6-6 Evaluation of impact significance

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 ‘test of significance’ is made.

A systematic process should be followed in evaluating significance, distinguishing between ‘as predicted’ and ‘residual’ 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 – 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 ‘facts’ and values and between EIA and decision-making. First, a technical judgement must be made of the extent to which mitigation will reduce ‘as predicted’ 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 – 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.

World Bank guidelines for environmental sustainability
Environmental Aspects of Bank Work, (OMS 2.36), para 9(a) states:
Source: World Bank 1991

“The Bank endeavours to ensure that each project affecting renewable natural resources (e.g., as a sink for residues or as a source of raw materials) does not exceed the regenerative capacities of the environment.”

Output Guide

Waste emissions from a project should be within the assimilative capacity of the local environment to absorb without unacceptable degradation of its future waste absorptive capacity or other important services.

Input Guide

Harvest rates of renewable resource inputs should be within regenerative capacity of the natural system that generates them; depletion rates of nonrenewable resource inputs should be equal to the rate at which renewable substitutes are developed by human invention and investment.

Examples of threshold tests for environmental acceptability
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?

July 26, 2006 Uncategorized — brendan @ 3:01 pm

4 Comments »

  1. keep up the standard you guys. the entire course is professionally prepared and presented in a high level but simple way. quite enjoyable

    Comment by Ezra Pedzisai — November 28, 2007 @ 11:58 pm

  2. I enjoyed reading this module. It is well summarized which is good in giving first insight to the field. But I feel it needs some revisions for there are some replications and inconcictencies making confusion. Also, better if add methodologies ofvarious impact assessment. For example, methodlogies of air quality and climate assessment, of noise assessment, of soil degradation…

    Comment by solomon — March 20, 2008 @ 10:01 pm

  3. Some confusions:
    1. Are all significant impacts acceptable subject to mitigation?
    2. Is there any impact are not acceptable that is not possible to mitigate or too costly to mitigate?

    Comment by Joba — September 18, 2009 @ 2:24 pm

  4. A review of EIAs for offshore oil & gas complained that magnitude of predicted environmental change from residual impacts relative to the baseline is too often used as the arbiter of significance. To my mind the notion that ‘magnitude x importance = significance’ is not helpful, firstly because it suggests that impact significance can be determined mathematically (I am sceptical about that), and secondly because ‘importance’ is very subjective. Screening may or may not establish what is important in the offshore environment where there are few human receptors to voice a subjective opinion. In this case, it is not surprising that significance has traditionally represented predicted change in the EIAs.

    Comment by Peter Scott-Wilsom — January 18, 2011 @ 7:17 pm

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