Acid precipitation in the Great Lakes - southeastern Canada region

Over the past 20 to 25 years the acidity of rain and snow has been increasing in many areas throughout the world, including the northeastern United States and southeastern Canada. This increased acidity, which has been attributed to increased emissions of sulfur dioxide and other acid-forming compounds from industrial and urban sources may have a number of dertimental effects. For example, observations in northern Europe and the northeastern U.S. have shown the decline or disappearance of freshwater fish populations, damage to forest vegetation, leaching of both plant and soil nutrients, and deterioration of buildings. Acid precipitation is usually defined as rain or snow with a pH value of less than 5.6, which is the expected value of pure water in equilibrium with ambient concentrations of carbon dioxide. Although some argue that this designated pH value should be higher, because of the presence in precipitation of sea salts and dust, the ''threshold'' value of 5.6 is still normally used. The pH values of a number of common materials are tabulated. Given the logarithmic scale of pH unit, a decrease of pH from, say, 5.6 to 4.6 represents a tenfold increase in acidity. Therefore, when we say that the pH of precipitationmore » in the past 20 years has decreased one to two units, we are talking about a tenfold to one hundredfold increase in acidity.« less