|Environmental impact assessments (EIAs) are an important element of the planning process by which environmental considerations are integrated into decision-making procedures for measures that may have adverse (environmental) effects.275 The overall aim of an EIA is to provide a basis by which to come to an informed decision through a thorough analysis of anticipated environmental impacts – revealing the main risks of the project and providing pathways for modifications of the plan to mitigate adverse (environmental) effects. The development of an EIA has been encouraged or required by various international instruments.276
While the UN Watercourses Convention does not directly require the planning state to carry out an EIA, it nevertheless suggests that if a state which might be affected by the planned measure asks the planning state to provide an EIA, the latter would have to comply with this request if the former bears the costs.277 Additionally, it could be maintained that the assessment and evaluation of possible environmental impacts of a new project on a transboundary watercourse is an inherent prerequisite for complying with the customary obligation not to cause significant transboundary harm278 (see Article 7).
The International Court of Justice (ICJ) supported this view in its judgement of the Pulp Mills Case by linking the interstate notification of planned measures to the satisfaction of the due diligence obligation to prevent significant transboundary harm. It established that conducting an EIA ‘may now be considered a requirement under general international law’ with regard to activities which ‘may have a significant adverse impact in a transboundary context.279’ The importance here is the argument of the court that the duty to notify, linked with the duty to conduct an EIA, exists in customary international law. Hence, it applies to all states – irrespective of the existence of such obligations in the relevant transboundary water agreements.280 However, due to the fact that this obligation is rather open and imprecise, it leaves considerable room for debate as to which elements to include in an EIA.281 An example of an international agreement which provides further information can be seen in the 1991 UNECE Convention on Environmental Impact Assessment in a Transboundary Context.282
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