A hidden reservoir of 40,000 cubic kilometers of fresh water

Countries & Cities · Published 30 September 2026 Swipe more like this →

In short: Deep beneath the soil of Argentina, Brazil, Paraguay, and Uruguay lies one of the world's largest aquifer systems. Protected by a heavy layer of volcanic rock, this massive reservoir remains largely isolated from the surface, creating a vital but vulnerable source of fresh water for millions.

The Guarani Aquifer is a massive geological formation spanning 1,200,000 square kilometers. Its scale is immense: it contains an estimated 37,000 cubic kilometers of water, with a thickness reaching up to 800 meters. The system is primarily composed of fluvial and aeolian sandstones from the Triassic and Jurassic periods. What makes it unique is its protective 'cap': over 90% of the area is covered by early Cretaceous basalts from the Serra Geral Formation.

This basalt layer acts as an aquitard. Because it has low permeability, it serves as a natural seal, isolating the aquifer from the vadose zone. This physical barrier is crucial because it protects the water from common surface threats like evaporation and direct contamination. However, the system is not entirely sealed; water enters through 'outcrops' where erosion has exposed the underlying sandstone. These specific points are where the aquifer's recharge occurs and where the risk of chemical infiltration is highest.

Managing such a vast, cross-border resource requires intense international cooperation. Since the aquifer spans four different nations, a unified management strategy is essential. Recent initiatives, such as the Strategic Action Plan (SAP 2025), involve over 2,000 participants and focus on 25 strategic actions to address climate impact and ecosystem services. This collaborative framework aims to ensure that the water is managed sustainably and equitably across all borders, balancing industrial use with environmental protection.

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