The Guarani Aquifer System (GAS) is one of the largest subterranean freshwater reservoirs on the planet, hidden beneath the surface of South America. This immense hydrological feature serves as a vast natural water bank for millions of people. Its size and capacity underscore its importance in the context of growing global water scarcity. The system is a complex geological formation whose preservation requires international cooperation.
Defining the Guarani Aquifer System
The Guarani Aquifer System is a massive transboundary groundwater reserve spanning approximately 1.2 million square kilometers across southeastern South America. This underground body of water is shared by four nations: Argentina, Brazil, Paraguay, and Uruguay. Brazil holds the largest share, covering about two-thirds of the total area, underlying parts of eight Brazilian states.
The aquifer is named in honor of the Guarani indigenous people who historically inhabited the overlying region. Before it was unified under the Guarani name, different sections were locally known by various names, such as the Botucatu Aquifer in Brazil and the Tacuarembó Aquifer in Uruguay and Argentina. Its recognition as a single, interconnected system was a crucial step in promoting international cooperation for its management. The estimated total volume of water stored within the system ranges from 30,000 to 40,000 cubic kilometers.
Geological Composition and Hydrology
The physical structure of the Guarani Aquifer is composed primarily of porous sandstones deposited during the Triassic and Jurassic periods, roughly 200 million years ago. The main water-bearing layer is the Botucatu Formation, which consists of highly permeable, well-sorted aeolian sandstones that originated in an ancient desert environment. The high porosity of these sands allows them to store vast quantities of water like a sponge.
The aquifer system’s depth below the surface varies significantly, from zero in outcrop areas where it meets the surface to more than 1,800 meters in its deepest, most confined sections. Over most of its extent, the sandstone is sealed by a thick cap of low-permeability basaltic rocks from the Cretaceous Serra Geral Formation. This basalt layer acts as an aquitard, confining the water under pressure and providing a high degree of protection from surface contamination.
Water enters the aquifer through a natural process called recharge, which is extremely slow. Recharge occurs mainly through direct infiltration of rainwater in the unconfined outcrop zones, which are primarily located in Brazil, eastern Paraguay, and northern Uruguay. Additionally, in some areas, water can leak indirectly through fractures and faults in the overlying basalt layers. Hydrologists estimate the annual recharge rate to be in the range of 166 to 250 cubic kilometers, meaning most of the water is considered fossil water that renews over millennia.
Primary Uses and Regional Importance
The Guarani Aquifer System is a fundamental source of freshwater, supporting major population centers and significant economic activity across the four sharing countries. Millions of people, concentrated in more than 100 cities across the region, rely on the aquifer for their municipal drinking water supply. The water is often of high quality and can be accessed relatively easily, especially in the shallower, unconfined zones.
Beyond human consumption, the aquifer’s waters are utilized for extensive agricultural irrigation, particularly in the fertile regions of Brazil and Argentina. This water sustains crops and livestock, directly supporting the regional agricultural economy and food security. Industrial applications, including manufacturing and processing plants, also draw upon the reliable supply provided by the GAS.
A unique use of the aquifer is for thermal tourism, especially in parts of Uruguay and Argentina, where the water is naturally heated due to the Earth’s geothermal gradient at greater depths. In these areas, the water temperature can range from 33°C to over 65°C, making it suitable for hot springs and spas. The consistent availability of this water underpins the economic stability and continued population growth of the entire region.
Transboundary Governance and Protection Efforts
Managing a resource of this magnitude that crosses four sovereign national borders presents unique political and legal challenges. Each country maintains national sovereignty over the portion of the aquifer within its territory, which necessitates a coordinated framework for its shared use and protection. The uneven distribution of the aquifer and differing national needs for water further complicate the management landscape.
A major step toward unified management was the signing of the Guarani Aquifer Agreement (GAA) by the four nations in 2010, which officially entered into force in 2020. This agreement established a framework for cooperation on the sustainable use and conservation of the shared resource. The GAA aims to coordinate technical and scientific efforts and ensure that the utilization by one country does not cause significant harm to the others.
The aquifer faces two primary threats: contamination and over-exploitation. Contamination is a concern in the unconfined recharge zones, where agricultural runoff carrying pesticides and fertilizers, or industrial waste, can infiltrate the water supply. Localized over-exploitation is a problem in areas with high population density, such as parts of São Paulo state in Brazil, where intensive drilling can cause water levels to decline. The Strategic Action Program (SAP) provides a long-term plan to monitor, regulate, and protect this transboundary resource.