The Washington Department of Fish and Wildlife (DFW) detected trace amounts of opioids in marine life around Puget Sound, CBS News reported Thursday.

Mussels are often used in researching levels of pollution and toxins in lakes and rivers. As filter feeders, the animals ingest whatever small particulates are floating through the water and can later be tested for their diet.

Researchers at the DFW took “clean” mussels and placed them in 18 locations across Puget Sound. Several months later, the researchers returned to the spots and harvested the mussels for testing. Of the 18 mussel colonies, three tested positive for trace amounts of opioids, according to CBS News.

Opioids, along with various other drugs, travel from human waste into bodies of water through wastewater management systems. Wastewater is filtered and cleaned before it is dumped, but drugs slip through the filtration systems. Small amounts of drugs inevitably make it into marine environments around population centers.

“It’s telling me there’s a lot of people taking oxycodone in the Puget Sound area,” DFW Biologist Jennifer Lanksbury told KIRO 7. (RELATED: 20 Percent Of Americans Have Known Someone Suffering From Opioid Addiction)

The research was done as part of project to test waters around Puget Sound with mussels every two years. The levels of chemicals found in the most recent test are thousands of times lower than what would be a therapeutic dose, according to the Puget Sound Institute.

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