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Pesticides from illegal cannabis are contaminating California waterways, survey says

A small cannabis plant in the dirt with a white residue all over it
Integral Ecology Research Center
A cannabis plant in an illegal grow site covered with pesticide spray

A newly published study confirms for the first time that heavy pesticide use on illegal cannabis grows in Northern California is contaminating local waterways.

Researchers with the Humboldt County-based conservation group Integral Ecology Research Center have previously published studies showing that sensitive species such as the Pacific fisher and northern spotted owl are at risk of poisoning from chemicals used at these grow sites.

Illegal cannabis growers were already known for their liberal use of sometimes banned pesticides on grows hidden deep on public lands in Northern California.

Now, the IERC has completed a six-year study to confirm those pesticides were leaching into downstream waterways.

The findings, published last month in the peer-reviewed Water Quality Research Journal, focused on illegal cannabis grows on public lands. They were done in collaboration with the U.S. Forest Service, UC Davis and the California Department of Fish and Wildlife.

“We get about 50% of our freshwater resources in California from headwater streams on National Forest Service lands," said lead author Ivan Medel. "So that is a major concern that these sites are present within those areas.”

These pesticides – including the highly-toxic chemicals Diazinon and Carbofuran, the latter banned in the U.S. – leach into the ground during the growing season, and then wash away once the rain comes.

Medel said researchers collaborated with law enforcement to find out where illegal cannabis grows were discovered. Once the sites were raided, scientists placed special devices to sample the water downstream and throughout the watershed to detect chemicals.

“We’re talking about locations that are deep in the forest," Medel said. "They’re in highly remote and rugged areas. Standard soil or water mitigation techniques that you would do on private property, they wouldn’t be feasible in these locations.”

Medel said resources need to be invested into stopping these cannabis grows before they’ve started. Earlier this year, California announced it's expanding a seasonal illegal marijuana eradication program into a year-round effort. Instead of focusing solely on finding and stopping illegal grows, the new program will look at other underlying causes to prevent these illicit operations from starting in the first place.

This study only confirms these chemicals are getting into nearby streams. Medel said researchers are now focusing on looking at the long-term effects of these pesticides in the ecosystem.

After graduating from Oregon State University, Roman came to JPR as part of the Charles Snowden Program for Excellence in Journalism in 2019. He then joined Delaware Public Media as a Report For America fellow before returning to the west coast.