WESTMINSTER — After seeing two dogs battle cancer she believes was caused by playing in a dog park next to the former Rocky Flats nuclear weapons factory, Denver resident Alesya Casse has started a campaign to educate dog owners about the area’s history.

“Dog owners are so passionate about the dog park but may not realize how close it is to a former nuclear weapons factory with a history of severe environmental contamination,” the 35-year-old corporate paralegal said. “I don’t want to try to close the park but just educate people about radioactive material in the area and let them decide if they want their dogs using the park.”

The Westminster Hills Off-Leash Dog Park on 105th and Simms is part of the 15,000-acre Colorado Hills Open Space, which is due east across Indiana Street from the former nuclear weapons plant. The plant operated from 1952 to 1992. Since 2007, the site has been a national wildlife refuge.

VIDEO: Westminster Dog Park

Casse said she grew up in Boulder and heard stories about Rocky Flats, including two major plutonium fires in the 1950s and 1960s , and leaking barrels of radioactive waste, along with former employees who developed cancer and health issues after working there..

Plutonium levels in the area of the dog park have been measured from three to 40 times the level of normal background radiation found along the Front Range. But officials from the Colorado Department of Public Health and Environment and a host of other federal, state and local organizations say the levels are magnitudes smaller than anything that could have a discernible impact on human health.

A 2011 test of plutonium levels along Indiana Street commissioned by the Rocky Mountain Peace and Justice Center showed levels of plutonium isotopes ranging from 0.019 picocuries per gram of soil to 1.579 picocuries per gram, the same level of contamination measured 40 years ago during the heyday of production.

Carl Spreng, an environmental protection specialist and Rocky Flats project manager for the state health department, said that even at 10 picocuries per gram, a refuge worker with no protective gear could spend 18 years working eight hours a day on the site and would have a one in a million chance of developing cancer that would not have occurred otherwise.

“Everyone knows someone across the street or has a family member who got cancer, and the natural tendency is to find someone or something blame …” Spreng said. “There have been millions of analyses of radionuclides, and the risk assessments show that the levels both on and off site are protective of human health and environment and that’s the bottom line for us.”

Casse said she was spurred to act after seeing a friend’s dog develop bone cancer in his leg at the age of 4 after playing in the dog park almost every day. The cancer spread through his body over a period of four years and the dog died in 2008.

Around the same time, another friend’s dog that had spent years playing in the park survived two bouts of cancer at the ages of 3 and 5.

“It was tragic,” Casse said. “The realization that this probably came from spending so much time in the dog park didn’t come right away, but it was staring us right in the face all along.”

She has held a series of workshops and spends a few afternoons every week handing out fliers at the park.

LeRoy Moore, a longtime nuclear watchdog with Rocky Mountain Peace and Justice Center, contends the site still poses a risk to health.

While state studies have shown little to no plutonium exposure to animals living in the wildlife refuge, Moore said an ongoing concern is burrowing animals that can bring plutonium-contaminated soil to the surface, where it could be distributed to surrounding areas, including the dog park.

“There have been numerous studies, including one recently done by Columbia University, that have shown one particle of plutonium can create cancer in a mammal,” Moore said. “That’s very, very different than saying protection standards have determined an area is safe.”

Bill Johnson, a retired veterinarian who owned three animal hospitals in Colorado and California — including one in northern Arvada downwind from Rocky Flats — said he’s not surprised to hear that dogs who played in the park have developed cancer.

“My biggest concern when I came back to Arvada to practice was seeing all these dogs with cancer,” Johnson said, noting two of his dogs developed cancer after living in the area. “I saw more in one week than I would in a few months in California.”

On a recent Sunday morning, the parking lot of the dog park quickly filled, forcing people to park along Simms Road. The bustling park saw lines of dog owners walking their dogs against a stunning view of the Rocky Mountains to the west.

Reactions were mixed when dog owners were asked if they had concerns about walking their dogs less than a mile from a defunct nuclear weapons facility.

“I tend to believe what the scientific data shows, and I think people tend to go off of emotions sometimes in these situations, so I have no problem letting my dogs run around here,” said Tim Fitzgerald, who was out with his dogs Buster and Lex.

Leigha Rose-Medina wasn’t so sure. The Westminster resident said it’s hard to trust what the government says about a site with a history of cover-ups.

“I only come out here every now and again, and after hearing there’s still radioactive contamination in the area, I don’t really feel safe bringing my dogs out here,” Rose-Medina said.

Casse said she is talking to new residents, many of whom are moving into developments popping up near Rocky Flats and may not know about the area’s history.

“There’s billions in development deals and new construction in the area,” Casse said, “and one of my goals is to bring awareness of the costs this may bring to the health of animals and humans.”

Austin Briggs: 303-954-1729, abriggs@denverpost.com