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January
2003

Vol. 1 No. 3

Snow, Road Salt and the Chesapeake Bay

Featured Article

The average winter in the Chesapeake Bay Watershed results in about 15 days with measurable snowfall, sleet, or freezing rain (this number varies for individual locations in the watershed). Even a small amount of wintry weather can create headaches for commuters who drive along the 200,000-mile network of roads that connect communities across the Chesapeake Bay. It is not surprising that local and state highway agencies make Herculean efforts to remove snow and ice from roads and roadways quickly so our society can keep moving. Increasingly, they rely heavily on salt, sand and other deicers to keep roads open and safe. Let's take a closer look at what happens to the salts and other chemicals that are applied to the roads and what is known about their impact on the environment.

Road Salt Applications

a photo of road saltRoad salting is a pretty recent phenomenon in our region. Prior to the 1970s, sand and other abrasives were the primary weapons of choice to attack snow and ice on our nation's roads. With the advent of new spreaders and increased road traffic, most highway agencies have shifted toward heavier use of road salt during the winter months. Annual road salt use has gradually increased over the last two decades and currently fluctuates between 10 and 20 million tons per year nationwide, depending on how severe the winter is. Despite the fact that much of the Chesapeake Bay watershed is situated below the traditional "Snow Belt", it still accounts for much of the road salt used in the country (About a third of all road salt used in the U.S. is applied to states in the Mid-Atlantic region). Although other alternative products are available and being used elsewhere, rock salt is the most commonly used in the Chesapeake Bay Drainage Area.

Within the Chesapeake Bay region, about 20 tons of road salt is applied to each mile of four-lane highway during an average winter. While exact statistics are not available for the total amount of road salt used across the watershed, we can conservatively estimate that about 2.5 million tons are applied each year. No matter how you look at it, this is a lot of salt. To put this in perspective, consider that if it all this salt were dissolved in a container of fresh water, it would be equivalent to over 15 billion gallons of seawater. Or to put it another way, the entire volume of the tidal Chesapeake Bay (51 billion cubic meters) typically contains about 250 million tons of sodium chloride at any given time.


Salt Drives the Chesapeake Bay
The Chesapeake Bay is an estuary, which means that it is influenced both by the freshwater from its tributary rivers and salt water from the ocean. It is the contrast between the two types of water that drives the circulation of the Chesapeake Bay. Ocean water has an average salinity of about 35 parts of salt per thousand parts of water. Freshwater, on the other hand, has virtually no salt (less than 50 parts of salt per million parts of waters). Consequently, when the denser ocean water enters the mouth of Bay, it tends to sink and creep along the bottom of the Bay. Fresh water is much more buoyant as it enters from the top of the Bay, and tends to travel along the top of the Bay. Throw in the tides, some wind, and the rotation of the earth, and the basic circulation of the Chesapeake Bay is created (see Figure 1)

a map of salinity in the Bay
Figure 1:
Sample of a computer-generated model of salinity levels (in parts per thousand) in surface and bottom waters of the Chesapeake Bay.
Arrows indicated the direction and magnitude of water flow.

Courtesy of the Virginia Institute of Marine Science.
© 2002 STORMCENTER COMMUNICATONS, INC. - ALL RIGHTS RESERVED

The Chesapeake Bay rarely freezes over, because the Bay is salty. Although the surface of the Bay has frozen enough on rare occasions in the past to allow people to walk across it, the Bay will never freeze completely. Read the Watershed Radio feature Bay Freezes to learn more.  

When the Snow Melts, Streams Get Salty


Chloride is one of the main components of road salt and dissolves easily in water. As a result, there is virtually no way to remove chloride once it is added to the snow, becomes part of the resulting runoff and makes its way into the watershed. Chloride moves freely and easily through both surface and groundwater on its way to the Bay. Road salting is thought to be the primary source of chlorides in our waterways. The highest chloride levels are recorded in melt-water runoff near salt depots, with lower concentrations further down the water pipeline.

Table 1. Chloride concentrations in melt-water runoff near selected places nationwide.

Source

Chloride Level, in parts per million (ppm)

Salt storage areas

50,000 to 80,000

Highway melt-water runoff

5000 to 20,000

Snow piles in parking lots

5000 to 15,000

Street melt-water runoff

2000 to 4000

Urban streams in winter

1500 to 2500

Normal Freshwater

20 to 50

Ocean Water

25,000 to 30,000

In addition, road salt contains many impurities. As much of 2 - 5% of road salt consists of other elements, such as phosphorus, nitrogen, copper and even cyanide. A form of cyanide is added to road salt as anti-caking agent (about 0.01% by dry weight). Under certain conditions, it can be transformed into free cyanide, which can be very harmful to humans and aquatic life. Scientists have measured cyanide levels in urban streams ranging from 3 to 270 parts per billion (ppb) for short periods of time as a result of road salting (toxicity to aquatic life begins at 20 ppb).

Melting Snow Packs: Not Exactly as Pure as the Driven Snow

Fresh snow is beautiful and relatively pure. After a while, however, the snow pack gets gray and dirty in urban areas, particularly along the roadside. Road slush, salt spray, airborne pollutants, street dirt, and trash all accumulate in the snow pack over days and weeks. When the snow pack melts, it releases many pollutants to the waterways, including sediments, nutrients, zinc, copper, lead and hydrocarbons and chloride. During the melt, pollutant concentrations in storm water runoff are among the highest seen all year. 

Figure 2. Old "polluted" snow pack (foreground) that has become gray and dirty from the addition of slush, salt spray, dirt and other pollutants (note cleaner snow pack in background).

Figure 2. Old "polluted" snow pack (foreground) that has become gray and dirty from the addition of slush, salt spray, dirt and other pollutants (note cleaner snow pack in background).
© 2002 STORMCENTER COMMUNICATONS, INC. - ALL RIGHTS RESERVED

Impacts of Road Salt

Generally, the presence of chlorides in our drinking water is not a major public health concern. Our tongues don't generally detect saltiness until chloride levels exceed 250 parts per million (ppm). Water utilities routinely report a peak in complaints about the taste of drinking water during winter melt events. However, since we only get about 2% of our daily salt intake from drinking water, the extra sodium and chloride are not usually a major health problem. We get about 98% of our salt intake from the foods we eat, so it makes more sense to pass on the French fries, rather than a glass of tap water.

Did you know…

Between 10 and 20 million tons of salt are used each year to deice roads nationwide.

Within the Chesapeake Bay region, about 20 tons of road salt is applied to each mile of four-lane highway during an average year.

The entire volume of the tidal Chesapeake Bay (51 billion cubic meters) typically contains about 250 million tons of chloride at any given time.

The impacts of chloride and melt-water pollution on aquatic life, however, can be much more severe. A growing body of research has lead Canada to recently designate road salt as an environmental toxin, and look for ways to reduce its use without compromising road safety. To learn more, listen to a recent Watershed Radio feature.

So, what are the impacts associated with chlorides in the environment?

To start with, chloride can be harmful to many forms of aquatic life at concentrations of about 1,000 ppm. Unfortunately, chloride levels are above this level in many small streams and wetlands, at least for short periods of time in the winter. Recent research has documented the impacts of chlorides on stream, lake and wetland ecosystems in the Snow Belt states, but few studies have examined these impacts in the Mid-Atlantic region.

Melting roads create an artificial "salt lick" that attracts both birds and mammals. In the past, natural salt licks were often considered the best hunting grounds since wildlife crave salt in their diet. Wildlife biologists have recently observed that deer, elk, moose and other mammals lick salt from roadsides where they often become road kill. The same effect can be seen with small birds, such as finches, whose cravings for roadside salt have earned them the dubious nickname as "grill birds" in northern regions of the country.

Salting the Earth

High salt levels are frequently measured in roadside soils. The saltiest soils occur within a few feet from the roadway, but the influence of salt can extend quite far from where it was applied- as far as 100 feet from a major highway and 50 feet from a two-lane road (salt is transported by spray from fast moving cars and trucks). High salt levels are usually observed in lawn soils within five or ten feet of sidewalks and driveways that are salted.

Many plant species are extremely sensitive to high soil chloride levels, and may be killed, dieback or fail to germinate under these conditions (see the list in What You Can Do section). Highway researchers have noted that as many as ten percent of trees found along road corridors have been harmed by road salt. You may have seen a similar effect at the beach; look at what happens when salt spray there reaches plants that are not salt-tolerant.

Excessive road salt also damages human-built infrastructures, including concrete bridge decks and parking structures and contributes to the corrosion of metal surfaces (such as the undersides of older cars). The Transportation Research Board estimates that the cost of these damages for the U.S. exceeds four billion dollars each year.

Towards a Low Salt Diet

The road salting that keeps us moving in the winter clearly has a large economic and environmental cost. Practices to minimize the amounts of salt applied to roads are available, and researchers are continuing to evaluate some promising alternative deicing products. Highway agencies have begun to take the salt problem seriously, and are working hard to develop new technologies to reduce its environmental impacts. Examples include the construction of salt domes to safely store salt, use of calibrated spreaders to apply the right doses, improved driver training, more sophisticated forecasting methods to treat roads at the proper time and the designation of low salt application zones near environmentally sensitive areas. Homeowners can also make better choices in how they use deicing chemicals. See the What You Can Do section for a series of practical tips.

The Envirocast Newsletter is a joint project of the National Environmental Education & Training Foundation (NEETF) and the Center for Watershed Protection(CWP) and is produced in partnership with StormCenter Communications. It was developed under Cooperative Agreement No.830502010 awarded by the U.S. Environmental Protection Agency. The views expressed in this document are solely those of NEETF, CWP and StormCenter Communications. EPA does not endorse any products, commercial services or links mentioned in this newsletter.
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