SEATTLE — A federally funded study has confirmed, not surprisingly, that marijuana use went up in Washington state after its first legal pot stores opened in 2014. In fact, consumption appeared to double, at least in one major city, over three years — a conclusion scientists reached by way of the unglamorous work of analyzing raw sewage.

The research entailed driving to two sewage treatment plants that serve the 200,000 people of Tacoma. The scientists would pick up a cooler full of frozen wastewater samples, thaw them and analyze them using liquid chromatography and mass spectrometry.

They were looking for THC-COOH, a substance produced when the body metabolizes THC, the main ingredient in marijuana that gets you high. THC-COOH is excreted mostly in people’s urine.

Their findings: Consumption of THC doubled from December 2013 to December 2016.

The researchers said they can’t tell whether the increase means more people are using weed or the same old users are consuming more of it. Another possible explanation is that the marijuana, edible treats and extracts sold legally in stores are more potent than what was commonly available on the black market.

Another important conclusion from the study, published this week in the journal Addiction, comes from Dr. Dan Burgard. He led the study, which was conducted by a team of researchers at University of Puget Sound and University of Washington. He says that, as far as the data is concerned, western Washington is a world leader in cannabis consumption. He says:

“We are part of an international study… with 60-80 other cities around the world. And according to wastewater, the Puget Sound area has the highest cannabis use per capita, even over Amsterdam.”