understanding of these levels.  Studies show that in Vermont the level of development in a watershed influences chloride concentration:  streams in areas of higher development experience higher chloride concentrations. Therefore streams in urban areas of Vermont, situated in areas of high development and road density, are most likely to be affected by chloride.

Streams in highly developed areas of the Mad River Valley, where road densities approach those of more obviously urban areas, are possibly being impacted by road salt, or might be in the near future.  Currently, FMR is investigating the available chloride data from small headwaters watersheds where there is significant development.  Doug Burnham, in charge of biological water quality assessments and monitoring activities at the Vermont Department of Environmental Conservation, has been examining chloride data collected throughout the state.  He notes that scientists have seen "significant increases" in chloride in small watersheds with ski area development.  Burnham encourages FMR to be vigilant about chloride contamination considering the existing development patterns and pressures.

Reducing Contamination from Road Salt

Better Backroads is a program of the Vermont Department of Environmental Conservation that focuses on working with municipalities to implement best management practices for maintaining rural roads.  The program focuses on controlling erosion through technical assistance and grants.  Best practices suggested include: installation of rock lined ditches, stabilization of culvert headers and eroding back slopes, and the stabilization of roadside banks.

However, following best management practices for controlling erosion alone will not reduce dissolved chloride in waterways.  There are several alternatives to sodium chloride, including calcium chloride and calcium magnesium acetate; these options are expensive and also are limited due to the different temperatures at which they are effective.  Generally, he cost makes use of these alternative prohibitive, except in situations where very sensitive habitats are being impacted.

One important way to protect water quality is to reduce the amount of salt used each season as much as possible while still keeping roads clear.  Municipalities must also focus on proper salt storage to prevent stream and groundwater contamination.   Ways to reduce salt use include use of calibrated salt spreaders on trucks, prioritizing roads and creating salt reduction areas, mixing salt with sand, or using brine--a solution of salt in water.  Brine has been used with success in Maine, New Hampshire and Iowa.  Use of brine instead of solid salt reduces the total amount of salt used by about 25%, but specialized equipment is needed to mix and dispense the solution.

For More Information

Guidelines and Best Management Practices for Road Salt Use:

EPA Road Maintenance Strategies to Protect Water

Environment Canada’s Implementation Guide for the Environmental Management of Road Salts

The Center for Environmental Excellence’s Environmental Stewardship Practices, Procedures and Policies for Highway Construction and Maintenance




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