The concentrations of legacy contaminants are decreasing quicker than previously reported in three of the Great Lakes, but stayed virtually the same in two other lakes.
The scientists published their findings in the journal Science of the Total Environment. “These are very positive results. The lakes are improving and slowly cleaning themselves up,” states Thomas Holsen, a co-author and from Clarkson University.
Even with these decreases, it will be another 20 to 30 years until the contaminants in the Great Lakes decline enough to the point that fish from them can be consumed. Older contaminants are being replaced by newer ones, which are mainly flame retardants that are building up in the fish and wildlife.
Polychlorinated biphenyls (PCBs), the pesticide DDT and other banned compounds dropped about 50% in fish in Lakes Michigan, Ontario and Huron from 1999 to 2009. There were no significant changes in Lakes Superior and Erie.
The status quo of Superior and Erie, which have had lower levels of contaminants that the other lakes isn’t surprising because Superior is big, deep, and cold, so changes will take longer. Lake Erie’s walleye have a shorter food chain that the trout in other lakes, so the contaminants don’t build up as much.
PCBs are decreasing at the slowest pace, at about 3 to 8% per year, while DDT declined between 11 to 16% per year. The pesticide mirex, which has been only detected in Lake Ontario, has decreased from 15 to 25% per year.
This is substantially faster than previously estimated. Between 1980 and 2003, PCBs and DDTs decreased 2 to 4%. Between 2000 and 2003, they remained relatively stable.
All of these compounds were phased out in the 1970s after they began building up in the environment. Since they are slow to break down, they continue to persist in the lakes’ sediments and are still accumulating in fish and other wildlife.
Bans have eliminated the manufacture and use of these compounds; they are mostly getting into the lakes through what’s circulating in the air.
Thanks to atmospheric deposition, the chemicals move from the air to the Earth’s surface. Flame retardants that have replaced polybrominated diphenyl ethers (PBDEs) in furniture and other consumer products are showing up more frequently in the region.
The old are just being replaced by something new. The concentrations of new flame retardants like Firemaster 550 are doubling every two years.
[via Scientific American]