A study recently published in the “Water Quality Research Journal” by U.S. Forest Service Law Enforcement and Investigations ecologists and partners discovered surface water contamination below illegal cannabis grow sites.
Previously, Forest Service ecologists documented the environmental threat illegal grow sites posed to terrestrial wildlife, including the federally protected northern spotted owl and Pacific fisher. However, little was known about the threats illegal cannabis grow sites pose to nearby surface waters and its wildlife. 
After six years of monitoring, illegal and banned pesticides have been unexpectedly detected in surface waters below these sites.
National forest lands support over 50% of California’s freshwater, 75% of California’s fish and wildlife, and 62% of native plants. The surface water in national forests provides critical aquatic and riparian habitat for many species, plus clean water to rural communities, agriculture, municipalities and Indigenous tribes.
“The results of this study were surprising and further highlight the need for the Forest Service to disrupt these clandestine sites and monitor their impacts to conserve the public’s natural resources for our and future generations to enjoy,” said Dr. Mourad Gabriel, co-author and the Regional Wildlife Ecologist for Law Enforcement and Investigations, Pacific Southwest Region. Mourad also serves as the Trespass Cultivation Ecology, Safety and Reclamation program lead.
“In California, national forests play a vital role for many wildlife, fish and plant species,” Gabriel added. “Studies like this allow us to continue our proactive role in removing pesticides off the lands we manage before fires, wildlife or weather create further surface water contamination.”
The study, conducted on four national forests in California, used new polar organic chemical integrative samplers to detect water-soluble pesticides commonly used on illegal grow sites. The samplers were deployed at four critical areas near illegal sites and at a larger watershed scale. Pesticides — including banned carbofuran and diazinon — were detected at 11% of downstream sampling stations during the first wet season following site eradication.
Gabriel emphasized that Forest Service Law Enforcement and Investigations proactively removed over 100 pesticide containers from 56 illegal cannabis cultivation sites in 2022 before they could pose a risk to watersheds on national forest lands. “Our findings in this study show the need for proactive management of trespass cannabis cultivation sites to reduce or eliminate surface water contamination for humans, fish and wildlife,” Gabriel concluded.
This study was conducted by an interdisciplinary team of scientists that also included the Integral Ecology Research Center, University of California Davis and California Department of Fish and Wildlife. 
Funding for the study was provided by the U.S. Fish and Wildlife Service Traditional Section 6 Grant, U.S. Department of Agriculture, Forest Service Law Enforcement and Investigations, Plumas National Forest, National Fish and Wildlife Foundation Grant and Trinity County.