Please Pass on the Salt

As your winter boots — or even worse, your tires — slide along icy roads, environmental conservation may be the last thing on your mind. But not for Karen Snetselaar, Ph.D., professor of biology at Saint Joseph's University in Philadelphia. She explains the impact of road salt on the biological world — especially in your tap water.

As a plant biology professor at Saint Joseph’s University, I’m responsible for a small rooftop greenhouse. Several years ago in the early spring I noticed some plants were yellow, so I gave them a shot of fertilizer. To my surprise, instead of recovering their green color, most of the plants died a few days later. After considering many possibilities, I figured out that the plants didn’t need fertilizer at all: They were yellow because I was watering them with salty water.

The water for the greenhouse comes right out of the tap. Why was it so salty?

Here in Philadelphia drinking water comes from the rivers, like it does for many cities. The water treatment system removes sediments and disease-causing organisms, but it can’t change the concentration of dissolved minerals. The levels of sodium, chloride and other minerals vary from week to week, but over the year average levels are quite acceptable and the water is completely safe for drinking. However, because of all the salt used to treat roads, parking lots and sidewalks after snowstorms, sodium and chloride levels may go way up for short periods of time.

So now, a couple days after it snows, I start testing the tap water. When the chloride level goes over 200 ppm, I use distilled water or melted snow on susceptible plants, like beans, citrus and corn. I work in a science building that has special water treatment capability, so I can haul distilled water to my plants for those few days in the year when the salt levels are high. But I wonder how many plants are lost every winter to puzzled gardeners, bedding plant producers and houseplant devotees.

It might seem strange that water that is safe for people to drink could kill plants. The problem is that plants have to take up all the mineral nutrients they need in dissolved form, through their roots. If there is too much sodium or chloride, the plants have trouble getting other essential minerals, like phosphorus and potassium.

And it’s not just greenhouse plants that are damaged by too much salt. Even if they aren’t killed outright, trees, grasses and other plants adjacent to roadsides can be severely stressed by salt, making them more susceptible to disease and drought.

There is evidence that the problem is even more widespread. Nationwide, the use of salt for de-icing has increased steadily since 1940. The Pennsylvania Department of Transportation uses around 500,000 tons of salt each winter, and local municipalities use more. All this salt washes into the storm sewers and eventually into the streams and rivers. Numerous studies show that the amount of salt in many waterways is gradually increasing. Higher salt levels are being recorded — not just in winter, but in all seasons of the year. If current trends continue, the effects of excessive salt use could eventually impact crop production. Urban areas, where growing food has lately become of great interest, would seem to be especially vulnerable.

There is no one easy solution to this problem. We have become so accustomed to roads and sidewalks being cleared that it would be impossible to just stop doing it. And of course, nobody wants people to get hurt in car accidents and falls due to icy surfaces. Sand or cinders can sometimes be used instead of salt. In parts of the world that get lots of snow, people might not see the actual road surface for months: After every snow, more gritty material is applied and it may even be swept up in spring and reused. There are other ways to provide traction: Car tires can be outfitted with chains or studs, and cleats or other traction devices on shoes work well to prevent slips and falls. In a few places, busy roads, bridges and sidewalks are heated to melt the ice. But it isn’t likely that these solutions will catch on in regions that only have periodic snowstorms.

At least we could be more sparing in our use of salt. Maybe putting down salt on roadways before a large snow could be avoided — it will just be plowed to the side anyway. Applying salt after first plowing could reduce the amount used. I have watched a maintenance worker use a leaf blower to move excess salt from the sidewalk to the lawn after somebody went overboard in pre-treating the walkway, and people were actually slipping on the salt.

Perhaps if we think about salt as the toxic chemical that it is we’d be more likely to limit its use as much as possible, like we do with petroleum products and other toxins. Or, consider where road-salt comes from: Much of it is mined from deposits laid down when the continent was a large shallow sea. By digging up these deposits and adding them into our freshwater lakes and streams, we are ever so slowly converting our productive forests, fields and rivers back into salt flats, where only a few tolerant plants can thrive, and very few of them provide food for human consumption.