Once the impacts have been analysed, they are evaluated to determine their significance. As noted earlier, the attribution of significance begins early, during screening and scoping, and extends throughout the EIA process. There is a gradually ‘narrowing cone of resolution’ on questions of impact significance as more complete information becomes available. Following impact identification and prediction, impact evaluation is the formal stage at which a â€˜test of significanceâ€™ is made.
A systematic process should be followed in evaluating significance, distinguishing between â€˜as predictedâ€™ and â€˜residualâ€™ impacts. Step one involves evaluating the significance of ‘as predicted’ impacts to define the requirements for mitigation and other remedial actions (discussed in Section 7 â€“ Mitigation and impact assessment). Step two involves evaluating the significance of the ‘residual’ impacts, i.e. after mitigation measures are taken into account. This test is the critical measure of whether or not a proposal is likely to cause significant impacts. It is determined by the joint consideration of its characteristics (magnitude, extent, duration etc.) and the importance (or value) that is attached to the resource losses, environmental deterioration or alternative uses which are foregone (see the figure below).
Impact evaluation is a difficult and contestable exercise, which cuts across the fluid boundary between â€˜factsâ€™ and values and between EIA and decision-making. First, a technical judgement must be made of the extent to which mitigation will reduce â€˜as predictedâ€™ impacts. Second, a subjective value must be placed on the significance of residual impacts, using criteria and tests described below. Finally, the attribution of significance usually will influence final approval and condition setting; for example by indicating whether or not the impact of a proposal is acceptable or not.
However, this latter task overlaps with the responsibility of the decision-maker. The environmental acceptability of a proposal and the terms and conditions to be attached to its implementation must be weighed against other economic and social factors by the decision-maker. Further information can be found in Section 10 â€“ Decision-making.
Evaluation of significance should take place against a framework of criteria and measures established for the purpose. These may be defined in EIA legislation and procedure; for example, by definition of what constitutes an environmental impact and guidance on how to determine significance. Often specified criteria are listed to aid such evaluation; for example, environmental standards and thresholds, protected and sensitive areas, valued ecological functions and components and resource and land use capabilities. Where this EIA guidance is not available, it can be developed separately by adapting criteria and measures that are relevant to local circumstances and the type of proposals reviewed.
EIA guidelines related to significance fall into two main categories:
- emissions based, comprising standards for air and water quality, noise etc.
- environmental quality based, comprising significance criteria for valued ecosystem components or similar attributes.
Emissions based standards will be jurisdiction specific (although certain standards may be internationally recognised) and provide an objective, technical means of determining significance; for example the anticipated residual impacts either do or do not exceed the relevant standard. However, reliance on standards suffers from certain deficiencies and limitations. The relevant technical standard may be the subject of disagreement or public concern (e.g. blood lead levels, traffic noise levels, electromagnetic field strengths). In many cases, an appropriate technical standard will not be available for the evaluation of significance (e.g. ecological, social and visual impacts).
Environmental quality based criteria or thresholds are qualitative, broadly drawn and require interpretation. In this context, impact evaluation is a subjective exercise, linking scientific criteria to social preferences (as discovered through public involvement or SIA methods) and relating them to the environment and community affected. Some of the impact identification techniques discussed earlier in this topic have built in scales or weightings (and hence values) based on prior experience. When applying them, the criteria should be modified to take account of local value systems and traditional practices.
Additionally, some countries and international agencies have established environmental sustainability criteria and environmental acceptability rules against which evaluation can be conducted. For example, the World Bank input and output guidelines are meant to ensure that each project does not exceed the regenerative and assimilative capacities of the receiving environment (see the box below). In practice, as the Bank acknowledges, there is considerable difficulty in applying these guidelines and it has augmented them with other environmental and social safeguards. Rules for environmental acceptability and their relationship to significance thresholds based on Western Australian experience are described in the companion box below.
|Environmental Aspects of Bank Work, (OMS 2.36), para 9(a) states:||Source: World Bank 1991|
|Level of acceptability||Potential impact threshold||Source: Sippe 1999|
|Unacceptable||Exceeds legal threshold, e.g. quality standard|
|Increases level of risk to public health and safety above qualitative or quantitative criteria (e.g. in some jurisdictions an increased risk of death of 1 in a million per year|
|Extinction of biological species, loss of genetic diversity, rare or endangered species, critical habitat|
|Normally unacceptable||Conflict with existing environmental policies, land-use plans|
|Loss of populations of commercial biological species|
|Large-scale loss of productive capacity of renewable resources|
|May be acceptable only with minimization, mitigation, management||Avoidance of spread of biological disease, pests, feral animals, weeds|
|Taking of rare or endangered species|
|Some loss of threatened habitat|
|Normally acceptable||Some loss of populations and habitats of non-threatened species|
|Modification of landscape without downgrading special aesthetic values|
|Emissions demonstrably less than the carrying capacity of the receiving environment|
Aids and principles for evaluating significance
Key reference points for evaluating significance include:
- environmental standards, guidelines and objectives;
- level of public concern (particularly over health and safety);
- scientific and professional evidence for:
- loss/disruption of valued resource stocks and ecological functions;
- negative impact on social values, quality of life and livelihood; and
- foreclosure of land and resource use opportunities.
Guiding principles for determining significance include:
- use procedure and guidance established by the jurisdiction;
- adapt other relevant criteria or identify points of reference from comparable cases;
- assign significance in a rational, defensible way;
- be consistent in the comparison of alternatives; and
- document the reasons for the judgements made.
A test of significance can be applied by asking three questions:
- Are there residual environmental impacts?
- If yes, are these likely to be significant or not?
- If yes, are these significant effects likely to occur e.g. is the probability high, moderate or low?