One of the Canadian lakes sampled.
Photo: Joshua Kurek

The lakes of New Brunswick, Canada, carry a toxic legacy. Researchers have found traces of the pesticide DDT in sediments of five of its remote lakes, more than 50 years after the government stopped administered the chemical to fight outbreaks of the nasty spruce budworm. This is the same infamous chemical that inspired acclaimed environmentalist Rachel Carson to write of vanished birds and silent springs, which ultimately led to DDT being banned in the United States. It appears that the infamous pesticide remains in our environment long after it’s been used.

Published in Environmental Science and Technology Wednesday, the new study looks at the lake basins of Upsalquitch, Sinclair, California, Goodwin, and Middle Peaked Mountain lakes close to where authorities sprayed millions of acres of conifer forest with more than 6,000 tons of the dangerous chemical a year from 1952 to 1968. Ultimately, the chemical made its way into the nearby lakes, as the sediment cores the team of researchers examined showed.

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The sediments in our oceans and lakes carry the memory of all these environments have seen throughout their lifetimes. Sediments are constantly falling onto the seafloor or lakebed; entrapping chemicals and fossils to provide a glimpse of history. That’s why scientists refer to sediment cores (and ice cores!) as an environmental record.

The team took a sediment core at each lake between May and June 2016, which gave the scientists a snapshot of the lakes’ conditions from 1890 to 2016. The researchers saw peak DDT levels in the mid-1970s, which is in line with the date of application. The levels ranged from 139 parts per billion to 4,500 parts per billion in the total DDT found per sediment core. The Canadian Council of Ministers of the Environment placed the acceptable levels at nearly 5 parts per billion in freshwater environments.

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Even modern samples saw DDT concentration levels at all the lakes higher than what is considered safe for aquatic life. The most impacted lake saw sediments with DDT levels about 450 times higher than what’s acceptable, per the study.

And, of course, this has left its mark on the ecosystem. The zooplankton species that survived in the years after DDT entered the ecosystem were smaller-bodied ones, according to an analysis of fossils in the cores. These critters typically handle contamination better than their larger relatives. However, the researchers are unsure whether this shift had an impact on the broader food web, or what that impact might have been. Some scientists suspect a shift toward smaller zooplankton can contribute to algae blooms (like the horror show in Lake Erie or Florida beaches).

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The National Pesticide Information Center has given DDT an aquatic half-life of 150 years. It requires five half-lives to almost entirely degrade; that means DDT can be present in aquatic environments some 750 years after its application. So while the results aren’t totally surprising, they are sobering, especially given that DDT is known to be “highly toxic” to fish and “moderately toxic” to amphibians.

A 2010 study found that DDT is still being recycled from our oceans into our atmosphere and repeat. This study is just the latest reminder of how far reaching our toxic legacy will be.

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