[en] The IAEA, through the Technical Cooperation Project (RLA8036), supports the application of isotope tools as part of a large international project which addresses a number of key questions in hydrolgeology and groundwater uses of the Guarani Aquifer. Groundwater constitutes around 95% of all the planet's fresh and liquid water. It is used by more than two billion people in the world and is considered as the greatest strategic reserve for the next decades. About 26% of the available freshwater in the world is in South America, and more than 50% of its population is supplied by groundwater. Low vulnerability to short climate variations places groundwater in a privileged position when societies' sustainability is evaluated. In spite of this importance, in many of our countries, groundwater resources have not received, nor receive, the attention deserved. During the last decade, the interest in South America about groundwater has grown but the few available management schemes operating in most countries are no longer adequate. As a result of a lack of adequate management schemes, ground pollution and inadequate well-head protection measures, some overexploitation (greater water extraction than the aquifer's potential) and pollution signs have been observed in some aquifers. If actions are not taken to ensure the preservation of these resources and correction measures are not adopted in critical areas, we will place at risk public and private water supply schemes, mainly in urban centres. The actions to be adopted will depend on the existing technical knowledge of the aquifers an on the countries' initiatives