Hydrogeological units

The distribution of underground water resources in mainland Portugal is closely related to the geological actions that shaped our territory. In the Meso-Cenozoic basins, mainly occupied by detritical or carbonated rocks, little or nothing affected by metamorphism phenomena, are the most productive aquifers with the most abundant resources. The Ancient Massif, mainly composed of eruptive and metasedimentary rocks, generally has few resources, although some exceptions are noted, usually related to the presence of limestone massifs. The correspondence between the distribution and characteristics of aquifers and geological units had already been noted by several authors, having formed the basis for the establishment by INAG of four hydrogeological units, corresponding to the four large morpho-structural units in which the country is divided: Ancient Massif, also known as the Iberian Massif or Hesper Massif. •Western Mesocenozoic Orla, abbreviated to Western Orla. •Orla Mesocenozoic South, abbreviated as Southern Rim. •Tejo-Sado Tertiary Basin, abbreviated to the Tagus-Sado Basin.