Environmental Engineering Reference
In-Depth Information
seldom possible, since, in most cases,
environmental conditions had already been
altered by acid rain, by the time monitoring was
introduced.
The areas at greatest risk from acid rain in
the northern hemisphere are the pre-Cambrian
Shield areas of Canada and Scandinavia, where
the acidity of the rocks is reflected in highly acidic
soils and water. The folded mountain structures
of eastern Canada and the United States,
Scotland, Germany and Norway are also
vulnerable (see Figure 4.3). Most of these areas
have already suffered, but the potential for
further damage is high. Should the present
emission levels of SO
2
and NO
X
be maintained
for the next ten to twenty years, it is likely that
susceptible areas, presently little affected by acid
rain—in western North America and the Arctic
for example—would also suffer damage as the
level of atmospheric acidity rises.
scientific investigation of the problem has grown
rapidly. By the mid-1980s, a majority of scientists
accepted that a link existed between the emissions
of SO
2
and NO
X
and the acidification of lakes.
A minority remained unconvinced. Along with
politicians in the United States and Britain, they
continued to counter requests for emission
reductions with calls for more studies (Park
1987), despite an estimate that research over a
quarter of a century had resulted in more than
3000 studies in North America alone (Israelson
1987).
There is a tendency for all lakes to become
more acidic with time, as a result of natural
ageing processes, but studies of acid-sensitive
lakes suggest that observed rates of change in
pH values since the middle of the nineteenth
century have exceeded the expected natural rates.
The analysis of acid sensitive diatom species in
lake sediments has allowed pH-age profiles to
be constructed for some 800 lakes in North
America and Europe (Charles
et al.
1990). Most
of these profiles indicate that lake acidification
is a relatively recent phenomenon, with little
change in pH values indicated prior to 1850. In
almost a century and a half since then, pH values
in the lakes studied have decreased by between
0.5 and 1.5 units (see Figure 4.7). In contrast,
less sensitive lakes—those with adequate
buffering for example—showed little change.
Some of the increased acidity could be
explained by changing land use. In the
Netherlands, for example, agricultural practices
such as sheep washing and the foddering of ducks
kept levels of acidity artificially low in the past,
but as these activities declined, the acidity of the
lakes began to rise again (Charles
et al.
1990).
In south-west Scotland, reforestation has been
accompanied by increasing acidity because the
trees have a greater ability than the vegetation
they replaced to scavenge acid aerosols from
passing air streams (Mason 1990). Such
situations seem to be the exception, however, and
the general consensus is that declining pH values
are the result of increased acidic deposition since
the Industrial Revolution. Rising levels of copper,
zinc and lead in the lake deposits, plus the
ACID RAIN AND THE AQUATIC
ENVIRONMENT
The earliest concerns over the impact of acid rain
on the environment were expressed by Robert
Smith, in England, as long ago as 1852 (Park
1987), but modern interest in the problem dates
only from the 1960s. Initial attention
concentrated on the impact of acid rain on the
aquatic environment, which can be particularly
sensitive to even moderate increases in acidity,
and it was in the lakes and streams on both sides
of the Atlantic that the effects were first apparent.
Both LaBastille (1981) and Park (1987) credit
Svente Oden, a Swedish soil scientist, with
bringing the problem to the attention of the
scientific community, in a campaign which began
in 1963. Both also mention the work of Eville
Gorham in the late 1950s, in the English Lake
District, and the studies of Gene Likens and
Herbert Bormann in New Hampshire, dating
back to 1963. Gorham was also one of the first
to become involved in the study of acid lakes in
Canada, in the area polluted by smelter emissions
around Sudbury, Ontario (Gorham and Gordon
1960). From these limited beginnings, the
