Even as states legalise marijuana, some US officials are demanding tougher sentences for illegal potgrowers if they also invade public lands, kill native vegetation and wildlife, and spread toxic pesticides.
The officials' environmental concerns took centre stage at a hearing in Washington, DC, of the US Sentencing Commission, a government body that guides federal judges on penalties for convicted criminals.
Illegal marijuana crops often are grown by Mexican drug cartels that find it easier to plant on the US side of the border rather than smuggle in the drugs, say prosecutors and law enforcement officials.
Sometimes armed, the growers plant huge gardens in remote parts of national forests and parks, setting up clandestine camps, diverting streams for irrigation and spreading pesticides, some so poisonous they are banned in the United States, the officials say.
Problem could worsen
Christopher Boehm, assistant director in the US Forest Service's investigations arm, told the commission that the problem could become worse as marijuana acceptance increases, with growers trying to meet higher demand by expanding crops on public lands.
Marijuana is legal for medical use in 20 states, and Colorado and Washington State have legalized its sale for recreational use.
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The US Justice Department, joining the call for tougher environmental penalties, proposed increasing prison time by months or even years by treating illegal marijuana growers more like methamphetamine producers, who use toxic chemicals.
The proposal's impact would vary widely depending on the criminal history of each defendant.
In the US House of Representatives, Jared Huffman of California – where the bulk of marijuana is grown – has led a bipartisan drive for sentencing changes to consider environmental harm and has been joined by Utah Republican Orrin Hatch and California Democrat Dianne Feinstein in the Senate.
The lawmakers' call for harsher penalties contrasts with the Obama administration's move to slash jail time for federal drug defendants to try to cut the ballooning prison population.
"It may seem contradictory that someone in my position would on the one hand be arguing for stronger criminal penalties on this problem and simultaneously arguing for decriminalisation as the ultimate solution," Huffman told Reuters in a phone interview. "But that is the only responsive approach. It gets at the immediate problem and the longer-term solution."
In 1996, California passed a law that allows marijuana growth for medical use, but any cultivation on federal land is considered a crime under the Controlled Substances Act. The drug remains illegal under federal law.
That leaves state regulators stretched between trying to regulate the environmental practices of legal medical growing operations, while combating the illegal ones in the forests.
In the United States, the problem has grown in recent years. In an informational video, the US Forest Service said that in the 1990s rangers might find patches of a few hundred plants but now regularly discover huge operations with many thousands of plants.
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A single plant can use up to 15 gallons of water per day, the Forest Service says, and fish populations can be hurt when natural water sources are dammed or drained.
In 2012, nearly one million marijuana plants were found and destroyed at 471 sites on National Forest lands in 20 states across the country, lawmakers say.
State and federal agencies, as well as private land holders, often get stuck with the clean-up bill.
Depredation of public land
"(They) bring clothes and old tents and plastic waste and beer cans, and we have to clean all that crap up so the animals don't eat it and it doesn't pollute our rivers and lakes and streams," said Jay Green, an agent at the US Environmental Protection Agency's criminal enforcement division in California. "They should be held accountable for that."
In the past, prosecutors have used different measures to try to win restitution awards when growers are arrested, charging them with depredation of public land or violations of federal pesticide regulations in addition to the drug charges.
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What about legal growers?
Federal environmental regulators are in a bit of a quandary also about policing legal growers.
In the "Emerald Triangle," private land cultivated with marijuana doubled between 2009 and 2012 as growers from around the country flocked to the area in a "green rush" to supply medical marijuana dispensaries, said Scott Bauer from California's Department of Fish and Wildlife.
Some medical marijuana growers – with outside fields or indoor greenhouses – use a large amount of water and fertilisers but are not subject to the same federal rules as traditional farmers, said the EPA's Green.
He said the chemical run-off from legal plots can flow into storm drains or sewers, which for a conventional farmer would require a permit from the EPA. But because marijuana is illegal under federal law, the EPA is not allowed to permit a pot farm.
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