Page 114 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

78 part I The energy–atmosphere System
F cus Study 3.2 Pollution
Acid Deposition: Damaging to Ecosystems
acid deposition is a major environmen- tal issue in some areas of Canada, the United States, europe, and asia. Such deposition is most familiar as “acid rain,” but it also occurs as “acid snow,” and in dry form as dust or aerosols. government estimates of damage from acid deposi- tion in Canada, the United States, and europe exceed US$50 billion annually.
acid deposition is causally linked
to serious environmental problems: declining fish populations and fish kills, widespread forest damage, changes in soil chemistry, and damage to buildings, sculptures, and historic artifacts. regions that have suffered most are southeastern Canada, the northeastern United States, Sweden, norway, germany, much of east- ern europe, and China.
Acid Formation
The problem begins when sulfur dioxide and nitrogen oxides are emitted as by- products of fertilizers and fossil fuel com- bustion. Winds may carry these gases many kilometres from their sources. Once
in the atmosphere, the chemicals are converted to nitric acid (HnO3) and sulfu- ric acid (H2SO4). These acids are removed from the atmosphere by wet and dry deposition processes, falling as rain or snow or attached to other particulate matter. The acid then settles on the landscape and eventually enters streams and lakes, carried by runoff and ground- water flows.
The acidity of precipitation is mea- sured on the pH scale, which expresses the relative abundance of free hydro- gen ions (H+) in a solution—these are what make an acid corrosive, for they easily combine with other ions. The
pH scale is logarithmic: each whole number represents a tenfold change.
a pH of 7.0 is neutral (neither acidic nor basic). Values less than 7.0 are in- creasingly acidic, and values greater than 7.0 are increasingly basic, or alka- line. (See Figure 18.7 for a graphic representation of the scale.)
natural precipitation dissolves carbon dioxide from the atmosphere to form
carbonic acid. This process releases hydrogen ions and produces an average pH reading for precipitation of 5.65. The normal range for precipitation is 5.3–6.0. Thus, normal precipitation is always slightly acidic. Scientists have measured precipitation as acidic as pH 2.0 in the eastern United States, Scandinavia, and europe. By comparison, vinegar and lemon juice register slightly less than 3.0. in lakes, aquatic plant and animal life perishes when pH drops below 4.8.
Effects on Natural Systems
More than 50 000 lakes and some
100 000 km of streams in Canada and
the United States are at a pH level below normal (that is, below pH 5.3), with several hundred lakes incapable of supporting any aquatic life. acid deposition causes the release of aluminum and magnesium from clay minerals in the soil, and both of these are harmful to fish and plant communities.
in addition, relatively harmless mer- cury deposits in lake-bottom sediments convert in acidified lake waters to form highly toxic methylmercury, which is deadly to aquatic life and moves through- out biological systems. local health advisories are regularly issued in two Canadian provinces and 22 U.S. states to warn those who fish of the methylmercury problem.
acid deposition affects soils by killing microorganisms and causing a decline in soil nutrients. in new Hampshire’s Hub- bard Brook experimental Forest (www .hubbardbrook.org/), an ongoing study from 1960 to the present found that half the nutrient calcium and magnesium base cations were leached from the soil; excess acids are the cause of the decline.
Damage to forests results from de- ficiencies in soil nutrients. The most ad- vanced impact is seen in forests in eastern europe, principally because of the area’s long history of burning coal and the den- sity of industrial activity. in germany and
▲Figure 3.2.1 Acid deposition blight. Stressed forests on Mount Mitchell in the appalachians. [Photo by Will and Deni Mcintyre/Science Source.]
84° 83° 37°
KENTUCKY
TENNESSEE 36°
35°
GEORGIA
84° 83°
82° VIRGINIA
81°
37°
36°
NORTH CAROLINA
Mt. Mitchell
0
40 80 KILOMETRES
SOUTH CAROLINA
35°
and city rankings of the worst and the best, see www.lung .org (enter “city rankings” in the website search box). No similar large-scale study has been conducted in Canada, but an air pollution study titled Burden of Illness pub- lished in 2004 found that 1700 premature deaths and 6 000 hospital admissions annually in Toronto are related
to ground-level ozone. Children accounted for the high- est number of those hospital admissions.
VOCs, which react with nitrogen oxides to form photochemical smog, include a variety of chemicals— the outdoor pollutants from gasoline and combustion at electric utilities, as well as indoor pollutants emitted