SEATTLE — The federal government has agreed to pay for an analysis of sewer samples to determine levels of marijuana use in Washington state.

The University of Puget Sound said Monday 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 legalization.

The university says Burgard has adapted a 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.