The Guarani Aquifer System is a massive groundwater body underlying large areas of Brazil, Paraguay, Uruguay and Argentina, with a thickness of 50–600 m (averaging about 250 m). It is one of the world's largest sandstone aquifers. The mainly weakly-cemented sandstones were formed by aeolian, fluvial and lacustrine continental deposition during the Triassic–Jurassic period and are overlain by extensive Cretaceous basalt lava-flows. The system is totally storage-dominated, with recharge amounting to only about 0.2% of the estimated 30 000 km3 of water stored. Using 14C and81Kr techniques, it was possible to confirm extremely slow flow rates, with groundwater older than 730 000 years BP in some parts of São Paulo State, Brazil. The vast regional freshwater storage contrasts sharply with localized active flow systems of recharge areas, which are strongly impacted by land-use change. The aquifer is the best known and most exploited in São Paulo State (80% of total extraction) and the experience of groundwater use for the supply of Ribeirão Preto and São José do Rio Preto (both with populations of over 0.5 million), together with one transboundary urban area, will be summarized.

Thematic collection: This article is part of the Hydrogeology of Sandstone collection available at: https://www.lyellcollection.org/cc/hydrogeology-of-sandstone

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