Cannabis is a demanding plant. When legalization efforts across the country began to take root, environmentalists sounded the alarm. An article in Nature noted that a single cannabis plant can require up to 22 liters of water every day. The article also pointed to one study’s finding that “one kilogram of cannabis in an indoor farm is associated with 4,600 kilograms of carbon dioxide emissions — roughly equivalent to the emissions from 3 million cars.” 
California now has about 10,000 cultivation permits. Because cannabis cultivation efforts have been stunted in the underground market for decades, researchers and growers are still figuring out the best ways to raise their plants. Some states require growers to commit to following environmental regulations. In California, for example, permit holders are forbidden from diverting surface water during the dry season.
The concerns over cannabis’ environmental impact range from worries over a dwindling water supply to fears that wildlife could be affected. A researcher at the Integral Ecology Research Center in Blue Lake, California, determined that a local fisher population was being harmed by a rodenticide commonly used by illicit cannabis growers. 
According to Jennifer Carah of the water program at The Nature Conservancy of California: “The black market is not going away… but to the degree that we can entice growers into the legal market, their agricultural practices can be regulated like other agricultural crops, which will go a long way to addressing potential environmental impacts.”
Many of the potential problems in the industry can be solved by aggressive legislation. It’s been argued that cannabis plants use a similar amount of water as other commonly grown plants like tomatoes and grapes. However, black market growers compound the issue by setting up their operations in remote, dry areas that can’t support the farm’s needs without harming the local watershed.
As a consumer who cares about the environment, your best option is to purchase legal cannabis.
-  Mills, E. “The carbon footprint of indoor Cannabis production”, Energy Policy, Volume 46, July 2012, pp. 58-67. [journal impact factor = 4.880; cited by 26 (ResearchGate]
-  Gabriel, M. W., et al. “Anticoagulant Rodenticides on our Public and Community Lands: Spatial Distribution of Exposure and Poisoning of a Rare Forest Carnivore.” PLoS ONE, vol.7, no.7, 2012, e40163. [journal impact factor = 2.776; cited by 69 (ResearchGate)]
Image source: Cannabis Business Times