Winter Road Salt Impacts Aquatic Ecosystems

Photos courtesy of the Illinois RiverWatch Network.

As winter approaches, some of us look forward to the first snowfall of the season. Waking up to see a blanket of clean, white powder across the landscape can be a refreshing and renewing experience. Others of us might dread the first snow as we anticipate delayed or cancelled school days and traffic jams as drivers re-learn how to operate their vehicles on slushy pavement.

To help us with our winter travels, transportation agencies spread salt on roads and highways to melt snow, sleet, and ice that would otherwise accumulate. Residents and businesses add salt to sidewalks and parking lots. Most of this salt is large chunks of sodium chloride (NaCl), the same salt we use to season our food. Every year, more than 20 million tons of salt is spread on roads across the United States (source USGS).

While our use of salt is effective at improving conditions for transportation, the introduction of salt is problematic for infrastructure, water supplies, and wildlife. Road salt is corrosive to concrete and metal, causing an estimated $5 billion in damage nationwide to roads, bridges and automobiles each year (source USEPA). Much of the salt is carried into groundwater and surface waters which may be used as a drinking water source. If the salt makes it into a drinking water supply, it can cause taste issues and potentially pose health risks for individuals who are on a salt-limited diet. Municipalities attempting to use chloride-contaminated water as a drinking water source may be forced to use expensive desalinization processes to meet EPA drinking water standards.

Most of the road salt that dissolves in meltwater is carried into a stormwater system which quickly sends the contaminated water into a nearby stream. This can cause spikes in the concentration of chloride in the stream which can harm the fish and invertebrates that live there.

Freshwater animals have evolved to live in an ecosystem that is very low in salts. Chloride concentrations are a commonly used measurement of the salinity, or saltiness, of water. Most pristine freshwater streams would likely have less than 30 milligrams of chloride per liter (mg/l), while ocean water has around 20,000 mg/l. The State of Illinois Water Quality Standards consider chloride concentrations higher than 500 mg/l detrimental to aquatic life.

Aquatic invertebrate animals like damselflies, snails, and mussels are sensitive to chloride. In the same way that drinking ocean water has detrimental effects on people, living in salty water will harm freshwater invertebrates. Some of the more sensitive invertebrate species may die in moderately salty water; however, even species that survive will likely face consequences, including a decrease in their growth rate or reduced reproductive capacity. Because invertebrates are a major food source for many fish, frogs, and birds, these changes in the size and number of invertebrates can have drastic impacts on the composition and function of a stream ecosystem.

A recent collaborative study involving citizen scientists in the St. Louis region found that chloride concentrations in urban and suburban areas are often higher than allowed by water quality standards. The Illinois RiverWatch Network is currently seeking volunteers to start measuring chloride levels during the winter in streams across Illinois. To learn more about this project, visit: www.ngrrec.org/riverwatch/chloride-monitoring or contact riverwatch@lc.edu.

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Danelle Haake has been the Illinois RiverWatch Director and Stream Ecologist for the National Great Rivers Research & Education Center since 2020. She began her career in biology as a citizen scientist and has continued working with citizen scientists throughout her career, including during her tenure as a Restoration Ecologist with Missouri Botanical Garden and her doctoral studies at St. Louis University.