Page 170 - Materials Chemistry, Second Edition

L1644_C04.fm Page 142 Tuesday, October 21, 2003 3:13 PM

The main function of risk assessment is the overall protection of the environ-
ment. However, certain assumptions are made concerning the aquatic environment
that allow an uncertainty extrapolation to be made from single-species, short-term
toxicity data to ecosystem effects. It is assumed that ecosystem sensitivity depends
on the most sensitive species and that protecting ecosystem structure protects
community function. These two assumptions have some important consequences.
When the most sensitive species to the toxic effects of a chemical in the laboratory
is established, extrapolation can subsequently be based on the data from that
species. Furthermore, the functioning of any ecosystem in which that species exists
is protected, provided the structure is not sufficiently distorted to cause an imbal-
ance. It is accepted that protection of the most sensitive species should protect the
structure.
With regard to the assessment of impacts that affect ecosystems, E-R or damage
functions have been developed for acidic deposition on natural and semi-natural
terrestrial ecosystems. That means it accounts for the impact corresponding to the
sub-area of protection (see Chapter 3) biodiversity and natural landscapes. Currently
the most widely applicable approach for the analysis of pollutant effects on terrestrial
ecosystems is the critical load/ level approach. The following definitions of the terms
critical load and levels were given by UN-ECE (1991).

Critical load: The highest deposition of acidifying compounds that will not cause
chemical changes leading to long-term harmful effects on ecosystems structure and
function according to the present knowledge.
Critical levels: The concentration of pollutants in the atmosphere above which direct
adverse effects on receptors, such as plants, ecosystems or material may occur accord-
ing to the present knowledge.

Critical loads have been defined for several pollutants and ecosystems. However,
they cannot be used directly to assess damages per se, rather they simply identify
the areas where damages are likely to occur. In the present study the relative
exceedance weighted (REW) ecosystem area approach is used to assess the envi-
ronmental impact. The contribution of a specific source of pollutants to the exceed-
ance of critical loads for ecosystems is analyzed by taking into account predefined
background conditions. The relative exceedance factor fRE is the contribution of
the concentration increase ∆c due to the emission to the height of exceedance of the
critical load divided by the critical load C itself (see Expression (4.2)). The REW
CL
2
ecosystem area indicator is expressed in km and is obtained by the multiplication
of f by the ecosystem exceeded area A where the critical load is exceeded by
EE
RE
the pollutant (see Expression (4.3)) (IER, 1998).
f = ∆c/ C CL (4.2)
RE
REW = f * A EE (4.3)
RE

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