Road Salt and Aquatic Environments: The Detrimental Effects of the North’s Favorite Deicer

With a colder-than-average January this year, we northerners saw our fair share of road salt to keep us safe while in transit. If you didn’t see the thick mounds of salt pebbles lining the asphalt, then you certainly noticed the cakey, white powder it left on your car or the paleness of the salt-laden cement. Now, as daytime temperatures are beginning to quickly rise with the coming of an unusually early spring, salt is still being sprinkled on highways and sidewalks alike to keep these paths free from overnight ice. While snow, safe ice, and consistent cold temperatures are becoming more of a question with each passing winter, one thing remains a constant here in the North: salt.

Early Road Salt Use

The first use of road salt in the United States was in New Hampshire around 1940. Prior to 1940, drivers used snow tires and chains to gain more traction while driving. Additional safety measures included sand, and of course plowing. Back then, people didn’t expect roads to be perfectly clear; as long as roads were passable, they could use their other traction devices to surmount any lingering ice and snow.

When other states began to catch on to the decreased crash rates in New Hampshire, they too began making use of this “new” technology. They learned that by using salt, they could lower the freezing point of water on top of frozen surfaces, raising its temperature and melting the surrounding ice and snow. The salt being used in the 1940’s, though, was hardly revolutionary; it was nothing more than granular sodium chloride, the unpurified version of the stuff found on your kitchen table at home. Still, road salt was gaining nationwide traction (no pun intended), and even began to be used by the U.S. highway system as they expanded in the 1950’s.

Road Salt Today

During the winter of 1941-1942, just 5,000 tons of salt were used across the United States. That number skyrocketed through the late 20th century, helped greatly by studies done in 1972, 1976, and 1993 that proved the effectiveness of deicing salts on road safety. Today, that meager 5,000 tons is up to an astounding 20 million tons annually, with little sign of slowing down.

Though sodium chloride still makes up the bulk of that figure, magnesium chloride and calcium chloride have also started to gain popularity.

Damage to the Environment: General Overview

In the 1970’s, Americans finally opened their eyes to the possibility that road salt was causing harm to their land and water. Unfortunately, by that time US cities and towns were already dependent on salt, and there was no feasible way to end its use. Even today, after years of studies on the harmful effects of deicing salt, transportation departments across the nation continue to dump the stuff on roads.

37% of the drainage area in the Lower 48 has experienced increasing salinity over the past 50 years. One study found that 84% of urban streams in the sample area had rising chloride levels (the major contaminant from salt), and 29% exceeded federal drinking safety guidelines for at least part of the year. What we have found is streams in areas with little to no snowfall don’t have the same issues with high chloride. Additionally, urban streams surrounded by more roadways than their rural counterparts have far higher chloride concentration than those in the sticks. All of this data suggests that road salt, not other contaminants, is causing chloride pollution and water salinization across the North.

Once salt has been spread, it typically has two paths it follows: infiltration into groundwater or direct contamination of waterways. In the former, even though the water is stored for greater periods of time before it enters our rivers and lakes, it slowly leeches into them throughout the year, leading to high salinity and chloride concentrations even during the summer.

Effects on Riparian Zones

Once salt pollutes groundwater, it is sucked up by plants through their roots for use in their life processes. Saltwater is a hypertonic solution, meaning its solute concentration (the dissolved salt) is greater than that found in the cells of these plants. If you remember back to high school biology class, cells want to remain in constant equilibrium. To balance out the solute concentrations between the saltwater and the cell’s interior, the cell will release water, causing it to shrivel, and oftentimes die.

This is bad news for streamside riparian zones, which are often considered the filters of the natural world. As plants succumb to the hypertonic saltwater, riparian zones decrease in size, therefore decreasing their capacity to filter water that enters lakes and streams. The amount of salt that reaches our waterways increases each year as riparian zones diminish.

Effects on Plankton

Not only is vegetation on land negatively impacted by road salt use, but so are aquatic organisms once salt runoff reaches the water. 40% of urban streams in the US have chloride levels that exceed safe guidelines for aquatic life, from tiny zooplankton to large predatory fish. When salt enters freshwater lakes or streams, it creates a dense layer at the bottom that depletes oxygen, detrimental to numerous species of zooplankton, insects, amphibians, and fish.

The effect on aquatic organisms starts at the bottom and works its way up the food chain. Increased sodium chloride concentration has been proven to decrease zooplankton abundance. Trophic cascades frequently follow the loss of these microscopic organisms, including an increase in the abundance of phytoplankton like blue-green algae, the same algae that keeps you from swimming in local ponds during the summer.

Effects on Insects

Moving up a trophic level, road salt has also been found to have negative impacts on macroinvertebrates, something fly fishers have certainly noticed. A study conducted in the Schuylkill River basin found a correlation between increased chloride concentration and decreased macroinvertebrate mass, including mayflies, caddisflies, and stoneflies, favorite bugs of fly anglers. In another study, it was found that certain caddis species particularly susceptible to increased salinity had 50% mortality rates over just three days of increased salt concentrations.

While all these statistics may be alarming, they are difficult for us to truly picture. Many fly fishers in the western United States understand the effects of road salt on insect populations on a much more personal level, however. The Logan River in northern Utah was once home to some of the most legendary salmonfly hatches in the entire country. The salmonfly, a species of stonefly that commonly reaches two-and-a-half inches in length, provided trout and other species in the river a year-round, protein-packed food source. Today, salmonflies have been absent in the river for half a century.

So what changed to cause the demise of a once prolific insect in the Logan? The Logan Canyon highway follows the Logan River for much of its length. Being a major roadway in an area that receives a large amount of snowfall during the winter, the highway was at the forefront of the road salt craze that swept the country, and continues to be salted to this day. While it hasn’t been proven, road salt is thought to be the leading factor for the loss of the salmonfly through the mid-20th century, and why restoration efforts today continually fail.

Effects on Fish

With such a profound impact on the bottom of the food chain, it only makes sense that salt contamination negatively affects larger aquatic organisms, like fish. One study looked at the effects of sodium chloride, magnesium chloride, and calcium chloride on the growth and development of rainbow trout fry. The results were alarming. While magnesium chloride had no effect, sodium chloride, the most prolifically used of the three substances, reduced length by 9% and mass by 27% at high concentrations. Furthermore, calcium chloride, often hailed as a more environmentally-friendly option to sodium chloride, reduced length by 5% and mass by 16% at moderate concentrations, and length by 11% and mass by 31% at higher concentrations. Quoting the study, ““Scaled to the population level, the reduced growth caused by NaCl [sodium chloride] and CaCl2 [calcium chloride] at critical early-life stages has the potential to negatively affect salmonid recruitment and population dynamics.”

With a number of other stressors already affecting fish populations, especially those found in urban and suburban environments, the last thing fish need is chloride contamination to stunt their growth. Whether road salt will cause the demise of certain fish populations as it has done to some insect populations remains unclear; one thing is for certain, though: it most definitely isn’t helping already struggling, environmentally crucial fish species.

Road Salt Alternatives

With the environmental consequences of road salt becoming so clear, there must be emerging alternatives, right? Well yes, but none of them are perfect.

One of the more popular substitutes, and one that my home town began implementing this year, is a salt brine mixture. Rock salt typically runs from $62 to $65 a ton. When used as a stand-alone road treatment, my town uses about 49 tons of rock salt. When mixed with water to create a salt brine, however, only 18 to 22 tons are needed, thereby reducing economic and environmental costs. Moreover, salt brine can be applied to roads up to 24 hours in advance, a significant improvement over rock salt, which can be brushed or blown off the road if applied too early.

Unfortunately, salt brine only works as a pre-treatment. Traditional rock salt must still be used once snow has fallen, or if temperatures fall below 10 degrees. Also, it shouldn’t be ignored that salt brine still contains sodium chloride, which will eventually make its way into nearby lakes and rivers.

Another up-and-coming alternative is a 99% biodegradable beet juice mixture. Made from sugar beet byproducts and a small amount of salt, beet juice deicers are sticky and do a good job of staying on roads for multiple storms. Like rock salt, they melt ice efficiently and keep our roads safe.

Unsurprisingly, there are some downsides to beet juice. First, it too contains salt, though in much smaller quantities than other alternatives. Secondly, it has been found to be harmful to some insects, including mayflies, a species already under environmental duress.

The Bottom Line

While other factors are often blamed for environmental damage, road salt is an often-overlooked, yet pertinent, element of environmental degradation. In aquatic environments, it harms riparian zones, zooplankton, macroinvertebrates, amphibians, fish, and other species that are already struggling to survive. While alternatives have emerged that seem to be less environmentally and economically costly, none of them are perfect. Although its nice to imagine a future in which we can have both, we are now forced decide what we value more: public safety or the fate of the environment.