One can identify two major categories of uncertainty in EIA: data (scientific) uncertainty inherited in input data (e.g., incomplete or irrelevant baseline information, project characteristics, the misidentification of sources of impacts, as well as secondary, and cumulative impacts) and in impact prediction based on these data (lack of scientific evidence on the nature of affected objects and impacts, the misidentification of source-pathway-receptor relationships, model errors, misuse of proxy data from the analogous contexts); and decision (societal) uncertainty resulting from, for example, inadequate scoping ofimpacts, imperfection of impact evaluation (e.g., insufficient provisions for public participation), 'human factor' in formal decision making (e.g., subjectivity, bias, any kind of pressure on a decision maker), lack of strategic plans and policies, and possible implications of nearby developments.
Some consequences of increased pollution of air, water, and soil occur abruptly or over a short period of time. Such is the case, for instance, with the outbreak of pollution-induced diseases, or the collapse of an ecosystem as one of its links ceases to perform. Avoiding or preparing for such catastrophes is particularly difficult when occurrence conditions involve uncertainty.
In spite of almost global attraction of the critical load concept, the quantitative assessment ofcritical load values is connected till now with some uncertainties. The phrase 'significant harmful effects' in the definition of critical load is of course susceptible to interpretation, depending on the kind of effects considered and the amount of harm accepted. Regarding the effects considered in terrestrial ecosystems, a distinction can be made in effects on (Figure 5):
• soil microorganisms and soil fauna responsible for bio-geochemical cycling in soil (e.g., decreased biodiversity);
• vascular plants including crops in agricultural soils and trees in forest soils (e.g., bioproductivity losses);
• terrestrial fauna such as animals and birds (e.g., reproduction decrease);
• human beings as a final consumer in biogeochemical food webs (e.g., increasing migration of heavy metals due to soil acidification with exceeding acceptable human daily intake, etc.).
In aquatic ecosystems, it is necessary to consider the whole biogeochemical structure of these communities
Figure 5 A simplified biogeochemical food web in the terrestrial ecosystems.
Receptor I I Compartment
Figure 6 A simplified biogeochemical food web in the aquatic ecosystems.
and a distinction can be made accounting for the diversity of food webs (Figure 6):
• aquatic and benthic organisms (decreased productivity and biodiversity);
• aquatic plants (e.g., decreased biodiversity, eutrophication);
• human beings who consume fish or drinking water (surface water) contaminated with mobile forms of heavy metals due to acidification processes (e.g., poisoning and death).
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