International Court of Justice reaffirms the importance of international customary law in environment disputes

In its latest Judgement of 1 December 2022 in dispute over the Status and Use of the Waters of the Silala (Chile v. Bolivia) the Court acknowledged the position of the parties that a river crossing their borders is considered as an international watercourse and is governed in its entirety by customary international law on non-navigational uses of international watercourses.

While refusing to satisfy on the claims of either party, the decision contributes further to the development of international law of treaties. The Court reaffirmed in its decision the importance of multilateral codifications, even not in force between the parties to the dispute, as the source of codification of universal norms of international customary law.

Since neither State was a party to the 1997 United Nations Watercourses Convention (UNWC) (§54), the Court decided the case on the basis of customary international law providing further clarity on which provisions of the said convention can be considered of customary nature.  It sided with Bolivia confirming the obligation to notify and consult only arise where there was a risk of significant transboundary harm. The court acknowledged that the Commentaries to the Draft Articles on the Law of Non Navigational Uses of International Watercourses prepared by the UN International Law Commission failed to impose the lower threshold than the one contained in Article 12 of the UNWC. The Court then reviewed the facts in the case and concluded that Chile had failed to allege or demonstrate any harm resulting from Bolivia’s planned measures on the Silala (§127).

The Judgement might have significant impact to the predating dispute between Lithuania and Belarus on the failure by the latter to consult and consider alternative sites for the building of nuclear power plant in Astravets on transboundary river Neris (Viliya) about 16 km from the border and very close to Lithuanian capital. While extensive arguments on Espoo convention were submited, little attention was given to the customary requirements in projects presenting significant threat to neighboring countries and the environment.




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