A new study from Statistics Canada draws data from an unexpected source to suggest Halifax has the highest rate of cannabis consumption per capita among five urban centres across the country.

The national statistics agency launched its study in March, using a relatively new technology called wastewater-based epidemiology.

Researchers analyzed wastewater from 15 treatment plants across Halifax, Montreal, Toronto, Edmonton and Vancouver to produce statistics on cannabis use.

The results suggest that between March and August, Halifax had the highest rate of consumption per capita, at 1,310 micrograms per person per week, with Montreal coming in second at 976.

Toronto came in at 451 micrograms per person per week, Edmonton at 416, and Vancouver at 288. In total, the areas surveyed represent nearly 8.4 million Canadians.

StatCan noted there were often large disparities in the amount of cannabis detected month-to-month, and stressed that its results are considered "preliminary and experimental.

"Statistics Canada is still exploring the benefits and limits of using WBE to produce statistics on drug use," a recent report posted on StatCan's website reads.

"Subsequent tests and results will allow the agency to better assess the long-term feasibility of using wastewater as an official data source."

The study was launched amid concerns that Canadians may be under-reporting their cannabis use due to stigma around the drug a reluctance to disclose purchases from "non-regulated suppliers."

"One consequence of under-reporting is that the size of the black market for cannabis will be similarly underestimated," the report, published last Thursday, states.

"As a result, without a direct measurement of cannabis consumption, the reduction of the black market for cannabis, one of the objectives of the legalization of non-medical cannabis, will be hard to track."

The report said that when people consume cannabis, their bodies process it into metabolites that are later eliminated from the body and can show up in wastewater.

It said this method of testing has been used in Europe since 2007 to report on people's drug consumption in large cities.

StatCan said sampling will continue through the spring of 2019.