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by Tiernan Henry
UW Sea Grant Water Quality Specialist
- Contaminants in the Great Lakes
- Map showing the International Joint Commission 43 Areas of Concern.
- Critical Pollutants in the Great Lakes
- Where contaminants come from
- How they are deposited
- PCBs and the Great Lakes
- Where they end up
- Effect on the food chain
- Contaminant research
- The future
- Ordering the print version
More about Toxic Contaminants and Air Toxics in the Great Lakes Region can be found on the Great Lakes Information Network
The Great Lakes cover about 94,000 square miles in the United States and Canada, collectively comprising 20 percent of the world's surface fresh water. Although huge, the lakes and surrounding basin are vulnerable to substances transported through the air from around the world. Once they enter the Great Lakes, many of these substances will remain in the system for extended periods. (It takes about 200 years, for example, for water to "flush" through Lake Superior.) Along with rain, snow and dust travelling on the global "atmospheric conveyor belt" come potentially harmful substances called contaminants. Any substance not naturally present in the environment, or that is present in environmentally harmful concentrations, is considered a contaminant. Scientists, policy makers and the public are concerned because many contaminants are toxic and pose risks to human and environmental health.
There are 362 contaminants in the Great Lakes that have been found in measurable amounts. However, only 11 contaminants are deemed critical or priority pollutants by the Water Quality Board of the International Joint Commission (IJC). The IJC, created by the United States and Canada under the Boundary Waters Treaty of 1909, advises the U.S.and Canada on water resource issues.
Critical pollutants are found in levels that impair specific beneficial uses because of their presence in the lakes and ability to bioaccumulate. Impaired beneficial uses include restrictions on dredging, algal blooms, beach closings, loss of habitat and degradation of species in or around the lakes.
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Where contaminants come from
Airborne contaminants originate from "point" and "non-point" sources. Point sources include smoke stacks, waste outlets and discharge pipes. Because they can be turned on and off, these sources can also be directly monitored. Non-point sources are harder to monitor. Rainfall in agricultural or urban areas, for example, can wash contaminants such as pesticides or road salt from a wide area into streams and lakes. It can be extremely difficult to identify individual sources of these pollutants.
Atmospheric contaminants are deposited by wet and dry deposition. In wet deposition, particles or gases are rained or washed out of the atmosphere and deposited on land or water. In dry deposition, particles are blown onto the lakes from land or dissolved in the water from the air. They can also be carried great distances. The 1986 Chernobyl nuclear power plant explosion sent radioactive particles into the upper atmosphere, where they ultimately circled the planet in 11 days.
Rain or snow that falls on the land surface of the basin can be modified by a variety of processes before entering the lakes. Contaminants can be trapped in the soil, broken down or neutralized. Pollutants falling on the lakes are unaffected by these processes.
PCBs and the Great Lakes
Of all the critical pollutants transported through the air and deposited on the lakes, perhaps the best example--and certainly one of the most harmful--is PCBs.
PCBs (polychlorinated biphenyls) are a group of synthetic chemicals that were widely used in industrial applications before their manufacture was banned in 1976. Fifty-eight percent of all PCBs manufactured are still in closed systems or are being stored, awaiting disposal. They are the principal toxic chemical found in Great Lakes fish. Exposure to small doses has been shown to be harmful to fish and wildlife populations in the region. Although the effects are less clear, PCBs are suspected of having the potential to cause a variety of human health problems.
PCBs were first detected in the Great Lakes basin in the mid-1970s. Later studies revealed that they were present in both snow and rain throughout the basin. More recently, PCBs and toxaphene (an agricultural pesticide) have even been discovered in fish in a small lake on Isle Royale -- a remote wilderness island in western Lake Superior -- providing conclusive proof of atmospheric transport.
Disposing of PCBs still presents a serious problem. They can leak from disposal sites, leach into ground water, run off in surface water or evaporate. Unfortunately, many types of PCBs don't break down easily or quickly, remaining mobile in the environment for years.
The IJC estimates that atmospheric deposition accounts for the majority of PCB inputs to the upper Great Lakes. According to Mass Balancing of Toxic Chemicals in the Great Lakes: The Role of Atmospheric Depositions, a 1988 EPA report prepared by William Strachan and Steve Eisenreich, the atmosphere brings 90 percent of the PCBs to Lake Superior, 58 percent to Lake Michigan and 63 percent to Lake Huron. The figures for the lower Great Lakes are much smaller but may still be significant. The majority of inputs of PCBs to Lakes Erie and Ontario is from other sources, including the upper lakes. Eisenreich says recent information continues to support the relative role of the atmosphere for Lakes Superior and Huron. The relative role of atmospheric deposition of toxics in Lakes Michigan may be somewhat less and is under active investigation.
Contaminants entering the Great Lakes do not necessarily remain in the water. They can be "removed" by sedimentation, volatilization or biological breakdown. They may also be removed through chemical reactions with water or light. While some contaminants gradually settle into lake bottom sediments, a number of things -- small animals foraging for food, storm waves, floods, dredging, propeller turbulence, etc. -- can cause sediment disturbance, resuspending contaminants in the water. Airborne contaminants can also settle and "ride" on the water surface, evaporate and be carried to yet another area of the lakes, or farther. This process is called "leapfrogging."
The major Great Lakes environmental health concern is the accumulation and upward concentration of contaminants in the food chain. Many contaminants are very small. They can easily become attached to fine sediment or nutrients, which are then filtered and eaten by zooplankton and invertebrates. These creatures are eaten in large numbers by small fish that, in turn, are eaten by large numbers of larger fish. While small amounts may be excreted, most contaminants are stored in the fatty tissue of fish -- a process called bioaccumulation.
Concentrations of PCBs and other persistent toxic chemicals in some large, older, fatty lake fish near the top of the food chain -- trout and salmon -- have reached levels a million times higher than those found in open water. Not only do contaminants bioaccumulate in fatty tissue, they increase in concentration as they move up the food chain, which is a process known as biomagnification. Because of these high PCB concentrations, certain larger fish have been judged unfit for human consumption. Fish-eating birds have also suffered health and reproduction problems. The bald eagle is one of the most visible examples. Declines in bald eagle coastal populations have been associated with high concentrations of PCBs, DDE and dieldrin in the tissues and eggs of the birds.
Contaminant research
Obtaining accurate measurements of contaminant inputs can be difficult. Rain falling over a small area can be accurately measured, but as the area increases, the uncertainty in estimates also increases. The sheer size of the Great Lakes magnifies the problem -- it is just not feasible to sample everywhere. Random factors such as inclement weather make the process even more complex and expensive. Understanding the way contaminants move within the Great Lakes system is complicated by these measuring and sampling difficulties. Nonetheless, air deposition monitoring should help provide a clearer picture about pollutant transport and deposition. Research such as the recently completed Green Bay/Fox River Mass Balance Study, which looked at the sources, transport and fates of contaminants-- specifically PCBs--in the Green Bay food chain, should aid in the development of strategies for reducing and eliminating contaminants and provide insights to contaminant behavior elsewhere in the Great Lakes.
The EPA's Toxic Release Inventory (TRI) is another means of determining relative amounts of pollutant-loading from industrial point sources. Manufacturers are required to report to federal and state government agencies the amounts of more than 300 chemicals they release to the environment. Pollutants released directly to the air, water or land, or those that are moved to off-site treatment and disposal sites, must be recorded and reported. An inventory of these contaminants, their sources and where they were released is compiled and published annually.
For example, the 1991 estimated total releases of lead in the Lake Michigan Basin were 54,702 pounds in the air, 10,288 pounds in the water and 96,199 pounds on land.
Unfortunately, these TRI figures are estimated rather than measured quantities. However, the TRI does show the importance of the atmosphere as a medium for contaminant transport. The IJC has estimated that 99.5 percent of the lead entering Lake Michigan, 97 percent entering Lake Superior; and 94 percent entering Lake Huron comes from the air. Atmospheric lead inputs to the lower Great Lakes, Lake Erie and Lake Ontario, are 39 and 50 percent, respectively. There is more extensive contaminant-loading from sources located between the lower lakes, which accounts for the relative difference.
On the whole, inputs of contaminantsto the Great Lakes have decreased dramatically over the past decade. The Clean Air Act Amendments of 1990 and private sector environmental efforts have both made a significant impact on the input of airborne contaminants. In particular, atmospheric PCB depositions have declined, though levels are still higher than expected and remain significant. The transport and deposition of DDT and mercury are increasing.
Long banned in the U.S., DDT is still used in Mexico, Central America and Asia. Scientists believe much of the DDT now being detected in rain samples over the Great Lakes most likely originated from these sources, and, to a lesser extent, from old stockpiles in the United States. The recent rise in the atmospheric transport and deposition of mercury may be due to an increasing use of waste disposal incinerators in the United States. As landfills fill or close, incineration is becoming an increasingly popular method of waste disposal.
Ultimately, the rise in pollutants -- and decline in PCBs -- shows that airborne contaminant control, including legislation, is not just a local or even regional issue. Atmospheric contaminant transport proves that, indeed, local actions do have global consequences.
LaMPs Education Project
To order a printed version of Airborne Contaminants and the Great Lakes (complete with map), contact:
University of Wisconsin Sea Grant Institute
Goodnight Hall, 1975 Willow Drive
Madison, WI 53706-1177
U.S.A.
Telephone (608) 262-0905
Fax (608) 262-0591
Email linda@seagrant.wisc.edu
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