The Guaraní Aquifer System

Background

In the late 1980's when we were working as Program Officials at the International Development Research Center of Canada we had the opportunity to develop a research project on one important aquifer in Latin America. 

We already knew the existence of several connected aquifers in the Southern Cone of South America (Botucatú in Brazil, Tacuarembó in Uruguay and Argentina, Misiones in Paraguay). 

We proposed and organized meetings to coordinate the studies and activities of these aquifers in each of the countries and, finally, we proposed that all of them could be integrated into a single aquifer system suggesting the name "Guarani Aquifer System" in homage to the Guaraní nation whose ancestral territory coincided with the great aquifer. The name was finally internationally accepted.

Description of the aquifer.

The Guaraní Aquifer System is contained in Jurassic-Cretaceous sandstones (180-120 M. years).

These sands were deposited in a extensive and prolonged arid environment that gave rise to the formation of numerous dunes (desert of Botucatú).

About 120 million years ago many volcanoes along large cracks in the crust erupted and the lavas gradually covered most of the sandy desert.

In that period the separation of South America from Africa and the formation of the Atlantic Ocean took place.

With time vegetation grew on the lavas, soil formed, rivers flowed, sand was deposited in the riverbeds, and gradually the current relief was formed.

The ancient sands of the desert have consolidated into sandstones and presently they are generally covered by basaltic lavas. However, in some areas, mainly where the lavas did not reach, the sandstones outcrop forming sandy soils and undulating reliefs...

 This occurs in the southern section of the aquifer, near Tacuarembó, Uruguay, in the northern section, near Riberao Preto in the state of São Paulo in Brazil and in Itapúa in Paraguay.

That is why when it rains in Tacuarembó, in Riberao Preto or in Itapúa, some of the rain infiltrates the sands into the underlying aquifer..

The infiltrated water flows to the lower parts of the basin below the basalt until it is discharged near the Paraná river valley, probably in the muddy depressions near the great rivers (for example, the Iberá estuaries).

The Guaraní Aquifer System is composed of several distinct domains

1) A complex domain of great extension in the north (Brazil and Paraguay) which appear as an independent basin with its edges in the sierras and hills of Paraguay and Serra do Mar of Brazil.

2) A smaller domain to the south that descends in the form of a ladder or inclined plane from Uruguay to the west,

3) A third domain is in the zone of transition that extends from Artigas in Uruguay to Uruguayana in Brazil and Posadas in Argentina.

 4) A fourth domain is observed in the marginal zones of its northern and western borders.

As an example, if a well is drilled in Guaviyú (Uruguay) or Concordia (Argentina), the water rises in the well, spilling on the surface with high temperatures (45-47 ° C). This is called thermal artesianism.

The southern section of the Guaraní Aquifer occupies about 100,000 km2, of which half are in Uruguay and the other half in Argentina.

The northern section of the aquifer occupies 80% of the basin (1 million km2) and extends from Misiones in Argentina to Mato Grosso and São Paulo.

The rains that fall in the east of São Paulo and in Paraguay to the west, filter through the sandstones and flow to the center of the basin below the upper Paraná.

There the basin acquires its maximum depth (several thousands of meters), the sandstones its greater thickness, the upwelling more power and the temperatures are higher (until 60º).

The southern section occupies 10% of the total basin

Another 10% is occupied by the threshold separating the two sections and other marginal areas.

The southern section recharges its aquifer mainly by the direct infiltration of rainwater and runoff over a total area of ​​about 4,500 km2.

If an infiltration rate of 10% is estimated, the approximate annual recharge would be 600 million m3 per year or 20 m3 per second. With an infiltration rate of 20% and adding an indirect recharge inferred through basalt and other formations the recharge per second would be about 80 m3.

These flows are similar to the average flow of the Santa Lucia river of Uruguay

For the entire basin there is direct recharge in 75,000 km2.

With an average annual precipitation of 1,500 mm and an infiltration rate of 10%, the total annual recharge of the entire S.A.G. Would be about 11 billion m3. This is equivalent to an approximate recharge of 350 m3 per second. With 20% infiltration rate plus indirect recharge, it would be 700 m3 /This flow is similar to or slightly higher than that of the Negro River of Uruguay or the Ibicuy River in Rio Grande do Sul and 5-10 times below the Uruguay River.

Compared with the larger watercourses of the region the availability of water from the Guaraní Aquifer System would be relatively limited.

However it has several advantages:

A) It is distributed in broad geographical areas (no driving)

B) In the central region it presents conditions that arise (does not require pumping)

C) It has high temperatures (35 to 60 Cº)

D) Normally it has potable quality and is very little vulnerable to contamination

On the other hand it has some disadvantages:

A) The perforations of the artesian-thermal sector are deep and therefore onerous

B) Defects of construction of the wells can cause upheaval loss or thermalism

C) Excessive exploitation of nearby wells may result in loss of pressure and disappearance of upwelling condition.

D) Recharge zones are vulnerable to change in land use (eg afforestation can reduce infiltration rates)

Another disadvantage is its international character

That makes it more difficult to agree to make aquifer management decisions, to protect recharge areas, to authorize or not to drill new wells, to control flows and quality.

Appropriate management would enable sustainable exploitation

This requires:

A) Geological and hydraulic characterization of the aquifer, including accurate determination of recharge, transit and discharge zones.

B) Limitation of the number of wells according to said characterization to maintain pressure, upwelling, thermalism and quality.

Strategies for sustainable exploitation (cont.)

C) Determination of optimal flows in wells and their control

D) Use of thermal water mainly for thermal purposes.

Strategies for sustainable exploitation (cont.)

F) Protection of recharge zones to ensure optimum infiltration rates.

G) Develop a coordinated regulatory framework in all countries where the aquifer is located.

H) Coordination of actions and measures to achieve a harmonious and sustainable management of the Aquifer System.

The importance of an international management project

It has recently been possible to create a coordination mechanism to carry out the necessary studies and to define strategies for the future.

In that sense, the Guarani Aquifer System Project opened a possibility to move towards a participatory management of the resource that should allow its valorization and sustainable use.

The pilot areas studied in detail are 4:

1) Pilot Area Ribeirão Preto

2) Itapuá Pilot Area

3) Salto-Concordia Pilot Area

4) Rivera-Livramento Pilot Area