The Guarani Aquifer, located beneath the surface of Argentina, Brazil, Paraguay, and Uruguay, is the second largest known aquifer system in the world and is an important source of fresh water. Named after the Guarani people, it covers , with a volume of about , a thickness of between and and a maximum depth of about . It is estimated to contain about of water, with a total recharge rate of about 166 km³/year from precipitation. It is said that this vast underground reservoir could supply fresh drinking water to the world for 200 years. However, at closer inspection, if the world population were to stay at an equilibrium of about 6.96 billion, not even taking into account that babies need less water than grown adults, this figure reaches 1600 years, allowing about 9 liters per day per person. Due to an expected shortage of fresh water on a global scale, which environmentalists suggest will become critical in under 20 years, this important natural resource is rapidly becoming politicised, and its control becomes ever more controversial. 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 . Research and monitoring of the aquifer in order to better manage it as a resource is considered important, as the population growth rate within its area is relatively high — resulting in higher consumption and pollution risks. The countries over the aquifer are also the original four Mercosur countries. Recharge of the aquifer The recharge distribution is related to precipitation rate. Its complexity is due to the indirect relation between