What is Acid Rain?
Acid rain is the end result of a process that begins when sulphur dioxide and nitrogen oxides are released into the atmosphere (referred to as emissions) from sources such as coal- and oil-fired power plants, smelters (mostly copper and nickel), motor vehicles, and other human-related sources. Natural phenomena, like volcanic eruptions and forest fires, also produce sulphur and nitrogen oxides. Once in the air, these gases undergo chemical reactions to form acids or acidifying sulphates or nitrates that may be carried hundreds of kilometres, before eventually falling to earth as dilute solutions of sulphuric acid and nitric acid in the form of rain, snow or fog (these forms are often referred to as wet deposition), or on the surfaces of fine particles (dry deposition). Because rain is only one of the means by which the acid reaches the earth's surface, scientists often prefer to speak of acid deposition rather than acid rain (but for convenience, the term acid rain is often used in this summary, but always in the broader sense of all forms of acid deposition).
The pH scale is a commonly used measure of acidity or alkalinity (a measure of acid-neutralizing or buffering capacity). It runs from 0 (highly acidic) to 14 (highly alkaline). A pH of 7 is neutral. Because the scale is logarithmic, each change of 1 pH unit represents a ten-fold change in acidity or alkalinity. Vinegar has a pH of 3 while milk of magnesia has a pH of 10. Normal rain, with a pH of 5.6, is slightly acidic because of reactions between rainwater and atmospheric carbon dioxide. The pH of acid rain typically lies between 4 and 5.
Damage caused by acid deposition affects lakes, rivers, forests, soils and buildings with profound impacts also observed on fish and wildlife populations. Furthermore, sulphur dioxide in the air can combine with other pollutants and water to form fine particles that can increase the risk of serious health complications for people with heart and respiratory disease. The haze that these particles form also contributes to visibility reductions, especially in parts of central and eastern Canada. In addition, sulphate particles contribute to climate change by scattering incoming sunlight and stimulating cloud formation, thus causing local and regional cooling.
Acid rain is a global problem. It is a major concern in North America, parts of Europe, particularly Scandinavia, and it is a developing issue in China and other industrialized areas of Asia. Because the emissions that cause acid rain often cross national borders, acid rain is an important issue on the international agenda.
The Acid Rain Problem in Canada - History and Current Status
The problem that acid rain posed to Canadian lakes and forests was identified in the 1960s, but it was not until the late 1970s that governments funded research to determine the extent of the problem in Canada. It was not until the 1980s, however, that Canadian governments began to act on the issue. Since then, much has been done to control the sources of acidifying pollution, but the problem is still far from resolved.
Actions to reduce acid deposition have focussed largely on sulphur dioxide
emissions, mainly because these have so far played a much larger
role in acidification than nitrogen oxides. The first major international
agreement to deal with acid rain was negotiated under the auspices
of the United Nations Economic Council for Europe in the early 1980s.
This agreement, to which Canada is a party, called for a 30% cut
in sulphur dioxide emissions. A major step was taken in 1985 when
the Canadian federal government and the seven easternmost provinces
launched the Eastern Canadian Acid Rain Program. The intention of
this program was to reduce sulphur emissions in these provinces
to 50% of 1980 levels by 1994. By the end of 1995, Canadian emissions
were 43% lower than they were in 1980 (see
emissions).
It was hoped that these cuts would reduce the deposition of sulphates by rain and snow from levels as high as 40 kg per hectare per year to no more than 20 kg per hectare per year. This "target load", based on the best information available at the time, was expected to protect aquatic environments. However, because half of the acid rain in eastern Canada came from American sources, the cooperation of the United States (U.S.) government was also needed to achieve this target. This cooperation was achieved with the passing of amendments to the U.S. Clean Air Act in 1990, which specified deep cuts in sulphur emissions by the year 2000 and further cuts by 2010. Under the Canada-United States Air Quality Agreement, signed in 1991, the U.S. is more than halfway to its reduction target and is expected to meet the 2010 deadline (see table). In contrast, nitrogen oxide emissions have remained relatively constant over the past 20 years.
The Canada-Wide Acid Rain Strategy for Post-2000, adopted in October 1998 by the federal, provincial, and territorial Energy and Environment Ministers, builds on the success of the 1985 Eastern Canada Acid Rain Program. Yet, the strategy reveals that acid rain will, despite the progress made, continue to damage sensitive ecosystems even after full implementation of current Canadian and U.S. control programs.
The Concept of "Critical Loads"
Although the 20 kg per hectare target is expected to be achieved by 2010, many sensitive lakes and forest areas will still remain vulnerable to acidification. That is because lakes and soils in different regions vary considerably in their capacity to neutralize or buffer the acids they receive. To assist policy makers in developing more precise deposition targets for the future, scientists have developed the concept of "critical load". This is an estimate of the amount of deposition that a particular region can receive without significant damage to its ecosystem. Essentially, it depends on the quantity of acid-neutralizing bases, such as calcium and magnesium salts, in the region's water and in the surrounding rocks and soils. Some regions, such as those with plenty of limestone, have an abundant natural supply of these bases, and, hence, a large capacity to neutralize acid. Other regions, such as those where granite predominates, have a much smaller supply of bases and thus have a lower neutralizing capacity. Acidification of ecosystems results when the supply of incoming acids exceeds the neutralizing capacity of the local environment. The acidity should decline when the situation is reversed.
The critical load for a lake ecosystem is the maximum yearly amount of acid deposition that will allow 95% of the lakes within the region to maintain a pH (a measure of acidity) of 6 or more. Estimates of critical loads for wet sulphate deposition in aquatic ecosystems in eastern Canada range from more than 20 kg of sulphate per hectare per year in the most tolerant areas to less than 8 kg per hectare per year in the most sensitive regions. These highly sensitive areas are found chiefly in the Canadian Shield areas of central Ontario, eastern Quebec, and the Atlantic Provinces. Interpolated maps of measured (S1:1982-1986 and S2:1990-1993) and modelled wet sulphate deposition in eastern North America show that levels of deposition have generally decreased and will continue to do so under the current legislated sulphur emission cutbacks, specifically after Canadian controls (S3: ca 40% cut by 1994, achieved) and after U.S. controls(S4: ca 40% cut by 2010, on target) (see diagram). Southern Ontario typically receives the highest sulphate deposition, but a broad region of central Ontario and Quebec receives high levels as well. Under each of the emission reduction scenarios, improvements occur in most regions, except that Atlantic Canada receives relatively little benefit until the 2010 emission cutbacks, indicating that much of the Atlantic Canada deposition problem comes from US emissions. Much of southern Ontario and Quebec will continue to receive 15-20 kg per hectare per year of wet sulphate deposition, which is below the target load set in the mid-1908s, but which exceeds the critical loads for many sensitive waters in this region. In some parts of southern Quebec and southern New Brunswick, sulphate deposition will continue to exceed the critical loads by more than 10 kg per hectare per year.
Surface Water Acidity: Are Present Control Measures Enough?
There are approximately 800,000 water bodies in that part of southeastern Canada that are affected by acid. Of these, the majority are small (less than 5 hectares in size) found mostly in sensitive areas in the provinces of Quebec and Ontario [see table]. In Ontario, 43 per cent of the Canadian Shield portion of the province (roughly 540,000 square kilometres) currently receives over 10 kg per hectare per year wet sulphate deposition. This area alone contains more than 170,000 water bodies (of which 90 per cent are less than 20 hectares in size) and support an estimated 192,000 pairs of nesting ducks and loons. These small lakes and wetlands are especially vulnerable to the effects of acid rain, and are of particular concern because they represent important habitats for wildlife, including water-dependent birds.
Using a database compiled from nearly 5000 lakes sampled across eastern Canada between 1980 -1995, the general distribution of pH and Dissolved Organic Carbon shows that approximately 31 per cent of the area surveyed (nearly 440,000 square kilometres) supports lakes with pH above 7, 43 per cent (600,000 square kilometres) supports pHs between 6 and 7, while 26 per cent (324,000 square kilometres) supports lakes below the critical pH 6 threshold. Regionally, parts of central Ontario, much of southern Quebec, and Atlantic Canada, are characterized by low pH (below 6) (see chart).
Given the current emission targets (after full implementation by the year 2010), the total area of eastern Canada where deposition is expected to exceed the critical load covers almost 800,000 square kilometres, includes about 95,000 lakes, and extends from central Ontario through southern Quebec and across much of the Atlantic provinces within these areas, populations of many species of fish and other aquatic organisms will disappear entirely from some lakes and be severely reduced in others. The resulting decline in species richness, which is defined as the number of species per lake, is estimated to be between 6 per cent and 15 per cent for fish. Altogether, it is estimated that continuing acidification of these lakes will result in a net loss of nearly 162,000 fish populations alone.
Extending protection to all of these vulnerable ecosystems will require further deep cuts in sulphur dioxide emissions in Canada and the United States; an estimated additional 75% cut beyond the current 2010 target would be necessary to bring wet sulphate deposition levels below the critical loads for virtually all aquatic ecosystems in eastern Canada.
What do we know about the current status of acidity in the water bodies of eastern Canada? Information about the acidity of surface waters (lakes, rivers, streams, wetlands, etc.) comes from a network of federal and provincial monitoring stations located throughout this area. The data collected by the various monitoring programs show that improvement of the quality of surface waters has been slow and uneven.
A study of 152 lakes in southeastern Canada indicates that only 41 per cent are less acidic today than they were 20 years ago, 50 per cent have not changed, and 9 per cent are actually more acidic. Central Ontario is the only region in which there has been a significant decline of acidity in the majority of lakes, and this is largely due to the considerable reduction in sulphur dioxide emissions from smelters in the nearby Sudbury area.
Sulphate concentrations have declined on the majority of lakes in Ontario and Quebec but not in the Atlantic provinces. Changes in lake acidity have also varied from region to region and have been generally modest. With many lakes and wetlands in the region receiving twice as much sulphate as they can tolerate, models predict that up to one quarter of the lakes in eastern Canada will remain chemically damaged after 2010.
Why are we not seeing more improvement in lake acidity? One reason is that it is too soon for a major recovery to be apparent. Emission reductions have been fairly recent, and the processes that lead to ecosystem recovery take time to occur. In addition, many lakes continue to receive more than their critical load of sulphate. For these lakes, the reductions in deposition that have occurred so far have not been enough. This is especially true in the Atlantic Provinces, where many lakes have only a very weak capacity to neutralize acids. But even where deposition has fallen below the critical load, the following factors may slow the pace of recovery.
- In some lakes there has been a decline in concentrations of acid-neutralizing bases, similar to that seen in precipitation - a phenomenon scientists are still trying to understand. As a result, the capacity of these lakes to neutralize acids has decreased. The same is happening in forest soils, where decades of acid rain have leached away calcium and magnesium leaving them less able to neutralize surface water before it reaches lakes and streams.
- Inactive sulphur that has accumulated in wetland and soils as a result of past deposition can be reactivated to form acidifying sulphates. This typically occurs during periods of drought, which is becoming more common due to climate change. When the rains return, the sulphates are flushed into surrounding lakes, thus increasing their acidity and delaying their recovery.
- Nitrogen oxides are beginning to play a greater role in acidification, particularly in south-central Ontario and southwestern Quebec. This development is disturbing, since it threatens to undermine the benefits that have been achieved through controls on sulphur dioxide.
- When acidification is combined with other stressors, as it often is, the impact on ecosystems may be magnified considerably. The cumulative effects (or interactions between stressors) of environmental stresses such as mercury, depletion of the ozone layer leading to increased ultraviolet radiation and climate change are among those whose combined impacts on the environment are difficult to predict. [see diagram].
Lake pH and Biodiversity
For the lakes of eastern Canada, the pH of water is a reliable indicator of biodiversity. From an acid rain perspective, healthy lakes are those with pH above 6 and generally moderate to high acid-neutralizing capacity (ANC = buffering capacity), so they are able to withstand acidic inputs with few changes in aquatic fauna (see chart). Damaged lakes are those with limited to moderate buffering, generally between pH 5 and 6, which support either a different mix of species, or fewer species, than lakes above this critical threshold. Further acidic inputs will usually increase the damage in these lakes. Acidic lakes are those with pH below 5 and with no ANC. These lakes have substantial to severe damage to their aquatic food chains, almost always having lost populations of fish and many acid-sensitive invertebrates. These water bodies generally retain a simple, acid-tolerant food web dominated by large, predatory insects.
The various species that inhabit our lakes, rivers and wetlands vary in their ability to tolerate acidity. Mild acidification will affect only the most sensitive species, but as acidity increases, more species will disappear. Across eastern Canada, species loss curves derived from various data sources support this general pattern. Crustaceans, like clams and crayfish, have very little tolerance because their shells contain calcium compounds which are dissolved by acidified water. Effects on crustaceans are noticeable even at a pH of 6.5, which is only slightly acidic. Fish have somewhat more tolerance, but soon encounter difficulties as the pH drops below 6. Trout, for example, begin to experience reproductive problems at a pH of 5.5 and have difficulty surviving below a pH of 5. Some living things, however, are much more tolerant of acidic water. Blackfly larvae, for instance, appear to thrive in acidified environments. Populations of benthic plants (those on the lake bottoms) also tend to increase with acidification.
Clearly, though, acidification primarily reduces the variety of life inhabiting a lake and alters the balance among surviving populations. Changes in the mix of species inhabiting water bodies will also affect birds and other species farther up the food chain, as some kinds of food resources become scarcer and others become more abundant.
Scientists cannot say whether species that have disappeared from an acidified
lake will ever return - even if pH levels return to normal. However,
near Sudbury, Ontario, where sulphur dioxide emissions from local
smelting plants have declined dramatically, invertebrates are reappearing
and fish popultions once extirpated from area lakes, are being successfully
established naturally or through restocking. Similar effects are
being witnessed in Europe, where sulphur dioxide reductions began
a decade earlier than in North America. Restoring lakes damaged
by acid rain is a long-term process, but scientists expect that
more strict emission-reduction targets will help put these ecosystems
on the road to recovery.
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