By Karena DiLeo, Stewardship Coordinator
Every winter as inclement weather approaches, deicing chemicals are liberally applied to roads and walkways. As a result, half of the streams in northern metropolitan areas experience elevated levels of sodium and chloride from the meltwater that washes downstream1. Sodium chloride (NaCl), the most common deicer, has been shown to increase salinity in freshwater ecosystems and decrease water quality in human water supplies1. Since these chemicals can be retained in the soil for up to 5 months, elevated concentrations of sodium and chloride can be present in surface and groundwater throughout the year2.
A study of the Philadelphia region, which primarily relies on the Schuylkill and Delaware Rivers for drinking water, found the highest levels of sodium and chloride in the water supply following periods of snowmelt. With elevated concentrations found in areas of dense development upstream, these levels continued throughout the year as contaminated groundwater entered small streams during the summer5.
Sadly, the influx of sodium and chloride into freshwater habitats degrades the ecosystem, reduces water quality, and decreases both species’ abundance and richness1. Scientists have found this increase in salt is detrimental to wetland vegetation, amphibians, plankton, insects and other macroinvertebrates and promotes a shift towards salt tolerant species 2,4.
Vernal pools are ephemeral wetland habitats, or landscapes that are wet in late fall and winter and dry during the summer months. This seasonality makes them especially susceptible to salt-polluted water from snowmelt 4. Vernal pools provide essential breeding and foraging grounds for wildlife, with several species of amphibian completely dependent on these pools for reproduction. The influx of road salts has been shown to reduce egg and larval survival in spotted salamanders and wood frogs and increase frequency of malformations in other amphibians6. These effects are greatest in vernal pools within 50 meters of the roadway, with evidence that on-going annual salt influx may cause local populations to even go extinct6.
Unfortunately, deicing chemicals not only affect our water, but also wash from roadways and leach from snow piles into soils3. Once salt is in the ground, it has been shown to significantly change the soil’s chemical composition and raise the pH, with elevated levels of sodium detected up to 50 meters from the roadway3. While these elevated levels may be detrimental to all biota in the soil, the impact on the growth, health, and survival of many tree species has been well documented. Sodium interferes with the trees’ ability to absorb nutrients and water from the soil which leads to deficiencies in the leaves. In turn, these deficiencies weaken photosynthesis (the process plants use to create energy) and can cause premature aging3.
These deicing chemicals also become airborne as heavy traffic or winds spray the salts on surrounding vegetation. This can lead to reduced cold hardiness of leaf and flower buds making them more susceptible to late-season frosts3.
The application of deicing chemicals to our roadways is necessary for safe travel in the wintertime; however, thoughtful and judicious application, along with consideration of alternate methods for deicing, can have a significant impact on our water quality and surrounding ecosystems.
For best practices and alternative deicing options, please visit Penn State Extension: https://extension.psu.edu/watershed-friendly-deicing
1. Hintz W.D. & R.A. Relyea (2017) Impacts of road deicing salts on the early-life growth and development of a stream salmonid: Salt type matters. Environmental Pollution 223:409-415
2. Robinson H.K., Hasenmueller EA., and L.G. Chambers (2017) Soil as a reservoir for road salt retention leading to its gradual release to groundwater. Applied Geochemistry 83:72-85
3. Equiza M.A., Calvo-Polanco M., Cirelli D., Senorans J., Wartenbe M., Saunders C., and J.J. Zwiazek (2017) Long-term impact of road salt (NaCL) on soil and urban trees in Edmonton, Canada. Urban Forestry & Urban Greening 21:16-28.
4. Vigil J.P. and M.S. Schuler (2024) Salt pollution reduces turbidity, dissolved organic matter, and cyanobacteria in experimental vernal pool communities. Science of the Total Environment 931:172948
5. Cruz, Y. D., Rossi, M. L., & Goldsmith, S. T. (2022). Impacts of road deicing application on sodium and chloride concentrations in Philadelphia region drinking water. GeoHealth, 6, e2021GH000538. https://doi. org/10.1029/2021GH000538
6. Karraker N.E. and J.P. Gibbs (2011) Road deicing salt irreversibly disrupts osmoregulation of salamander egg clutches. Environmental Pollution 159(3):833-835
