Washington state may soon be able to determine people’s levels of marijuana use by analyzing an unlikely source.

Cities’ sewage.

In late 2013, Dan Burgard, an associate professor of chemistry at the University of Puget Sound, started collecting wastewater samples to look for traces of marijuana use. The National Institutes of Heath has now chipped in $120,000 so Burgard and his team can complete a three-year study that will look at how pot use changed after legalization.

“Sewage treatment plants do a great job of collecting wastewater from an entire city and so you can very quickly figure out drug trends within a city by looking for those drug metabolites,” Burgard says.

Specifically, the process requires looking for the metabolite of the active ingredient in marijuana, THC. From there Burgard’s team is hoping to find changes in marijuana consumption between 2013 and 2017.

“We started collecting wastewater in December of 2013, and that was after we had legalized recreational marijuana in the state of Washington but before the stores had opened. So that gave us about an eight month window before the stores had opened,” Burgard says. “That will give us our baseline.”

This type of analysis would theoretically enable his team to conduct a community drug test without survey bias.

The university says Burgard has adapted the 10-year-old method developed in Italy for retrieving and analyzing wastewater. He and his students plan to test 110 random, one-liter samples taken from two wastewater treatment plants in Western Washington each year.

The research is being done in collaboration with Caleb Banta-Green, a senior scientist at the University of Washington’s Alcohol and Drug Abuse Institute.

AP contributed to this report.