Road Salts in Your (Ground) Water

As the latest of what seems like an endless conveyor belt of winter storms hits the middle and eastern portions of the US, it’s worth considering not only the record amount of snow that has fallen in many places, but the vast amounts of road salt that have been used this winter.  One estimate of last winter’s road salt use was 17 million tons*, and with more snowfall this winter than last we have likely surpassed that.

For much of the US, a common sight this past winter. (Source)

For much of the US, a common sight this past winter. (Source)

*In fact, based on the estimate of 17 million tons of road salt, at about 12–15 tons per truck and about 25 feet per truck length, the total length of trucks deployed last winter was over 30 million feet.  That’s a line of trucks, bumper to bumper, from New York City to Los Angeles and back!

When it comes to road salt, there are four main types.  Sodium chloride (rock salt) is the most common  and is basically a larger form of table salt—with a few chemical extras.  Potassium chloride is less common than sodium chloride, but works about the same way and in the same temperatures.  Calcium chloride is the most pricey, but the one that works well into subzero temperatures.  And finally, magnesium chloride is the brine often sprayed on roads prior to a winter storm and also works to melt ice below about 15F where sodium and potassium chloride are less effective.

But regardless of the type of salt used, the vast majority of them end up somewhere in the watershed (often in streams or in groundwater).  And while a little salt in the river might not be that big of an issue, these salts dissolve into their constituent ions and have been documented to dramatically increase the sodium and chloride ion load in most places where they were measured.

Most road salts eventually end up as runoff into local waterways. (Source)

Most road salts eventually end up as runoff into local waterways. (Source)

In fact, increased road salt usage likely drives part of the reason that sodium and chloride levels in surface waters have increased over the decades.  But how might these levels continue to increase both in recent years—when salt usage has not increased—and during the summer months?  The explanation for this is the salt load in groundwater has continued to build up over time, and as groundwater feeds many streams throughout the year, these streams get a salinity bump long after the plows are put away.

But this is The Fisheries Blog, so we need to bring this back to how road salt impacts fish and aquatic life.  Unfortunately, the answer is we don’t know.  At the levels we’ve seen over the past few decades, it’s likely that outside of short-term, high-runoff events, increased sodium and chloride levels don’t harm fish populations in a measurable way (or, at least not in a way that we measure).  Often invertebrates and amphibians are more sensitive to changes in salinity, and this is an area of active research.

Freshwater and saltwater fish have very different osmotic demands. (Source)

Freshwater and saltwater fish have very different osmotic demands. (Source)

What can be said is that all fish osmoregulate—that is, they balance the ionic needs (e.g., salt) of their body with their environment.  For example, saltwater fish drink a lot of water and pee concentrated urine, while freshwater fish drink much less water, and pee less concentrated urine. And while there are many exceptions, fish species are designed to operate best within a certain salinity range. Moving outside of the range first results in compromised health, but can easily result in death.  This is why so many freshwater fish cannot enter the ocean!

Overall, the use of road salts is great thing.  Rock salt alone represents an inexpensive and effective method of making our roads and sidewalks much safer, resulting in countless saved lives.  So let’s not abandon road salts anytime soon!  But it’s important to remember that whatever we put on the ground will end up in our water—even if it gets stored in our groundwater for months or years and reemerges much later!

Steve Midway

For more on road salts in the environment, check out:

Long-Term Sodium Chloride Retention in a Rural Watershed: Legacy Effects of Road Salt on Streamwater Concentration

Short radio piece 

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