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Lake Superior Recycles Its PCBs

Clouds

Despite a ban, PCBs (polychlorinated biphenyls) still enter Lake Superior through rainfall and diffusion from the air to the water.

People usually think recycling is good for the environment. But it’s not such a good thing where Lake Superior and PCBs (polychlorinated biphenyls) are concerned. Sea Grant researchers have found that instead of burying PCBs in its sediment like smaller freshwater lakes do, Lake Superior converts PCBs that settle to its murky depths into fish food. That may be why even though PCBs are declining in Lake Superior’s water and fish, they still contaminate the fish at relatively higher levels than expected.

Jeff Jeremiason, currently a scientist with the Minnesota Pollution Control Agency, was one of four Sea Grant researchers who conducted a study focused on the role that Lake Superior’s bottom, or benthic region, plays in recycling PCBs. Along with principle investigator, Steve Eisenreich of Rutgers University, Jeremiason examined how Lake Superior differs from smaller freshwater lakes in processing PCBs.

“Our research validates that PCB levels have been steadily dropping in Lake Superior since the chemical was banned in 1976,” said Jeremiason. In fact, levels fell 20 percent each year from 1984 to 1991. PCBs were once used in many industrial products, including coolant for electrical transformers and in dyes and printing inks. Despite the United States’ ban, they still enter Lake Superior through rainfall and diffusion from the air to the water.

Jeremiason and his colleagues measured PCB levels in the detritus that settled into sediment traps, 4-8 inch diameter plexiglass tubes with attached bottles and funnels. They placed approximately 40 sediment traps at five locations in the lake at various depths. As they analyzed the sediment, the researchers recorded trends in concentrations, examined different levels of contaminants in the water column, and hypothesized how their findings might cause higher PCB concentrations in the Lake Superior food web.

They discovered that the benthic region plays an important role in hindering the permanent burial of PCBs in the sediment. The benthic region in Lake Superior is a murky world of dead, PCB-contaminated algae and waste from zooplankton and fish who have eaten contaminated algae. It’s a source of food for amphipods, which in turn are food for bottom-dwelling fish. These bottom-dwellers are preyed upon by sport fish.

The researchers called this process ‘recycling.’ “That’s because this process reintroduces PCBs back into the water column, increasing the residence time of PCBs in the lake, and possibly increasing exposure to the fish,” said Jeremiason. “The lake rapidly moves PCBs from the surface via settling particles to the bottom, but it’s very inefficient at incorporating PCBs into the sediment. Most of the PCBs in Lake Superior don’t get buried in the sediment at the bottom of the lake,” said Jeremiason. “They get recycled.”

This recycling process in the benthic region is not as significant in smaller bodies of freshwater. There, PCBs have a good chance of sinking to the bottom, and, if the benthos remains undisturbed, the chemicals are effectively buried.

“Benthic recycling in Lake Superior could be a major entry point for PCBs into the food web,” said Jeremiason. This may explain why Lake Superior fish have higher PCB levels than fish in other smaller freshwater lakes with higher PCB levels in the water. The oligotrophic nature of Lake Superior, with its scarcity of food and nutrients, makes the benthic region an important food source.

How much of the chemical is being recycled? “Twenty-six thousand kilograms of PCBs were lost from the lake from 1980 to 1992, mainly via volatilization from the lake surface,” said Jeremiason. “That’s 26 metric tons.” If not for efficient recycling, more PCBs would have been buried in the sediments, rather than re-emitted back to the atmosphere. Estimates indicate that Lake Superior still contained about two tons of suspended PCBs in 1992, plus about five tons buried in the sediments. Jeremiason said that worldwide, PCB levels are slowly dropping as the chemical is buried or degraded into the environment.

Jeremiason’s results were published in the journal Environmental Science & Technology. To find out more, you can order a reprint of the article, PCB Decline in Settling Particles and Benthic Recycling of PCBs and PAHs in Lake Superior from Sea Grant. Look on the publications page and our journal reprint order form, under JR 442.


By Gail Trowbridge
April 1999

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