This post was edited on 2/17/2017 thanks to some clarification and feedback from MassDOT.
As winter progresses, wet roads will give way to icy roads, and these roads will be treated with de-icing agents. I don’t think many would argue against de-icing roads and parking lots as a necessary precaution to keep drivers safe. However, there are negative environmental impacts to certain de-icing methods that should be addressed and, when possible, mitigated.
When thinking about the methods and substances used to prevent ice formation and increase traction, it is necessary to consider the different types of roadways being treated. For example, major highways are not, and should not, be treated in the same way that low-traffic residential roads and sidewalks are treated.
The most widely used de-icing agent in the United States is sodium chloride (NaCl), commonly known as road salt. Sand is also a commonly used substance.
Salt has the property of lowering the freezing point of water, making it more difficult for ice to form on roads. While this is good news for drivers, runoff generated when it rains or from snow melt becomes concentrated with salt. This runoff is discharged directly into natural waterways through storm drain systems, but also makes its way into soil, groundwater, and onto vegetation.
Water that contains saline runoff becomes denser than water without the increased salt. The result is that the denser water will settle into the lowest, slowest parts of waterways, such as lakes and ponds. This prevents mixing of the upper and lower layers of water. The bottoms of these waterbodies can become depleted of dissolved oxygen and can lead to mass fish kills and the death of other aquatic life. This has been recorded in other parts of the country, however, no recorded instances of this have occurred in Massachusetts.
The chloride ions in road salt are also toxic to aquatic life, as there are no natural processes through which chloride can be “broken down” or “taken up”. The only way chloride is removed is through the process of dilution. Additional negative impacts of road salt include increased sodium in drinking water supply, dehydration of vegetation, alteration of soil chemistry, damage to infrastructure, and health concerns for pets and wildlife that are inclined to lick anything salty.
Road salt usage has greatly increased in the United States since the 1940s, while urban development also continues to increase each year, expanding the amount of impervious surface available to create runoff. In 2014, 19 million tons of salt was used in the US for de-icing purposes, which accounted for 43% of all salt sales nationally. It is a bit harrowing to wonder how much of that salt ended up in our surface and groundwater.
There are a number of alternative de-icers such as calcium chloride, magnesium chloride or (our favorite) calcium magnesium acetate that are appropriate in residential or commercial settings and that are somewhat more environmentally benign. However, on high-speed and high-traffic volume roadways, salt is still by far the most cost-effective, efficient, and widely used de-icer.
To mitigate the environmental impacts of de-icers, especially road salt, the MassDOT Environmental Services Planning Report refers to a number of techniques, including “surface pre-wetting, closed-loop controllers, friction meters, weather data, segmented plow blades, and training”. These techniques not only serve to improve water quality, but to help keep drivers safe. All of these techniques focus on reducing the amount of salt that is applied, so that its just enough and just at the right time, and no more.
One alternative de-icing agent to salt is sand, although calling sand a “de-icer” is misleading since it does nothing to melt snow but does provide better traction on low-speed and low-traffic roadways. Sand is not a good alternative to salt on highways because the speed of cars actually grinds down the sand, creating ineffective and potentially dangerous driving conditions. In addition, adding sand to roads can also be damaging to water quality. Sand can clog up stormwater infrastructure (such as catch basins) or be swept into waterways, increasing turbidity and sedimentation.
In some cases, eclectic de-icing agents have been used to treat surfaces, including beet juice and cheese brine. Still, these substances come at a cost. For example, beet juice increases the amount of nutrients and organic matter in water, which can result in dissolved oxygen crashes, unless there are facilities to capture and remove the nutrients from the runoff once everything melts.
One thing is clear: the appropriate de-icing method for each road or parking lot depends on a number of factors. Since each method has the potential to cause a different type of damaging effect on water quality, using a combination of de-icing agents isn’t necessarily a good option either. Some roadways are equipped with best management practices (BMPs) which are able to remove the sand and nutrients from runoff, however, they are not able to remove salt from runoff.
The most important thing to remember about de-icing agents – do not apply more than is necessary in order to mitigate negative effects on water quality. MassDOT and municipalities use various methods to work towards this, but it is certainly something that many residential and commercial property owners could improve upon, especially utilizing proper storage methods.
The effect of road salt has not been thoroughly investigated in the Neponset Watershed. However, it is something that we would like to look into next winter, using both conductivity and dissolved oxygen loggers.
Meghan Rauber, Field Sampling Coordinator