The Guaraní Aquifer System

Background

In the late 1980s, when I was working as Program Officer at the International Development Research Center, I had the opportunity to develop a research project on an important aquifer in Latin America. We already knew of the existence of several aquifers connected 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 be integrated into a single aquifer system for which we suggest the name "Guaraní Aquifer System" in homage to the Guaraní nation whose Ancestral territory coincided with the great aquifer. The name was finally accepted internationally.

Shallow description of the aquifer.

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

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

About 120 million years ago, many volcanoes erupted along large cracks in the crust that gradually covered the sands.

After 20 M. years the sands were covered by basaltic lava

In that period there was the separation of South America from Africa and the formation of the Atlantic Ocean.

The vegetation grew on the lavas, soil was formed, rivers flowed, more sand was deposited in the channels, and gradually the current relief that we all know was gradually formed.

The ancient sands of the desert have been consolidated in sandstones and are generally covered by basalts but in some places they emerge.

This occurs for example in the southern section of the Aquifer, near Tacuarembó, Uruguay, in the northern section, in Riberao Preto in the state of São Paulo in Brazil or in Itapúa in Paraguay.

So when it rains in Tacuarembó, in Riberao Preto or in Itapúa, part of the rain infiltrates the sands.

The infiltrated water flows to the west and south below the basalt until it is discharged near the Paraná river valley, probably in the swampy depressions near the great rivers (for example, the estuaries of Iberá).

The Guaraní Aquifer System is composed of several differentiated domains

1) A complex domain of great extension in the north (Brazil and Paraguay) that has the shape of a basin with its edges in the mountains and hills of Paraguay and the Serra do Mar of Brazil.

2) A smaller domain to the south that descends as a stairway or inclined plane from Uruguay to the west,

3) A third domain is in the transition zone 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 north and west edges.

As an example it is observed that if a well is drilled in Guaviyú (Uruguay) or Concordia (Argentina) the water rises through the well spilling on the surface with high temperatures (45-47 Cº). This is called thermal arts.

The southern section of the Guaraní Aquifer occupies some 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 in eastern São Paulo and in Paraguay in the west seep through the sandstones and flow towards the center of the basin below the upper Paraná course.

There the basin acquires its maximum depth (several thousand meters), the sandstone its greater thickness, the upwelling more power and the temperatures are higher (up to 60º).

The southern section occupies 10% of the total of the basin

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

The southern section recharges its aquifer mainly by the direct infiltration of rainwater and runoff in 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 the 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 at 75,000 km2.

With an average annual rainfall of 1,500 mm and an infiltration rate of 10%, the total annual recharge of the entire S.A.G. It would be about 11,000 million cubic meters. This corresponds to an approximate recharge of 350 m3 per second. With 20% infiltration rate plus indirect recharge, it would be 700 m3 / s

 This flow is similar or slightly higher than that of the Negro River of Uruguay or the Ibicuy River in Rio Grande do Sul and 5-10 times lower than the Uruguay River.

If you compare it with the largest water courses in the region

The water availability of the Guaraní Aquifer System would be relatively limited.

However, it has several advantages:

a) It is distributed in large geographical areas (no driving required)

b) In the central region presents upstream conditions (does not require pumping)

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

d) Normally it has drinking 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 burdensome

b) Defects in the construction of the wells can cause loss of upwelling or hydrotherapy

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

d) The recharge zones are vulnerable to land use change (eg afforestation can decrease infiltration rates)

Another disadvantage is its international character

That makes it more difficult to reach an agreement to make decisions about the management of the aquifer, to protect the recharge zones, to authorize or not the drilling of new wells, to control the flows and the quality.

Appropriate management would allow sustainable exploitation

This requires:

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

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

Strategies for sustainable exploitation (cont.)

c) Determination of optimum flow rates in wells and control thereof

d) Use of thermal water mainly for thermal purposes.

Strategies for sustainable exploitation (cont.)

f) Protection of recharge zones to ensure optimal 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

Recently, a coordination mechanism has been created to carry out the necessary studies and define strategies for the future.

In a possibility to move towards a participative management of the resource that should allow its valorization and sustainable use.

The pilot areas studied in detail are 4:

1) Ribeirão Preto Pilot Area

2) Itapuá Pilot Area

3) Salto-Concordia Pilot Area

4) Rivera-Livramento Pilot Area