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Determining the Significance of Impacts

Impact significance places value on impact magnitude. It relates to the importance society places on the resources affected, the importance of the impact to system stability and sustainability.

Determining the significance of impacts can be one of the most difficult aspects of an impact assessment. Quantification of impact magnitude is typically an objective, scientifically based process. But once impact magnitude is determined, the analyst must decide on the significance of those impacts. Decision makers typically consider only the significant impacts in deciding among alternatives. Significant impacts will be the focus of mitigation measures and possible adjustment of project features.

Factors to consider in determining impact significance often parallel those factors used to determine impact magnitude. These factors include but are not limited to:

  • Area of Influence: Impact significance is often directly related to the size of the area affected. An example would be the acres of land disturbed.

  • Percentage of Resource Affected: The greater the percentage of a resource affected, the more significant the impact.

  • Persistence of Impacts: Permanent or long-term changes are usually more significant than temporary ones. The ability of the resource to recover after the activities are complete is related to this effect.

  • Sensitivity of Resources: Impacts to sensitive resources are usually more significant than impacts to those that are relatively resilient to impacts.

  • Status of Resources: Impacts to rare or limited resources are usually considered more significant than impacts to common or abundant resources.

  • Regulatory Status: Impacts to resources that are protected (e.g., endangered species, wetlands, air quality, cultural resources, water quality) typically are considered more significant than impacts to those without regulatory status. Note that many resources with regulatory status are rare or limited.

  • Societal Value: Some resources have societal value, such as sacred sites, traditional subsistence resources, and recreational areas. Note that some of these resources also have regulatory status.

To ensure an objective determination of impact significance, it is best to establish significance levels prior to determining impact magnitude. Predetermined significance criteria allow an objective comparison of alternatives and a determination of the anticipated effectiveness of mitigations. It is often useful to establish these criteria in consultation with regulatory or resource management agencies.

If regulatory criteria exist (e.g., air quality criteria, water quality criteria, radiation exposure criteria), these can serve as benchmarks against which impacts can be measured. Exceedances of criteria would be considered significant. Impacts would not be considered significant if no exceedances occurred.

In the absence of regulatory criteria, establishing criteria for insignificant and significant impacts will rely on professional judgment, but these should be well-defined in the assessment. Criteria often need to be established separately for each resource. Some agencies have established significance criteria that are applied to all resource areas. Some examples are those used by the U.S. Nuclear Regulatory Commission:

  • Small: Environmental effects are not detectable or are so minor that they will neither destabilize nor noticeably alter any important attribute of the resource.

  • Moderate: Environmental effects are sufficient to noticeably alter important attributes of the resource but not to destabilize them.

  • Large: Environmental effects are clearly noticeable and are sufficient to destabilize the resource.

A similar set of criteria was used by the Bureau of Land Management in assessing the significance of impacts resulting from oil shale and tar sands development on wildlife:

  • Small Impact: This is an impact that is limited to the immediate project area, affects a relatively small proportion of the local population (less than 10%), and does not result in a measurable change in carrying capacity or population size in the affected area.

  • Moderate Impact: This is an impact that extends beyond the immediate project area, affects an intermediate proportion of the local population (10 to 30%), and results in a measurable but moderate (not destabilizing) change in carrying capacity or population size in the affected area.

  • Large Impact: This is an impact that extends beyond the immediate project area, could affect more than 30% of a local population, and could result in a large, measurable, and destabilizing change in carrying capacity or population size in the affected area.