Overestimation of mitigation leads to underestimation of residual impacts

Abstract

An essential part of environmental impact assessment (EIA) is the proposal of mitigation measures, whose effectiveness has a decisive influence on the magnitude of the residual impacts of the proposals. However, the effectiveness of mitigation is not always well known, and sometimes it is even used in a manipulative way. This paper presents a sample of effectiveness values for some mitigation measures commonly proposed in EIA reports, based on information from the academic and grey literature. The aim is to show that the effectiveness of mitigation measures can be highly variable, and may even lead to undesirable effects, and to highlight how this can influence the assessment of residual impacts. As examples, climate change mitigation may have undesired effects; revegetation effectiveness is overrated at least in some regions; wildlife protection measures may be over-optimistic; and sometimes there are complaints after the implementation of mitigation measures for odour, noise or vibration because beneficiaries expected greater effectiveness. Overestimating mitigation effectiveness leads to ignoring the mitigation hierarchy and underestimating residual impacts, which may distort decision-making. The effectiveness of mitigation is often site-dependent and species-dependent, so a lot of information is needed. Feedback from environmental follow-up is necessary, and EIA authorities should make an effort to collect and disseminate this information. Academics, practitioners and environmental authorities need to work hand in hand in this regard.