Thanks to strict regulations imposed in the past 2 decades, industries in Western nations have steadily slashed their emissions of sulfur dioxide, the major cause of acid rain. But no waterways in North America have yet become less acidic, according to a study in tomorrow's Nature, suggesting that other pollutants may be preventing their recovery.
Burning coal and other fossil fuels liberates sulfur and nitrogen compounds, which in the atmosphere react with water, oxygen, and sunlight to form acids. Falling in rain and snow, the acids, scientists began to realize in the 1970s, were lowering the pH of many lakes and streams. In some regions, such as the Adirondacks in New York, the acidity became so strong that it began to wipe out fish populations.
Laws aimed at reducing sulfur emissions came on the books in Europe in 1985 and in the United States as a 1990 amendment to the Clean Air Act. To gauge the laws' effectiveness, a team led by John Stoddard of the U.S. Environmental Protection Agency analyzed 15 years' worth of data on surface sulfur concentrations, soil chemistry, and alkalinity in 205 lakes and streams across North America and Europe. Reductions in sulfur emissions have indeed led to a strong trend of declining sulfur concentrations in waters throughout the Northern Hemisphere, they found.
"The good news is, the laws are having their intended effect," says team member Steve Kahl, an environmental chemist at the University of Maine. In Europe, as sulfur concentrations went down, the waters became less acidic.
But the picture is more complicated in North America. At most of the test sites, lower sulfur concentrations did not lead to less acidic waters, particularly in three areas--south-central Ontario, the Adirondack and Catskill mountains, and the Midwest--possibly due to the Midwest's large number of coal-fired utility plants, which produce the lion's share of sulfur emissions.
The problem may lie in the soil near these waters, which has a paucity of minerals, such as sodium and calcium, that act like antacid tablets and buffer the acids. According to Kahl, it's possible that nitrogen emissions--which also contribute to acid rain and are less strictly curtailed under current regulations--may produce enough acid to offset the action of these natural buffers.
"It may be decades before some of these waters are suitable for fish again," notes Alan Jenkins, a chemist at the Institute of Hydrology in Wallingford, United Kingdom. "We need to do more in terms of sulfur and nitrogen reduction, and we still have a lot to understand about the recovery process."