Acid Deposition 237© America/Alamysmokestacks of coal-fired power plants. The resulting
decrease in acid deposition prevents surface waters and
soil from becoming more acidic than they already are.
Rainfall in parts of the Midwest, Northeast, and Mid-
Atlantic regions is less acidic today than it was two decades
ago, as a result of governmental policies that require
cleaner-burning power plants and the use of reformulated
gasoline. Many power plants in the Ohio Valley switched
from high-sulfur to low-sulfur coal. However, solving one en-
vironmental problem often creates others. While the move
to low-sulfur coal reduced sulfur emissions, it contributed
to the problem of global climate change. Because low-sulfur
coal has a lower heat value than high- sulfur coal, more of
it must be burned—and more CO 2 emitted—to generate
a given amount of electricity. Low-sulfur coal also contains
higher levels of mercury and other trace metals, so burning
it adds more of these hazardous pollutants to the air.
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This mountain stream in the Adirondack Mountains of New York may be acidic because
years of acid precipitation have altered soil chemistry.
However, a 1991 agreement between the United
States and Canada, along with legislation within each
country, has reduced acid deposition by as much as
35 percent in some areas.
Despite the fact that the United States, Canada, and
many European countries have reduced sulfur emissions,
acid precipitation remains a serious problem. Acidified
forests and bodies of water have not recovered as quickly
as hoped. Trees in the U.S. Forest Service’s Hubbard
Brook Experimental Forest in New Hampshire, an area
damaged by acid deposition, have grown little, even fol-
lowing two decades of declining emissions. Many north-
eastern streams and lakes, such as those in New York’s
Adirondack Mountains, remain acidic (}ÕÀiÊ °£n).
A likely reason for the slow recovery is that the past 30
or more years of acid rain have profoundly altered soil
chemistry in many areas. Essential plant minerals such