Thousands of used butane cans used to process concentrated marijuana dumped in the forest in Humboldt County, California
Pollution from illegal marijuana farms deep in California’s national forests is far worse than previously thought, and has turned thousands of hectares into waste dumps so toxic that simply touching plants has landed law enforcement officers in the hospital.

The volume of banned or restricted pesticides and illegally applied fertilisers in the woods dwarfs estimates by the US Forest Service in 2014, when a top enforcement official testified that the pollution was threatening forest land in California and other states.

California accounts for more than 90% of illegal US marijuana farming, with much of it exported to other states from thousands of sites hidden deep inside forested federal land, and more on private property, law enforcement officials said.

The state is still developing a licensing system for growers even though legal retail sales of the drug will begin next year, and medical use has been allowed for decades.

Ecologist Mourad Gabriel, who documents the issue for the Forest Service as well as other state, local and federal law enforcement agencies, estimates California’s forests hold 41 times more solid fertilisers and 80 times more liquid pesticides than Forest Service investigators found in 2013.

Growers use fertilisers and pesticides long restricted or banned in the US, including carbofuran and zinc phosphide. In previous years, it was commonly sold fertilisers and pesticides that were used illegally, law enforcement officials said.

Exposure to the pesticides has sent at least five law enforcement officials and two suspects to hospitals with skin rashes, respiratory problems and other symptoms, court documents and state data show.

Use of any chemicals in national forests is against federal law, as pesticides have killed sensitive species and fertilisers can cause algae blooms and bacteria problems in rivers and streams.

According to unpublished data, Gabriel, who has visited more than 100 sites in California and is widely considered the top expert on toxics at marijuana farms, calculated that federal land in California contains 332000 kilograms of solid fertiliser, 14000kg of concentrated liquid fertilizer and 5700kg of toxic pesticides.

“We’re getting contamination over and over again at those locations,” said Gabriel, as toxins move from unsafe containers into the soil and water.

At sites that state officials said they had cleaned up completely, his team found 30-50% of the chemicals were still there.

“They are like superfund sites,” said assistant US attorney Karen Escobar, whose Fresno office has filed numerous marijuana-related environmental damage cases.

Superfund sites are those targeted by the US government for hazardous waste clean-up because of the risk to human health or the environment.

The most toxic sites cost as much as $100000 to clean up, leaving taxpayers with a bill that could reach $100million or more in California alone.

“These places aren’t safe to go into,” said state assemblyman Jim Wood, who has pushed for clean-up funding.

Use of toxics has grown over the past three to four years, and chemicals have been found at sites in Oregon and Washington as well, said Chris Boehm, the Forest Service’s assistant director for enforcement and investigation.

As California moves to license growers, officials plan to regulate the use of chemicals.

But rules can only be enforced against those who cultivate pot legally. - Reuters