5) Brazilian Northeast; aquifer systemsi
- Crystalline aquifers,
Fracture Systems
Crystalline aquifers in semi-arid regions are normally fracture aquifer systems. In the Northeast, fracture aquifers of this type are prevalent. They represent more than half of the total area of the region. In these aquifers, good well locations are difficult to find, productivity is normally low to very low (i.e. less than 10,000 liters per hour) and groundwater storage small.
Crystalline aquifers in semi-arid regions are normally fracture aquifer systems. In the Northeast, fracture aquifers of this type are prevalent. They represent more than half of the total area of the region. In these aquifers, good well locations are difficult to find, productivity is normally low to very low (i.e. less than 10,000 liters per hour) and groundwater storage small.
Although
the stored volumes are limited and discontinuous, they can be found
in large areas with scarce rainfall, and therefore they may
represent the best (sometimes the only one) option for water supply
in many rural communities and small towns. The maitimize its exploitation or even increase their productivity (i.e. through construction of groundwn problem is how to
find suitable sites for productive water wells. Appropriate criteria
and tools need to be utilized (or eventually developed) in order to
opater damsii).
Weathered
mantle systems
In
some areas, crystalline bedrock is covered with a weathered mantle
several meters thick (5 to 30 meters). In these areas well locations
are easier to define and yields are higher (i.e. up to 20-30,000
liters per hour). These systems are found mainly in the more humid
peripheral zones of the Northeast (i.e. coastal region, Maranhão,
Southern Bahia, etc).
- Sedimentary aquifers
Sedimentary
aquifers are by far the most productive groundwater resources of the
“Nordeste”. They are found in some larger sedimentary basins
(Parnaiba-Maranhão;
Jatoba-Tucano-Reconcavo, São
Francisco), along the coastal margin (Potiguar-Paraiba-Recife;
Sergipe-Alagoas) and in some interior basins (i.e. Araripe and other
smaller basins). There are also important aquifers in modern
alluvial deposits (fluvial accumulations along present river
valleys). The main sedimentary aquifers are described in the
following pages.
The
Parnaiba-Maranhão basin is a huge tectonic feature with an area of
nearly 0.5 million km2.
It comprises two large hydrographic basins drained by the Itapicurú
and Parnaiba rivers, plus some peripheral areas draining to other
rivers (i.e. the Jaguaribe hydrographic basin in the East). The main
aquifers are the Serra Grande, the Cabeças, the Poti-Piauí, the
Corda-Grajaú and the Itapecurú aquifer systems.
- Serra Grande aquifer system
The
Serra Grande formation is located in the base of the
Parnaiba-Maranhão sedimentary basin outcropping in the Eastern part
of this region. Its thickness is about 500 meters and the recharge
area is estimated at 30,000 km2.
The productive member of the system is composed of coarse to medium
grain-size sandstone sediments, with conglomerate levels. It may be
confined or unconfined. Well yields are about 6,000 to 13,000 liters
per hour depending of confining conditions. It possesses
calcium-sulfate water with satisfactory drinking quality. It is
appropriate for irrigation in low lixiviation soils.
i
Sources:
- J.L. Gomew Zoby, Fernando de
Oliveira, Brasilia, D.F., may 2005; Agencia Nacional de Aguas,
Cadernos de - Recursos Hídricos “Panorama da Qualidade das Aguas
Subterráneas no Brasil”;
- Zoneamento geoambiental do
Estado do Maranhão, Ministerio dde Planejamento, Orçamento e
Coordenação, IBGC, Diretoria de Geociéncias, M.Lamartin Montes,
Salvador, 1997;
- Website do Goberno de Piaui,
Bacia do rio Longa;
- João Suassuna;
Potencialidades Hídricas do Nordeste Brasileiro, Seminario de
Tropicologia, 2003, Recife (Anais).
ii
Dams that are build as “walls” obstructing groundwater flow,
producing a rise in groundwater levels. This added water may feed
the crops roots or generate springs in selected areas.
iii
Source: Panorama da Qualidade das Aguas Subterraneas no Brasil,
Agencia Nacional De Aguas, Cadernos de Recursos Hídricos, José
Luiz Gomes Zoby, Fernando Roberto de Oliveira, Brasillia, DF, Maio
2005.
- Cabeças aquifer system
This
system is also located in the Parnaiba-Maranhão sedimentary basin.
It outcrops in the East of the basin, west of the Serra Grande
Formation. The Cabeças formation, with a thickness of about 300 m.,
is composed of fine and coarse sandstones, siltstones and shale. It
may be confined or unconfined. Well yields are 12,000 to 50,000
liters per hour depending on confining conditions. Normally
groundwater in the Cabeças system is weakly mineralized (300
mg/liter), with Ca-Mg-chloride composition. In depth, water can be
more saline, up to 2,600 mg/liter (490 to 707 meters). Locally, Serra
Grande and Cabeças groundwater can be mixed.
- Poti-Piaui aquifer system
This
extensive system, which also belongs to the Parnaiba-Maranhão
sedimentary basin, possesses a thickness of up to 230 meters and is
composed of fine and medium sandstones with lenses of shale and
siltstones. It outcrops in Piaui, Para and Tocantins with a recharge
area of 117,000 km2.
It is confined or unconfined. Well yields vary from 18,000 to
100,000 liters per hour, depending on confining conditions. Water
quality is good, with low mineral content (average: 200 mg/liter).
- Corda- Grajaú
The
Corda aquifer, which is also part of the Parnaiba-Maranhão
sedimentary basin, is found in Maranhão, Tocantins and Piaui, with a
total area of 35,000 km2.
It is composed of medium sandstones and conglomerates. Thickness is
100 to 200 meters. It is confined or unconfined. Well yields average
15,000 liters per hour. Water quality is good (less than 400 mg/liter
mineral content).
- Itapecurú aquifer system
The
Itapecurú formation is the unconfined upper unit in the
Parnaíba-Maranhão sedimentary basin, located in the West of the
basin. It outcrops in Maranhão and Pará and it is composed of fine
to coarse sandstones, with lenses of shale and clayey siltstones. The
recharge area is very extensive (203,000 km2)
and well productivity about 12,000 liters per hour. Water quality is
good.
IV)
Potiguar-Paraiba-Recife sedimentary basins
The
Potiguar- Paraiba- Recife sedimentary basin is an elongated coastal
structural strip (600 km long, 30 km wide), related to the
continental tectonic margin. It extends from Ceara to Pernambuco and
it is filled with Cretaceous, Tertiary and Quaternary formations,
some of which are good aquifer systems. The main ones are Açu,
Jandaira, Beberibe, Barreiras and other related alluvial aquifers.
- Açu aquifer system
The
Açu formation is the oldest and deepest unit (down to 900 m deep in
Ceará) of the Apodi Group in the Potiguar Basin. It is mainly
composed of sandstones (white or reddish, gravelly or clayey) with
intercalation of shales). Permeability may decrease locally because
of the clayey lenses. Well yields may reach up to 80,000 liters per
hour in Rio Grande do Norte, but production is somewhat less in
Ceara.
- Jandaira aquifer system
The
Jandaira limestone aquifer system is located in the Potiguar
sedimentary basin in the states of Ceará and Rio Grande do Norte
where it is used for irrigation (chapada do Apodi, region of
Barauna). Mineral content is relatively high (1500 to 2400 mg/liter)
due to dissolution of calcite
and
dolomite which are principal minerals in the formation rocks, and its
composition is of sodium chloride type.
- Beberibe aquifer system
The
limestone Beberibe system is found in the states of Pernambuco and
Paraiba, with an area of 318 km2.
Although the aquifer is of drinking quality, processes of saline
intrusion have been observed. This is due to inflow of seawater or
movement of saline water from the Boa Viagem aquifer,
It is
heavily exploited in the Recife Metropolitan Region, although
over-pumping has increased salinity for more than 20% in the Recife
area.
- Barreiras aquifer system
The
Barreiras group is widely extended throughout the Northeast from Para
to Espiritu Santo. It is a Tertiary heterogeneous formation,
including sandstones, conglomerates, silty and clayey layers, etc.
Water in Barreiras aquifer is magnesium-sodium chloride type evolving
to sodium-calcium chloride and sodium and calcium bicarbonate, with
low to very low mineral content (figures of 40 mg/liter are common).
In coastal areas over-pumping has led to increased salinity due to
seawater intrusion.
Barreiras
is used for water supply in Natal metropolitan area (Rio Grande do
Norte), in the North of Recife metropolitan area and in several other
coastal towns.
V)
Alagoas-Sergipe sedimentary basin
Groundwater
can be mainly found in the Tertiary Barreiras aquifer system (see
above) and in coastal and alluvial sediments. Both are used for water
supply and irrigation. Other aquifers contained in older formations
also exist but they are less important.
VI)
Araripe sedimentary basin and other interior smaller basins
The
basins of the interior of the Northeast, of which the Araripe basin
is the largest, are pre-Tertiary sedimentary basins with their own
particular geological historyi.
The
Araripe sedimentary basin, of Mesozoic age, appears as a elongated
plateau (chapada)
160 km long East-West and 50 km wide (North-South direction) in the
boundary between the states of Piauí, Ceará and Pernambuco.
The
lowest sequence is represented by the Cariri formation of Paleozoic
age. On top of this formation, discordantly, there is a Jurassic
sequence (formation Missão Velha) and a Cretaceous group (Exu and
Santana formations). Missão Velha and Exu are the main aquifer
systems of the basin.
- Aquifer Cariri
The
Cariri formation is located in the Cariri depression and Araripe
“chapada” geosystems in the central part of the “nordeste”
region. It is the lowest unit of the Araripe sedimentary group. It is
sthickness is 100 m. Water belongs to the chloride-bicarbonate type,
suitable for human consumption. Well yields are about 100,000 liters
per hour.
- Missão Velha aquifer system
The
system is related to the formation of the same name, which is
believed to be of Jurassic age.
i
Mario Luis Assine; Paleocorrentes e paleogeografia na bacía de
Araripe, nordeste do Brasil; Revista Brasileira de Geociencias, 24
(4) 223-232; dezembro 1994
This formation is
found in two areas of irregular shape to the northeast and east of
the Araripe “chapada”.
It is composed of siltstones and fine sandstones in the lower part,
passing to medium and coarse sandstones in the upper section. The
thickness is about 250 m.
The Missão Velha
water possesses low mineral content averaging 150 mg/liter, although
iron contents can be slightly high. In spite of that limitation, it
can be used without restrictions for human consumption. It is
exploited for water supply in Juazeiro do Norte, Crato and Barbalha
in Ceará, and in a lesser degree in Piauí.
- Exu aquifer system
The Cretaceous Exu
formation is the top of the Araripe sedimentary basin and possesses a
maximum thickness of 300 m. It outcrops in Ceara, Pernambuco and
Piaui, with an area of about 6,400 km2.
It is composed of coarse fluvial sediments with relatively high
permeability. Exu water is used for domestic supply. Locally, quality
has been affected due to the construction of “barreiros”
for rainwater collection.
d) Other smaller
basins:
In addition to
Araripe there are other small sedimentary basins in the interior of
the Northeastern region but their productivity is low or restricted
to local production. The main ones are the following:
- Rio do Peixe, São José de Belmonte, Serra dos Martins: composed of narrow strips of fine and medium sandstones, with intercalations of coarse sandstones, conglomerates, siltstones, claystones, etc. Depth of productive wells is less than 100 m. and yields average 7,000 liters per hour.
- Iguatu-Icó- Lima Campos: composed of a basal coarse sandstone formation (Quixoá formation) passing to medium sandstone and finer sediments in the top. Local productivity and good water quality.
- Other basins include the following: Lavras de Mangabeira, Varzea Alegre, Pau dos Ferros, Cedro Carnubeiras.
VII)
Jatoba-Tucano-Reconcavo sedimentary basins
The
Jatoba-Tucano-Reconcavo basin is the largest coastal basin in the
Eastern Atlantic façade of Brazil. It is an elongated basin
extending for more than 300 km from Eastern Pernambuco to
Southeastern Bahía with a width of 30 to 50 km. It is filled with
Cretaceous and Tertiary sediments covering older Paleozoic
formations, which outcrop in the extreme North of the basin. The main
aquifers are Marizal and São Sebastião.
- São Sebastião aquifer system
The São Sebastião
aquifer is a very important, extensive and deep aquifer of the
Reconcavo basin with a recharge area of 6,780 km2
i.
It is 3,000 meters thick (including other underlying formations)
and is saturated with freshwater down to 1,000 meters. Well yields
may reach 30,000 liters per hour and water quality is good, with
mineral contents less than 200 mg/liter. It is used for water supply
in the Salvador Metropolitan Area.
- Marizal aquifer system
The
Marizal aquifer system, mainly composed of sandstones, is located at
the top of the Reconcavo sedimentary basin, extending for about
10,000 sq. km along the lower course of the river São Francisco down
to the Atlantic coast. Because of its large outcropping area,
shallowness, low mineral contents, moderate to high wells yields and
unconfined conditions, it is
i
Source: Avaliação do grau de internalização da proteção da
agua subterranea no Pólo Industrial de Camaçari; João Severiano
Caldas da Silveira, Jr.,U.F.de Bahia, MEPLIM, Salvador, 2004.
widely
utilized throughout the lower São Francisco region. However, it may
be vulnerable to contamination in the outcropping areas, particularly
in the Salvador Metropolitan Region.
VIII)
São Francisco sedimentary basin
The
São Francisco sedimentary basin occupies the southwestern part of
the hydrographic basin, extending westwards into other hydrographic
realms.
The main
aquifers that are found in this area are the Bambui aquifer system
and the Urucuia-Areado aquifer system.
- Bambui aquifer system
The
Bambui aquifer system is located in the São Francisco sedimentary
basin extending through Minas Gerais, Bahia, Tocantins and Goiás.
It is contained in a Cambrian sequence of limestones and
meta-sediments. The aquifer develops in karstic fracture systems and
possesses a recharge area of 182,000 km2.
In the karstic region of Bambui, water quality is good, with chemical
composition of calcium-bicarbonate and mineral content about 210-220
mg/liter. In the karstic-fracture region, water types are
magnesium-bicarbonate, sodium-bicarbonate or both combined. There are
some limitations for drinking purposes, due to excess fluoride
(locally).
It is
extensively exploited in various regions and used for irrigation in
the Verde Grande sub-basin of the São Francisco hydrographic basin
and in the Plato de Irece, in Bahia.
b)
Urucuia-Areado aquifer system
The
Urucuia-Areado aquifer system is located in a peripheral zone of the
Northeast. It is a large unconfined aquifer which is found to the
West of the middle course of the river São Francisco. It has been
extensively used for irrigation in the western part of Bahia. Water
quality is good, mainly of the calcium-bicarbonate type, with low
mineral content, suitable for drinking purposes.
IX)
Alluvial aquifer systems
There
are numerous alluvial aquifer systems in the Northeast of Brazil.
They are normally shallow and, in most cases, thicknesses do not
exceed 50-100 m. Hydraulic conditions are generally unconfined. Water
quality is variable. Some wells have low mineral content, some
higher, often due to processes of irrigation return. In general, they
have moderate to high vulnerability to contamination.
They
are widely used both for water supply in urban and rural communities
and for irrigation.
i
Representacão Esquematica dos Principais Aquiferos Brasileiros,
MMA,2003
