Demystifying Science | Acid Rain News

Every week, our journalist answers readers’ scientific questions.

Why don’t we hear about acid rain anymore? Twenty or thirty years ago, this rain threatened our lakes and rivers. This is no longer the case today? – Yves Poirier

It is less talked about because the situation has improved a lot. But the effects of acid rain are still being felt.

“In the 1980s, we heard about the decline of maple groves,” explains Louis Duchesne, a forest engineer with the provincial Ministry of Natural Resources and Forests, who last fall published a summary of progress in the fight against acid rain.

Acid rain was formed by the interaction between sulfur and nitrogen oxides, emitted by coal-fired factories and power plants, and rain. The sulfuric and nitric acids created in the clouds damaged metal infrastructure and some buildings, especially those made of limestone. They also affected fish in the lakes.

Initially, it was thought that the acids caused the leaves to fall. “But eventually, we realized that the effect was happening in the forest soils,” says Duchesne. “The basic elements in the soils were counteracting the acids, then being leached into the streams. That led to soil depletion.”

The leached elements are “basic cations”, potassium, calcium and magnesium.

Only the very northern regions, which had less acid rain, and those with very clayey and rich soils, such as the Gaspé, were spared. In the maple groves, the maples gave way to beeches, which were less affected by the impoverishment of the soil.

When will it stabilize?

Sulphur and nitrogen oxide emissions have fallen by 78% and 61% respectively since their peak in the 1980s. Measurements in maple groves taken by the Ministry show that soil pH has increased from 4.2 to 5.2, which is 10 times less acidic, because the pH scale is logarithmic. For example, a pH of 4.2 is similar to that of a tomato.

“We thought it would stabilize at 5, we are surprised,” notes Mr. Duchesne. “It is the legacy of 40 years of atmospheric pollution, we do not know where the pH will stabilize.”

Maple trees that were affected by acid rain continue to grow more slowly, but those that were born more recently are more vigorous, according to the Quebec forest engineer. The maple grove dieback monitoring program was abandoned about ten years ago, given the improvement.

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