This chapter presents the Guarani Aquifer System (GAS), its location and its characteristics, and discusses its management, since it is located in four countries, Argentina, Brazil, Paraguay, and Uruguay. The GAS is a transboundary aquifer, which goes beyond political boundaries, watersheds, and hydrogeological provinces. After briefly highlighting the technical aspects of the GAS, an overview of the groundwater management legislation of each country will be offered, with emphasis on Brazil. International Environmental Law and international treaties and agreements involving this territory will also be discussed, to present the characteristics of the Guarani Aquifer Agreement formulated by these four MERCOSUR countries, which, although not in force, provides for the protection of this environmental good. Finally, it is proposed to create an urban environmental planning tool to protect the GAS recharge areas, innovating the legal system to unite environmental law with urban law in search for GAS protection. This special environmental zoning tool will be called ZEA.