Environmental Encyclopedia 3
Acid rain
are observed in central and northern Europe. Generally, the
greater the population density and density of industrializa-
tion the lower the rainfall pH. Long distance transport,
however, can result in low pH rainfall even in areas with
low population and low density of industries, as in parts of
New England, eastern Canada, and in Scandinavia.
A very significant portion of acid deposition occurs in
the dry form. In the United States, it is estimated that 30–
60% of acidic deposition occurs as dry fall. This material is
deposited as sulfur dioxide gas and very finely divided parti-
cles (aerosols) directly on the surfaces of plants (needles and
leaves). The rate of deposition depends not only on the
concentration of acid materials suspended in the air, but
on the nature and density of plant surfaces exposed to the
atmosphere and the atmospheric conditions(e.g., wind speed
and humidity).
Direct deposition of acid cloud droplets can be very
important especially in some high altitude forests. Acid cloud
droplets can have acid concentrations of five to 20 times
that in wet deposition. In some high elevation sites that are
frequently shrouded in clouds, direct droplet deposition is
three times that of wet deposition from rainfall.
Acid deposition has the potential to adversely affect
sensitive forests as well as lakes and streams. Agriculture is
generally not included in the assessment of the effects of
acidic deposition because experimental evidence indicates
that even the most severe episodes of acid deposition do not
adversely affect the growth of agricultural crops, and any
long-term soil
acidification
can readily be managed by addi-
tion of agricultural lime. In fact, the acidifying potential of
the fertilizers normally added to cropland is much greater
than that of acidic deposition. In forests, however, long-
term acidic deposition on sensitive soils can result in the
depletion of important
nutrient
elements (e.g., calcium,
magnesium, and potassium) and in soil acidification. Also,
acidic pollutants can interact with other pollutants (e.g.,
ozone
) to cause more immediate problems for tree growth.
Acid deposition can also result in the acidification of sensitive
lakes and with the loss of biological productivity.
Long-term exposure of acid sensitive materials used in
building construction and in monuments (e.g., zinc, marble,
limestone, and some sandstone) can result in surface corro-
sion and deterioration. Monuments tend to be the most
vulnerable because they are usually not as protected from
rainfall as most building materials. Good data on the impact
of acidic deposition on monuments and building material is
lacking.
Nutrient depletion due to acid deposition on sensitive
soils is a long-term (decades to centuries) consequence of
acidic deposition. Acidic deposition greatly accelerates the
very slow depletion of soil nutrients due to natural
weather-
ing
processes. Soils that contain less plant-available calcium,
7
magnesium and potassium are less buffered with respect to
degradation due to acidic deposition. The most sensitive
soils are shallow sandy soils over hard bedrock. The least
vulnerable soils are the deep clay soils that are highly buffered
against changes due to acidic deposition.
The more immediate possible threat to forests is the
forest decline
phenomenon that has been observed in forests
in northern Europe and North America. Acidic deposition
in combination with other stress factors such as ozone, dis-
ease and adverse weather conditions can lead to decline in
forest productivity and, in certain cases, to
dieback
. Acid
deposition alone cannot account for the observed forest de-
cline, and acid deposition probably plays a minor role in the
areas where forest decline has occurred. Ozone is a much
more serious threat to forests, and it is a key factor in the
decline of forests in the Sierra Nevada and San Bernardino
mountains in California.
The greatest concern for adverse effects of acidic depo-
sition is the decline in biological productivity in lakes. When
a lake has a pH less than 6.0, several
species
of minnows,
as well as other species that are part of the food chain for
many fish, cannot survive. At pH values less than about 5.3,
lake trout, walleye, and smallmouth bass cannot survive. At
pH less than about 4.5, most fish cannot survive (largemouth
bass are an exception).
Many small lakes are naturally acidic due to organic
acids produced in acid soils and acid bogs. These lakes have
chemistries dominated by organic acids, and many have
brown colored waters due to the organic acid content. These
lakes can be distinguished from lakes acidified by acidic
deposition, because lakes strongly affected by acidic deposi-
tion are dominated by sulfate.
Lakes that are adversely affected by acidic deposition
tend to be in steep terrain with thin soils. In these settings
the path of rainwater movement into a lake is not influenced
greatly by soil materials. This contrasts to most lakes where
much of the water that collects in a lake flows first into the
groundwater
before entering the lake via subsurface flow.
Due to the contact with soil materials, acidity is neutralized
and the capacity to neutralize acidity is added to the water
in the form of bicarbonate ions (bicarbonate alkalinity). If
more than 5% of the water that reaches a lake is in the form
of groundwater, a lake is not sensitive to acid deposition.
An estimated 24% of the lakes in the Adirondack
region of New York are devoid of fish. In one third to one
half of these lakes this is due to acidic deposition. Approxi-
mately 16% of the lakes in this region may have lost one or
more species of fish due to acidification. In Ontario, Canada,
115 lakes are estimated to have lost populations of lake trout.
Acidification of lakes, by acidic deposition, extends as far
west as Upper Michigan and northeastern Wisconsin, where
many sensitive lakes occur and there is some evidence for