Of all the winter pollutants, road salt, which lowers the freezing point of water, is the most concerning because it is particularly harmful to freshwater organisms that haven’t evolved to tolerate salt water, experts say.

“The reason this is an environmental issue for aquatic ecosystems is that salt specifically, is toxic,” said Andrew Hrycyna, water quality program manager at the Mystic River Watershed Association.

Hrycyna said the Environmental Protection Agency sets threshold levels for exposure to chloride, a part of salt that dissolves in water. Over time, such exposure can stunt growth and reproduction in fish and aquatic insects, while sudden exposure can cause them to die instantly.

Data collected for over 20 years by the Mystic River Watershed Association show an upward trend in chloride levels in freshwater sites across the waterbodies. The group measures “specific conductance,” a measurement of the electrical conductivity of water which is directly proportional with salt content, and has documented rising levels not only in winter but year round, suggesting that groundwater itself is becoming saltier.

A 2016 study by the United States Geological Survey found similar increases in chloride concentrations across rural and urban parts of New England, reinforcing concerns that the use of road salt has long-term regional impacts.

In Alewife Brook, a tributary of the Mystic River, chloride concentrations spiked sharply during snowmelt and rain events, monitoring during winter storms has shown. In some cases, levels briefly exceeded acute toxicity thresholds for freshwater organisms. Alewife Brook is now officially listed as “impaired” by chloride on Massachusetts’ Integrated List of Waters.

“The big problem comes not from the fact the snow melts, it’s the sheer volume [of salt] we’re using,” Hrycyna said. “We know we have to put some salt down to keep people safe, but can you put less and keep people equally safe?”

In most neighborhoods, storm drains are directly connected to nearby bodies of water through an underground pipe network. Anything on the street — from sand and soot, to dog waste and plastic debris — can be swept into rivers during snowmelt.

“It’s still precipitation,” said Chris Mancini, executive director of Save The Harbor/Save The Bay. “It’s still landing on urban streets, and where it melts, it’s carrying whatever it’s captured into storm drains over land into the water.”

Some cities, like Cambridge, are experimenting with brine, salt dissolved in water and sprayed onto roads, which allows for precise application and reduced overall use.

But there are other pollutants.

That black, crusty layer visible on plowed snow contains accumulated car exhaust particles, microplastics, sand and bacteria from pet waste. In the Charles River, elevated bacteria levels have the potential to make people sick. Nutrient pollution — including phosphorus from fertilizers, detergents and even organic matter like fallen leaves — can lead to over-fertilizing the river.

“It’s essentially like dumping fertilizer into the river, and you just get way too much plant growth, and it can actually overwhelm the river’s ecosystem,” said Julie Wood, climate resilience director at Charles River Watershed Association.

Advocates say the solution lies in redesigning how cities handle runoff.

Mancini said Boston’s Deer Island treatment plant and stormwater tunnels help manage intense runoff, storing and gradually releasing water to avoid overwhelming the system.

Green stormwater infrastructure, first pioneered in Maryland in the 1990s, is another part of the solution. These systems, which can take the form of sunken gardens, tree pits or underground infiltration tanks, capture runoff before it reaches storm drains. Soil and plant roots filter pollutants while water slowly seeps into the ground.

“That’s what we’re always pushing for both rainfall runoff and the snow melt,” Wood said. “Just natural filtration before it reaches the river.

Wood said salt remains harder to filter because it dissolves easily and can damage plants used in green infrastructure systems. That makes reducing salt use at the source especially critical. Mancini suggested the use of environmental friendly salt as an alternative.

There is, however, one environmental benefit to heavy snowfall. Wood said slow, gradual snowmelt allows water to recharge groundwater supplies, something intense rainstorms often fail to accomplish.

Still, advocates say the sight of blackened snowbanks offer a powerful visual reminder.

“Snow is just revealing to our eyes the pollution that’s always there year-round,” Wood said. “That black stuff we see on the snow is not only there in the winter, it’s just only visible in the winter.

Aayushi Datta can be reached at aayushi.datta@globe.com.