Road salt and stormwater ponds don’t mix
Seasonal patterns of salt accumulation and flushing. Water below the red line indicates a
"salt blob." Photo: Jacques Finlay
Jacques Finlay is new to road salt research, and, unsurprising for those who have been following the issue, he does not come bearing good news.
“I’m here to add to your list of worries about road salt,” Finlay told the crowd at the 18th Annual Road Salt Symposium in February.
Finlay presented his research on stormwater ponds—those small ponds scattered around the Twin Cities and the state that are supposed to filter runoff from roads and other surfaces before it enters our freshwater streams, rivers, and lakes.
“They’re primarily put into service, modified or created, to improve water quality by trapping contaminants and controlling floodwaters,” Finlay said. Unfortunately, they’re not working as they should.
Finlay is a professor in the University of Minnesota’s Department of Ecology, Evolution and Behavior and is also associated with the University’s Saint Anthony Falls Laboratory and the Institute on the Environment. His lab “investigates the ecology of freshwater ecosystems, and their interactions with surrounding natural and human-altered landscapes.”
According to the Minnesota Pollution Control Agency, the Twin Cities metro area applies an estimated 365,000 tons of road salt each year, and roughly 78 percent of that is either transported to groundwater or remains in local lakes and wetlands. Research shows that salt concentrations are increasing in water bodies across the state.
This is a problem because road salt pollutes water with chloride, which negatively impacts fish and other aquatic life.
‘That’s a lot of salt’
In 2018, Finlay’s team surveyed 38 stormwater ponds. In the shallow ponds (those less than three feet deep) chloride levels were evenly mixed. The levels were above water-quality standards, but not extreme given their proximity to roadways, according to Finlay. However, in the deeper ponds, road salt accumulated in the bottom.
“It’s accumulating at such high levels in some ponds that we need a new frame of reference,” Finlay declared. “That new frame of reference, unfortunately, is seawater.”
In one of the six deeper ponds, chloride levels near the bottom of the pond were approaching those of salt water. “That’s a lot of salt sitting there in the bottom waters of these ponds.”
Road salt may contribute to phosphorous pollution, which
turns freshwater ponds and lakes into green soups of algae.
Photo: Jacques Finlay
Chloride impacts
Snow melt adds chloride to stormwater ponds in the spring and late winter, and the chloride can stay in the ponds for months—longer than Finlay expected. Because the salty snowmelt entering these ponds is heavier than pure water, the salt sinks to the bottom, forming a salty layer at the bottom of the ponds, with fresher water on top.
Chloride pollution has three main impacts on stormwater ponds:
- Chloride toxicity: Finlay said that the levels of chloride in many of these ponds is “way above the standard for aquatic life.” In addition, there is some evidence that warmer summer temperatures enhance the toxicity of chloride.
- Heavy metals: Salt also acts as an extractant, or mobilizer, for metals bound in the sediment at the bottom of stormwater ponds. High levels of salt can increase the concentrations of toxic heavy metals in these ponds.
- Mixing: Stormwater pond waters are supposed to mix well, but road salt impairs this mixing. Ideally, these ponds would be mixing often to ensure oxygen reaches the sediments. “They’re mixing very, very infrequently,” Finlay explained. This lack of mixing means that there are very low levels of dissolved oxygen in the bottom waters of the water profile, which is bad for animal and plant life.
From dragonflies to frogs, Finlay described these salty ponds as “ecological traps.” “Many of the organisms that might use these ponds don’t know that,” Finlay said.
Phosphorus pollution
In addition to chloride, road salt may impact phosphorus pollution, famous for turning freshwater lakes and ponds into scummy, green soups of algae.
“Stormwater ponds are used to trap that phosphorus before it gets into our lakes, but some early evidence that we’re seeing, in the Twin Cities at least, is that one in three ponds are not doing their job,” Finlay explained.
"[Salt is] accumulating at such high
levels in some ponds that we need
a new frame reference. That new
frame of reference, unfortunately, is
seawater."
—Jacques Finlay
Phosphorus should be sinking to the bottom of stormwater ponds, where it is stored in the sediment. Instead, it is recirculating in the water column, where it can move downstream and into our lakes. We do not yet understand the role that road salt plays in phosphorus very well, but previous work suggests that it may have an impact, Finlay said.
Potential solutions
Finlay sees two main potential solutions for dealing with stormwater ponds’ salt problem. First, the obvious: reduce road salt inputs. He said the focus should be on reducing inputs to particularly sensitive ponds.
The second potential solution is to better manage the stormwater ponds themselves. Many of the ponds they surveyed need to be excavated and retrofitted so that they better dilute and flush salty water. In addition, he highlighted the need for more research and stakeholder engagement.
—Paul McDivitt, LTAP freelancer