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What is pH?:
The term pH refers to the free
hydrogen ions (electrically charged atoms) in water and is
measured on a scale from 0 to 14. Seven is considered neutral
and measurements below seven are acidic while those above it
are basic or alkaline. Every point on the pH scale represents
a tenfold increase over the previous number. Thus, pH 4 is 10
times more acidic than pH 5 and 100 times more so than pH 6.
Similarly, pH 9 is 10 times more basic than pH 8 and 100 times
more basic than pH 7.
Rain
Water Acidity:
Rain is naturally acidic because carbon dioxide, found
normally in the earth's atmosphere, reacts with water to form
carbonic acid. While "pure" rain's acidity is pH 5.6-5.7,
actual pH readings vary from place to place depending upon the
type and amount of other gases present in the air, such as
sulphur oxide and nitrogen oxides.
The acid in acid rain comes from two kinds of air pollutants--
sulphur dioxide (SO2) and nitrogen oxides (NOx).
These are emitted primarily from utility and smelter
"smokestacks" and automobile, truck and bus exhausts, but they
also come from burning wood. When these pollutants reach the
atmosphere they combine with gaseous water in clouds and
change to acids--sulphuric acid and nitric acid. Then, rain
and snow wash these acids from the air.
Affected Countries:
The effects of acid rain have been recorded in parts of the
United States, the late Federal Republic of Germany,
Czechoslovakia, the Netherlands, Switzerland, Australia,
Yugoslavia and elsewhere. It is also becoming a significant
problem in Japan and China and in southeast Asia. Rain with a
pH of 4.5 and below has been reported in Chinese cities
("Japan and China").
Indian
perspective:
Sulphur dioxide emissions were reported in 1979 to have nearly
tripled in India since the early 1960s, making them only
slightly less than the then-current emissions from the Federal
Republic of Germany.
General Impacts of Acid rain:
Acid rain kills
aquatic life, trees, crops and other vegetation, damages
buildings and monuments, corrodes copper and lead piping,
damages such man-made things as automobiles, reduces soil
fertility and can cause toxic metals to leach into underground
drinking water sources. Acid rain affects lakes, streams,
rivers, bays, ponds and other bodies of water by increasing
their acidity until fish and other aquatic creatures can no
longer live.
Aquatic plants grow best between pH 7.0 and 9.2 (Bourodemos).
As acidity increases (pH numbers become lower), submerged
aquatic plants decrease and deprive waterfowl of their basic
food source. At pH 6, freshwater shrimp cannot survive. At pH
5.5, bottom-dwelling bacterial decomposers begin to die and
leave un-composed leaf litter and other organic debris to
collect on the bottom. This deprives plankton--tiny creatures
that form the base of the aquatic food chains, so that they
too disappear. Below a pH of about 4.5, all fish die. As
un-decomposed organic leaf-litter increases, owing to the loss
of bottom-dwelling bacteria, toxic metals such as aluminum,
mercury and lead within the litter are released. Other metal
flows into the water from the soils in the surrounding
watershed.
These toxic metals are bad for human health; high lead levels
may harm people who drink such water and people who ingest
mercury in tainted fish suffer serious health problems. Most
of the frogs and insects also die when the water reaches pH
4.5.
Acid rain harms more than aquatic life. It also harms
vegetation. The forests of the Federal Republic of Germany and
elsewhere in Western Europe, for example, are believed to be
dying because of acid rain. Scientists believe that acid rain
damages the protective waxy coating of leaves and allows acids
to diffuse into them, which interrupts the evaporation of
water and gas exchange so that the plant no longer can
breathe. This stops the plant's conversion of nutrients and
water into a form useful for plant growth and affects crop
yields.
Perhaps the most important effects of acid rain on forests
result from nutrient leaching, accumulation of toxic metals
and the release of toxic aluminum. Nutrient leaching occurs
when acid rain adds hydrogen ions to the soil which interact
chemically with existing minerals. This displaces calcium,
magnesium and potassium from soil particles and deprives trees
of nutrition.
Toxic metals such as lead, zinc, copper, chromium and aluminum
are deposited in the forest from the atmosphere. The acid rain
releases these metals and they stunt the growth of trees and
other plants and also that of mosses, algae, nitrogen-fixing
bacteria and fungi needed for forest growth.
Solution:
The only cost-effective solution to the problem, according to
many people, is to reduce emissions at their point of origin.
Anyone investigating acid rain should update these figures.
Environmental analyst and writer Sandra Postel of Worldwatch
Institute says: "Forest effects do not stop at the forest
boundary, but ripple to groundwater, streams and lakes which
receive acids and metals that break from the forest cycle.
Humanity's intimate connection to these forest systems ensures
that it will not escape feeling the effects of their demise".
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