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Contaminated Sediments and the Great Lakes

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by Tiernan Henry
UW Sea Grant Water Quality Specialist
 
 

Contents

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Situation

The Great Lakes are among the largest bodies of surface fresh water on Earth. Despite their size, the lakes are still susceptible to the effects of contaminants. Any substance that is not naturally present in the environment, or that is present in environmentally harmful concentrations, is considered a contaminant. Contaminants are washed into the lakes via rivers and streams, or through pipes and discharge outlets. Lakes are also exposed to contaminants via atmospheric depositions.

The International Joint Commission (IJC), created by the United States and Canada to advise both governments on Great Lakes water issues, has designated 43 Great Lakes Areas of Concern (AOCs). These AOCs are highly contaminated. Contaminated sediments have been identified as a problem in 42 of them. A Critical Pollutants List comprised of the most serious 11 pollutants has been drawn up by the IJC.

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Contaminated sediment deposition

Contaminated sediments, primarily in harbors and industrialized segments of rivers and estuaries, contain pollutants that have been entering the lakes for decades. As rivers and streams enter the lakes, they slow down and lose their ability to carry sediments. The heavier material drops out first; finer silts and clays are carried farther from shore. And because many contaminants are associated with different sediment types, contaminant distribution is often linked to sediment deposition. In addition to river mouths and harbors, three different Great Lakes "sedimentation zones" have been identified. They are:

non-depositional zones
scoured regions where sediment does not permanently accumulate
depositional zones
regions of high sedimentation
transitional zones
regions with thin layers of recently deposited materials

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Why the Great Lakes are particularly susceptible

The Great Lakes contain 20 percent of the world's surface fresh water. Every year less than one percent flows out. The lakes have long water retention times - the time span for water to move through the system - that make them nearly closed systems. Due to a relatively low outflow rate, contaminants don't leave the lakes quickly. A fraction of some contaminants can be "removed" from the system by volatilization, which is a form of evaporation. A fraction can be broken down in the water and another fraction buried in bottom sediments. The rest can be removed by the slow flushing of the system.

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Where they end up

Until permanently buried, contaminants can be stirred up in the sediment through a variety of processes, including storms, biological activity, dredging, ship and boat movement, and wave action. Coupled with the long water retention times, contaminants may remain active for long periods.

Many lake contaminants break down quickly. Some, however, are long-lasting, or "persistent." Persistent contaminants generally accumulate at higher levels in organisms than in the surrounding water. In open water, contaminant levels are often difficult to detect. And persistence of contaminants can lead to contaminant levels harmful to the ecosystem. Several factors make the Great Lakes particularly susceptible to the effects of persistent contaminants:

  • nearly closed system
  • long water retention time
  • low sedimentation rates
  • low biological productivity
  • low suspended solids concentrations
  • the presence of a largely self-contained fish and wildlife
  • population dependent on the lakes for food and water
  • heavy concentration of industries and large population centers
  • surrounding the lakes
  • huge surface areas susceptible to airborne contaminants.

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Contaminants and the food chain

The Great Lakes food chain's lowest level is occupied by phytoplankton -- microscopic plants that absorb their necessary nutrients from the water. As phytoplankton absorb nitrogen and phosphorus, they also collect contaminants and are eaten by zooplankton, which, in turn, are eaten by progressively larger fish. Phytoplankton probably get most of their contaminants from the dissolved fraction of contaminants in the water column, but some can also be transferred in the suspended fraction.

Although pollutants may be excreted by a fish, most of the persistent contaminants are stored in the soft, fatty tissue and gradually build up, or bioaccumulate. Persistent contaminant concentrations in older, larger lake fish such as lake trout and salmon may be more than a million times higher per unit weight than concentrations in the surrounding lake water. The process by which a contaminant increases in concentration as it rises in the food chain is called biomagnification.

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PCBs

Harmful effects of PCBs (polychlorinated biphenyls) on fish and fish-eating birds have been documented. PCB effects on humans are less well understood. Because of the known impact on wildlife, there is justifiable concern about the effects of PCBs on human health. Scientific and medical researchers suspect that prolonged exposure to small doses of PCBs can contribute to a variety of human health problems, including developmental problems in children, liver damage and cancer. For this reason, health advisories caution against eating large amounts of large Great Lakes fish. The U.S. Food and Drug Administration has determined that fish with PCB levels greater than two parts per million (ppm) pose a human health risk. PCBs accumulate in the body fat of humans just as they do in fish, and most are not passed out of the body. However, indications are that if the population heeds the health advisory, individuals can be reasonably assured that they are adequately protected.

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The high costs of cleanup

Based on a preliminary review of Superfund case studies, the U.S. Environmental Protection Agency (EPA) recently concluded that the benefits of remediating Great Lakes sediments are at least as great as the costs. This comparison held even when the only benefits considered were avoiding human cancer caused by not eating contaminated lake fish. The EPA also concluded that the benefits would far outweigh costs if economic values could be assigned to non-cancer health effects and environmental impacts. Nonetheless, remediation costs are huge. Treatment and disposal costs are also extremely high. The cleanup of just one AOC can cost millions of dollars and take several years to complete.

Under the terms of the Great Lakes Water Quality Agreement, developed by the IJC, U.S. states and Canadian provinces must develop Remedial Action Plans (RAPs) to fully restore polluted AOCs. The Great Lakes states and Ontario have primary RAP development responsibilities. RAPs must be based on an ecosystem approach, which looks not only at the affected area, but also at pollution sources and pathways. Because the Great Lakes ecosystem is essentially closed, such an approach is especially important.

RAPs are developed in three stages. The first stage looks at impacts and stresses. The second stage develops remedial actions and implementation methods. The third stage establishes how impaired uses have been restored.

There is no specific cleanup strategy for contaminated sediments. Remediation varies depending on contaminant type and toxicity, contaminant distribution in the sediment, and surface area, depth and volume of the contaminated layer. The potential for sediment transport and the movement of contaminants from sediments to the water or aquatic community are also factors.

Controlling pollution sources before remediation begins is, in many cases, the most important part of sediment cleanup. Otherwise, millions of cleanup dollars may be wasted. However, several factors are involved in the cleanup decision-making process:

  • Is contaminated sediment contributing to severe effects on the ecosystem?
  • Are sediments posing an immediate risk for spreading harmful contamination?
  • Are sediments likely to remain in place, or is there a chance they will be transported downstream or farther offshore?
  • What is the remediation time frame, and is there potential for further contaminant remobilization during remediation?
  • What are the costs, and how feasible are alternative treatment and remediation options?

Contaminated sediments dredged from harbors and estuaries can not simply be dumped farther out in the lakes, nor can the severely contami-nated sediments be placed in ordinary landfills. From burying contaminants in sealed landfills to super-heating sediments where contaminants are completely destroyed, various treatment options are available. Unfortunately, they're all very expensive.

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The Waukegan case

In Waukegan Harbor, Ill., almost one million pounds of PCBs were removed from the harbor and surrounding land. Before cleanup began, PCB levels in some harbor sediments were as high as 500,000 ppm. In other words, half of certain harbor sediments were PCBs. By the time cleanup was completed, PCB concentrations were lower than the 50 ppm limit set by the Toxic Substance Control Act. Specifically, more than 12,500 tons of contaminated harbor sediment were treated, 32,000 cubic yards of sediments dredged from the harbor, and about 32,000 gallons of contaminated fluids removed. In total, 97 percent of the PCBs were removed from the contaminated soil and sediment, though the harbor is still listed as an AOC and testing continues. PCB-contaminated sediment was treated in a heated "desorption" unit that removed most of the organic contaminants. The treated soil was placed in three nearby landfills that will be monitored indefinitely by the EPA. Total cleanup costs for the three-year project were $21 million.

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The Green Bay/Fox River Mass Balance Study

The recently completed Green Bay/Fox River Mass Balance Study was undertaken to determine the feasibility of using a mass-balance approach for assessing contaminant sources, transport and ultimate fates in the Great Lakes food chain. The mass-balance model for PCBs is, essentially, an equation: PCBs entering the system minus PCBs leaving equal PCBs stored, transformed or degraded. This approach is ideal for setting priorities and guiding management and regulatory policies.

The study revealed, among other things, that most PCBs presently entering Green Bay via the Fox River come from river bottom sediments, which currently contain an estimated 53,000 pounds of PCBs. Many of these PCBs were originally discharged from industrial operations in the Fox River Valley and became incorporated in the sediments. Some of these continue to slowly leak back into the system.

The EPA and Wisconsin Department of Natural Resources are currently undertaking field studies for a more focused study of the Fox River in order to prioritize specific sites for remediation.

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The future

Further studies and cleanup activities are planned for the Great Lakes. Although contaminant inputs are decreasing, decades-old contaminated sediments continue to be a costly problem. Research projects like the Green Bay/Fox River study should help scientists and city managers determine cost-effective remediation and cleanup efforts, and better understand the location, transport and makeup of contaminated sediments.

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To order a printed version of 
Contaminated Sediments and the Great Lakes,
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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last updated 22 February 2001

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