On 1 January, Proposition 64 was welcomed with open arms and Happy New Year blunts by many Californians.

To bring in the new year, the state had become the latest in the US to legalise the drug for recreational use, following years of partial legalisation for medical purposes.

Even before full legalisation kicked in, California was already the cannabis capital of the US. “California produces the majority of the marijuana that the US consumes, legal or illegal,” said Dr Mourad Gabriel, an ecologist at the University of California, Davis.

While this status has made California popular among cannabis enthusiasts for decades, it has been accompanied by a creeping environmental disaster.

Hidden in California’s extensive forests are thousands of illegal cannabis farms. At these sites the desire to maximise production and profit and the need to grow in discrete areas that are far from ideal for agriculture, has driven growers to extreme measures.

The lack of oversight means many of these sites are doused in toxic chemicals that are working their way into the ecosystem and poisoning California’s wildlife. “Overuse of pesticides is just absolutely rampant,” said Dr Gabriel.

As many cannabis growers are already operating outside of the law, they do not feel compelled to stick to state regulations concerning chemicals and excessive use is not the only concern. On illegal farms run by cartels, scientists have found traces of fertilisers and pesticides like carbofuran that have been banned in the US for years.

In pictures: 4/20 Marijuana world rallies Show all 26 1 /26 In pictures: 4/20 Marijuana world rallies In pictures: 4/20 Marijuana world rallies A man wears a marijuana leaf mask during the annual 4/20 cannabis culture celebration at Sunset Beach in Vancouver, British Columbia AP In pictures: 4/20 Marijuana world rallies A lady smokes marijuana on Parliament Hill on 4/20 in Ottawa, Ontario Getty Images In pictures: 4/20 Marijuana world rallies James Reed smokes a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies People sign a 4/20 sign on Parliament Hill on in Ottawa, Ontario Getty Images In pictures: 4/20 Marijuana world rallies A woman smokes marijuana on Parliament Hill on 4/20 in Ottawa, Ontario. Polling released showed strong support in Canada for a government drive to legalise recreational use of marijuana, but many would like the proposed minimum age for consumption to be raised. Sixty-three percent of respondents told the Angus Reid Institute they support legalisation Getty Images In pictures: 4/20 Marijuana world rallies A man smokes marijuana during the annual 4/20 marijuana rally on Parliament Hill in Ottawa, Ontario, Canada Reuters In pictures: 4/20 Marijuana world rallies Demonstrators smoke marijuana during the '4/20 Santiago' rally in favour of legalisation in front of the La Moneda presidential palace in Santiago, as part of the Global Marijuana March which is being held in hundreds of cities worldwide Getty Images In pictures: 4/20 Marijuana world rallies People play with a mock marijuana joint during a 4/20 party to demand legalisation and to celebrate marijuana culture outside the Senate building in Mexico City, Mexico Reuters In pictures: 4/20 Marijuana world rallies Adam Eidinger, co-founder of DCMJ, hands out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC Getty Images In pictures: 4/20 Marijuana world rallies In pictures: 4/20 Marijuana world rallies Police arrest Rachel Ramone Donlan after she handed out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC AFP/Getty Images In pictures: 4/20 Marijuana world rallies Police arrest Rachel Ramone Donlan after she handed out free marijuana joints to DC residents who worked on Capitol Hill as part of the 1st Annual Joint Session to mark '4/20' day and promote legalising marijuana on Capitol Hill in Washington, DC Getty Images In pictures: 4/20 Marijuana world rallies Thousands of people gather to smoke marijuana during the '420 Santiago'rally in front of the La Moneda presidential palace in Santiago Getty Images In pictures: 4/20 Marijuana world rallies People attend the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Kevin Barron and Lasean Moore of Raleigh, North Carolina, share a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Nic Ruhl takes a pull on a giant hand rolled joint at precisely 4:20pm MDT during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Various cannabis paraphernalia on display at a vendor's stall during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies Mo Banez, of Austin, Texas, lights a joint during the Denver 420 Rally at Civic Center Park in Denver, Colorado Getty Images In pictures: 4/20 Marijuana world rallies A man displays a large container of cannabis during the Denver 420 Rally at Civic Center Park in Denver, Colorado. The rally, held annually, is a celebration of both the legalisation of cannabis and cannabis culture. Colorado is one of twenty-six U.S. states along with the District of Columbia that has legalised the use of cannabis either recreationally or medically AFP/Getty Images In pictures: 4/20 Marijuana world rallies Sitting in small groups on mats shaded by trees in the Rose Garden just across from the Knesset, participants lit up as the clock struck 4:20 for the local version of the traditional worldwide April 20 pro-marijuana events, known as '420' rallies Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem during a rally at the Rose garden Getty Images In pictures: 4/20 Marijuana world rallies Sitting in small groups on mats shaded by trees in the Rose Garden just across from the Knesset, participants lit up as the clock struck 4:20 for the local version of the traditional worldwide April 20 pro-marijuana events, known as '420' rallies Getty Images In pictures: 4/20 Marijuana world rallies An Israeli girl poses with a mock marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current laws Getty Images In pictures: 4/20 Marijuana world rallies aelis pass around a marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current laws Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem during a rally at the Rose garden, to celebrate 420 and to express their defiance of current law Getty Images In pictures: 4/20 Marijuana world rallies An Israeli smokes a marijuana joint in Jerusalem on April 20, 2017 during a rally opposite the Knesset to celebrate 420 and express defiance of current laws AFP/Getty Images

In their work, Dr Gabriel and his colleagues have found endangered owls and weasel-like mammals called fishers showing signs of poisoning by rodenticides. These chemicals are applied to prevent rats from eating sugar-rich cannabis sprouts and when ingested they cause brain damage and internal bleeding.

Besides working their way up the food chain and poisoning animals, toxic chemicals are seeping into the water supply. Here they have the potential to harm more wildlife, as well as livestock and people.

But as the cannabis industry seeks legitimacy, a chance presents itself for a crackdown on the illegal chemicals that currently prop up much of California’s cannabis production and a move towards a greener solution.

“I talk to a lot of growers who are permitted or who want to come into regulatory oversight and some of them are seeking organic status, they want to be sustainable,” said Dr Gabriel.

With legalisation, economics and market pressure could ultimately “weed out the majority” of black market cannabis production, according to Dr Gabriel. This would pave the way for a state-wide cannabis industry that complies with chemical regulations. However, the dissipation of the black market will “not take place overnight”.

Dr Reggie Gaudino is chief science officer at Steep Hill Labs, a medical cannabis research facility that aims to “empower cultivators, dispensaries, manufacturers and consumers with a transparent understanding of science”.

He says regulation of the industry, along with proper enforcement and education on alternative pest management methods, could result in an overall reduction in pesticide use.

According to Dr Gaudino, there is growing awareness about the need to protect the environment while growing cannabis: “As the industry matures the community does become aware,” he explained.

“There are several movements in the industry towards reducing the use of chemicals,” he said, mentioning the Clean Green growing programme, which allows growers to brand their product as grown without pesticides while still passing microbial testing.

However, some in the state are a little less optimistic about the future of California’s environment. Stephen Frick, assistant special agent in charge of the Forest Service, thinks the legalisation of cannabis can only be bad news for the state’s wildlife.

“I expect more people moving into the state for the sole purpose of marijuana production,” he said. “We will see more people and more marijuana production on both private and public lands.”

Instead of purely green intentions, Mr Frick is concerned these new growers have another motivation: money.

The issue, he said, is that the state lacks the capacity to deal with those who grow their cannabis using illegal chemicals. The stakes are just too high.

Oregon is producing three times more marijuana than it can consume

“Who is going to enforce the pesticide regulation in the state? What is the punishment for those caught with illegal pesticides?” Mr Frick asked.

“The millions of dollars in illicit marijuana production here in California far outweigh any fines imposed by the state for using these illegal and/or legal pesticides in an unlawful manner.

“Until California gets serious about making clean water and our environment a priority over legalising marijuana, pesticides will continue to be abused by growers regardless of the impact they have on our resources,” said Mr Frick.

In the densely forested Humboldt county, on California’s north-west coast, a report released in November revealed there are around 15,000 grow sites. Of these, only 2,300 have applied for legal permits, of which only 91 were permitted.

“So that demonstrates clearly that the majority of cultivators on private lands still want to be in the black market,” said Dr Gabriel. “Probably because of the high profit margin in the black market versus the regulated industry.”

Dr Gabriel agrees there is a lack of capacity to deal with the illegal cannabis growers and their harmful chemicals. He estimates there are no more than 300 biologists and law enforcement agents hired to address the problem of illegal pesticide use by the state’s cannabis farmers.

“That may seem like a lot of personnel, but we have upwards of 13,000 unpermitted grows that need to be inspected, enforced or eradicated in addition to the permitted grows that need inspecting in one county along,” he said.

With 58 counties in California, that’s a lot of farms that require monitoring.

“We can put regulations on the books, but it’s the question of how you enforce them, and how you standardise that enforcement,” said Dr Gabriel.

Many have referred to the burgeoning cannabis economy as a “green rush”, an opportunity for motivated entrepreneurs to legitimise a previously illegal practice and bring in a huge amount of money in the process.

However, Dr Gabriel warns that unless proper attention and respect are paid to the environment, the “green rush” analogy may be accurate in a less positive way.

“When we look at it as conservationists, what we don’t want to happen is something similar to the gold rush,” he said.

The gold rush brought enormous wealth to California, but also brought environmental devastation. Rivers were dammed, land was torn up and forests were logged to support the demand for the precious minerals.

“We are still currently spending lots of public funded dollars to restore the ramifications from the gold rush that happened 150 years ago, said Dr Gabriel.