On November 8, 2016, four additional U.S. states (California, Massachusetts, Nevada, and Maine) legalized the use of recreational marijuana and four more relaxed medical marijuana laws. This is effectively creating a new industry in United States, one that looks set to rival all but the largest of current businesses with projected income expected to exceed that of the National Football League by 2020. In Colorado sales revenues have reached $1 billion, roughly equal to that from grain farming in the state and a third higher than residential construction,(1) an industry with strict environmental monitoring procedures.
The few studies that have investigated specific practices associated with marijuana cultivation have identified potentially significant environmental impacts due to excessive water and energy demands and local contamination of water, air, and soil with waste products such as organic pollutants and agro-chemicals(2, 3) (see Figure 1). Cannabis spp. require high temperatures (25–30 °C for indoor operations), strong light (∼600 W m–2), highly fertile soil,(2) and large volumes of water (22.7 l d–1 per plant,(3) around twice that of wine grapes(3)). A study of illegal outdoor grow operations in northern California found that rates of water extraction from streams threatened aquatic ecosystems(3) and that water effluent contained high levels of growth nutrients, as well as pesticides, herbicides and fungicides, further damaging aquatic wildlife.(3)