(CNN) Shellfish in the Puget Sound, an inlet of the Pacific Ocean along the northwest coast of Washington, tested positive for the prescription opioid oxycodone.

But that wasn't all, according to Washington Department of Fish and Wildlife biologist Jennifer Lanksbury. In the midst of a national opioid crisis, the opioid may be the most attention-grabbing contaminant found, but it could be the least worrisome.

The mussels also contained four kinds of synthetic surfactants -- the chemicals found in detergents and cleaning products -- seven kinds of antibiotics, five types of antidepressants, more than one antidiabetic drug and one chemotherapy agent.

Surfactants, in particular, are "known to have estrogenic effect on organisms, so they affect the hormone system of some animals in an estrogenic way, such as feminizing male fish and making female fish reproductive before they're ready," Lanksbury explained.

Scientists have not studied whether mussels are harmed by oxycodone. However, the presence of this drug in the mollusk speaks to the high number of people in the urban areas surrounding the Puget Sound who take this medication, said Lanksbury.

"A lot of the pharmaceuticals are probably coming out of our wastewater treatment plants. They receive the water that comes from our toilets and our houses and our hospitals, and so these drugs, we're taking them, and then we're excreting them in our urine so it gets to the wastewater treatment plant in that way," Lanksbury said. "Some people, unfortunately, flush their drugs down the toilet, and that's a huge source of these pharmaceuticals."

"The doses of oxycodone that we found in mussels are like 100 to 500 times lower than you would need for an adult male therapeutic dose," she said. "So you would have to eat 150 pounds of mussels from these contaminated areas to even get a small dose. But just the fact that it's present tells us it is getting into our waters, at least in urban areas."

The study findings suggest toxic contaminants are entering the food web of the greater Puget Sound, especially along shorelines nearest Seattle and other urban areas.

"What this is telling us is some of this stuff is coming out of our wastewater treatment plants and so we need to do a better job either at controlling the sources or trying to reduce the exposure in the Puget Sound," said Lanksbury.

The results are from a special small-scale study. Every other year, she and her colleagues monitor fish and shellfish from the Puget Sound -- specifically herring, English sole, Chinook salmon and most recently mussels.

"Mussels have a simpler system than fish, and that makes them great for monitoring," Lanksbury said.

Fish can metabolize some chemicals, but the mussels do not, so in many cases, they are better at revealing contaminants in the water. To test the water, Lanksbury and her team get clean mussels and put them in antipredator cages. Citizen science volunteers stake the cages to the inner tidal area of the Puget Sound at low tide, and the scientists collect them after several months.

The group started mussel monitoring in winter 2013 and conducted two additional surveys in 2016 and 2018.

During their biennial reviews , the group routinely tests samples for a suite of contaminants: polychlorinated biphenyls ( PCBs ); polybrominated diphenyl ethers ( PBDE s), which are flame retardants; polycyclic aromatic hydrocarbons ( PAH s), which are chemicals resulting from the combustion of fossils fuels; chlorinated pesticides, including dichlorodiphenyltrichloroethane ( DDT ) compounds; and six metals: lead, copper, zinc, mercury, arsenic and cadmium.

Recently, Lanksbury and her colleagues had access to extra funding.

"We decided it was important for us to start looking for ' contaminants of emerging concern ,' " she said. This term refers to pharmaceuticals and personal care products -- including prescription drugs, detergents, shampoos and microplastic beads -- that are increasingly being detected in waterways, such as the Puget Sound.

"We sent 18 samples (of mussels) to a laboratory up in Canada and asked for a suite of pharmaceutical and personal care products," Lanksbury said. "When that data came back to us, we found oxycodone in three of those 18 samples."

One of the samples came from the shoreline of Seattle, and the two others came from near Bremerton, she said.

"So to us, that says that the oxycodone problem is specific to the urban waters of the Puget Sound. All of the other areas tested did not have oxycodone.

"All of our species indicate where contamination is coming into the Puget Sound," she explained. "Most of the shorelines of the Puget Sound are pretty clean. It's these highly urbanized locations where we're starting to get concerned about the levels of pharmaceuticals and personal care products."

The population of the Puget Sound is slated to double over the next 10 to 20 years, Lanksbury noted, and a high proportion of that population is expected to live on the shore. Urban centers across the country are growing, as well.

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"It's a nationwide problem," Lanksbury said.

A study conducted by the US Geological Survey found measurable amounts of one or more medications in 80% of the water samples drawn from 139 streams in 30 states.

Still, she is hopeful because wastewater treatment mechanisms have improved, and improvements continue to be made. And the public is becoming aware of the problem.

Meanwhile, Seattle residents need "to keep in mind that what they do at home, what they put on their lawns, what they flush down the toilet ends up in the Puget Sound," she said. "The Puget Sound is a jewel in Washington, and if we all work together to keep it clean, we can make great strides."