SEATTLE » The federal government is chipping in money for a three-year pilot study using sewage samples to determine levels of marijuana use in two Washington cities — research that could help answer some key questions about pot legalization, according to the University of Puget Sound.

The school said Monday that the National Institutes of Health is chipping in $120,000 so Dan Burgard, an associate chemistry professor, can conduct a three-year study that will look at how per-capita pot use changed after Washington’s first legal pot shops opened last July.

The research, based on methods first developed by scientists in Italy in 2005, involves analyzing wastewater samples for levels of metabolites produced when the body processes drugs.

Burgard previously used it to test campus wastewater to determine whether students were using more "study drugs" such as Ritalin and Adderal during mid-terms and finals. In one finals period at the university, he found an eight-fold increase over such stimulant levels from the first week of school.

Burgard began collecting marijuana data in December 2013 — after voters passed legalization in 2012 but eight months before legal pot shops began opening.

The upcoming study is aimed at helping determine whether the opening of pot shops increase a community’s marijuana use, whether data from the wastewater correlate to what people answer in surveys about their marijuana use, and whether weekday or weekend marijuana use has increased.

"We’re trying to get a sense of the type of user," Burgard said. "If there’s more use on the weekends, maybe that’s more recreational. But if Sunday to Thursday use goes up as much, that might be a public health concern, with habitual users using a lot more."

The data could also show how much of the illicit black market for marijuana the state’s legal stores are capturing, by comparing the wastewater data with the state’s close tracking of marijuana sales. If sales figures continue to rise, but the wastewater levels show that overall pot use is flat, that would indicate that people are getting their marijuana at legal stores instead of on the black market.

But if sales figures rise and the sewer evidence of pot use also rises, that could indicate that people are still buying on the black market — and that legalization has increased overall use in the state without displacing much of the black market.

Burgard declined to say which two cities in Washington will be tested until the study is complete.

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.