Environmental Engineering Reference
In-Depth Information
rain. Decline is not widespread, and is present
even in areas where acid precipitation is minimal
(Abrahamsen et al. 1989).
Damage to existing trees is only part of the
problem. The future of the forests is at stake
also. Natural regeneration is no longer taking
place in much of Central Europe, and even the
planting of nursery-raised stock provides no
guarantee of success (Tomlinson 1985).
Developments such as these raise the spectre of
the wholesale and irreversible loss of forest
land. Such is the importance of the forest
industry to many of the regions involved, that
this would lead to massive economic disruption,
and it is therefore not surprising that calls for
action on acid rain from these areas became
increasingly more strident—perhaps even
desperate—through the 1980s (Pearce 1982b;
Piette 1986). Although acid gas emissions have
declined in Europe and North America in the
last decade, allowing some limited recovery in a
number of aquatic ecosystems (Last 1989),
there is little evidence of similar improvements
in terrestrial ecosystems. This may reflect the
longer response time associated with the greater
ecological complexity of the latter, but it may
also support the claims of many scientists that
there are no well-established physical links
between acid rain and forest decline (Haines
and Carlson 1989; Pitelka and Raynal 1989).
The decline of European forests, for example, is
now seen as a multifaceted problem in which
acid precipitation may only be a minor
contributor along with tree harvesting practices,
drought and fungal attacks (Blank et al. 1988).
In contrast, Ulrich—who first formulated the
processes by which acid rain was thought to
cause forest damage (see, for example, Ulrich
1983)—accepts the contributions of other
factors, but continues to claim that they only
come into play fully once soil acidification has
made the forest vulnerable (Hauhs and Ulrich
1989). Until these differences are resolved, and
the role of acid rain is better understood there is
no guarantee that the greater control of acid
emissions will have the desired effect of saving
the forests.
ACID RAIN AND THE BUILT
ENVIRONMENT
Present concern over acid rain is concentrated
mainly on its effect on the natural environment,
but acid rain also contributes to deterioration in
the built environment. Naturally acid rain has
always been involved in the weathering of rocks
at the earth's surface. It destroys the integrity of
the rock by breaking down the mineral
constituents and carrying some of them off in
solution. All rocks are affected to some extent,
but chalk, limestone and marble are particularly
susceptible to this type of chemical weathering.
Inevitably, when these rocks are used as building
stone, the weathering will continue. In recent
years, however, it has accelerated, in line with
the increasing acidity of the atmosphere.
Limestone is a common building stone,
because of its abundance, natural beauty and ease
of working. Its main constituents are calcium and
magnesium carbonates, which react with the
sulphuric acid in acid rain to form the appropriate
sulphate. These sulphates are soluble, and are
washed out of the stone, gradually destroying
the fabric of the building in the process. Further
damage occurs when the solutions evaporate.
Crystals of calcium and magnesium sulphate
begin to form on or beneath the surface of the
stone. As they grow, they create sufficient
pressure to cause cracking, flaking and crumbling
of the surface, which exposes fresh material to
attack by acid rain (Anon. 1984). Limestone and
marble suffer most from such processes.
Sandstone and granite may become discoloured,
but are generally quite resistant to acidity, as is
brick. Acid damage to the lime-rich mortar
binding the bricks may weaken brick-built
structures, however (Park 1987). Structural steel
and other metals used in modern buildings may
also deteriorate under attack from acid rain
(Ontario: Ministry of the Environment 1980).
By attacking the fabric of buildings, acid rain
causes physical and economic damage, but it does
more than that; it also threatens the world's
cultural heritage. Buildings which have survived
thousands of years of political and economic
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