More than 6,280 tons of DDT were sprayed on the forests of New Brunswick, Canada to fight insect outbreaks in the 1950s and 1960s. Now, 50 years after the chemical was banned, scientists have discovered it lingering in the sediments of remote lakes at levels high enough to potentially alter entire aquatic food webs.

The research, published in the journal Environmental Science & Technology, collected samples from five lakes in New Brunswick in the spring of 2016, taking sediment cores dating as far back as 1890. Unsurprisingly, levels of DDT peaked in the 1960s and 1970s. But the team of Canadian scientists found that the uppermost layers of sediment — deposited in the last few years — still had levels of DDT and its toxic breakdown products well above 5 parts per billion, the limit for what is considered safe by the Canadian Council of Ministers of the Environment. On average, modern-day sediments in the lakes had levels 16 times higher than the safe limit; one lake measured 450 times the limit, Smithsonian reported.

“What was considered yesterday’s environmental crisis in the 1950s through [the]1970s remains today’s problem,” lead author Josh Kurek, an assistant professor in geography and the environment at Mount Allison University, said in a statement. “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.”

The scientists also found that the presence of DDT has altered the composition of zooplankton in the lakes. Fossilized remains of Cladocera, or water fleas, show a shift from large-bodied to small-bodied zooplankton species — which are generally more tolerant of contaminants — beginning in the 1950s. For example, populations of water fleas in the genus Daphnia, a key part of lake food webs, significantly declined in recent decades as DDT levels increased. According to the National Pesticide Information Center, DDT has an aquatic half-life of 150 years and is “highly toxic” to aquatic animals, including fish, impacting systems such as the brain and heart. The authors of the new study note that aquatic organisms in these New Brunswick lakes “may contain high levels of DDT within their tissues.”