Environmental Engineering Reference
In-Depth Information
to chronic renal failure (Abrahamsen et al.
1989). Heavy metals such as copper, cadmium,
zinc and mercury, liberated from soil and
bedrock by acid rain, may eventually reach the
human body via plants and animals in the food
chain or through drinking water supplies. The
corrosion of storage tanks and distribution
pipes by acidified water can also add metals to
drinking water: the liberation of lead from lead
piping or from solder on copper piping is a
particular concern. Although quality control in
water treatment plants can deal with such
problems (Ontario: Ministry of the
Environment 1980), many areas subject to acid
rain depend upon wells, springs and lakes,
which provide an untreated water supply. This
may expose users to elevated levels of such
metals as lead and copper and although
individual doses in all of these situations would
be small, regular consumption might allow the
metals to accumulate to toxic levels.
declined by nearly 40 per cent since 1970
(Caulfield and Pearce 1984), and, in Canada,
emissions decreased by 45 per cent between 1970
and 1985 (Environment Canada 1991), yet the
reduction in aquatic or terrestrial acidity
downwind from these areas remains less than
expected. The discrepancy may be explained in
part by the continuing rise in NO
X
emissions,
offsetting the decline in SO
2
, but it is also possible
that the link between reduced acidity and
ecological recovery is not linear. In that case, a
specific reduction in acid emissions might not
bring about an equivalent reduction in
environmental damage (Park 1991). In Lake
Oxsjon in Sweden, for example, the pH fell from
6.8 to 4.5 between 1968 and 1977. Despite the
marked reduction in SO
2
emissions from Britain
and Sweden in the decade or so since then, the
pH has only recovered to 4.9 (Mason 1990).
Reductions of a similar magnitude have been
identified in some Scottish lochs (Last 1989).
Surveys in lakes in Ontario indicate that pH
values may rise by small but significant amounts
soon after acid input is reduced, but it may be
10 to 15 years before aquatic biota respond to
the reduced acidity (Havas 1990). This indicates
that natural recovery does seem to be possible,
but that it is a slow process, taking much longer
than the original acidification. It may also
indicate that some direct human input will be
necessary, either to initiate the recovery process
or to speed it up.
One possible input is the addition of lime,
which would produce an immediate reduction
in acidity, and allow the recovery mechanisms
to work more effectively. Lime has been used as
a means of sweetening acid soils for many years,
and may be the reason that in areas of acid soils
agricultural land is less affected by acid rain
than the natural environment. In areas where
natural regeneration is no longer possible, the
restoration of the original chemical balance of
the soil by liming and appropriate fertilizer
application might allow reforestation to be
successful.
The same situation might apply in the aquatic
environment. Simply re-stocking an acidified
SOLUTIONS TO THE PROBLEM OF ACID
RAIN
Although the cause and effect relationship
between emissions of SO
2
and NO
X
and acid
rain damage is not universally accepted, most of
the solutions proposed for the problem involve
the disruption of that relationship. The basic
approach is deceptively simple. In theory, a
reduction in the emission rate of acid forming
gases is all that is required to slow down and
eventually stop the damage being caused by the
acidification of the environment. Translating
that concept into reality has proved difficult,
however.
The reduction in emissions of SO
2
and NO
X
is a long-term solution, based on prevention. It
is also a solution in which the environment itself
has a major role. As emissions decline, it must
adjust until some new level of equilibrium,
reflecting the decreased acidity, is attained. There
is concern that in some areas the damage has
gone too far to be reversed completely, and there
is currently some support for this point of view.
For example, SO
2
emissions in Britain have
