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==Environmental & Health Impacts==
==Overview==
Between 5 and 7 million tonnes of salt is applied every year in Canada for winter maintenance of roads and other paved surfaces, making it one of the most ubiquitous contaminants in urban environments. NaCl- is the most common de-icer applied for winter maintenance, comprised of 40% sodium and 60% chloride. Sodium chloride rock salt is often treated with liquid MgCl2 and CaCl2 to reduce the effective temperature range of salts. Liquid brines comprised of NaCl-, MgCl2 and CaCl2 or a combination of these products are increasingly being used on roads for anti-icing.
[[File:(LSRCA Logo) Atherley Narrows annual chloride concentrations (1971 - 2020).png|thumb|700px|A graph showing increasing average levels of chloride found in Atherley Narrows, (located between Lake Couchiching and Lake Simcoe), over the past few decades, due in part to increased use of rock salt in parking lots, roadways and commercial and residential properties. From 2005 - 2020 the amount of chloride increase per year has doubled when compared to 1971 - 1986 (1.26 mg/L per yr. vs. 0.63 mg/L per yr.) (LSRCA, 2021). It is estimated that by 2120 the average level of chloride within the the Lake Simcoe watershed will exceed the 120mg/L guideline set by CWQG. (LSRCA, 2018)<ref>LSRCA. 2018. Parking Lot Design Guidelines: Municipal Policy Templates to Promote Salt Reduction in Parking Lots. https://www.lsrca.on.ca/Shared%20Documents/Parking-Lot-Design-Guidelines/Parking%20Lot%20Design%20Guidelines.pdf.</ref>]]
[[File:(LSRCA Logo) Atherley Narrows annual chloride concentrations (1971 - 2020).png|thumb|700px|A graph showing increasing average levels of chloride found in Atherley Narrows, (located between Lake Couchiching and Lake Simcoe), over the past few decades, due in part to increased use of rock salt in parking lots, roadways and commercial and residential properties. From 2005 - 2020 the amount of chloride increase per year has doubled when compared to 1971 - 1986 (1.26 mg/L per yr. vs. 0.63 mg/L per yr.) (LSRCA, 2021). It is estimated that by 2120 the average level of chloride within the the Lake Simcoe watershed will exceed the 120mg/L guideline set by CWQG. (LSRCA, 2018)<ref>LSRCA. 2018. Parking Lot Design Guidelines: Municipal Policy Templates to Promote Salt Reduction in Parking Lots. https://www.lsrca.on.ca/Shared%20Documents/Parking-Lot-Design-Guidelines/Parking%20Lot%20Design%20Guidelines.pdf.</ref>]]
==Impacts on the Environment, Human Health and Built Infrastructure==
While salt is needed to keep roads safe in the winter, it is highly corrosive and toxic to freshwater wildlife at relatively low concentrations. Some of the impacts of salt on infrastructure, human health and the environment include the following:
===Freshwater wildlife===
Just as we depend on air with the right makeup of oxygen, freshwater species – like fish, frogs, mussels, salamanders and zooplankton – need water with the right balance of chloride to survive. Having adapted to low levels of chloride in their habitats, increased levels begin to disrupt their basic functions – such as regulating their water content (osmoregulation) and breathing. Studies have shown widespread effects of salt on ecosystems at all trophic levels from biofilms to fish species. Specific effects vary based on exposure concentrations, and may include reductions in fecundity, size, shape, growth and abundance (Hintz and Relyea, 2019)<ref name="example1">Hintz, W.D. and Relyea, R.A. 2019. A review of the species, community, and ecosystem impacts of road salt salinisation in fresh waters. Freshwater biology, 64(6), pp.1081-1097. https://www.researchgate.net/publication/331991752_A_review_of_the_species_community_and_ecosystem_impacts_of_road_salt_salinisation_in_fresh_waters</ref>
===Vegetation===
Salt affects vegetation in various ways. Salt in soil water generally makes it more difficult for roots to take up water. This phenomenon mimics drought conditions for the plant and underlies the recommendation for salt tolerant plants in LID practices. If passing traffic sprays salty water onto [[plants]] it can reduce cold hardiness in buds and new twigs. These then become more susceptible to freezing, mortality or deformation. In high enough concentrations, sodium and chloride can also be directly toxic to plants. In some species the ions are absorbed by the plant and build up in the leaves causing them to die.
===Soils===
Dissolved sodium ions may replace calcium and magnesium ions in soil minerals, with negative effects on soil structure, hydraulic properties and fertility. Salt can also cause trace metals to be leached from the soil and into groundwater (e.g. Norrström and Bergstedt, 2001<ref>Norrström, A.C., Bergstedt, E. 2001. The impact of sodium from de-icing salts on colloid dispersion and base cation pools in roadside soils. Water Air and Soil Pollution. Vol. 127. pp. 281-299. https://www.researchgate.net/publication/226123793_The_Impact_of_Road_De-Icing_Salts_NaCl_on_Colloid_Dispersion_and_Base_Cation_Pools_in_Roadside_Soils</ref>; Norrström, 2005<ref>Norrström, A.C. 2005. Metal mobility by de-icing salt from an infiltration trench for highway runoff. Applied Geochemistry. Vol. 20, pp. 1907-1919. https://www.researchgate.net/publication/248337128_Norrstrom_A_C_Metal_mobility_by_de-icing_salt_from_an_infiltration_trench_for_highway_runoff_Applied_Geochemistry_20_1907-1919</ref>). Excess salt (chloride) in soils can also result in decreased [[Flow through media|hydraulic conductivity]], which can lead to increased occurrences of surface runoff, erosion and ultimately anaerobic soils, where [[plants]] would have a hard time establishing themselves (Shannon et al. 2020<ref>Shannon, T.P., Ahler, S.J., Mathers, A., Ziter, C.D. and Dugan, H.A., 2020. Road salt impact on soil electrical conductivity across an urban landscape. Journal of Urban Ecology, 6(1), p.juaa006. https://academic.oup.com/jue/article/6/1/juaa006/5741239?login=true</ref>).
===Human Health===
Salt has an unpleasant taste in drinking water at chloride concentrations above 250 mg/L. It also poses risks to people with hypertension or to those who have experienced congestive heart failure. Sodium from winter salts can also be dangerous for people on sodium restricted diets. These aesthetic and human health impacts of salt, combined with its high mobility in soils, has led to recommendations to limit implementation of infiltration-based LID in [https://sustainabletechnologies.ca/app/uploads/2021/10/2016-2020-SWQ-Report-v11_FINAL_AODA-FA.pdf source water protection areas]. This is of particular relevance for communities that utilize groundwater as their primary source of drinking water.
===Terrestrial Wildlife===
Ingestion of road salts has been associated with mammalian and avian behavioral and toxicological effects. Indirect effects of plant cover reductions or shifts in plant communicates can also negatively impact wildlife habitat (Fay and Shi, 2012)<ref>Fay., L. and Shi., X. (2012). Environmental Impacts of Chemicals for Snow and Ice Control: State of the Knowledge. Water Air Soil Pollut 223:2751–2770. http://www.dot.state.mn.us/research/RFP/Lit/LS526a.pdf</ref>. Deer, moose and other mammals attracted by salt on highways can increase the risk of vehicle collisions. Furthermore, various bird species can be be sensitive to even small amount of rock salt ingestion. These birds may be attracted to roads dude to salt deficient diets and foraging for "grit" (which aids in the birds digestion by grinding up hard to breakdown items). A 2005 study conducted by Bollinger, et al.<ref>Bollinger, T.K., Mineau, P. and Wickstrom, M.L., 2005. Toxicity of sodium chloride to house sparrows (Passer domesticus). Journal of wildlife diseases, 41(2), pp.363-370. https://bioone.org/journals/Journal-of-Wildlife-Diseases/volume-41/issue-2/0090-3558-41.2.363/TOXICITY-OF-SODIUM-CHLORIDE-TO-HOUSE-SPARROWS-PASSER-DOMESTICUS/10.7589/0090-3558-41.2.363.pdf</ref> discovered that salt grain amounts similar in size foraged as grit by birds could contain lethal amounts of sodium for these birds. Ingestion of even a small number of road salt granules by some bird species (songbirds, etc.) could be a lethal dose as a result of sodium (Na) toxicity.
===Built Infrastructure===
Chloride based deicers are highly corrosive and can cause significant damage to vehicles, steel bridges, railings and other metal infrastructure. They can also cause damage to concrete structures by accelerating corrosion of re-bar within the concrete, which poses a problem for bridges, parking lots above underground garages or other contexts where the concrete contains metals. The salts may also react with the cement paste causing deterioration of the cement matrix in poured and pre-cast concrete pavers. Magnesium chloride (MgCl2) is particularly reactive in this regard, resulting in a decrease in concrete compressive strength (e.g. Shi 2009<ref>Shi, X., M. Akin, T. Pan, L. Fay, Y. Liu and Z. Yang (2009). Deicer Impacts on Pavement Materials: Introduction and Recent Developments. Open Civil Engineering Journal 3. https://opencivilengineeringjournal.com/VOLUME/3/PAGE/16/FULLTEXT/</ref>, Shi et al, 2009)<ref> Shi, X., L. Fay, Z. X. Yang, T. A. Nguyen and Y. J. Liu (2009). Corrosion of Deicers to Metals in Transportation Infrastructure: Introduction and Recent Developments. Corrosion Reviews 27(1-2): 23-52. https://westerntransportationinstitute.org/wp-content/uploads/2016/08/4W1095_Intro_Developments.pdf</ref>.
Salt contamination in freshwater (freshwater salinization) is a major concern to Ontarians wellbeing as it can lead to:
Salt contamination in freshwater (freshwater salinization) is a major concern to Ontarians wellbeing as it can lead to:
Furthermore, salt can contribute to both biodiversity and habitat loss for numerous species. In Ontario, road salt was identified as one of the threats to drinking water under the Clean Water Act, 2006 - as well as a known toxin to wildlife species<ref>Government of Ontario. 2006. Clean Water Act, 2006, S.O. 2006, c. 22. https://www.ontario.ca/laws/statute/06c22.</ref>. Salt can impacts bird species, many plants and trees growth ability, and decrease size, function and fecundify in fish, mollusks (snail, mussels, etc.), amphibians and benthic invertebrate species.
Furthermore, salt can contribute to both biodiversity and habitat loss for numerous species. In Ontario, road salt was identified as one of the threats to drinking water under the Clean Water Act, 2006 - as well as a known toxin to wildlife species<ref>Government of Ontario. 2006. Clean Water Act, 2006, S.O. 2006, c. 22. https://www.ontario.ca/laws/statute/06c22.</ref>. Salt can impacts bird species, many plants and trees growth ability, and decrease size, function and fecundify in fish, mollusks (snail, mussels, etc.), amphibians and benthic invertebrate species.
A recent literature review by [https://www.researchgate.net/publication/331991752_A_review_of_the_species_community_and_ecosystem_impacts_of_road_salt_salinisation_in_fresh_waters Hintz and Relyea (2019)]<ref>Hintz, W.D. and Relyea, R.A. 2019. A review of the species, community, and ecosystem impacts of road salt salinisation in fresh waters. Freshwater biology, 64(6), pp.1081-1097. https://www.researchgate.net/publication/331991752_A_review_of_the_species_community_and_ecosystem_impacts_of_road_salt_salinisation_in_fresh_waters</ref>,
A recent literature review by [https://www.researchgate.net/publication/331991752_A_review_of_the_species_community_and_ecosystem_impacts_of_road_salt_salinisation_in_fresh_waters Hintz and Relyea (2019)])<ref name="example1" />.
discusses the impacts of road salt on local ecosystems and found that road salts negatively affect species at all trophic levels, from biofilms to fish species, but the concentration of road salt where adverse effects were observed varied and the effects themselves ranged from:
discusses the impacts of road salt on local ecosystems and found that road salts negatively affect species at all trophic levels, from biofilms to fish species, but the concentration of road salt where adverse effects were observed varied and the effects themselves ranged from:
*Reductions in fecundity, size and shape of various species
*Reductions in fecundity, size and shape of various species