Environmental groups are accusing Six Flags Magic Mountain of polluting the Santa Clara River with huge volumes of contaminated water and allowing trash with its logos to spill into the Southern California waterway and toward the ocean.

The allegations were made in a letter sent to the Valencia theme park last week by a coalition of environmental groups, whose investigators say they found alarming levels of pollutants in water sloshing out of the facility’s storm water outfalls into the nearby waterway during rainstorms.

Santa Monica Baykeeper, Wishtoyo Foundation and Friends of the Santa Clara River contend the popular Southern California theme park has for years been fouling one of the region’s largest river systems, putting wildlife and swimmers at risk through “frequent and high volume discharges [that] carry bacteria, metals, toxins, sediments and other pollutants into the Santa Clara River, its Estuary, and the Pacific Ocean.”

The groups intend to sue Magic Mountain for violating water pollution laws in 60 days if it does not significantly reduce pollution getting into the waterway, which flows 45 miles downstream from the park before emptying into the sea between Ventura and Oxnard, according to the letter.

In a statement, Six Flags Magic Mountain said it had not had a chance to review the letter but “is concerned about the environment and feels a responsibility to improve the storm water process” as it strives to meet stringent requirements of the law.

“In the last six years,” the statement said, “the Park has spent hundreds of thousands of dollars to not only lower the amount of storm water discharge but to improve the quality of the storm water discharged.”

The theme park came under scrutiny last year after a tip from a park employee who expressed concern that Magic Mountain’s practice of washing down its midways, bathrooms and restaurants at the end of the day was washing pollutants into the river, the environmental groups said. Community members also complained about pollution and trash in the waterway.

The groups took water samples of runoff flowing from the amusement park’s two storm water channels and a pipe leading from the facility. Three rounds of tests revealed excessive levels of copper, zinc, aluminum, iron, lead, and titanium and also showed the presence of mercury, oil and grease, bacteria and other effluent, the groups said. They say the park’s storm water monitoring reports show similar pollution dating back to at least 2007.

“Our investigation revealed that their discharges were polluting the Santa Clara River much more severely than we could have imagined,” said Jason Weiner, staff attorney for the Wishtoyo Foundation’s Ventura Coastkeeper Program.

On visits to the site the groups alleged that they also discovered and photographed a large amount of Six Flags-labeled trash around its discharge channels and downstream, including plastic drink bottles, wristbands and midway prizes such as rubber sports balls.

“The sheer quantity of trash, there’s no other way to put it: It was just disgusting,” Weiner said.

Though it is unclear where in the 260-acre theme park the pollution could be coming from, the letter ticked off a list of possibilities — runoff from the park’s irrigation system, its parking lots or its backstage equipment areas; the water used to wash its midway, or even overflow from its lakes, ponds or the Hurricane Harbor waterpark.

And some of the particulate metals detected in the water, the groups contend, could be coming from the park’s roller coasters and thrill rides as they grind along their tracks.

The coalition’s letter, a precursor to a lawsuit under the federal Clean Water Act, seeks civil penalties of $37,500 for each day of each violation. But the preferred solution, the groups said, would be for the park to do something about the runoff before it leaves the site.

Under a permit from the Los Angeles Regional Water Quality Control Board, Magic Mountain is allowed to discharge millions of gallons of storm water on rainy days provided it does not exceed pollution limits.

But the park has a history of violations.

It has paid more than $1 million in fines over the last decade for releasing polluted runoff, water board records show. The park has reported discharging water with excessive levels of effluent, copper, lead, chlorine, bacteria and oil and grease into the river more than 400 times since 1995.

tony.barboza@latimes.com