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As Salt Coats Snowy Roadways In Winter, Freshwater Ecosystems Pay a Heavy Price | Audubon

The titanic Whiteface Mountain looms over the Village of Lake Placid, a light snow dusting the peak by early October. The smooth surface of Mirror Lake reflects the tranquil autumn scene, but beneath is a problem that’s weighed heavily in this densely-forested upstate New York region.

Come winter, crews cover streets with road salt, which, sooner or later, washes into the lake. In 2014 scientists learned the lake’s chloride concentration—an indicator of salt pollution—was almost 160 times higher than Adirondack lakes without nearby paved roads. Heavier than freshwater, a briny layer accumulated at the bottom, says Brendan Wiltse, senior research scientist at the Adirondack Watershed Institute. This impeded the regular mixing of oxygen, he says, stifling fish such as lake trout and other aquatic life. 

Around the United States, scientists are increasingly worried about rising levels of salt in freshwater wetlands, rivers, and lakes, even in much larger bodies like the Great Lakes. “Essentially, we’re blanketing the Earth with salt,” says Bill Hintz, an ecologist at University of Toledo’s Lake Erie Center. 

Sodium chloride or rock salt, a rough version of table salt, is the most common deicing salt. It works by lowering water’s freezing point, melting snow and ice more quickly. Relatively cheap and proven to drastically reduce accidents, salts have been applied liberally on roads, parking lots, and sidewalks for decades. In snowy regions, home to about 70 percent of the U.S. population, road salt use has almost tripled since the 1970s. 

The costs have become clear. In 2005 University of Maryland geologist Sujay Kaushal co-authored one of the first studies to sound the alarm, predicting that many surface waters in the Northeast could become toxic for freshwater life and human consumption within the next century. Road salt is a major cause of what he calls salinization syndrome, but fertilizer, wastewater, and mining-related salts are other big contributors. “We rely on salts for just about everything,” says Kaushal. Climate change adds to the burden, causing sea-level rise that pushes salt inland and worsening droughts that intensify evaporation. 

Current safety levels may not be enough to protect sensitive freshwater organisms, especially those at lower rungs of food chains that birds and other wildlife rely on. In a 2022 study Hintz and an international team of scientists showed that, at a majority of 16 sites, more than half of zooplankton populations died off when chloride levels were at or below thresholds established in Europe, Canada, and the United States. Almost half the sites also saw increases in algae, which consume precious oxygen, block sunlight to plants below, and can grow into harmful blooms. 

“When these freshwater ecosystems reach a certain chloride concentration, we see a lot of different sensitive organisms disappear from those systems and that triggers ecological change,” Hintz says. Aquatic plants and insects such as mayflies, freshwater crustaceans, and frogs are also vulnerable. 

Each ecosystem responds differently. But the U.S. EPA’s chloride threshold—a non-binding recommendation created in the 1980s and twice as high as Canada’s—applies to all. “What we need to do is lower those thresholds for a lot of lakes and understand the regional context that contributes to species decline in those areas,” Hintz says.

In some places, saltier wells, reservoirs, and other drinking water sources raise the stakes, especially for people on sodium-restricted diets. And once salinization takes hold, it can take decades to undo. Salt-related corrosion also damages infrastructure—cars, bridges, and pipes—and mobilizes toxic metals like lead and mercury. Victoria Kelly, environmental monitoring program manager at the Cary Institute of Ecosystem Studies, points to one example with dramatic consequences: In 2014, the city of Flint, Michigan, switched its drinking water supply to the Flint River, where chloride levels were higher than its previous source, Lake Huron. When corrosion inhibitors weren’t used, lead and other contaminants began to show up in the city’s drinking water, triggering a health crisis. 

Today there are few inexpensive, nature-friendly alternatives to road salt, says Hintz. That’s why he and other experts call for officials to invest in best practices that cut salt use and limit pollution, such as pre-treating roads with brine, improving storage to control runoff, and using modern plows. Many cities and states, including Wisconsin and Minnesota, are already taking action. 

A few years ago, towns and villages around Mirror Lake began to reduce salt use, and environmental groups spearheaded efforts asking residents and businesses to chip in. This year, testing revealed progress: Mirror Lake turned over in the spring—meaning its bottom waters mixed with the surface—for only the second time since 2016. “The lake is mixing more than it was in the past as a result of salt reductions,” says Wiltse. “There’s still a long way to go.” 

As a society, Hintz thinks we need to ask some difficult questions about the price we’ll pay to keep so many paved surfaces snow-and-ice free. It may be time to ask people to drive more slowly or stay home in bad conditions, rather than expect clear roads every winter, he says. No matter what, action is needed now, says Kelly, because ecosystems won’t recover right away: “That legacy effect is going to carry on for years to come.” 

This story originally ran in the Winter 2022 issue as “The Road Salt Conundrum.” To receive our print magazine, become a member by making a donation today.

Snowy Towns are Weaning Themselves Off Road Salt this Winter | Time

This winter’s first snowstorm will be an adjustment for some residents in Davenport, a city of 100,000 on Iowa’s side of the Mississippi river, which averages nearly 30 inches of snow over the season. Like many towns across the U.S. and Canada, Davenport dumps a hefty amount of rock salt across its roadways each year, melting ice and keeping conditions easy for drivers. But this year, residential streets on the northwestern side of town will be left salt-free.

The trial, which may see residents have to drive a few blocks over snowy roads to reach major arteries, is Davenport’s attempt to “find a balance” between road safety and a growing list of problems associated with road salt, says public works director Nicole Gleason.

Ice-free roads may be good for drivers, but scientists warn that salt is seeping into lakes and rivers, including the Mississippi, killing wildlife and posing health risks to humans. Salt also corrodes asphalt and metal, causing some $5 billion in damage each year to roads and cars. And it lures deer and moose onto highways to lick it up, triggering accidents.

And yet, North Americans are addicted to road salt. Road crews have been pouring the stuff in ever greater quantities since the 1950’s, when cars and highways began to proliferate across the region. According to the U.S. Geological Survey, the amount of salt used on U.S. roads ballooned from 1 million tons in 1954, to 10 million tons in 1985, to around 24 million tons a year by 2019, as drivers demanded increasing levels of safety and convenience. “Fifteen years ago, it wasn’t common practice to expect dry pavement after snow,” Gleason says. “But somehow this idea has taken over that everyone should be driving like it’s summer in the winter.”

The quest for an alternative to salt

As the case against road salt has become firmer over the last decade, scientists and local governments have led a quest to find a less harmful alternative. But success has been limited. Chemical solutions have proven expensive and carry their own environmental risks, like decreasing oxygen levels in water or damaging foliage. Sand works for driveways or small areas where it can be easily swept up, but if used on a larger scale, it can clog drains, contribute to fine particle air pollution, or damage vehicles. Cheese brine has been used to melt snow in Milwaukee, Wis., since 2013—handily using up waste from the local dairy industry. But the available quantities only make a small dent in salt needs. Beet juice has proved effective in many places, but it releases sugar into waterways, fuelling harmful algae blooms which are just as dangerous for wildlife as salt is. As a result, it can only really be used as an additive to salt.

“People always want a silver bullet, but this is a multi-dimensional problem with a lot of pieces,” says Xianming Shi, a leading researcher on alternative deicing methods and professor at Washington State University. “I don’t think we will find a magic solution.”

With dreams of fully replacing salt on hold, many states and cities are focusing on reduction. Public information appeals can help. For example, this winter Sudbury, a city of 300,000 in Ontario, is giving out plastic cups in a bid to change residents’ salting habits on driveways. The 12 oz cups, the label notes, hold enough salt to safely de-ice 10 sidewalk squares, or 500 square feet—that’s far less than many people think they need. The cups also remind people not to bother salting when it’s colder than -12°C (10.4°F), because it simply won’t work.

Road crews cut back

Officials are also trying to make it easier for road crews to cut back on salt. Minnesota is a leader here. Since 2016, the state has run a “smart salting” program to train public road crews and private maintenance workers to apply salt without wasting any, helping organizations cut their usage by between 30% and 70% per the state pollution control agency. Minnesota also has policies limiting the deployment of salt on residential streets, like Davenport is trialing. The state’s legislature is now considering a bill that would protect professional salt-appliers and homeowners from legal liability for accidents if they use too little—a factor advocates say has prevented people from using salt sparingly in the past.

The most effective way to cut back on salt may be a more fundamental change in how it’s applied. Spraying brines—roughly one part salt to three parts water—on roads in the hours before snow starts to fall prevents ice from forming in the first place. That proactive approach reduces salt needs by anywhere between 23% and 70%. There are challenges: you need very accurate weather forecasts, and the equipment for mixing and transporting brines is more expensive than for conventional salting. But over the last five years the rising cost of salt and growing awareness of environmental threats has convinced many cities to use brine over salt where possible, including New York, Des Moines and Philadelphia.

For Davenport, which already tries to brine where possible and incorporates 5% beet juice to its deicing solutions to replace some of its salt needs, the next frontier is trying to change public expectations around how clear roads need to be. Judith Lee, the city council member who proposed the trial, says some constituents have expressed anger about the idea of not salting. “They thought we were going to let them slide around on ice, and of course that’s not the case,” she says, noting that road crews will still attend to the trial area if dangerous conditions develop. “But when we talk through all the reasons we want to cut back, people start to understand.”

Shi, the researcher, hopes more local and state governments will find ways to curb their salt addiction. If they can’t, 50 years from now the sodium content of many water sources could reach levels unsafe for human consumption, he warns. “Convenience for this generation—driving from point A to point B fast—could mean our grandkids are drinking salty water.”

Impact of road salt on Cambridge drinking water raising concern - CambridgeToday.ca

The drinking water supply in the city of Cambridge has higher levels of sodium than recommended by Health Canada according to regular water quality reports taken by the region.

The Region of Waterloo website states that regular sodium levels in drinking water should be below 20 mg/L. Anything higher must be reported to public health.

A major cause of the increase is the way roads are treated and the type of salt that is used over the winter months. It's an issue the region is working on.

“We have elevated salt in our drinking water supply water,” Eric Hodgins, manager of hydrogeology and water programs for the Region of Waterloo said.

“We’ve been advocating for the protection of the environment and drinking water.”

It's important to note that the 20 mg/L threshold is a low bar, Hodgins says.

It’s primarily a concern for people who suffer from severe hypertension, congestive heart failure or are on a sodium-restricted diet. 

Doctors are made aware when the levels pass the threshold and know to advise patients who would need to consider water sodium levels when it comes to their overall daily intake.

“Everybody should be considering the sodium levels if they need to be concerned,” Hodgins said.

“We’re required to report it to public health. The doctors know in Cambridge that the water is likely exceeding the health threshold because it’s fairly low.”

The region is working on ways to limit the amount of salt that is used on roadways, including the use of a liquid form that requires a lot less volume. The liquid is something the city is already using where possible as rock salt loses it’s effectiveness when it's colder than seven or eight degrees below freezing. Occasionally the city, who is responsible for laying the salt, will use a sand and salt mixture for freezing rain events but that can have negative impacts on storm sewers in urban environments.

“We want to increase the use of this liquid form,” Hodgins said.

“If you put it on in advance of a storm as anti-icing, you’re putting less on the road because when the snow falls, it’s already there. If you’re putting rock on snow, it needs to make it’s way through to the road, as it’s going it’s melting but it needs to get to the pavement. You have to put more on to get it to the road surface. The rock crystals isn’t what melts the ice, the rock has to come in contact with water that breaks crystals down in liquid forms.”

The other issue is cost. Rock salt is the cheapest and the liquid alternative requires the rock to be processed and broken down.

“The mines pump out rock salt and that then needs to be made into the liquid,” Hodgins said.

“It’s not feasible for us to tell transportation to stop putting rock salt down. It’s not an argument we can win, but we can reduce. The liquid form can reduce total output by about half, we've seen it on private parking lots and pilots that have been done.”

The cost increase doesn’t stop with the processing, as different trucks are required to spread a liquid than what are used to spread the crystals.

On top of the need for some residents to be cautious about the sodium levels in the water, salt being pumped into the environment is a hazard for animals and other creatures.

Water Canada points out that excess salt gets washed away into streams and lingers in the soils, groundwater and storm water ponds. 

Birds can eat salt crystals, resulting in dehydration and death. For aquatic animals and organisms, high levels can result in decreased size, limit their ability to reproduce and in high enough concentrations can be lethal. 

Many plants and trees also can’t grow if the conditions are too salty.

It’s currently a tough spot for the region to be in as they grapple with cost, the importance of safety in the form of clear roadways in the winter and managing health concerns and the environment.

What they can’t control is the use of salt on private properties and parking lots, where an excess is often spread due to liability concerns for the owners.

“The parking lots are a totally different issue because there’s no standard they have to meet, they're all operated different based on the contractor,” Hodgins said.

“There's so much more over-application on parking lots. They don’t have the same traffic so they need to put more down to achieve the same outcome. What were seeing is a really significant over application.”

Despite the obstacles, Hodgins is confident there is a solution.

“We’re trying to find out cost differences in our pilot projects of liquid versus rock salt,” he said.

“Our roads team is moving in the right direction.”

New campaign underscores impact of road salt on environment - Barrie News (barrietoday.com)

Eventually, these chemicals enter the natural water system, including municipal drinking water wells and surface water intakes, which can impact our drinking water sources, Conservation Ontario says.

Conservation Ontario and local source protection authorities and regions have launched a 15-week public information campaign about how to "salt responsibly" this winter.

Protecting Ontario’s water sources is a critical step in bringing safe municipal drinking water to Ontario residents, they say. 

“The objective of the campaign is to raise awareness of the road salt issues and to promote salt reduction and better road salt management (winter chemicals) while striking a balance with human safety when travelling,” said Deborah Balika, Conservation Ontario’s source water protection manager, in a newsletter.

Road salt enters the environment in several ways. Snow gets plowed to the road shoulder and meltwater either infiltrates through soil into the groundwater or runs off into drains and creeks or to stormwater management facilities. 

Eventually, these chemicals enter the natural water system, including drinking water source protection and vulnerable source protection areas (municipal drinking water wells and surface water intakes), which can impact our drinking water sources.

As well, climate change is resulting in more extreme weather patterns that may result in an increased use of winter maintenance chemicals. 

To help create awareness about salty situations across the province, a Salt Responsibly Sticker campaign was developed by Conservation Ontario and a small working group. More than 8,000 salt bins located in vulnerable drinking water areas will have information stickers applied to them.

“This outreach program is a great way to bring attention to the connection between the activities we do on land can have impact on our lakes, rivers, streams and groundwater” and spread the word about the importance of source water protection,” Balika said.

New education tools include social media posts and an online mapping application about the impacts of road salt across Ontario. 

Drinking water protection zones are areas around municipal drinking water sources, where extra protective measures help to reduce risk and keep drinking water safe and clean. Ontario’s municipal drinking water sources include groundwater (underneath our feet in aquifers, drawn through municipal wells); and surface water (such as Great Lakes and rivers). 

Drinking water source protection is one of several barriers, or ‘lines of defence,’ that help to protect drinking water in the province. Other barriers of protection include monitoring, distribution, and the Three Ts  — treatment, testing and training of water operators. 

Drinking water source protection is possible in Ontario through the Clean Water Act, 2006. Local source protection committees include representatives of many interests. These committees have developed source protection plans at the local level and the plans have been approved by the Province of Ontario. The source protection plans include policies that reduce risk to our municipal drinking water sources in order to keep drinking water safe and clean for Ontarians. 

To learn more about drinking water source protection in Ontario, visit the Conservation Ontario source water protection webpage and the Province of Ontario source protection webpage. 

City of Greater Sudbury campaigns for residents to use less road salt | CBC News

A 

 lot of people use too much salt to melt ice on their driveways, according to the city of Greater Sudbury.

To educate people on how to use road salt more responsibly, the Greater Sudbury Public Library has started to hand out 12-ounce, or around 340 ml, cups, which would contain enough salt to cover 10 sidewalk squares or around 500 square feet.

"The messaging we're trying to give is to use salt responsibly," said Jennifer Babin-Fenske, the climate change co-ordinator with the city of Greater Sudbury.

"And when you cover an area, you're not covering it like a blanket with salt, you're supposed to use it to help with a little bit of the ice formation."

Babin-Fenske said it's important to use as little salt as possible in the winter because it can be corrosive and damage the environment in large quantities.

"You know, it's harmful for the environment," she said.

"It can corrode things like your concrete walkways and things like that. And a lot of people are really concerned about their pets and their paws."

Babin-Fenske added that road salt is only effective at melting ice at temperatures below around -12 C. She said people should consider alternatives like sand, for traction.

Babin-Fenske said the city follows a salt management plan for winter maintenance and only uses salt on 25 per cent of the city's roads.

Class 4-6 roads, which are rural and residential streets, only get snowplows and sand for traction. Main arterial roads and secondary collector routes are salted before and after a winter storm, if the outside temperature is not too low.

City contractors are also certified through the Winter Salt Management Program.

According to the program's website, people should shovel their driveway well before adding any salt or "de-icing material."

Once the driveway is cleared, they should only sprinkle small amounts of salt, or a salt alternative, on icy areas.

Awareness campaign underscores impact of road salt on environment - Newmarket News (newmarkettoday.ca)

Conservation Ontario and local source protection authorities and regions have launched a 15-week public information campaign about how to "salt responsibly" this winter.

Protecting Ontario’s water sources is a critical step in bringing safe municipal drinking water to Ontario residents, they say. 

“The objective of the campaign is to raise awareness of the road salt issues and to promote salt reduction and better road salt management (winter chemicals) while striking a balance with human safety when travelling,” said Deborah Balika, Conservation Ontario’s source water protection manager, in a newsletter.

Road salt enters the environment in several ways. Snow gets plowed to the road shoulder and meltwater either infiltrates through soil into the groundwater or runs off into drains and creeks or to stormwater management facilities. 

Eventually, these chemicals enter the natural water system, including drinking water source protection and vulnerable source protection areas (municipal drinking water wells and surface water intakes), which can impact our drinking water sources.

As well, climate change is resulting in more extreme weather patterns that may result in an increased use of winter maintenance chemicals. 

To help create awareness about salty situations across the province, a Salt Responsibly Sticker campaign was developed by Conservation Ontario and a small working group. More than 8,000 salt bins located in vulnerable drinking water areas will have information stickers applied to them.

“This outreach program is a great way to bring attention to the connection between the activities we do on land can have impact on our lakes, rivers, streams and groundwater” and spread the word about the importance of source water protection,” Balika said.

New education tools include social media posts and an online mapping application about the impacts of road salt across Ontario. 

Drinking water protection zones are areas around municipal drinking water sources, where extra protective measures help to reduce risk and keep drinking water safe and clean. Ontario’s municipal drinking water sources include groundwater (underneath our feet in aquifers, drawn through municipal wells); and surface water (such as Great Lakes and rivers). 

Drinking water source protection is one of several barriers, or ‘lines of defence,’ that help to protect drinking water in the province. Other barriers of protection include monitoring, distribution, and the Three Ts  — treatment, testing and training of water operators. 

Drinking water source protection is possible in Ontario through the Clean Water Act, 2006. Local source protection committees include representatives of many interests. These committees have developed source protection plans at the local level and the plans have been approved by the Province of Ontario. The source protection plans include policies that reduce risk to our municipal drinking water sources in order to keep drinking water safe and clean for Ontarians. 

To learn more about drinking water source protection in Ontario, visit the Conservation Ontario source water protection webpage and the Province of Ontario source protection webpage. 

LETTER: Let's get smarter about winter salt usage - Guelph News (guelphtoday.com)

GuelphToday received the following letter from Karen Rathwell regarding excessive use of road salt in the winter.

Our first snowfall came early and no doubt there were lots of smiles on faces when children woke up, but the smile on my face disappeared when I ventured downtown and saw firsthand the copious amount of salt on the doorsteps of RBC and BMO.

I appreciate businesses are anxious about citizens falling on slippery payment at their threshold, and looking for easy money by suing someone else for their
mishap, but drenching pavement in salt is not the answer.

We live in a groundwater city and need to protect this pristine resource along with our beautiful creeks and rivers. There are so many environmental concerns right now, assault on our Greenbelt, heat waves, species loss, soil degradation, drought, and flooding, but we still need to take time to rethink our overuse of winter salt.

This is something we can really do something about personally and locally. If you’re not sure if it's important, try drinking a glass of salt water! Perhaps we need to rethink the legislation for slip and fall litigation? Maybe we need to look more closely at what the responsibility of the individual should be?

If the streets and walkways are icy, citizens need to plan for it or possibly postpone the outing. Wearing a hat, boots, and thick winter coat only makes sense, as does wearing grippers on the bottom of boots or bringing a walking pole, cane or walker. Most importantly, for the frail and elderly, find a companion to help keep you steady.

This should not be the salt’s job! It’s impossible to salt everywhere anyone might step so planning to reduce your personal risk and protecting yourself if you should fall, need to play a bigger part.

Let’s stop pouring salt over our children’s future.

Karen Rathwell, Guelph

Winter pet hazards: Is road salt toxic to cats and dogs? - Vancouver Is Awesome

When cold, wintry weather lands in Vancouver, a dusting of road salt often covers every paved surface.

The salt is certainly a nuisance when it stains our new suede boots or gets tracked into the house but is it also dangerous to our four-legged friends?

Dogs spend time stepping on the salt crystals when out for their daily walks and most don't have the benefit of shoes. 

When dogs or cats come home with road salt on their paws and set about licking themselves, can that salt - also known as ice melt - cause them harm? 

According to Dr. Ko Arman, a veterinarian and board director at the Society of BC Veterinarians: Yes, road salt is toxic to cats and dogs.

"This is definitely a valid concern," Arman tells V.I.A in an email. "The good news is that it's rare for pets to ingest sufficient quantities of salt to cause systemic symptoms, but it is still important to maintain awareness about the issue."

What symptoms should pet owners look for if their pet has ingested road salt?

Arman says that signs of road salt ingestion will be mild and are localized to the paws and mouth. Paw pad irritation and some excess salivation (from licking paws) can be observed with mild exposures, or if there has been prolonged contact then painful ulcerations of the pads and mouth can occur.

"If a pet has ingested a sufficient amount of an ice-melt product like road salt, the best course of action is to seek immediate veterinary care," says Arman. "Systemic symptoms may include vomiting, diarrhea, decreased appetite, excess thirst, lethargy, incoordination, tremors, seizures, and eventually coma and even death can take place."

How to prevent your pet from ingesting road salt

Cleansing and drying your pets' paws and belly fur after coming in from the outdoors is a good preventative measure to avoid paw and mouth irritation from salt and other chemicals they may have come into contact with on the roads as well as prevent any ingestion that may occur from self-grooming, according to Arman. 

Additionally, little booties provide great paw protection if your dog will tolerate them.

If not, Arman suggests that pet balms can also be used as protective coating for paw pads, "but owners need to be careful not to let their pets lick and ingest the products: be sure to clean them off once home from a walk outside," she says.

She adds that pet owners should also make sure they use pet safe ice melts on their own walkways. And as a bonus tip warns that anti-freeze is also a winter pet hazard. "Pet owners should always buy pet safe anti-freeze for their vehicles - never use anti-freeze that contains ethylene glycol, a compound that is highly toxic to pets," she says.

B.C. researchers investigating impact of road salt on Pacific salmon | Globalnews.ca

B.C. university researchers and scientists are looking into the impact that road salt may be having on local Pacific salmon.

Researchers at the University of British Columbia have teamed up with Simon Fraser University, British Columbia Institute of Technology and Fisheries and Oceans Canada scientists to monitor salt levels in more than 20 streams around the Lower Mainland.

“We know that Pacific salmon are in decline and we don’t know all the factors involved. Could too much salt in their streams be a cause?” questioned Chris Wood, a UBC professor.

“Even though adult salmon live in salt water, they grow up in fresh. When they’re ready for salt water, their whole body has to change to adapt.”

Wood continued, “There’s evidence that quite moderate salt levels at young ages have caused mortality and stunted growth.”

The research team will be measuring salt levels and review the data every two months.

“While things like climate change are known causes of salmon decline, salt could also be a factor,” said Patricia Schulte, a UBC professor.

“We know road salt use in Canada is increasing at about 2.5 per cent per year. In Vancouver, the city has 3,000 tonnes of salt in its yards for winter maintenance on streets and sidewalks.”

Schulte said as snow and ice melt, salt from roads can run into streams and may be affecting salmon.

The five-year project will be funded by a Natural Sciences and Engineering Research Council grant and will specifically be looking at Chum and Coho salmon.

Woods said community members can do their part to help local salmon by using only a little amount of salt when salting driveways and sidewalks.

Do road salts trigger eutrophication? Reviewing twenty years of water quality data in an urban lake. | Water Institute Research | University of Waterloo (uwaterloo.ca)

Introduction

Urbanization and associated land use change is a global phenomenon with often negative impacts on water quality and aquatic ecosystems. In particular, urban development of watersheds has been blamed for worsening eutrophication in receiving lakes and wetlands. Increased loadings of nutrients, especially of phosphorus (P), are generally assumed to be the main driver of eutrophication. A high availability of P stimulates dense algal growth that, in turn, may severely deplete dissolved oxygen (DO). In the worst case, the development of a so-called anoxic “dead zone” may result in die-offs of most organisms, such as fish and plants. 

In cold temperate regions, urbanization is typically accompanied by growing winter applications of deicing road salts that inevitably lead to the salinization of local waterways, lakes, and groundwater aquifers. A direct link between salinization and eutrophication of lakes has not yet been demonstrated, however. This study investigates water quality changes in a Canadian lake over a two-decade period. The long-term multi-variate dataset is used to delineate and explain general trends in water quality using a variety of statistical methods. The main outcome is that rising salinity is responsible for the observed intensification of eutrophication-like symptoms, with very low DO levels in the deeper waters of the lake persisting over increasingly longer periods of time during the year. Yet, no evidence was found for an increase in the external loading of P from the watershed to the lake.  

Methodology

Lake Wilcox is a small kettle lake located in Richmond Hill, Ontario with a watershed area of 2.39 km² (Figure 1). The originally forested watershed has undergone significant development since the early 1900s when the land was converted to agriculture, followed by urban development since the 1980s. As a result, land surface imperviousness is now over 60% (Figure 2a). In the 1980s, harmful cyanobacteria became an issue and an aeration system was installed but later deactivated. In the early 2000s, several stormwater management (SWM) systems were installed in the lake’s catchment to address deteriorating lake water quality.  

A variety of datasets for the period of 1996 to 2018 were collected for the study, including water quality monitoring data, climate data, and surveys and satellite images of land use. The dissolved oxygen (DO) time series data served to calculate values for the Anoxic Factor (AF), a measure of the changes in the intensity of deoxygenation of the lake’s deeper waters. Interannual variations in the stability of the water column stratification were assessed by calculating the water density from measured temperature and salinity data and comparing the Brunt-Väisälä or buoyancy frequency depth profiles for the month of August, that is, when the thermal stratification of the lake is highest.   

Multiple statistical methods were used to characterize and interpret water quality trends. Temporal changes were assessed with the Mann-Kendall test (Figure 3) and Principal Component Analysis helped to group correlated variables and identify the main drivers of water chemistry changes. A Multiple Linear Regression model was also developed to explore the relationships between the principal components and the following potential predictor variables: watershed imperviousness (as a measure of urbanization), lake chloride (Cl−) concentration (as a measure of deicing salt usage), external P loading to the lake, water column stratification (using the Brunt-Väisälä frequency), and climate parameters (including, measures of temperature conditions and the intensity of extreme precipitation events that can play an important role in pollutant wash-off in urban areas).

Outcomes

Although the external P inputs to Lake Wilcox have been generally declining, the lake is showing increasing eutrophic symptoms, most visible by the lengthening of the annual period during which the lower part of the lake (i.e., the hypolimnion) experiences low DO concentrations (i.e., hypoxic to anoxic conditions). The chloride concentration (and, hence, salinity) trend was explored as a potential explanatory variable because of the large increase in the lake water Cl⁻ concentrations (Figure 2b) at the same time hypoxia expanded (Figure 4).

During the study period, the water chemistry of Lake Wilcox underwent significant changes, especially in terms of the concentrations of Cl⁻, DO, total phosphorus (TP), and dissolved inorganic P (DIP). Changes in the ratios DIP:TP and DIN:DIP ratios (where DIN refers to dissolved inorganic nitrogen) were also observed. Chloride concentrations increased by a factor of 4, likely due to the increasing application of deicing agents as the watershed urbanized and connectivity to stormwater management infrastructure increased. While the results revealed decreasing trends for the external TP load to the lake and the TP concentrations in the lake, they showed little change in the DIP concentration, meaning that the lake DIP:TP ratios increased over time. This is important because DIP represents the most bioavailable fraction of TP. The longer annual periods of DO depletion of the hypolimnion and the corresponding increase of the AF likely intensified the P loading from the bottom sediments to the water column. It is well known that recycling of P from sediments, also known as internal P loading, is more efficient when the overlying water is anoxic.

The results of the study show that the increasing salinity of Lake Wilcox due to the growing impervious land cover and accompanying increase in road salt application in the watershed has led to a strengthening of the lake's water column stratification. In turn, this has increasingly isolated the hypolimnion from the atmosphere and, hence, accelerated the depletion of DO and amplified the role of internal P loading, relative to the external loading. These changes in DO and P cycling are common symptoms of eutrophication. In Lake Wilcox, however, they appear to be more linked to the lake’s progressive salinization than to a higher external loading of P from the watershed.

Conclusions

By increasing water density and stabilizing the stratification of the water column, salinity emerges as a major driver of changes in water quality in Lake Wilcox. These changes include the expansion of hypoxia and anoxia in the hypolimnion and the increased contribution of internal loading to the lake’s P budget. This is the first known study that directly links salinization with lake eutrophication symptoms.

Salinization, in addition to climate warming, may be an overlooked driver of downward trends of hypolimnion DO concentrations in lakes worldwide. In cold-temperate regions, urbanization is usually accompanied by increased application of road salts as indicated by rising Cl⁻ concentrations observed in urban lakes of North America and northern Europe. The study further suggests that efforts to reduce eutrophication of water bodies in urbanizing areas by controlling external nutrient loadings may be offset by the enhanced internal P loading as DO depletion intensifies.

Historical use of road salt in urban watersheds also created chloride legacies in soils and groundwater. These legacies could continue to supply excess Cl⁻ to receiving lakes even if winter salt applications were reduced or altogether halted. Protecting the health of urban lakes therefore calls for integrated management strategies and requires further work to understand the contributions of climate change, urbanization and salinization, and their interactions, to changes in lake water quality.

Radosavljevic, J., Slowinski, S., Shafii, M., Akbarzadeh, Z., Rezanezhad, F., Parsons, C. T., Withers, W., Van Cappellen, P. (2022). Salinization as a driver of eutrophication symptoms in an urban lake (Lake Wilcox, Ontario, Canada). Science of The Total Environment, 846, 157336. doi.org/10.1016/j.scitotenv.2022.157336