Guarani Aquifer

Located beneath the parts of Argentina, Brazil, Paraguay, and Uruguay, it covers 1,200,000 km² (460,000 sq mi), with a volume of about 40,000 km³ (9,600 cu mi), a thickness of between 50 m (160 ft) and 800 m (2,600 ft) and a maximum depth of about 1,800 m (5,900 ft). It is estimated to contain about 37,000 km³ (8,900 cu mi) of water, with a total recharge rate of about 166 km³/year from precipitation.

It was named by uruguayan geologist Danilo Antón after the Guarani people, an indigenous group that inhabited roughly the same area.

Geology of the aquifer

The Guarani Aquifer consists primarily of fluvial sandstones of the Piramboia Formation and aeolian sandstones of the Botucatu Formation, which were deposited during the Triassic and Jurassic periods. Over 90% of the total area is overlain by early Cretaceous basalts of the Serra Geral Formation. The basalt's low permeability allows it to act as an aquitard, providing a high degree of containment. This greatly reduces the rate of infiltration and subsequent recharge, but also isolates the aquifer from the vadose zone, subsequent surface-associated losses due to evaporation and evapotranspiration, and potential contamination.

The erosion exposes pieces of sandstone on aquifer's banks, which are called outcrops. This is where the rain comes in and also where the contamination might happen. In general, the aquifer comprises a variety of quartz sand, well-rounded format, and has low clay content. Granulometric data indicate that over 50% of the grains have a diameter between 0.125 and 0.250 millimetres (125 and 250 μm; 4.9 and 9.8 thou).