Demystifying Science | Pollution in Rain

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“There are a few studies on the subject, but not in Montreal,” explains Benoit Barbeau, a drinking water specialist at Polytechnique Montréal. “Rainwater everywhere on the planet has a human signature in terms of biological and chemical pollution. But it depends on local conditions. Studies in Asia, for example in India, have high contamination rates. But if you are north of Mont Tremblant, it is probably low.”

Rain “washes” the atmosphere of its contaminants, explains the civil engineer from Polytechnique. And water droplets can agglomerate around chemical particles of pollutants. “It would be interesting to see what effect last year’s forest fires had on the contamination of rainwater,” says Mr. Barbeau.

Is it dangerous to open your mouth to drink contaminated rain? “Unless it rains cats and dogs like during the monsoon in Vietnam, we’re going to take 5 to 10 milliliters at the most, so I don’t think so,” says Barbeau.

However, rainwater collected in a container should not be drunk directly, unless you take it out at the start of the rain and bring it back in quickly. “If you leave a tank outside, there will be pollution deposits over the days, and microbial contamination from birds and animals,” says Mathieu Lapointe of the École de technologie supérieure (ETS).

Mr. Lapointe is a specialist in “runoff water,” which flows into waterways after passing through the land. In particular, he develops technologies for treating agricultural runoff water.

Perfluorinated

In 2022, a European study published in the journal Environmental Science & Technology concluded that rainwater often contained more perfluorinated compounds than the then-current U.S. standards (which have since been tightened).

Perfluorinated chemicals, also known as PFAS, are often dubbed “forever pollutants” and are used in a host of consumer products, from stoves and stain-resistant fabrics to firefighting foams.

“It depends on the measurement methods, probably also on the weather,” explains Sébastien Sauvé, a chemist at the University of Montreal who has worked extensively on perfluorinated compounds.

Faced with water shortages in some parts of Canada, provincial and federal reports assessed the contamination of rainwater tanks 10 to 15 years ago. In 2013, a report from the Canada Mortgage and Housing Corporation listed the types of treatments needed for different uses of rainwater: washing clothes, toilets, gardens.

The 2013 report noted that Texas requires government buildings to harvest rainwater, and that in Germany, Belgium and Japan, the retention and use of rainwater is encouraged, in part to reduce pressure on sewers.

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