Scientists who track pollution have discovered traces of the pain reliever oxycodone in some mussels in Seattle’s Puget Sound.

Washington Department of Fish and Wildlife obtained clean mussels from Penn Cove on Whidbey Island and put them in different areas to test for water contamination, KIRO-TV reported this week.

Scientists worked with the Puget Sound Institute to analyse the data and discovered three out of 18 locations came back positive for trace amounts of oxycodone.

Two were near the historic naval shipyard district and one was in Elliot Bay near Harbour Island in Seattle.

Puget Sound Institute scientist Andy James, who assisted with the study, said the areas where the oxycodone-tainted mussels were sampled are considered highly urbanised and are not near commercial shellfish beds.

“You wouldn’t want to collect [and eat] mussels from these urban bays,” he said.

He also said the oxycodone was found in amounts thousands of times lower than a therapeutic dose for humans.

Scientists usually find chemical compounds in Puget Sound waters, ranging from pharmaceuticals to drugs such as cocaine, but this is the first time that opioids have been discovered in local shellfish, according to the Puget Sound Institute.

“It’s telling me there’s a lot of people taking oxycodone in the Puget Sound area,” said State Fish and Wildlife biologist Jennifer Lanksbury, who leads the monitoring studies.

The contamination is likely coming through wastewater treatment plants, she said, adding that the chemicals may be having an impact on fish and shellfish in those areas.

“Hopefully our data shows what’s out there and can get the process started for cleaning up our waters,” she said.

She says mussels at a restaurant or store are healthy because they come from clean locations.