Across Minnesota, chloride levels in lakes and rivers are on the rise. So far, the Minnesota Pollution Control Agency has identified 54 lakes with chloride pollution above state standards and more than 70 more that are at risk. Even though salt is common in our diet and needed for winter safety, it harms our lakes and rivers when too much tips the natural balance.
Wintertime brings a surge of chloride pollution from deicers to our waters. The most used deicers are made with sodium chloride and help remove ice by lowering its melting temperature. The chloride part of that chemical sticks around in our environment, causing chloride pollution.
Where does it go?
Much of the de-icing salts used on sidewalks, driveways, and roads washes into lakes and rivers where it dissolves into chloride and builds up salinity over time. A 2008 study by the University of Minnesota found that 78 percent of the chloride applied for winter maintenance in the Twin Cities metro area stayed here, either being transported to our groundwater or remaining in the local lakes and wetlands.
Groundwater is where 75 percent of Minnesotans and all Carver County residents get drinking water. When tested, 30 percent of shallow groundwater wells in the Twin Cites had chloride concentrations that exceeded the water quality standard.
What does chloride pollution do?
In lakes and rivers, high chloride levels are toxic to fish, aquatic insects, amphibians and the whole aquatic ecosystem. Our fish in Minnesota are used to freshwater, not saltwater, and too much salt disrupts osmosis, a cellular process that moves molecules through cells. High amounts of chloride cause water to leave the cells and can prohibit transport of other needed molecules into the cell. This is why seawater is toxic to organisms that consume freshwater, including humans. Over time fish, aquatic bugs and even some plants become stressed and/or die when exposed to higher levels of chloride.
Studies from the University of Toledo show that higher chloride concentrations reduce the growth and weight of fish, such as rainbow trout, and decrease zooplankton numbers. Zooplankton are small organisms that feed on phytoplankton (algae). They are also a critical food source for fish and other aquatic life. Smaller numbers of zooplankton in lakes could cause an increase in phytoplankton leading to more algae in the water. It affects more than fish. Research has also shown that high chloride levels dry out amphibian eggs reducing hatching rates, and affect the behavior, abundance and mortality of many aquatic bugs which are food for many fish and wildlife.
Too much chloride in a lake also affects the circulation of oxygen that happens during a natural process called “lake turnover.” Typically, lakes have layers in summer and winter because water temperatures at the surface are different than those at the bottom of the lake. Cold water is more dense than warm water. During spring and fall, water temperatures are more even, and so top-to-bottom mixing occurs and oxygen is distributed throughout the lake.
High chloride levels in freshwater lakes prevent this mixing. Chloride increases the density of water, and so as freshwater becomes saltier, it becomes heavier. The heavier chloride laden water sinks to the bottom of a lake and stays there, preventing lake turnovers and oxygen from circulating.
The problems go beyond the health of our waters. In the landscape, high chloride levels are toxic to plants, trees and lawns, and can change the soil chemistry so future plant growth is difficult. It can also harm pets who may lick it off their paws or drink salty snowmelt.
Lastly, it corrodes metal in vehicles and on bridges, and damages pavement on roads, garages, and sidewalks. According to the Environmental Protection Agency, maintenance, and repair of salt damage to roads, bridges and automobiles in the U.S. costs around $5 billion each year.
A permanent problem
Chloride is a permanent pollutant. Once in a lake or river, it’s impossible to remove it safely and cost-effectively. The only way to reduce chloride levels in water is to use less salt.
Monitoring results
The Carver County Water Management Organization began sampling for chloride levels in lakes in 2019. While most lakes are well below the standard for chloride of 230 milligrams per liter, many lakes are showing an increasing trend in chloride levels, putting them at risk. The lakes at greatest risk include Benton and Meuwissen in Cologne and Brickyard Clayhole, Grace, Jonathan, Hazeltine, Big Woods and McKnight in Chaska.
How to stay safe and protect water
We can stay safe from slips and falls and protect lakes and rivers. It’s all about using salt effectively and efficiently. You can have safe walkways while using less or no salt. Follow the tips below.
Shovel that snow
The best way to prevent ice is to shovel. Shovel early and shovel often. Remove the snow and there is less there to get compacted and turn to ice, thus reducing the need for salt.
Watch your scatter
When applying salt, watch your scatter. Scatter the salt widely, leaving space between grains of salt. A coffee mug of salt is enough for 60-70 feet of sidewalk, or two parking spots. A hand or push spreader helps get the right scatter.
Watch the temperature
Rock salt (sodium chloride) is NOT useful when the temperatures are below 15 degrees. Even at 15 degrees it can take an hour to melt ice. Shovel and use a little sand for traction if needed during cold temperatures.
Sweep up leftovers
Salt only works once it is dissolved. If you can see salt on your driveway hours after it has been applied, it isn’t doing any work. Sweep it up and use it again next time. Otherwise, it will just blow away or wash into storm drains with the next melt.
Madeline Seveland is an education coordinator with Carver County Water Management. She can be reachedat mseveland@co.carver.mn.us.
