By Sharon Bernstein

SACRAMENTO, California (Reuters) - Toxic chemicals from illegal marijuana farms hidden deep in California's forests are showing up in rivers and streams that feed the state's water supply, prompting fears that humans and animals may be at risk, data reviewed by Reuters show.

The presence of potentially deadly pollutants in eight Northern and Central California watersheds is the latest sign of damage to the environment from thousands of illegal cannabis plantations, many of them run by drug cartels serving customers in other states, according to law enforcement.

"I don't drink out of the creeks - and I used to," said Sergeant Nathaniel Trujillo, a narcotics expert with the sheriff's department of Trinity County, about 200 miles north of San Francisco. "I grew up drinking out of them."

California accounts for more than 90 percent of illegal U.S. marijuana farming. There are as many as 50,000 marijuana farms in California according to state estimates, and even though voters legalized the drug last November, only about 16,000 growers are expected to seek licenses when commercial cultivation becomes legal next year.

Many of the illegal growers use fertilizers and pesticides long restricted or banned in the United States, including carbofuran and zinc phosphide.

The chemicals have turned thousands of acres of forest into waste dumps so toxic that law enforcement officers have been hospitalized after inadvertently touching plants and equipment, and scores of animals have died.

The streams in which they have been detected are crucial sources of water for fish, vulnerable animals including the Pacific fisher and the Northern Spotted Owl and are used for drinking by people and cattle. Ultimately, the contaminated rivers and creeks flow into the massive water supply system relied on by the most populous U.S. state.

"Carbofuran is in the water, and it's not supposed to be," said Mourad Gabriel, an ecologist who works with law enforcement on marijuana contamination issues. "How are we going to mitigate something like that?"

Carbofuran poisoning can cause headaches, nausea, dizziness, vomiting, uncontrollable muscle twitching, convulsions and even death, according to the National Institutes of Health.

Poisoning by diazinon, another chemical Gabriel has found in streams, can cause difficulty breathing, weakness, blue lips and fingernails, convulsion and coma, the agency says.

Gabriel, who has visited more than 100 sites in California and is widely considered the leading authority on toxins at marijuana farms, said about half the streams he studied in eight watersheds in the state's prime pot-growing regions tested positive for contaminants.

In unpublished data seen by Reuters, Gabriel's testing showed carbofuran, diazinon and other chemicals were present downstream from pot farms in Kern County in Central California, Humboldt County on the state's northwestern coast, Mendocino County north of Santa Rosa and others. In some cases, the chemicals were present only in trace amounts.

Some streams tested positive more than a year after law enforcement cleared illegal grows from nearby land.

At Brush Mountain in Kern County, law enforcement shut down a growing operation in June 2014, Gabriel said. But testing the following November and December showed the presence of diazinon in a local stream.

CHEMICAL 'LAYER CAKE'

In further testing in February 2015, the stream appeared to be chemical-free. But chemicals showed up again the following year, Gabriel's unpublished data show, prompting him to speculate that it can take months or years for chemicals to migrate through the soil.

"It's like a layer cake," Gabriel said. "They put chemical on chemical on chemical. We'll find different chemicals in the water on different years."

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In another instance, a stream in Trinity County tested negative for pesticides in 2014 but positive in December 2016.

The state does not have a comprehensive testing program for marijuana contaminants, and little such work has been done at the local level, officials said.

But many people and animals rely on water from local streams. And some are growing concerned.

Patricia Young, whose family grazes cattle in Shasta County, said eight cows have died suddenly over the past three years near an irrigation channel they use for drinking.

Young said the family was worried the cows died from poisoning from marijuana farms in nearby woods, and they were testing the stream.

The chemicals have been found in game animals, including a quail Gabriel shot and ate with his family, and numerous deer and elk whose livers were tested in a study for the Mule Deer and Rocky Mountain Elk Foundations, he said.

Trujillo, the Trinity County sheriff's department narcotics sergeant, said his law enforcement dog, a Belgian Malinois named Johnny, almost died from pesticide poisoning after jumping into a reservoir at an illegal marijuana grow.

California is developing regulations for marijuana farms including rules about water quality and pesticide use, but widespread water testing is not included.

The federal government, which owns much of the land on which illegal marijuana grows are planted, has also not conducted extensive testing of streams near the toxic sites, officials said.

Matt St. John, executive officer of the North Coast Regional Water Quality Control Board in the heart of marijuana country, said his agency is planning to regulate pesticide use by marijuana farmers. But testing streams on a regular basis would be too expensive, he said.

Trinity County supervisors will decide on Sept. 19 whether to authorize a testing program along part of the Trinity River and its tributaries.

"Maybe six months down the road we'll say water quality wasn't affected all that much," said Trinity County Planning Director Leslie Hubbard. "But maybe we'll say we have a disaster on our hands."

(Reporting by Sharon Bernstein; Editing by Ben Klayman and Cynthia Osterman)