Puget Sound area uses more weed than Amsterdam — judging by its sewage, experts say

Sewage doesn’t lie.

That’s why a team of researchers from the University of Puget Sound and the University of Washington analyzed Puget Sound-area wastewater to estimate cannabis use from 2013 to 2016 — a timeframe that captured samples before and after the state legalized recreational use in 2014.

That team published a paper on its findings Tuesday in the journal Addiction, revealing that cannabis use has increased in the area and shifted away from the illegal market since legalization, according to a UW news release.

Researchers estimated the amount of psychoactive THC found in Puget Sound’s public wastewater ticked up 9 percent each quarter on average between December 2013 and December 2016. Meanwhile, cannabis sales shot up nearly 70 percent a quarter from stores that opened shop in August 2014 or later, according to the researchers. That 9 percent quarterly increase amounted to a doubling of weed consumption over the three years studied, researchers wrote in the findings section of the paper’s abstract.

“Given that wastewater represents a total population measure, these findings suggest that many established users switched very quickly from the illegal to the legal market,” Dr. Dan Burgard, a University of Puget Sound chemist and study co-author, said in a statement released by UW. “This is the strongest statement possible regarding displacement of the illegal market.”

In an interview with KING 5, Burgard put the western Washington area’s cannabis use in perspective as it compares to other areas.

“According to wastewater, the Puget Sound area has the highest cannabis use per capita, even over Amsterdam,” Burgard told the TV station, explaining that the researchers were part of a global team that studied 60 to 80 other cities.

Wastewater is also being used in Canada to study cannabis usage, NPR reports, and the method has been used previously in Italy and beyond to test for drugs like cocaine.

Caleb Banta-Green, a UW Alcohol and Drug Abuse Institute researcher and paper co-author, said in a statement that “surveys are subject to important biases and limitations, including potential changes in self-report as social norms change as well as very limited information on the amount of THC actually consumed.”

Sewage doesn’t have that limitation.

“With wastewater, it’s anonymous, and wastewater doesn’t have any reason to lie,” Burgard told KING 5.

To complete the research, the team collected full days worth of raw wastewater samples from two treatment plants and studied those samples for extremely low concentrations of drugs, according to the university news release. Those samples were gathered on 387 days across the three years that were analyzed.

The research was funded in part through a federal National Institutes of Health grant, the Associated Press reported in 2015.

Burgard has used the wastewater method before: He once demonstrated that students on a university campus were more likely to use “study drugs” Ritalin and Adderall when midterms and finals were being held — and discovered that during one finals period, study drug use jumped eight-fold, according to AP.

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Jared Gilmour is a McClatchy national reporter based in San Francisco. He covers everything from health and science to politics and crime. He studied journalism at Northwestern University and grew up in North Dakota.