Salt: The urban watershed's double-edged sword - Metropolitan Planning Council

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Salt: The urban watershed's double-edged sword

Image courtesy chuddlesworth Flickr CC

First, the white-out conditions. Then, the road salt, both inconvenient and environmentally taxing.

Just about everyone I talk to is ready for spring. It’s February in the Midwest and the sentiment is that we’re ready to see snow melt and enjoy longer days. But many of us don’t realize the significant environmental damage that will accompany our transition from winter to spring. It has to do with road salt and its impacts on our water.

Before explaining why road salt (mainly sodium chloride—it’s the chloride we're interested in here) is so harmful to ecosystems or even how it reaches our water resources, I believe the game-changing issue is this: Salt readily dissolves in water, but by doing so, breaks down into sodium and chloride ions. This releases chloride into water bodies, which then persists in the environment!

As salt-laden snow begins to melt or the snowfall turns to rain, chloride is finding its way into ground and surface waters, such as rivers, streams and reservoirs—and staying there.

“Chloride is one of our biggest urban pollution problems today damaging our built and natural environments,” says Stephen McCracken, director of watershed protection for the Conservation Foundation. McCracken is blazing the path on combatting chloride with the DuPage River Salt Creek Workgroup.

Unlike many other pollutants, chloride is not easily broken down, metabolized, taken up or removed from the environment. Even in rain events, chloride concentrations prevail. While there are actually different sources of chloride pollution, most water bodies in the Chicago region are experiencing elevated levels of chloride (on the rise since the 1960s) primarily due to road salt runoff, according to a 2012 report issued by the Illinois State Water Survey. 

So here we are in winter, scattering road salt on highways and sidewalks as a public safety measure. As salt-laden snow begins to melt or the snowfall turns to rain, chloride is finding its way into ground and surface waters, such as rivers, streams and reservoirs—and staying there. The end result is impeded water quality which threatens our ecosystem services as well as our drinking water supply.

Salt is the urban watershed’s double-edged sword!

As you may guess, ecosystems do not function as well as they should if too much chloride is present. Higher than normal levels of salinity in otherwise freshwater bodies create a toxic environment for fish and aquatic life, wildlife and surrounding plants. According to the U.S. Environmental Protection Agency, chloride at 230 milligrams per liter of water negatively affects aquatic organisms while 860 milligrams per liter of water kills many of them quickly. The amounts are surprisingly small; the latter is the equivalent of one tablespoon of salt dissolved in five gallons of water.

Even natural vegetation and green infrastructure like rain gardens along roadsides are in harm’s way, which I’ll explore more in the next post of this series. A report by the Cary Institute in New York shows that even relatively moderate levels of salt in water bodies can result in decreased reproduction in amphibians, plant browning and lower nutrient availability for plants and animals.

The take-away is that the increase in chloride concentrations in our water resources can be linked to our choices on the surface that are causing a tipping point in environmental damage. Because of this extensive and persistent harm to the environment, McCracken suggests that “Chloride is going to be one of the top urban water pollutants of concern over the next 50 years. It has an enormous environmental footprint, but it is also inherently very useful. We need to be clear about its costs and benefits and manage it accordingly.”

Ecosystems do not function as well as they should if too much chloride is present.

Looking into the environmental cost of chloride, the Minnesota Pollution Control Agency, as part of its Chloride Management Plan project, found that one estimate modeled by Fortin Consulting equated chloride’s environmental impact on surface water and forests to adding $60 million to the price tag of an annual batch of 350,000 tons of salt. As a reminder, the Chicago area applies more than 350,000 tons of salt every year on average.

Within our water supply management work, the Metropolitan Planning Council is collaborating with the City of Aurora and others in the Northwest Water Planning Alliance, on critical water resource management. If your well runs dry, isn’t big enough or just doesn’t exist, there is no cost-effective treatment technology to physically remove the chloride from water resources. 

It seems that protecting our environment and palatable drinking water means putting less chloride in our waters in the first place. In 2015, the Illinois Pollution Control Board arrived at a similar conclusion and indicated what that chloride limit should be for the Chicago Area Waterway System for the first time.

The new chloride water quality standard took effect in 2018, setting a limit of 500 milligrams per liter of water during winter months. MPC is committed to the water quality of Chicago’s rivers in part through Great Rivers Chicago, a comprehensive vision for our region's waterways.

The Metropolitan Water Reclamation District of Greater Chicago knows a thing or two about water quality. The Environmental Protection Agency asked the reclamation district to lead the coordination required between the various municipalities, public works departments, water experts and treatment processes to work toward meeting the new water quality standard for chloride. Tony Quintanilla, assistant director in maintenance and operations at the district working with the Chicago Area Waterways Chloride Initiative Work Group, sums up what this new standard means:

“Chloride in our waters boils down to improving environmental degradation to enhance the macro invertebrates that are fundamental to the food chain. Treatment of chlorides already in the water is not a viable solution; we are focusing our efforts on source reduction in order to meet this new standard.” 

Early signs of spring are in the forecast. Luckily, there are also signs of change in our attitude toward road salt. More efficient application can result in less polluted water, healthier ecosystems, protected drinking water in addition to more efficient government. Let’s keep shaking up our understanding of salt’s impacts in our region beyond the snowy days in February and March when it is laid down to combat ice.

version of this article first ran in 2016. 

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