LAUSR

Abstract Study region Latin America. Parana Basin. Argentina, Brazil, Paraguay, Uruguay. Study focus This article traces the trajectory of transboundary cooperation over the past fifteen years for the Guarani Aquifer System. A new insight for the region The Guarani Aquifer System is a transboundary aquifer shared by Argentina, Brazil, Paraguay, and Uruguay. It stands as one of the largest reservoirs of freshwater worldwide and is one of the few transboundary aquifers whose management is regulated by an international treaty, the Guarani Aquifer Agreement (GAA). The latter is also the first to refer in its preamble to the UN International Law Commission Draft Articles on the Law of Transboundary Aquifers. A first period (2002–2010) of positive collaboration in which the four countries actively moved forward towards a better understanding of the aquifer, culminated with the adoption of the GAA in August 2010. A second period (2010–2017) has been marked by a slowdown in transboundary cooperation, limited in this period to sporadic cross-border projects and initiatives linked to past and existing international projects. In this period, Argentina and Uruguay, and more recently Brazil have ratified the GAA. A third phase seems to be emerging in 2017 due to the possibility that finally Paraguay ratifies the GAA allowing it to enter into force. This article argues that in the future implementation of the agreement countries should build on the good practices, both substantive and institutional, stemming from the first period of transboundary cooperation.