Legacy contaminants are decreasing more quickly than previously reported in three of the Great Lakes, but have stayed virtually the same in two other lakes, according to new research.
“These are very positive results. The lakes are improving and slowly cleaning themselves up,” said Thomas Holsen, co-director of Clarkston University’s Center for the Environment and co-author of the study.
Even with the decreases, it will be 20 to 30 years until the decades-old contaminants in Great Lakes fish decline to the point that consumption advisories can be eliminated, Holsen said. In addition, the older contaminants are being replaced by newer ones, mostly flame retardants, that are building up in fish and wildlife.
Polychlorinated biphenyls (PCBs), the pesticide DDT and other banned compounds dropped about 50 percent in fish in Lakes Michigan, Ontario and Huron from 1999 through 2009, although there were no significant changes in Lakes Superior and Erie fish, according to the study to be published this month in the journal Science of the Total Environment.
The status quo at Superior and Erie – which have lower levels of contaminants than the other lakes – is not surprising, Holsen said. Lake Superior is big, deep and cold, so changes happen more slowly. And Lake Erie’s walleye have a shorter food chain than the trout tested in the other lakes, so the contaminants did not build up as much.
PCBs – industrial compounds that are the most prevalent pollutants tested – are decreasing at the slowest pace, at about 3 to 8 percent per year, while DDT declined about 11 to 16 percent per year. The pesticide mirex, which is only detected in Lake Ontario, decreased 15 to 25 percent per year.
That is substantially faster than previous rates for the contaminants: 2 to 4 percent annual decreases between 1980 and 2003. PCBs and DDT concentrations stayed relatively stable from 2000 to 2003.
“If you look at PCBs in the region, there are still very high concentrations in urban areas,” Holsen said.
All of the compounds studied were phased out in the 1970s after they began building up in the environment, particularly in the Great Lakes. Because they are slow to break down, they persist in the lakes’ sediments and still are accumulating in fish and other wildlife.
Fish consumption warnings remain throughout the basin for PCBs, which have been linked to an array of health effects, including cancer and reduced IQs in people.
PCBs also seem to affect the reproduction of lake trout. Their populations are rebounding as the PCB levels decline, but they face other threats, too, such as invasive species.
“There is significant evidence that contaminants contribute to a lack of reproduction in the lakes,” said Deborah Swackhamer, co-director of the University of Minnesota’s Water Resources Center. “PCBs and dioxin are going down in Lake Ontario and contaminants are going down in Superior, and you’re seeing more successful lake trout reproduction in these lakes.”
Since bans have eliminated manufacture and use of the compounds, they are now getting into the lakes mostly through what’s circulating in the air.
“We’ve tackled the low-hanging fruit,” said Ronald Hites, a professor at Indiana University’s School of Public and Environmental Affairs who specializes in air monitoring in the Great Lakes basin.
Through atmospheric deposition, the chemicals move from the air to the Earth’s surface. “The trends in the air are much the same. These old chemicals are decreasing,” Hites said. “Especially in the more remote regions like northern Lake Michigan and Lake Superior, it’s almost all atmospheric deposition.”