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Rock salt everlasting

winter thaw

Why is it salt turning up in streams in the summertime? Rock Salt lowers the temperature at which water freezes, so it's widely used in wintertime to prevent our roads from icing up. It's a boon for safety but there are environmental consequences.

Kelly: Rock salt was first used in the United States in New Hampshire, which began using it on an experimental basis in 1938. By the winter of 1941 to 42, a total of 5,000 tons of salt was spread on highways nationwide. And now, between 10 and 20 million tons of salt are used today in the United States.

Vicky Kelly is manager of the environmental monitoring program at the Cary Institute of Ecosystems Studies.

Kelly: We've been monitoring salt concentrations in streams since 1986, and one of the things that we've discovered is that we see salt in streams in the summertime. We know that we use rock salt in the wintertime. Why is it turning up in streams in the summertime? 

We did a study and what we discovered is about 95% of the total amount of salt that was entering the stream from the watershed that feeds the stream is from rock salt we use in the wintertime to de-ice and keep roads free of ice and snow.

Rock salt doesn't just wash off the roads and go into the stream and then run out to the sea during that wintertime. It actually penetrates the sub-surface. It goes through the soil and down into the groundwater, and that groundwater is what feeds the stream during the summertime.

Those elevated concentrations that we see in the summertime are from rock salt that has built up in the sub-surface, in the groundwater, over many, many decades.

Photo: Gary Lerude

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