Although salt occurs naturally in our environment, our use of road salt can have negative impacts to vegetation and freshwater aquatic ecosystems. Road salt sales and application have increased greatly since 1940. What we generally refer to as “salt” is an ionic compound called “Sodium Chloride” or “NaCl”. Whenever salt dissolves in water, it separates in to two different ions (charged atoms) Na+ (sodium) and Cl- (chloride). These ions can be stored in the soils and transported to the waterways over time. Storage in the soils can lead to damage to vegetation. A study by Kaushal et al. (2005) stated that a low concentration of 30 mg/l of Cl- in soils can be damaging to land plants. Continuing application of road salt can cause a baseline increase of Cl- in soils over time. Storage of road salt in soils can also lead to an increase in concentrations of salt ions in streams and aquatic environments from groundwater flow. Values as low as 250 mg/l of Cl- in water have been shown to be not potable for human consumption and harmful to freshwater life (Kaushal et al., 2005). Harm to aquatic organisms can include changes in mortality and reproduction of aquatic plants, invasion of salt water species in freshwater areas, and harm to invertebrates. To put the harmful threshold concentrations of 30 mg/l and 250 mg/l of chloride into perspective (30 mg Cl-= 49 mg NaCl; 250 mg Cl-= 411 mg NaCl), 49 mg is the mass of about 2.5 postage stamps and 411 mg is about half the mass of a paper clip. Imagine these tiny masses of salt dissolved in a 1 liter bottle of water. Now imagine that amount of road salt that is applied to your local highway during the dead of winter. See the problem? Although road salt isn’t applied everywhere, the areas it is applied to, the surrounding soil, and drainage of those areas can be adversely affected.
When salt applied to road ways is dissolved by water, the salt ions can enter the soil or the stream directly. Whenever the dissolved salt enters the soil the Na+ and Cl- become uncoupled from a process called ion exchange. Ion exchange occurs when the Na+and Cl- ions latch on to other ions of an opposite charge. Cl-is much more mobile in soils compared to Na+, which is often trapped in soils by ion exchange, and can act as a plant nutrient. When Cl-and Na+enter the stream directly, their concentrations are often similar. Road salt enters the stream directly from rain events and snow melting events during the winter time. When salt is dissolved in water and transported to soils and streams, toxic metals may also be transported by impurities in the salt or by ion exchange (Kaushal et al., 2005). A local waterway, Yellow Creek (a tributary to the Cuyahoga River), has been studied for salt concentrations in the stream during pre and post salt application. Data from this study suggests that the degree of development in a watershed is directly related to the amount of salt concentrations in the stream (Delaney and Peck, unpublished data).
Increases in impervious surfaces (roadways, rooftops, concrete, etc.) can be related to increases in road salt application, which in turn creates a less healthy environment land plants and aquatic ecosystems. If development and impervious surfaces continue to increase at the current rate in the North East US, baseline salinity values will exceed 250 mg/l in many rural streams within the next century (Kaushal et al., 2005). The effects of salinity will be more prevalent in urban streams.
Road salt application can affect land plants, aquatic habitats, and can contaminate drinking water sources. Some municipalities and organizations use chemical alternatives to road salt including potassium acetate (C2H3KO2) and calcium magnesium acetate (CaxMgy(C2H3O2)2(x_y)) (Jackson and Jobbágy, 2005). These chemical alternatives are much more expensive than road salt. Road salt is desirable because of its low cost and good functionality. As road salt continues to be applied to our roadways and development continues at its current rate in the NE US, baseline levels of salt will increase in our soils and streams. Though there is no need to ban road salt at the current time, alternative, cost-efficient deicing techniques should be further developed and utilized in the future in order to minimize the ecological impact of road salt application.
Blog Author: Adam Delaney, Natural Resources Intern
- Aquarius Systems: Surface Water Management Issues
- Encyclopedia Brittannica: Ion – Exchange Reaction
- Jackson, R.B., and Jobbagy, E.G., 2005, From icy roads to salty streams: Proceedings of the National Academy of Sciences of the United States of America, v. 102, no. 41, p. 14487-14488
- Kaushal, S.S., Groffman, P.M., Likens, G.E., Belt, K.T., Stack, W.P., Kelly, V.R., Band, L.E., and Fisher, G.T., 2005, Increased salinization of fresh water in the northeastern United States: Proceedings of the National Academy of Sciences of the United States of America, v. 102, no. 38, p. 13517-13520
- NHDES: Environmental, Health, and Economic Impacts of Road Salt
- Sodium and Chloride