The Arapey group

confining basalts of the Guarani aquifer in Uruguay

Resumo

With the increasing demand of drinking water for agricultural use, takes special importance the geological and hydrogeological knowledge of the infrabasaltic aquifer known as Guaraní Aquifer in the South Cone. The different stratigraphic concepts historically managed for the geological formations of the top of Gondwana Supergroup in Uruguay, carrying great volumes of groundwater with thermal potential, agricultural use (irrigation) and urban supply are discussed. Geological units of Guaraní Aquifer are described: Buena Vista, Cuchilla Ombú, Tacuarembó and Rivera Formations.

Geometry  and thickness of its confining unit - Arapey group - condition the economic possibilities of taking advantages using groundwater from Guaraní Aquifer. Such group has been divided - according to lithostructural criteria - on six formations for which a brief description is given.

A preliminary tridimensional geometry of Arapey group´s basaltic lava and its relation with Guaraní Aquifer is showed. This geometry is related to the basaltic thickness to cross in a borehole in order to obtain groundwater with an error margin of about 30 meters. This is very relevant, specialy in lithosol zones of the basaltic area, where the installation of irrigation systems would mean a sustancial agrarian improvement in these areas.