According to Yahoo!, an ecologist and consultant to the US Forest Service claims that illegal pot farming has produced contamination from liquid pesticides. The ecologist, Mourad Gabriel, says that forests in California now hold 80 times more liquid pesticides than they did merely 4 years ago (2013).
The Yahoo! Article says that illegal pot growers “use fertilizers and pesticides long restricted or banned in the United States, including carbofuran and zinc phosphide.” One estimate is that there are 4,000 illegal farms in one county (Trinity in Northern California) alone.
California is apparently in the process of attempting to establish regulations for chemical use on pot farms. The regulations are designed to promote the use of non-toxic chemicals. However, illicit pot growers may be no more inclined to follow the regulations than these growers are to follow the law in general. It is probably the case that severe legal punishment needs to be established for the illicit use of pot-growing chemicals poisoning the forests.
You can read the Yahoo! Article at https://www.yahoo.com/news/toxic-waste-u-pot-farms-alarms-experts-100944016.html.