We all know road salt is bad for plants and waterways, and that doesn’t even count what it’s doing to my car, but new research in New Hampshire based in part on citizen-science work says it’s worse than we realized, or at least more complicated.

An analysis of water samples collected all across New Hampshire shows that more road salt doesn’t merely make our surroundings saltier, it is also correlated with trace amounts of other metals moving out of soil into our streams and lakes and groundwater – including (uh-oh) arsenic and lead.

The levels that were found aren’t high enough to affect human health, but they remind us that when you’re spreading around road salt, less is definitely better.

“If we keep pouring salt on these soils, are we going to potentially push enough trace metals out that we are going to produce health concerns?” said Mark Green, an associate professor of hydrology at Plymouth State and a co-author on a new paper about the research.

I am interested in this work not just because of the findings, but because of its data source. It used water samples collected between 2010 and 2016 by scores of citizen-science volunteers at sites from the edge of Quebec to the Massachusetts line, part of an entirely different project to monitor other water quality throughout the state.

“A friend at the University of Pittsburgh does trace-metal chemistry, which we hadn’t planned on analyzing,” said Green. “There’s evidence in the literature that sodium can mobilize metals in soils, and … we got thinking, I wonder if that would show up in our data set?”

The data was useful partly because of an important detail: Water-quality samples are always taken at the same time on the same day of the month because changes in weather (especially rainfall) can affect bacteria levels in streams. Such snapshots remove the variable of time and made conclusions firmer.

“The thing that made this possible is that for 60-some people, every bottle was dipped within three hours of each other. If we hired technicians to do that, we just couldn’t afford it,” Green said.

Speaking as somebody who has collected plenty of river samples for water-quality studies (if you’re going to volunteer for a citizen-scientist project, I suggest finding one that lets you wade into streams before going to work in August) it is great to think that this work can produce what might be considered extra credit. A lot of great science has been produced over the centuries by taking existing material and using it for an entirely different purpose; maybe this will be another example.

As for the paper’s findings, researchers discovered various levels of correlation between patterns of road salt usage and the amounts of cadmium, copper, zinc, lead and arsenic in surface and ground waters.

“We were a little bit surprised. There has been some speculation about this over the last couple of years, and bench-top experimentation that has shown it, but at this scale it hasn’t been demonstrated before,” said Green, who is also a research hydrologist for the U.S. Forest Service.

The paper, whose lead author Jessica Wilhelm was getting her masters at PSU at the time, has a typically boring-sciencey headline, “Trace metals in Northern New England streams: Evaluating the role of road salt across broad spatial scales with synoptic snapshots.” Green cautions that it can’t say for certain road salt is causing tiny amounts of these metals to move from soil to water. “This doesn’t show causation,” he said, although it documents “a pattern that’s consistent” with causation.

He also pointed out that the detected levels of trace minerals in water are very low.

“We note in the paper that the concentrations are not above human health standards – although some meet aquatic-life-concerning criteria,” Green said.

Next steps would include more geochemistry studies about how exactly sodium and chloride compounds might be affecting how trace metals bind to dirt from bench lab work and site-specific analysis and research. This sort of detailed work is beyond us citizen-science fans but maybe somebody will figure out some clever way to make use of material or data that you and I have collected.

Either way, try to avoid using de-icer around your house or workplace. New Hampshire’s Department of Environmental Services has some suggested alternatives online at des.nh.gov/organization/commissioner/pip/newsletters/greenworks/documents/201701-greenworks.pdf.

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