Some Canadian lakes still store DDT in their mud

Five decades after DDT was last sprayed across Canadian forests, this harmful pesticide can still be found at the bottom of several lakes.

Researchers analyzed sediment from five lakes in New Brunswick, Canada, where airplanes spewed DDT to combat spruce budworm outbreaks before the insecticide was phased out circa 1970. Millions of kilograms of DDT were sprayed across the province, making it one of the most heavily treated forest areas in North America. Today, elevated concentrations of DDT and its chemical by-products persist in lake sediments in this region, researchers report online June 12 in Environmental Science & Technology. 

“This is a cautionary tale,” says study coauthor Joshua Kurek, an environmental scientist at Mount Allison University in Sackville, New Brunswick. “When we ban a substance, we tend to forget about it.” But these results indicate that decades-old pesticide sprays may have left a lasting impression on hundreds to thousands of lakes in eastern North America. 

Kurek and colleagues measured concentrations of DDT, as well as its toxic breakdown products DDE and DDD, in lake sediments dating back several decades. Most modern sediments from all five lakes exceeded the safety thresholds for aquatic life for DDT, DDE and DDD set by the Canadian government: 4.7, 6.7 and 8.5 micrograms per kilogram, respectively. 

The only lake that contained safe DDT levels in modern sediments still contained DDE and DDD concentrations that were 3.7 and 1.5 times as high as safety thresholds. In all five lakes, on average, DDE levels in modern sediments were about 16 times as high as the safety threshold (SN Online: 10/5/10). Still, the levels aren’t considered dangerous to people.