Probably residue from good Canadian weed!
Back in the 1990’s, I got a call from the RCMP. They needed to dispose of two tractor trailer loads of primo Colombian weed, seized from an old fishing trawler they caught. The weed was fresh cut from the fields and placed in burlap bags. The bags also contained tarantulas and other poisonous insects.
I arranged for them to burn the illegal weed in a pulp and paper mill bark burner with a very high stack. Only thing affected by the emissions were the seagulls.