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Critical pollutants are poisonous or "toxic" chemicals affecting the Lake Superior Basin They are called "critical" because their characteristics make them harmful to the health of the lake and its natural and human communities.

Nine critical pollutants, called the "Nasty 9" are the worst toxic chemicals affecting Lake Superior. We can group these 9 pollutants into 4 chemical "families".

While some critical pollutants, like mercury, also enter the environment from natural sources, most of the pollution from these toxic chemicals comes from human activities. The good news is that we can do something to eliminate these critical pollutants, reduce the environmental and human health threats. If we understand more about critical pollutants, we can find ways to stop them from entering the Lake Superior Basin and our communities.

superior basin
Investigate the location of critical pollutants to discover where you might be exposed to toxic chemicals in your community.



Meet the Four Families of
Lake Superior's "Nasty 9"
critical pollutants:

Persistent Pesticides

Identify the characteristics that make Lake Superior especially vulnerable to accumulations of toxic chemicals

Understand what are "critical pollutants", their sources, and ways to reduce or eliminate them in our communities

Understand the principle of "zero discharge" and how it can be achieved


lake superior
Just how big is Lake Superior? Follow the link on the map photo above, to find out.


Lake Superior is the second largest freshwater lake in the world, but the largest in terms of surface area. Only Lake Baikal in Russia can claim more volume of freshwater than Lake Superior.

How could such a huge body of water, containing over 3.2 billion gallons of beautiful crystal clear water, be affected by toxic chemicals that affect the health of people, animals, and plants. Wouldn't all that water dilute any pollution?

The key to understanding why Lake Superior is so vulnerable to the build up of critical pollutants is understanding the unique physical characteristics about Lake Superior and its relationship to people:

waveLake Superior is big and deep. Lake Superior is not a lake; it's an inland sea. It is the largest of the Great Lakes and has the largest surface area of any freshwater lake in the world. It contains almost 3,000 cubic miles of water. That's enough to cover North and South America to a depth of one foot of water!

The Lake's average depth is 500 feet with a deepest point of 1,332 feet. The lake stretches approximately 350 miles from west to east, and 160 miles north to south, with a shoreline almost 2,800 miles long.

It takes 191 years-- almost two centuries-- for water to flush through Lake Superior. This means that what we put into the lake, stays in the lake for a very, very long time.

people on shoreLake Superior is COLD. Ask any swimmer! The average water temperature is 40-degrees F (4C). Chemicals do not breakdown quickly in Lake Superior's icy cold waters. When persistent chemicals get into the Lake, they are not easily diluted or broken down to less toxic substances. As more and more of these persistent chemicals enter the lake, the amount of critical pollutants builds up, multiplying their harmful effects.

Water from Lake Superior eventually flows through to the other Great Lakes and empties into the Atlantic Ocean. Persistent chemicals that flow out of Lake Superior can be flushed through the entire Great Lakes system.

A simplified depiction of the Lake Superior food web

Lake Superior's food chain is simple. The food chain of the lake is simple. There are fewer species of plants and animals, so there is a more direct connection between the plants that can absorb pollutants, the animals that eat the plants, and the humans who eat both the plants and animals. Persistent toxic chemicals can bioaccumulate as they move through the Lake Superior food chain, eventually affecting human populations.

Lake Superior is a fragile ecosystem. The Lake Superior Basin is one of the most unique and fragile ecosystems in North America. It functions as the headwaters of the larger Great Lakes St. Lawrence Basin. Water that falls within the Basin eventually flows downstream into Lake Superior. Whatever we put on the land or in the streams within the Basin eventually is washed down to Lake Superior.

These natural characteristics of Lake Superior make it especially vulnerable to pollution caused by human interaction with the lake and its natural resources.


life chart
Diagram depicting the Ojibwa migration into the Lake Superior Region. Learn more about the Ojibwa people and the migration story by following the link on the picture.



People have lived around the Lake Superior Basin since 8000 B. C. Lake Superior's rich natural resources have provided food, shelter, and commerce for the people who call it their home.

According to their teachings, the Lake Superior Ojibwa Indians, or Anishinabe, followed the sacred Megis shell to the place where "food grows on the water.. The place was Lake Superior and the food was wild rice. Historians date their arrival in the Lake Superior region in the 1400's.



french explorers
Trading with Native people for beaver fur pelts; by artist Howard Sivertson

Fur Trade Era:
French explorers entered the Lake Superior region in the mid-1600's and found abundant populations of beaver and other fur-bearing animals living in Lake Superior's coastal wetlands. By the 1650's, an active trade with Native people was established for beaver fur pelts used to create top hats, which were the fashion in Europe. The beaver fur trade would last until the mid-1800's.

The Bigelow Sawmill on Lake Superior, Ashland, WI ca. 1890

When beaver felt top hats went out of fashion, the demand for beaver pelts declined. People turned to logging the great inexhaustible stands of Lake Superior's white pine forests for a new source of jobs and income.




cabin farm
After the great white pine forests were cut over in the late-1800's, immigrant families came to farm the land within the Lake Superior Basin.

White Pine Mining:
By the late 1800's, the great forests were cutover and gone. Disastrous wildfires that scarred the land followed.

With the great forests gone, there was an abundance of open land. The stage was set for European immigrants to come to the region to farm the cutover land. Towns and cities grew with their numbers. New businesses and industries sprang up to support the growing population.


minersMining "Red Gold":
At the same time, iron ore was discovered across the southern portion of the Lake Superior Basin. A boom was on for "red gold" and good jobs in Lake Superior's rich iron and copper mines. The "boom" would last until the 1960's when many of the region's deep iron ore mines would close.

Recreation continues to draw people to the Lake Superior region.

Each "era" of natural resource use has brought new people to the Lake Superior region. Lake Superior's natural resources and beauty continue to attract newcomers for recreation and tourism.

Today, compared to other areas of the United States, not many people live within the Lake Superior Basin. The Basin's population is only 600,000, about the size of the Milwaukee, Wisconsin. Population density is about 70 people per square mile in the United States portion of the Lake Superior Basin and 2 people per square kilometer in the Canadian portion.

Satellite image of Lake Superior at night showing population density by lighting intensity.

Most of the population is scattered in small communities and towns, on farms or rural forested areas, or on Native American or First Nation lands. There are only a few, moderate sized "cities" or urban industrial where populations are more concentrated.

How could such a small population contribute enough critical pollutants to hurt a lake as large Lake Superior?

The answer is rooted in Lake Superior's natural resources and the lake itself. The region's rich natural resources were used to provide the jobs and income needed to sustain its communities. The natural resources also provided products that society wanted.

However, in processing these natural resources into useful products our society needed, another by-product was sometimes created-pollution. Some of the critical pollutants that still haunt Lake Superior are left over from past industrial practices that ended long ago. Others are generated through improper disposal of home and business waste. Still others are carried in the atmosphere from areas thousands of miles from Lake Superior and deposited here when it rains and snows.

Once these critical pollutants enter Lake Superior, they may persist in its ecosystem for a very, very long time.



Past Industrial Practices
Many of critical pollutants found within the Lake Superior Basin got here as the result of past industrial practices. These toxic chemicals were used in manufacturing processes or were created as an industrial by-products or wastes.

In the past, there were few environmental pollution regulations. The poisonous effects of these chemicals on plants, animals, and humans and their effect on Lake Superior were not recognized.


Past industrial practices at the US Steel Blast Furnace in Duluth, MN released toxic pollutants into the St. Louis River Estuary and Lake Superior. Today the mill is gone, but the level of toxic pollutants remaining at the site is high enough for that it has been designated a Super Fund site. Click on mill to learn more.


Toxic chemicals were often disposed of by dumping them into the Lake or disposing of them on the land where run off from rain and snow would carry them into the Lake.

samplingThese practices resulted in thousands of pounds of critical pollutants entering the Lake Superior environment from these old industrial sources.

Today, there are fewer industries within the Lake Superior Basin and stricter pollution controls thanks to federal and state environmental laws. Many of the critical pollutants once used by industries are now banned. These factors have significantly decreased the amounts of new industrial pollutants entering the Lake.

Because of Lake Superior's unique natural characteristics toxic chemicals used by these old industries still remain in Lake Superior's water and in lake bottom sediments. The greatest concentrations are near old industrial sites and harbors. Contaminated areas called Areas of Concern left over from the old industrial sources, need to be properly cleaned up. Super Fund sites are designated Areas of Concern where special federal funding is used to clean up hazardous materials have been dumped or abandoned.

lake cities
Investigate more about Lake Superior's Areas of Concern.
Which contrubutes more toxic pollution to Lake Superior or your home environment: a backyard burning barrel or an industrial smokestack?

Our Homes and Businesses
It's easy to blame industry for pollution problems. Our homes and businesses may not be smoke stack industries, but they contribute "Nasty 9" toxic chemicals to the environment.

One of the greatest sources of toxic chemicals within the Lake Superior Basin is backyard burning barrels Toxic chemicals are released into the air we breathe when garbage is burned in burning barrels and small business incinerators.

Airborne poisons caused by the incineration of trash in burning barrels can "rain" down on Lake Superior and enter its ecosystem. Because burning barrel and home garbage incinerators burn at lower temperatures, they release up to 80 times more toxic pollutants into the air than a full-scale municipal incinerator, according to the U.S. Environmental Protection Agency. Municipal incinerators and industrial smokestacks usually come equipped with pollution emission control systems that are regulated to reduce pollutants, while backyard burning barrels do not.

If your family, school, or business uses a burning barrel, consider the consequences to your health and the environment's. Burning barrels make poison!

pollutant flow
Pathways for toxic chemicals to enter and cycle through the Lake Superior Basin

Atmospheric Deposition From Outside The Basin:
The primary route for the "Nasty 9" to enter the Lake Superior is through the atmosphere. This is called atmospheric deposition.

Unfortunately, the "Nasty 9" toxic chemicals are still produced and used in many countries. Even though they may be used thousands of miles away, toxic chemicals can be carried in the atmosphere and deposited on the Lake Superior Basin by precipitation or wind.

Keeping the "Nasty 9" out of Lake Superior means that we must support and participate in national and international efforts to reduce the use and emissions of these persistent toxic chemicals that bioaccumulate in our environment.



So if these toxic chemicals are so nasty, what's the solution. The answer is: totally stopping any way they can enter the Lake Superior Basin either through discharge into the water or through emissions of these chemicals into the air. This is called "Zero Discharge."

In 1991, the governments around Lake Superior agreed to work together on a zero discharge demonstration project to eliminate the Nasty 9 toxic chemicals from the Lake by 2010. Everyone's help is needed to reach this goal and kick the Nasty 9 out of Lake Superior forever!



Click on each of the four families of critical pollutants to investigate more about how each pollutant effects Lake Superior.

Meet the Four Families of Lake Superior's "Nasty 9" Critical Pollutants

The toxic chemicals in the Dioxin, PCB, and Persistent Pesticide families are also known as Persistent Organic Pollutants or "POPS". Mercury, because it is not organic, is not considered a POP, but it is a critical toxic pollutant. Learn more about POPs at



man on beach CREATE... your own service learning experience to protect and restore aquatic communities. This section provides you with a template to get started in developing your own service learning project.
girl with net ACT... Take action to help restore and sustain aquatic communities and learn about what others are doing in the Lake Superior Basin and your community. This section will give you hands-on things you can do to help!
group reflects REFLECT.... Share and celebrate your experiences with others. This section lets you share what you learned with others.


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