Acid Rain - A 'Solved' Environmental Problem

Adam Jeziorski   |   May 2021 

Acid rain (or more broadly acid deposition) is largely considered a solved environmental problem, and in many ways it is. One of the great environmental success stories of the past few decades is how scientific efforts in the 1970s and 1980s were able to identify the sources of acid rain (principally industrial SO2 and NOX emissions), and eventually convince policy makers to regulate emissions and lower acid deposition rates across North America and Europe.


The concerted push to stop acid rain was due to the profound negative impacts it had on plant and animal life. The direct impacts of increased acidity can eliminate sensitive aquatic species and terrestrial vegetation. As acid-sensitive species were lost, cascading effects throughout food webs could result in reduced fish and waterfowl populations. Increasingly acidic soils promoted the release of bound metals, resulting in downstream metal toxicity. Over time, the cumulative effects of acid rain were devastating to some regions.


Thirty years ago, Canada and the U.S. made important revisions to their respective Clean Air Act aimed at addressing acid rain in both countries, and this is an anniversary worth celebrating. Over the past three decades, the emissions that cause acid rain have been reduced to a fraction of their maximum levels. For example, the sulphur dioxide emissions from mining and smelting operations in Sudbury, Ontario were among the largest point sources of acid emissions on the planet, but today those emissions are only a tiny fraction (~2%) of the peak values of the 1960s. As a result, the Sudbury Superstack , once emblematic of the phrase “the solution to pollution is dilution”, is no longer in service and scheduled to be dismantled. At a continental scale, the emission reductions have allowed many acidified lakes to recover; however, the consequences of decades of acid rain are still affecting many regions.


The recovery that was anticipated to follow strict limits on acid emissions has been uneven. Although the acidification of lakes in North America has largely ceased, many lakes that were acidified have not yet recovered. The slow recovery is most notable in the lakes of softwater regions such the Canadian Shield. Even when chemical recovery is apparent (i.e. a rise in pH to pre-impact values), biological recovery often lags behind.


One factor contributing to the slow recovery of biological communities in lakes recovering from acidification is a decline in lakewater calcium (Ca) concentrations. Acid deposition accelerated release of Ca from watershed soils, and as the primary source of this Ca is often mineral weathering, its replenishment will require a soil formation time scale (i.e. centuries or even millennia). In lakes with naturally low Ca concentrations, the decline may have lowered levels below threshold values for keystone aquatic invertebrates, acting as an additional barrier preventing food webs from recovering. Concerns about low lakewater Ca have raised the possibility of using community wood ash recycling programs to restore watershed Ca. Studies examining this approach are now underway in the Muskoka-Haliburton region of Ontario .


The acid rain saga illustrates how an environmental issue can persist long after it has been ‘solved’, and in many cases the hope of a full ‘recovery’ may never be realized. Even as global attention has moved on to the threat posed by climate change, it is important to keep in mind how acid rain continues to shape ecosystems in a warming world.

Dr. Adam Jeziorski is a research scientist in the Paleoecological Environmental Assessment and Research Laboratory (PEARL) at Queen's University, where he uses the invertebrate remains preserved in lake sediments to track long-term environmental change and the response of aquatic communities to human activities.

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Sudbury Superstack.jpeg

The Sudbury Superstack.