Significant traces of the harmful pesticide DDT, or dichloro-diphenyl-trichloroethane, are still present in the sediment of Canadian lakes, nearly 50 years after the last drop was sprayed, a new study published in the journal Environmental Science & Technology has found.
One of the major uses of DDT was controlling insect outbreaks, like spruce budworm, in conifer forests across North America. Thousands of tons of the chemical were blanketed over forests using airplanes and helicopters—and a portion of it washed into the lakes. Bob Weber at The Canadian Press reports that in the province of New Brunswick alone, almost 6,300 tons of the stuff covered forests between 1952 and 1968.
That’s why the study’s lead author Joshua Kurek, an environmental scientist at Mount Allison University, and his colleagues wanted to measure just how much DDT still persisted in the lake sediments of remote bodies of water in the province. The team collected sediment samples from five lakes in May and June 2016, pulling up cores that went back to 1890.
Most of what the team found wasn’t too surprising. The amount of DDT in the sediment peaked in the 1960s and 1970s. But they were dismayed to find that the levels of DDT in the top layers of sediment were still significantly more than 5 parts per billion, which is considered acceptable by the Canadian Council of Ministers of the Environment. On average, the modern-day sediments were 16 times beyond that level, but one lake was 450 times over the limit. That means DDT and its toxic breakdown products, DDD and DDE, are still hanging around causing problems in those lakes—and likely many more throughout the continent.
“What was considered yesterday’s environmental crisis in the 1950s through 1970s remains today’s problem,” Kurek says in a press release. “Decades of intense insecticide applications to our conifer forests have left a lasting mark on these lakes—and likely many others in eastern North America.”
Yessenia Funes at Earther reports that it’s hard to quantify exactly what the current impacts of the DDT are on lake ecosystems. The authors note that populations of small water fleas in the genus Daphnia, an important species in the lake food web, have declined precipitously since use of DDT began. Other studies have shown that the disappearance of Daphnia has cascading effects on lake ecosystems:
Algae overgrowth begins which in turn reduces populations of prey fish available for top predators to eat.
Fossils in the sediment cores also show that as DDT levels rose, lake ecosystems became stressed, which then favored a smaller type of zooplankton that often flourishes in tough conditions. Currently, it’s unclear how a boom in smaller zooplanton has affected the rest of the ecosystem. Some researchers suspect, as Earther’s Funes reports, that this kind of zooplankton is often associated with toxic algae blooms. Kurek says he hopes to look at the DDTs continuing effects on bird and fish populations in a follow-up study.
Water ecologist John Smol at Queen’s University in Ontario, who not involved with the study, says this new research is important. “There’s this legacy effect 50 years later in the aquatic ecosystem that has important implications for lake ecology,” he tells The Candaian Press. “Nature is slow to pardon our mistakes and we’re overly optimistic.”
In fact, it will be quite a while before people in North America can stop thinking about DDT. Funes reports the chemical has an aquatic half-life of 150 years, meaning it will take about 750 years before the amount of insecticide trapped in lakes degrades by 97 percent.
The chemical is still being used in at least 19 nations—mainly indoors in the countries of sub-Saharan Africa to prevent the spread of malaria, a use sanctioned by the World Health Organization. But the chemical is still sprayed for agricultural purposes in North Korea and India. (As of 2014, India was still using over 3,000 metric tons of the stuff per year.)
While there’s not much scientists can do to remove the DDT besides waiting for the chemical to break down. Kurek tells Weber the research should help people to think about other environmental contaminants.
“You could substitute DDT with plastic pollution, with greenhouse gases, with salting on our roads,” he says, “[A]ny contaminant that you put in our environment over a massive region is going to have tremendous effects and sometimes surprise effects,” he says.