The application of road salt to highways, sidewalks, and parking lots has resulted in improved winter driving and walking conditions. But salt is easily dissolved. It leaches into groundwater or runs off into ponds and streams, where it can cause serious harm to freshwater plants, fish, amphibians, and invertebrates. In New Hampshire, researchers recently found that more than 10 percent of streams in the Merrimack River watershed have been negatively affected by high salt concentrations as a result of the salt applied to pavement in winter.
“Too much salt can harm a variety of freshwater organisms, and many of the plants and animals that form the base of the food web that support[s] fish are affected,” said Shan Zuidema, a research scientist at the University of New Hampshire’s Earth Systems Research Center. “We need as much good information as we can [get] to make informed decisions about how to trade off winter safety with preservation of our riverine habitats.”
To evaluate the effect of road salt on water quality in local streams, Zuidema led a team of scientists in creating a computer simulation based on an existing hydrological model to examine the factors that lead to variability in salt concentrations over time.
“There’s a very significant time lag associated with when we put down salt and when it gets into our waterways,” Zuidema said. He found the highest salinity in the summer because most salt leaches into the groundwater and isn’t released into rivers and streams for several months. In winter, when some salt runs off directly into waterways, there is usually enough pristine snow melting that it has a diluting effect on the salt.
The researchers’ findings indicate that winter and summer weather have a significant effect on how much salt enters local waterways, resulting in variations in salt content from year to year and season to season.
Unsurprisingly, salt concentrations are highest during years when winter precipitation is high because more road salt is applied. But Zuidema said that summer salt concentrations are driven by dry conditions, at least in the smaller rivers.
In years with dry summers, the only source of water in some streams is groundwater, which often has accumulated large quantities of road salt.
“We were quite surprised to see how different the effect of high-snow years was on large versus small rivers,” Zuidema said. “Our results suggest that if high-snow years are associated with wetter summers, large rivers have better water quality because cleaner parts of the watershed, like the White Mountains, dilute the poor water quality coming from more polluted streams. However, we didn’t anticipate that that effect would overwhelm the additional loading from road salt.”
The researchers plan to model the Connecticut River next and perhaps expand to other watersheds that have similar threats from road salt. Zuidema is especially interested in factoring in how the changing climate and changes in land-use patterns could affect road salt use and its impact on local rivers and streams.