Research Topics » Salting the Earth

By Mary Ann Cunningham, Associate Professor of Geography, Vassar College & Stuart Belli, Associate Professor of Chemistry, Vassar College
Published: December 9, 2009

As the days get shorter and colder, we stock up on bags of salt in preparation for winter snow and ice. Every year we battle to keep sidewalks, driveways, and roads cleared and safe for traveling. Spreading salt on roads and walkways to reduce ice is a common practice, as it is relatively cheap and saves lives and property. Road salt is effective because it lowers the freezing point of water, which causes the water to stay a liquid rather than freezing to ice.

Introduction

Salt is a necessary ingredient in our diets, and in small amounts it certainly isn’t toxic. However, just as it might not be healthy for our bodies to consume too much salt, it isn’t healthy for the environment either. Although it is important to have safe walking and driving conditions, excess road salt can have consequences for natural and built environments.

Excess salt:

• pollutes drinking water
• rusts our cars and bicycles
• corrodes sidewalks, steps, and buildings
• harms invertebrates and nutrient-cycling bacteria in streams
• lowers biodiversity
• kills vegetation

In fact, “salting the earth” refers to the ancient practice of spreading salt on enemies’ fields to make them incapable of growing crops.

Salt use in the United States has risen from 0.16 million tons in 1940 to more than 23 million tons in 2005 (Salt Institute 2009). In 2008, 71% of this was used for road de-icing, compared to only 5% for human consumption (Salt Institute 2009). This means that every year, we apply approximately 16 million tons of salt to roadways in the United States.

Although we often don’t think about it, all of this salt has to go somewhere. Salt enters streams when water from rain or melting snow washes it into stormdrains that lead directly to waterways (i.e. lakes, streams, rivers, or wetlands). Salty stormwater can also flow into pervious surfaces like soil, where salts can accumulate or flush through to groundwater systems. Salt in groundwater migrates slowly toward streams throughout the year, so that even during warmer months there are elevated levels of salt in the water (Howard & Haynes 1993).

Nearly all of the salt used on roadways is sodium chloride. While chloride can be naturally present in local streams in low amounts (through dissolution of bedrock), road salt is causing the levels of sodium and chloride to increase significantly, with potential consequences for the environment.

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Research

Water dissolves salt into its ionic components, sodium and chloride. We can measure the impact of road salt on streams by looking at the concentration of the chloride ion (Cl^-) or the conductivity of the water. Because chloride is present in such low amounts in regional streams, most of the chloride that we detect comes from road salt (Peters and Turk, 1981; Jackson and Jobbagy, 2005; Kaushal et al., 2005; Kelly et al., 2008).

Faculty and students from the Vassar College Environmental Research Institute collected summer and winter water samples from five small streams located in the Hudson Direct Drainage watershed:

• Sawkill
• Landsmankill
• Crum Elbow
• Fall Kill
• Casperkill

This research has shown that there is a significant relationship between impervious surface cover (i.e. roads, sidewalks, driveways, and buildings) and chloride concentrations in streams (Cunningham et al. 2009). This is largely due to the application of road salt on these surfaces during the winter. In all five streams, as the amount of impervious surface cover within the watershed increased the amount of chloride also increased - this trend is illustrated in the graphs below. There was a strong linear pattern for both rural watersheds (like the Sawkill) and urban watersheds (like the Casperkill) in both seasons. Winter and summer levels differed significantly, but in both seasons concentrations were elevated above natural levels (<6 mg/L).

(Graph from Cunningham et al. 2009)

Although road salt isn’t applied during the summer, high levels of chloride in warmer months indicate that groundwater is washing the salt into streams over time.

Other studies have described a benchmark threshold of 10% impervious surface cover, at which urbanization clearly affects streams (Wang et al. 2001, Kaushal et al. 2005). The surprise was that this linear pattern occurred in the most rural watersheds, not just in more urbanized areas (those with less than 10% impervious surface cover). These findings suggest that there is no meaningful threshold in impervious surface cover effects on the stream, and that development even in rural areas has an impact on water quality.

For most of the year, chloride levels are below EPA water quality standards of 230 mg/L. However, during the winter months some streams exceed this. The Casperkill Creek is known to have among the highest chloride concentrations of all streams in Dutchess County (Burns 2006). In winter, chloride concentrations in the Casperkill can exceed 1050 mg/L (Menking, unpub. data). These elevated levels are due to relatively high percentages of impervious surface cover where road salt is applied within the Casperkill watershed.

The graph above shows the average concentrations of chloride in the Casperkill Creek. CK5 is a site near the headwaters of the Casperkill, and the sites proceed downstream. The EPA cites a chronic exposure of 230 mg/L as being dangerous for aquatic organisms; concentrations between 50 and 100 mg/L are considered "sub-lethal," where signficant impairments can be seen in biotic indices, microbial processes, and associations within the stream. Many other waterbodies in Dutchess County have elevated levels of chloride, and the levels are only increasing over time.

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Management

Reducing the amount of road salt applied during the winter would have many benefits: it would save money for municipalities, limit the wear on vehicles, and lessen the environmental impact. Unfortunately, road salt inputs are very difficult to reduce because it has proven to be an effective way to remove ice from paved surfaces and reduce the risk of accidents. We have come to expect clear and safe roads in all seasons, especially as commuting is important for many residents in the area and public transportation options are often limited.

Reducing road salt impacts on watersheds may require attention to planning policies, as much as changes in salt use per se(Cunningham et al. 2009). These results suggest that stream health can be quickly threatened by sprawling development patterns. Cluster-style development reduces the overall length of roadways necessary to access neighborhoods, thereby decreasing the amount of impervious surface required and associated construction and maintenance costs. Because impervious surfaces have such a large impact on streams, encouraging local planning boards to plan cluster-style development and improve public transportation could be an important step in improving water quality.

Other steps that can be taken include:

• following NYSDOT guidelines for the application of snow and ice control materials
• using salting trucks fitted with salt distributers that are tied to vehicle speed (to avoid excessive amounts of salt being distributed at stop signs/red lights and to allow for more uniform application)
• using alternative de-icers with lower toxicity
• spraying salt water on roads rather than salt crystals to prevent the bonding of ice to pavement
• using sand to provide traction (from NYSDOT 2006, AASHTO 2009)

Local highway departments are already using some of these solutions, but more work should be done to implement them region-wide.

If you’re applying road salt to your driveway or walkway, be mindful of how much you use. Applying the salt as a briny liquid instead of a solid may also help improve its effectiveness.

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Methods

Stream water samples were collected at about five sites on each stream in January and June/July. Water samples from each site were filtered and tested for concentrations of chloride and other ions using an Ion Chromatograph (IC). We used GIS (geographic information systems, or computer mapping) to calculate the percentage of impervious surface cover (as well as population, road density, and other variables) upstream of each sample point. This information allowed us to examine relationships between land use factors and stream water quality.

For more information, contact This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

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References

• American Association of State Highway and Transportation Officials, 2009, "Compendium of Environmental Stewardship Practices in Construction and Maintenance, Chapter 8 - Winter Operations and Salt, Sand, and Chemical Management," accessed January 10, 2009. Available here.
• Booth & Jackson, 1997.
• Cunningham, M.A.; O'Reilly, C.M.; Menking, K.M.; Gillikin, D.P; Smith, K.C.; Foley, C.M.; Belli, S.L.; Pregnall, A.M.; Schelessman, M.A.; & Batur, P., 2009, "The Suburban Stream Syndrome: Evaluating land use and stream impairments in the suburbs," Physical Geography, v. 30, p. 269-284.
• Howard, K.W.F. & Haynes, J., 1993, "Groundwater contamination due to road de-icing chemicals - salt balance implications," Geoscience Canada, v. 20, p. 1-8.
• Jackson, R.B. & Jobbagy, E.G., 2005, "From icy roads to salty streams," Proceedings of the National Academies of Science, v. 102, p.14487-14488.
• Kaushal, S.S, Groffman, P.M., Likens, G.E., Belt, K.T., Stack, W.P., Kelly, V.R., Band, L.E., & Fisher, G.T., 2005, "Increased salinization of fresh water in the Northeastern United States," Proceedings of the National Academies of Science, v. 102, p. 13517-13520.
• Kelly, V.R, Lovett, G.M., Weathers, K.C., Findlay, S.G., Strayer, D.L., Burns,D.J., & Likens, G.E., 2008, "Long-term sodium chloride retention in a rural watershed: Legacy effects of road salt on stream water concentration," Environmental Science and Technology, v. 42, p. 410-415.
• Menking, unpub. data. Vassar College Environmental Research Institute.
• New York State Department of Transportation, October 2006, "Equipment Operator Snow & Ice Manual," Highway Maintenance Division.
• Peters, N.E. & Turk, J.T., 1981, "Increses in sodium and chloride in the Mohawk River, New York, from the 1950's to the 1970's attributed to road salt," Journal of the American Water Resources Association, v. 17, p. 586-598.
• The Salt Institute, 2009,"The Uses and Benefits of Salt," accessed November 2009.
• Wang, L., Lyons, J., Kanehl, P., & Bannerman, R., 2001, "Impacts of urbanization on stream habitat and fish across multiple spatial scales," Environmental Management, v. 28, p. 255–266.

Adapted from:

• Cunningham, Mary Ann; O'Reilly, Catherine M.; Menking, Kirsten M.; Gillikin, David P.; Smith, Kelsey C.; Foley, Catherine M.; Belli, Stuart L; Pregnall, A. Marshall; Schlessman, Mark A.; & Batur, Pinar. “The Suburban Stream Syndrome: Evaluating Land Use and Stream Impairments in the Suburbs.” Physical Geography, Vol. 30, No. 3: pp. 269-284.
• Cunningham, Mary Ann & Stuart Belli. Spring 2009. “Salting the Earth.” ScienceWorks@Vassar. Volume 5, Issue 1.
• Vassar College Environmental Research Institute. “Health of the Casperkill.” February 2009.

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