We can soon expect our annual doses of wintry weather. On average, we can expect measurable snowfall, sleet, or freezing rain just less than twenty days a year in the Bay 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 then, that local and state highway agencies make herculean efforts to quickly remove snow and ice from roads and freeways so our society can keep moving. Increasingly, they rely heavily on road salt and other deicers to keep roads open and safe. This article examines what happens to road salts applied to roads and their impact on stormwater and the aquatic environment.
Road salting is a pretty recent phenomenon in our region. Prior to the 1970’s, sand and other abrasives were the primary weapon of choice to attack snow and ice. With the advent of new spreaders and increased road traffic, highway agencies shifted toward heavier use of road salt in the winter. Annual road salt use has gradually increased over the last two decades, and now fluctuates between 10 and 20 million tons per year on a nationwide basis, depending on the severity of the winter. Despite the fact that much of the Chesapeake Bay watershed is situated below the traditional “snow-belt”, we apply a huge amount of road salt. About a third of all road salt used in the U.S. is applied to states in the mid-Atlantic region.
In our region, about 20 tons of road salt is applied to each mile of four lane highway, in a normal year. While exact statistics are not available for the total amount of road salt used, CSN conservatively estimates that about 2.5 million tons are applied across the Chesapeake Bay watershed each year. That’s a lot of salt.
Chloride is one of the main components of road salt, and is extremely soluble in water. As a result, there is virtually no way to remove chloride once it gets into the watershed. In reviewing national studies of BMP performance, I cannot recall a single instance where any kind of BMP showed any capability of removing chloride, and numerous instances where chloride removal was negative.
So, once chloride is applied to a road, it migrates easily through either surface water or groundwater en route to the Bay. Consequently, once snow melts, streams tend to get salty.The highest chloride levels are recorded in melt-water runoff near salt depots, major highways, snow piles in parking lots, street runoff and urban streams, as shown below:
|Runoff, Salt storage areas||50,000 to 80,000 mg/l|
|Highway melt-water runoff||5000 to 15,000 mg/l|
|Snow piles in parking lots||2000 to 4000 mg/l|
|Street melt-water runoff||2000 to 4000 mg/l|
|Urban streams in winter||1500 to 3000 mg/l|
|Urban streams in summer||50 to 400 mg/l|
|Normal Freshwater||20 to 50 mg/l|
|Chesapeake Bay||1000 to 10,000 mg/l|
|Mouth of Chesapeake||20,000 to 30,000 mg/l|
|Ocean Water||30,000 to 36,000 mg/ l|
The contribution of road salt to the increased salinization of urban streams and rivers in the mid Atlantic has recently been documented in a fine (and short) paper by Dr. Sujay Kaushal and his colleagues:
The researchers discovered that mean annual chloride levels were positively correlated with watershed impervious cover. In addition, they found it took many months to completely flush out chlorides from the streets to the stream. For example, chloride concentrations frequently exceeded 250 mg/l in the summer and fall months in Baltimore streams, which is above the chronic aquatic toxicity threshold.
In addition, road salt contains many impurities. As much of 2 to 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% dry weight). Under certain conditions, it can be transformed into free cyanide, which can be very harmful to humans and aquatic life. As much as two pounds of cyanide are deposited on a mile of four-lane highway through normal road salting concentrations. 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 begins at 20 ppb).
Many public health officials have traditionally held that the increased presence of chlorides in our drinking water is not a major drinking water concern. After all, our tongues can generally only detect saltiness in drinking water when chloride levels exceed 250 mg/l (which is why water utilities routinely report a peak in complaints about the taste of drinking water during winter melt events). The broader public health concern, however, is the other half of the road salt equation, Na or sodium. Bill Stack has reported that sodium levels are steadily rising in Maryland drinking water reservoirs, and may soon get to levels that might affect sensitive populations who need to restrict sodium (e.g., folks with hypertension).
A growing body of research has led Canada to designate road salt as an environmental toxin, and look for ways to reduce its use without compromising road safety. So, what are the impacts associated with chlorides in the aquatic environment?
To start with, chloride can be harmful to many forms of aquatic life at concentrations ranging from 250 to 1000 mg/l, depending on the species. Amphibians and some plants appear to be more sensitive, whereas most stream aquatic insects appear to be able to tolerate somewhat higher levels.
Chloride levels in the 250 to 3000 mg/l range are not uncommon in many small streams and wetlands, during melt events and extending into other seasons of the year. A growing body of research has documented the strong impacts of chlorides on stream, lake and wetland ecosystems in the snow-belt states, but few studies have examined these impacts in the Chesapeake Bay watershed.
High salt levels are frequently measured in roadside soils. The saltiest soils occur within a few feet from the blacktop, but the influence of salt can extend 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 streets, sidewalks and driveways. Many species of trees, shrubs and ground covers are extremely sensitive to high soil chloride levels, and may be killed, dieback or fail to germinate. Indeed, highway researchers report that as many as ten percent of trees found along road corridors are harmed by road salt.
The impact of adjacent road salt on plant survival is an important design factor for stormwater practices that rely on vegetative cover, such as bioretention, swales and constructed wetlands. When a stormwater practice drains a highway, street or parking lot that is heavily salted, then designers need to pick tough plant species that can tolerate high chloride levels. I am always reminded of this fact after I visited two bioretention cells built on the campuses of the University of Maryland and Villanova in the early spring.
The bioretention cells in College Park had taken a beating, with many of the plant species killed or taking much longer to start growing. Road salt pellets were still clearly visible in the parking lot that drained to the Maryland bioretention cells. By contrast, plant growth was vigorous in the early spring at the Villanova bioretention test sites, despite similar evidence of road salt deposits on the contributing parking lot. The difference, according to Dr. Rob Traver, was they accidentally picked plant species from the coastal plain that were much more salt-tolerant. The other thing I have noticed is that while enormous amounts of beer cans and bottles accumulated in the bioretention cells of both universities, Villanova students appear to drink higher-end beers than their Maryland counterparts
At any rate, I have compiled a list of common plant species that are at high risk from road salt. Perhaps some our landscape architect friends can expand this list, and also come up with a companion list of salt tolerant bioretention plant species.
|Landscaping Areas||Species at Risk from Salting|
|Deciduous Trees||Tulip polar, Green ash, Hickory, Red maple, Sugar Maple|
|Conifers||Balsam fir, White pine, Hemlock, Norway Spruce|
|Shrubs||Dogwood, redbud, hawthorn, rose, spirea|
|Grasses||Kentucky bluegrass, Red fescue|
Some plant species flourish in soils with high chloride levels. Two notable examples are cattails and Phragmites; two hardy and invasive wetland species that prefer brackish water and are often are ubiquitous in roadside swales and constructed wetlands. Since most swales and wetlands receive road runoff, designers should be conscious of choosing salt tolerant wetland species, and ensure there is a range of micro-topography to create plant diversity (see ChesNet StormNews # 5).
Highway engineers are acutely aware that road salt can do major damage to road infrastructure, such as bridge decks and paving. The same is probably true for stormwater infrastructure, but most designers don’t think about long term damage to concrete pipes, permeable pavers, and metal corrosion (e.g., valves, pipes, grates, and the like) that influence of the longevity of their practices.
The road salting that keeps our society moving in the winter clearly has a large economic and environmental cost. To date, no cost-effective alternative to road salting has emerged, although research efforts are continuing to evaluate some promising candidates.
Highway agencies have begun to take the salt problem seriously, and are working hard to develop new technology to reduce its environmental impact. Examples include the construction of salt domes to safely store salt, use of calibrated spreaders to apply the right dose, 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. Most highway agencies are now covered by MS4 stormwater permits, so they will soon be challenged to show how new road salting technologies can actually reduce chloride inputs to Bay watersheds.
Homeowners can also make better choices in using de-icing chemicals for their driveways and sidewalks. A series of practical tips on how to achieve a low salt diet can be accessed by clicking here: