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

Mexico: not enough water in a huge metropolis

Mexico: a thirsty city

 Danilo Antón

There are few cases in the world in which the physical environment has been so completely transfigured by urban development as it was in Mexico City. The valley of Mexico is a 9 600 square kilometre closed basin that is more than 2 200 metres above sea level, in the heart of the Mexican neovolcanic belt. Before the arrival of Europeans in 1521, the valley was a depression in whose bottom several large lakes had developed because of volcanic obstruction of their outlets about 700 thousand years ago. The lakes covered a total area of about 2 thousand square kilometres and were partly connected, especially during periods of high water. Three of the lakes contained fresh water — Chalco, Mexico, and Xochimilco — and the other three, brackish water — Ecatepec, Texcoco, the largest at 800 square kilometres, and Zumpango.

The area was, and to a certain extent still is, subhumid. Rainfall was probably slightly more than the current amount, which ranges from 600 millimetres per year at the bottom of the valley to 1 200 millimetres per year in the nearby mountains. The average temperature was relatively cool for the subtropic latitude at which the city is located, ranging from 8° to 15°C depending on the altitude. Soils were deep, highly fertile, and easy to work.

The land was completely covered by thick forests, particularly on the slopes of the mountains and highland areas. The plains in the valley, which were originally also covered by forests, were soon allocated for agriculture, and parts of the forest were cleared to make way for farms. In addition to the freshwater lakes, a large number of springs around the lakes and in the foothills of the nearby mountains provided considerable volumes of good-quality water.

Because of its abundant resources, the valley was occupied early by a number of indigenous peoples, who based their economy on locally domesticated crops and farm animals: corn, tomatoes, chili peppers, cacao, turkeys, dogs, honey bees, and fish. Because these people did not have draft animals or use the wheel, most trade was carried out by boat (or walking).

Several peoples successively inhabited and established political control over the lacustrine area during the few centuries before the arrival of the Europeans. The last group was the Aztecs, who arrived from the legendary land of Aztlán (probably in the northern arid territories) during the 14th century.

The Aztecs probably maintained a livelihood by fishing and trading with neighbouring groups. Gradually, they managed to build an island

in the centre of the Lake of Mexico on which a town developed: Tenochtitlán. Through alliances and wars, the Aztecs built an empire, and Tenochtitlán became a thriving city of several hundred thousand people. A bridge was built to connect the island with the mainland, and large boats transported people and merchandise. The Aztecs also built earth dikes to control flooding and to separate the brackish lakes from the fresh water. Aqueducts carried fresh water from springs to the city through the lake and along the dikes.

It is difficult to comprehend the extent of the changes that took place in the few centuries after the Spanish conquest. Today, the proud Tenochtitlán has disappeared, and only scattered archaeological remnants can be found. In its place stands the highly urbanized downtown area of Mexico City.

The Lake of Mexico is gone. In its place are several hundred square kilometres of urban neighbourhoods built on what used to be the lake bottom. A few canals and small lakes are the only remnants of Chalco and Xochimilco lakes. Like the southern lakes, the three northern lakes were gradually drained (beginning in 1786), and the former Texcoco Lake has become a vast flat plain on which little vegetation grows because of the highly alkaline soil (pH is over 10). An intricate maze of wells and pipes pump brine from the lacustrine sediments for sodium carbonate and sodium chloride extraction.

The old springs that provided water to the riverine populations are also gone. Now over 5 thousand wells draw more than 50 cubic metres of water per second from an average depth of 100 metres, causing the level of water in the aquifers to subside by as much as 1 metre per year. As a result of this overpumping and the compaction of the upper layers of sediments, widespread subsidence is occurring. The surface has dropped 6 metres in several places and, because of differential rates of subsidence, many structures have been weakened. This phenomenon has been exacerbated by frequent seismic activity, of which the most recent destructive example was the earthquake of September 1985.

The forests that used to cover the adjacent hills have practically disappeared, and widespread soil erosion occurs. Most former agricultural land has been covered by pavement, houses, and other urban

constructions. Quarries, which supplied construction materials, can be found throughout the region. Some have become garbage dumps, into which some of the annual 10 million tonnes of garbage is thrown. A significant portion of the garbage is dumped on the “shores” of the former Texcoco Lake, particularly in the south. Ciudad Netzahuatcoyotl, in that area, is a neighbourhood of 3 million people. Although recently established, this urban area is extremely degraded; developed areas alternate with garbage dumps and slums.

Water, which used to flow into the lakes, is channeled out of the basin, together with urban wastewater, through a system of canals and tunnels into the Gulf of Mexico hydrographic system. A number of pumping wells used to supply the city are located next to the canal (the Chalco Canal). Risks of contamination are obvious and, in fact, some wells had to be closed because of the presence of nitrates in the water.

The atmosphere of the valley has also changed. Emissions from 4 million vehicles and 25 thousand industrial establishments in a poorly oxygenated environment (because of the altitude) have transformed the air in Mexico City into one of the most unhealthy urban environments for human life, particularly near the downtown core.

Mexico City contains 21 million people, making it the largest urban centre in the world. Every year, its population increases by 750 thousand people, including both births and migration from the rest of the country. By the year 2000, the city will hold 29 million people (surpassing the population of Canada) and, by 2010, 38 million. If corrective measures are not taken, the city’s problems will continue to grow, and the ancient paradise may become one of the worst environmental nightmares of the 21st century.

The aquifer underlying the valley of Mexico is one of the key natural elements in Mexico’s environment. It provides the bulk of the water that makes the existence of the city possible. Although some water is brought in from the Lerma–Cutzamala basin, the volume is less than one-fifth of total requirements.

Any other option for bringing water from outside the valley is becoming impractical or too expensive. The Lerma–Cutzamala resources are almost exhausted, but using other basins (such as the

Balsas basin or the Amacuzac subbasin) may mean pumping water 1 200 to 1 500 metres upward and constructing long pipelines, storage reservoirs, and other expensive engineering works. Bringing this water into Mexico City will also deprive a number of communities that now depend on it for irrigation and other uses.

Mexico’s aquifer is contained in a number of Tertiary and Quaternary units with a thickness ranging from a few hundred metres to nearly 2 thousand metres. These units comprise a wide range of sedimentary materials. Continued volcanic activity produced huge volumes of pyroclastic material, which has been more or less reworked by fluvial action, and intercalated lava flows. During periods of volcanic activity, tuffs, breccias, ashes, and lava formed; at other times, alluvial and lacustrine action was more important. The main water-bearing layers are the Tarango formation and associated alluvia and the Cenozoic sequence of fractured pyroclastic and lava flows. These are covered by younger lacustrine sediments, confining the main aquifer.

The whole sequence can be up to 2 000 metres thick, but the lower 1 500 metres are more consolidated and less porous. The upper few dozen metres of the aquifer are too close to the upper lacustrine clays and continued pumping might produce dewatering and consolidation of these clays, causing subsidence. Therefore, the usable portion of the aquifer is generally between 100 and 500 metres underground.

The aquifer is recharged mainly in the mountain region (Sierra Chichinautzin in the south, Sierra Las Cruces in the west, and Sierra Nevada to the east). The total available recharge volume has been estimated to be 25 to 50% of precipitation: 25% in Sierra Las Cruces, 35% in Sierra Nevada, and 50% in Sierra Chichinautzin. Of these volumes, about half flows toward the valley of Mexico and the rest outward to other basins. An accurate figure for inflow to the aquifer itself is difficult to estimate (probably 30 to 40 cubic metres per second). However, it is certainly below 50 cubic metres per second — the amount being pumped out — because the water level is sinking.

Additional lowering of water levels will increase inflow from the Sierras because of an increase in gradient. This will not compensate for the deficit, however, particularly if pumping is increased. Precise forecasting of the aquifer’s reaction to prolonged extraction requires accurate modeling. Only recently has adequate information on the geometry and hydraulic properties of the reservoir been available. Modeling of the aquifer has been carried out at the Instituto de Geofísica, and it is expected to allow prediction of the actual potential of the groundwater resources of the valley.

It has recently become clear that the groundwater resources of the valley of Mexico are limited and that additional water will have to come from external sources. Such external sources are all found at elevations lower that that of the city. Therefore, tapping this water will not only require enormous energy consumption but will also deprive downstream communities of this vital resource. The bottom line is that the urban model of Mexico City is unsustainable. It has become too large for its territorial base. The city has not only run out of water, but also its air is heavily polluted, the local ecosystems have been destroyed or critically damaged, and the surrounding soils are under severe strain as a result of heavy urbanization. To check this continuous destruction of resources, radical policy shifts are essential. The window of opportunity to save Mexico City is rapidly closing.

From "Diversity, Globalization and the Ways of Nature", Danilo Anton, published by IDRC Books, Ottawa.

Geography
Aridity and poverty in the Brazilian Northeast

Danilo Antón

The Brazilian northeast is the poorest region in the Brazilian subcontinent. Insufficient rains and a rough  has produced a tough human environment. Poverty and emigration are common. From Piauí to the hinterland of the Bahia state, the Northeast has a shared history of exploitation, poor practices of farming, slavery and frustrated rebellions. However, the main limitations of the area relate to the climatic and ecological environment.

I Climateⁱ

The Northeastern region of Brazil, possesses a hot and humid climate with a short dry season along the coast, and a hot and semiarid climate in the interior. Much of the region is a transition from the dry hinterland to the more humid coastal plains.

In the proximity of the ocean (up to about 50 to 100 km from the shoreline) annual precipitations exceed 1000 mm (i.e. Alagoas, Pernambuco, Paraiba, Rio Grande do Norte) reaching more than 2000 mm in some ocean-facing slopes.

As described earlier, rainfall in semi-arid areas varies from 800 mm per year to slightly below 400 mm in the driest zones of the sertão, The dry season occurs from July through September. Rainfall takes place in the Southern Summer (January through March). However, actual precipitations during the rainy season may be very irregular.

Mean monthly temperatures have small variations throughout the region, and they are more influenced by altitude than sun exposure. Annual medium temperature in the plains varies between 24 C^o (coast) and 26 C^o (interior).

Clouds and fogs can form in the highest areas even during the dry season, reducing sun exposure to less than 5 hours per day in the planalto (i.e. plateau), while the plain areas are subject to higher sun exposure, up to seven hours daily or more.

II) Geology and geomorphology

Northeastern Brazil geomorphology has developed on a very ancient shield bedrock, with cratonic (stable) geological behavior. Sedimentary basins formed along the eastern and northeastern edges of the region (Sergipe- Alagoas and Potiguar-Recife sedimentary basins), and in the Northwest and West, the large Parnaiba- Maranhão sedimentary basin and the nearby São Luis and Barreirinhas sedimentary basins, and the São Francisco sedimentary basin in the Southwest.

There are also several smaller sedimentary basins inland, within the shield region, of which the largest is the Araripe basin (approximately 15,000 km², 200 km long and about 100 km wide) which is located in the boundary of the states of Pernambuco, Piauí and Ceará..

Old erosion surfaces are found next to the water divides of the main hydrographic basins (i.e. between the Tocantins and the São Francisco, between the São Francisco and the northeastern basins) forming “chapadas” (plateaus)ⁱⁱ. Chapada soils are normally shallow and, because they have suffered long geological lixiviation, their fertility levels are low. Vegetation in drier chapadas is caatinga (see section 3), while more humid chapadas next to the coastal plain may possess other types of ecosystems (agreste).

Old chapadas are separated by erosion river valleys, often filled with alluvial deposits containing local aquifers that are normally used for water supply and irrigation.

Downstream, alluvial plains become wider until they merge with the coastal Tertiary and Quaternary formations (dunes, beach, lower valley alluvial and estuarine deposits)

III) Soils

In the interior of the Northeast region soils are shallow, particularly in the erosion surfaces and slopes. In the upper chapada flat areas fertility is low due to lengthy lixiviation. In the lower areas (alluvial valleys) soils are deeper and more fertile (more productive), but they may have salinity problems (often induced by inadequate irrigation management). However, due to their medium to high fertility and the persistent warm temperatures, if water resources (i.e. irrigation) are available, farming productivity can be very important.

Brazilian Northest; Biological environment

I) Caatinga

Caatinga (“white bush” in tupi language) occupies about 730,000 km² (7% do Brazil) and can be found in all northeastern states: Maranhão, Piauí, Ceará, Rio Grande do Norte, Paraíba, Pernambuco, Sergipe, Alagoas and Bahia (and North of Minas Gerais)

Caatinga is formed by cactus, bushes and small trees adapted to dry and hot conditions. The main adaptations of caatinga species are thorns, fleshy and/or small leaves, which also tend to fall during the dry season, and well developed root systems.

There are two types of caatinga the “Caatinga Hipoxerófila” typical of less extreme climate, as for instance the coastal “baixada” (slopes), and the hyper-xerophile caatinga, found in the driest regions, with thorny bushes and trees, and cactuses.

II) Agreste

Agreste is a transition zone between “caatinga” and tropical forest, including more and larger trees than in caatinga, but less than in the tropical forest. It is composed of trees, bushes and an undergrowth of herbaceous plants.

III) Cerrado

Cerrados (from the Spanish word “closed”) are a typical ecosystem of the Brazilian interior. In the Northeast they are found in the Western plateaus of the “sertão baiano”. They are composed of low canopy trees with a bushy and grassy undergrowth, producing a dense configuration difficult to penetrate.

IV) Tropical forest (mata umida)

The mata úmida or tropical forest, composed of trees with tall canopies, is found in coastal areas and in mountain valleys and lower slopes in the proximity of the sea.

In a large measure, these forests have been eliminated (logged, burned), often to make place for plantation activities. The present extension of “mata úmida” is today restricted to steep slopes and protected areas (which are not very extensive in Northeastern Brazil).
(to be continued)

i Source:Guia Internet Brasil, 1999

ii As defined by Aziz Nacif Absaber; Summit surfaces in Brazil, Revista Brasileira de Geociencias, 30 (3) 515-516; Sept. 2000.