A growing number of states are legalizing or looking at legalizing marijuana. Oregon is set to join Washington and Colorado this fall when retail sales of recreational pot begin. States hope these legal, regulated markets will displace the illegal black market.

But here’s the tricky part: how can you tell?

The answer might just lie in the sewage system. That’s what researchers in Washington state are hoping. Dan Burgard of the University of Puget Sound in Tacoma recently received a grant from the National Institutes of Health to study THC metabolites — the byproducts of marijuana use — in waste water. Researchers say they can get a more accurate snapshot of an entire city’s drug use through sewer analysis.

Aeration basins at the wastewater treatment plant in Spokane, Washington. 

Jessica Robinson

Burgard already has a couple of freezers’ worth of samples collected during an eight-month period before pot stores opened in Washington. The $120,000 in federal grant money will allow him to study changes at two sewage plants on the western side of Washington state over a three-year period.“So rather than asking people about illicit or maybe taboo drug use in surveys — there’s no real reason why the sewage should lie,” Burgard says.

Think of it like an anonymous, community-wide drug test.

The idea is to compare consumption rates, as shown in wastewater, to numbers already collected by the state. Washington operates a marijuana tracing system that tracks each sale of pot, the size of the product and its potency.

“So if we know the number of grams of THC being sold, then we should be able to look at the sewage levels and say, ‘OK, well there’s this much that comes to the sewer, there’s this much that was sold,” Burgard says. The difference could reveal what’s happening to illegal sales.

For example, if state figures show legal sales are climbing, but the sewage indicates consumption is remaining steady, that could be a sign the black market is being displaced. On the other hand, if consumption goes up and sales go up at the same time, the legal market may just be operating alongside the existing black market, but more people are now taking part.

Burgard says the math isn’t perfect. For one thing, medical marijuana in Washington also has to be factored in. “But at least it would give us a sense of the new recreational market and how big of a piece of the pie it is,” says Burgard.

Here’s another question he’s hoping to answer: what day of the week are consumers using, not buying, marijuana? That would give some indication as to whether this new market is fueling a weekend pastime or a weekday habit.