Road salt pollution isn’t just a winter problem anymore

Delaware River watershed scientists and volunteers are sounding an alarm about a worrying trend in water quality samples from local streams: Freshwater streams are staying salty long after spring rains wash winter road salt away, indicating that road salt contamination in groundwater sources is increasing. 

John Jackson, a longtime senior research scientist at the Stroud Water Research Center, said he has dealt with salt for decades as an entomologist. He’s known for years that a highly developed city or suburban area is a predictor of a salty stream. 

Salt spikes are expected in such streams after winter melt events. The Tookany Creek in Pennsylvania, among other developed streams, has registered salt levels at or above sea water after a winter storm. 

Jackson said the problem is that, over time, those melt events have been systematically replacing the fresh groundwater with saltwater.

“It’s reached the point now that salt in the streams is year-round,” Jackson said in an interview in May.

The Delaware River basin’s groundwater provides up to 500 million gallons of water per day for human use, according to the Delaware River Basin Commission. Most of that water supply, around 54 percent, is used for the public water supply. About 22 percent is used for self-supplied domestic uses, and 8 percent for irrigation uses.

The Delaware River is the source of drinking water to more than 14 million people.

Jackson said salt levels in the Delaware River basin’s streams are “nowhere near” lethal to humans. However, salty groundwater has the potential to contaminate well water in the watershed, and it could have untold impacts on municipal infrastructure because of its corrosive nature on water pipes and roads. 

Rachel McQuiggan, a researcher at the Delaware Geological Survey, has found that some existing infrastructure is actually contributing to the salt pollution in groundwater. 

She and her colleagues studied several New Castle County, Del., stormwater management areas — usually small aquifers designed to dilute pollutants like salt before runoff sinks into the groundwater.

McQuiggan’s study showed that road salt isn’t diluting as it should — often diluting much farther away from the stormwater management area — and that it’s accumulating in the earth’s subsurface.

Stephanie Uhranowsky, the executive director of the Brodhead Watershed Association, noted that its stream watchers are still measuring high salt levels as late in the year as August. That reflects how long the effects can linger and how much salt seeps into lakes, creeks, ponds and groundwater, she said.

Jackson said melting and stormwater runoff events can trigger a lethal flush of chemicals for some macroinvertebrates and freshwater fish, but it’s been hard to study the long-term effects of elevated salt levels on freshwater fish.

He said salt is just one of several pollutants, including pesticides and fungicides, in a “chemical cocktail” that’s stressing the vulnerable organisms that call the watershed’s streams home.

“South of the canal in Delaware, groundwater is the primary source of freshwater, so it’s important to remember that what we put on the ground may find its way into an aquifer,” McQuiggan said.

Salt snapshots

The Stroud center has spent the last couple of years collecting data from around 120 “EnviroDIY” stream monitoring stations in the Delaware River Basin, according to Jackson. The stations are a reliable source of salt level data — viewable online at Monitor My Watershed — run by more than 200 volunteers from at least 50 different groups, schools and universities across the watershed. 

Stroud and volunteers have also done single-day “snapshot” events with hundreds of sampling points in Valley Creek, Darby Creek and Tookany Creek, among others, to provide a broader picture of the road salt problem in streams. 

The Stroud center in Avondale, Pa., is near White Clay Creek, a stream that’s exposed to a small, rural road without much salt use. Salt levels haven’t changed there since the 1970s, Jackson said, keeping at a base level of about 5 to 10 mg/L of chloride. That’s a typical salt level for a freshwater stream in the basin.

But he said streams near more developed areas are “moving the needle to the extreme,” with consistently elevated salt levels year-round.

Brandywine Creek, where the surrounding land is about 30 percent developed, has seen chloride levels increase about 10 times since the 1970s, with measurements reaching more than 50 mg/L. And in the Valley Creek, where the land is about 60 percent developed, levels have increased by more than 30 times, to more than 150 mg/L of chloride.

In his research over the last few years, Jackson said he’s measured samples frequently above 250 mg/L, and as high as 500 mg/L in watershed streams near urban areas.

“That’s all at baseflow separate from a snow event. That’s all groundwater contamination,” Jackson said. “That’s salty groundwater coming out and forming these tiny little streams in the landscape.”

Changing expectations

Jackson believes that adjusting salt use to avoid these consequences could require a culture change.

According to Jackson, the application of road salt boomed in the United States in the early ’90s, from an average of 12 million metric tons of salt applied per year in the 1970s through the early ’90s, to an average of 25 million metric tons from the early ’90s to today. 

People have come to expect clear roads no matter the weather, Jackson said, and it’s become an emblem of commitment to public safety. Any political candidate worth their salt could prove they kept roads clear during past winter storms. 

Salt has also become a liability issue. A good portion of salt use is from private applicators, like suburban homeowners associations, privately owned parking lots and businesses, Jackson said.

In a hypothetical world where salt would never be applied again, Jackson said it could take two decades to clear out all the salt that’s accumulated in stream water.

Even if we cut our road salt use in half, he said, the road salt levels in streams would still go up.

 “We’re just using so much,” he said.