Editing Hydrogeology of Guinea Bissau

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  Do you have more information on hydrogeology of Guinea Bissau? Please get in touch.
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'''Do you have more information on hydrogeology of Guinea Bissau? Please get in touch.'''
  
  
[[File:CC-BY-SA_logo_88x31.png | frame | This work is licensed under a [https://creativecommons.org/licenses/by-sa/3.0/ Creative Commons Attribution-ShareAlike 3.0 Unported License]]]
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Present-day Guinea Bissau was part of the the ancient Mali empire from the 13th to 17th centuries, and later part of the Gabu (or Kaabu) kingdom from the 16th to 19th centuries. From the 15th century, the coast became a focus of slave trading by Portuguese merchants, and the coast and inland areas were fully colonised by the Portuguese in the 19th century. An armed rebellion against colonial rule from the 1950s led to independence in 1974 as a republic. Independence fighters in Guinea Bissau were allied to an extent with those from another Portuguese colony, Cape Verde, although the two countries never unified. Since independence, Guinea Bissau has experienced ongoing political instability. Initially, government was highly centralised, with multi-party governance established in 1991. However, since independence no president has successfully served a full five-year term, and there has been a succession of military coups.  
  
Present-day Guinea Bissau was historically part of the Mali empire from the 13th to 17th centuries, and later part of the Gabu (or Kaabu) kingdom from the 16th to 19th centuries. From the 15th century, the coast became a focus of slave trading by Portuguese merchants, and the coast and inland areas were fully colonised by the Portuguese in the 19th century. An armed rebellion against colonial rule from the 1950s led to independence in 1974 as a republic. Since independence, Guinea Bissau has experienced ongoing political instability. Initially, government was highly centralised, with multi-party governance established in 1991. However, since independence no president has successfully served a full five-year term, and there has been a succession of military coups.  
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Guinea Bissau adopted the CFA currency in 1997. There is some potential for mineral exploitation and possibly offshore hydrocarbon exploitation, but their development has been impeded by political instability and armed conflict. The economy remains dominated by agriculture and fishing, with cashew nuts and groundnuts the most important export crops. GDP and HDI (human development index) are some of the lowest world-wide. Illegal drug trafficking is significant, with the country used as a transit point for drugs between South America and Europe.  
  
Guinea Bissau adopted the CFA currency in 1997. There is some potential for mineral exploitation and possibly offshore hydrocarbon exploitation, but development has been impeded by political instability and armed conflict. The economy remains dominated by agriculture and fishing, with cashew nuts and groundnuts the most important export crops. GDP and HDI (human development index) are some of the lowest world-wide. Illegal drug trafficking is significant, with the country used as a transit point for drugs between South America and Europe.
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A tropical country with high but seasonal rainfall, Guinea Bissau has relatively abundant seasonal surface water resources, but dry season water supplies are largely sourced from groundwater.  
 
 
Guinea Bissau has relatively abundant seasonal surface water resources, as a tropical country with high (but seasonal) rainfall, but dry season water supplies are largely sourced from groundwater.  
 
  
  
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Guinea-Bissau lies between the Fouta Djallon massif, of uncertain Palaeozoic age, and the Mesozoic-Cenozoic Senegal basin (UN 1988). It can be roughly divided into two geological units:  
 
Guinea-Bissau lies between the Fouta Djallon massif, of uncertain Palaeozoic age, and the Mesozoic-Cenozoic Senegal basin (UN 1988). It can be roughly divided into two geological units:  
  
* an eastern zone with predominantly clastic sedimentary Paleozoic rocks, and some Precambrian rocks; and
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* an eastern zone with predominantly clastic sedimentary Paleozoic rocks, and some carbonates of Silurian age; and some Precambrian rocks; and
* a western zone with mainly Cenozoic sediments of Cretaceous to Tertiary age, mainly of marine origin (University of Guelph).
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* a western zone with mainly late Mesozoic to Cenozoic sediments. The Cretaceous to Cenozoic sediments are mainly of marine origin (University of Guelph).
 
 
There is Quaternary alluvium infill in many valleys, and marine/coastal Quaternary unconsolidated sediments on coastal plains.
 
  
A buried sequence of Mesozoic sedimentary rocks is thought to rest on the Palaeozoic sequence, the equivalent of the 'intercalated continental' series known in Senegal. These do not crop out at the surface in Guinea Bissau, and so are not shown on the geology map. Little is known of these rocks in Guinea Bissau, as they are buried at depths of over 800 m (UN 1988). They are thought to include schists with some sandstone and limestone intercalations in the northeast, and a dominantly continental series of schists with intercalations of fine sandstone in the centre of the country. This series is 120 m thick at Safim and 330 m thick at S. Domingos (UN 1988).
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There is a buried sequence of earlier Mesozoic sedimentary rocks thought to rest on the Palaeozoic sequence, which is the equivalent of the 'intercalated continental' series known in Senegal. These do not crop out at the surface in Guinea Bissau, and so are not shown on the geology map. Little is known of these rocks in Guinea Bissau, as they are buried at depths of over 800 m (UN 1988). They are thought to include schists with some sandstone and limestone intercalations in the northeast, and a dominantly continental series of schists with intercalations of fine sandstone in the centre of the country. This series is 120 m thick at Safim and 330 m thick at S. Domingos (UN 1988).
  
  
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||Coastal sediments, including beach sands; river and coastal alluvium. Including sands, silts, and clays. These often overlie similar lagoonal-coastal Oligocene-Miocene sediments (see below).
 
||Coastal sediments, including beach sands; river and coastal alluvium. Including sands, silts, and clays. These often overlie similar lagoonal-coastal Oligocene-Miocene sediments (see below).
 
|-
 
|-
!colspan="4"|Tertiary to Upper Cretaceous sedimentary  
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!colspan="4"|Cretaceous - Tertiary sedimentary  
 
|-
 
|-
|Tertiary (Palaeocene-Eocene, Oligocene, Miocene); Cretaceous (Maastrichtian)
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|Maastrichtian, Palaeocene-Eocene, Oligocene, Miocene
||A sequence of largely marine, coastal or lagoonal sedimentary rocks, including limestones, marls, clays, silts, sands and phosphates. Most are of Tertiary age. A strip of Cretaceous rocks of Maastrichtian age lie along the eastern boundary of this western zone.
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||A sequence of largely marine, coastal or lagoonal sedimentary rocks, including limestones, marls, clays, silts, sands and phosphates.  
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The Oligocene-Miocene 'continental terminal' series comprises Oligocene lagoonal fine grained, clayey sandstones at the base, overlain by Miocene marine limestone-marls that are sometimes sandy (UN 1988).
  
 
The Palaeocene-Eocene sequence is dominantly marine, formed of sandy marl-limestone formations with dolomitic intercalations (UN 1988).
 
The Palaeocene-Eocene sequence is dominantly marine, formed of sandy marl-limestone formations with dolomitic intercalations (UN 1988).
  
The Oligocene-Miocene 'continental terminal' series comprises Oligocene lagoonal fine grained, clayey sandstones at the base, overlain by Miocene marine limestone-marls that are sometimes sandy (UN 1988).
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The Middle and Upper Cretaceous series is very thick (1360 m at S. Domingos and 600 m at Safim). The base of the sequence consists of schists with some limestone-dolomitic intercalations, overlain by dark shales with some sand, and terminating at the top in a thick sandstone layer of Maastrichtian (top Cretaceous) age (UN 1988). This top sandstone is up to 490 m thick in the northwest (S. Domingos) and 540 m in the west (Cangongue) (UN 1988).
 
 
The Cretaceous series is very thick (1360 m at S. Domingos and 600 m at Safim). The base of the sequence consists of schists with some limestone-dolomitic intercalations, overlain by dark shales with some sand, and terminating at the top in a thick sandstone layer of Maastrichtian (top Cretaceous) age (UN 1988). This top sandstone is up to 490 m thick in the northwest (S. Domingos) and 540 m in the west (Cangongue) (UN 1988).
 
 
|-
 
|-
 
!colspan="4"|Igneous  
 
!colspan="4"|Igneous  
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|-
 
|-
 
|Devonian
 
|Devonian
||Shales and sandstones over a large area in the east of the country, forming a northwest/southeast syncline.  
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||Shales and sandstones over a large area in the east of the country, forming a northwest/southeast syncline. The base of the series consists of the sandstones of Lower Devonian age in the Cusselinta-Saltinho area, which are mostly well consolidated micaceous and feldspathic sandstones. Overlying this is the Upper Devonian Bafata Group, comprising argillaceous schists with intercalactions of fine grained quartz sandstone.  
 
 
At the base of the series are Lower Devonian sandstones, seend in the Cusselinta-Saltinho area. These are mostly well-consolidated micaceous and feldspathic sandstones.  
 
 
 
Overlying this is the Middle to Upper Devonian Bafata Group, comprising argillaceous schists with intercalactions of fine grained quartz sandstone.  
 
 
|-
 
|-
 
|Silurian
 
|Silurian
||The Buba Group: mostly sandstones with some organic rich/carbonaceous black shales. Drilling in the southeast of the country showed very compact black, carbonaceous schists with fine grained sandstone intercalations, and some doloritic layers. In the northeast, schists interbedded with dolerite were seen, with varying indications of metamorphism, and fine grained, clay-rich sandstones at the top at the transition to the overlying Devonian rocks.  
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||Rocks of the Buba Group, including sandstones with some organic rich/carbonaceous black shales. Drilling in the southeast of the country showed very compact black, carbonaceous schists with fine grained sandstone intercalations, and some doloritic layers. In the northeast, schists interbedded with dolerite were seen, with varying indications of metamorphism, and fine grained, clay-rich sandstones at the top at the transition to the overlying Devonian rocks.  
 
|-
 
|-
|Ordovician and Cambrian
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|Cambrian-Ordovician
 
||Sandstones, shales, conglomerates and rare limestones. These are found in the northeast of the country, overlying older Neoproterozoic metamorphic rocks.  
 
||Sandstones, shales, conglomerates and rare limestones. These are found in the northeast of the country, overlying older Neoproterozoic metamorphic rocks.  
 
The Ordovician rocks distinguished on the geology map are dominated by the Canjufa-Canjadude series of quartz-arenites. The other dominant Ordovician series is the Gabu sandstone series, thought to be mostly steeply dipping at up to 50 degrees. At its base is a white, coarse grained sandstone unit up to 170 m thick, overlain by less compact, sandier strata of considerable thickness, and then by a fine grained sandstone unit that is 10-30 m thick (UN 1988).
 
 
 
Cambrian rocks are dominated by fine grained sandstones and shales, including shales of Pirada and Canquelifa, schist-sandstones of Cantari, and the younger Caium sandstones of Upper Cambrian age.
 
Cambrian rocks are dominated by fine grained sandstones and shales, including shales of Pirada and Canquelifa, schist-sandstones of Cantari, and the younger Caium sandstones of Upper Cambrian age.
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Ordovician rocks are dominated by the Gabu sandstone series, which is thought to be mostly steeply dipping at up to 50 degrees. At its base is a white, coarse grained sandstone unit up to 170 m thick, overlain by less compact, sandier strata of considerable thickness, and then by a fine grained sandstone unit that is 10-30 m thick (UN 1988).
 
|-
 
|-
!colspan="4"|Precambrian (Neoproterozoic) metamorphic complex
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!colspan="4"|Precambrian metamorphic complex
 
|-
 
|-
 
|Neoproterozoic  
 
|Neoproterozoic  
||A volcanic and metasedimentary complex, including schists, quartzites and metavolcanic rocks. There are few outcrops - these mostly in the far north-east - as they generally lie below fairly thick younger geological formations.  
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||A volcanic and metasedimentary complex, including schists, quartzites and metavolcanic rocks. There are few outcrops of these rocks, as they generally lie below fairly thick younger geological formations.  
 
|-
 
|-
 
|}
 
|}
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====Unconsolidated====
 
====Unconsolidated====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||Description
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|Aquifer||Description
 
|-
 
|-
|Quaternary to Tertiary
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|Tertiary (Miocene) to Quaternary 
||Low to Moderate Productivity
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||Coastal/marine silty or clayey sands, with small banks of Miocene limestone, and laterite. Found over most of the central and western regions. These are usually unconfined, providing low storage and low borehole yields, although yields can vary from place to place and from season to season (UN 1988). Yields are directly linked to rainfall and evapotranspiration.
||Mostly coastal/marine sands, with small banks of Tertiary (Miocene) limestone. Laterite is found over most of the central and western regions. These aquifers are usually unconfined and provide low storage and low borehole yields, although yields can vary from place to place and from season to season (UN 1988). Thicker and coarser grained sediments may form locally higher productivity aquifers.
 
  
Groundwater is subject to pollution and is often highly mineralised.
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Groundwater is subject to pollution and is often highly mineralised
 
|}
 
|}
  
 
====Sedimentary - Mixed Intergranular and Fracture flow====
 
====Sedimentary - Mixed Intergranular and Fracture flow====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||Description
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|Aquifer||Description
 
|-
 
|-
 
|Tertiary (Palaeocene-Eocene-Oligocene)
 
|Tertiary (Palaeocene-Eocene-Oligocene)
||Low to High (Variable) Productivity
 
 
||These marine sands, sandstones, and limestones form an important aquifer, which is buried below ~175-200 m, depending on the area. There are no major low permeability beds in the sequence, and so the whole unit behaves as a single aquifer, which is confirmed by groundwater levels (piezometry) and groundwater chemistry. The sands and sandstones are likely to be dominated by intergranular flow, and the limestones by fracture flow.  
 
||These marine sands, sandstones, and limestones form an important aquifer, which is buried below ~175-200 m, depending on the area. There are no major low permeability beds in the sequence, and so the whole unit behaves as a single aquifer, which is confirmed by groundwater levels (piezometry) and groundwater chemistry. The sands and sandstones are likely to be dominated by intergranular flow, and the limestones by fracture flow.  
  
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The highest mineral content (1000 ppm) is seen in the northwest coastal zone. Sulphate concentrations and hardness are higher than in the Maastrichtian aquifer (UN 1988).
 
The highest mineral content (1000 ppm) is seen in the northwest coastal zone. Sulphate concentrations and hardness are higher than in the Maastrichtian aquifer (UN 1988).
 
|-
 
|-
|Cretaceous
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|Cretaceous  
||Low to High (Variable) Productivity
 
 
||The main Cretaceous aquifer is the thick sandstone bed of Maastrichtian age, at the top of the Cretaceous sequence, which is an aquifer of major importance in Guinea Bissau. It has been explored and exploited most intensively and at shallowest depths close to its outcrop zone in central and southern regions, and by a number of deeper boreholes 200-260 m deep on the island of Bissau and at Farim (UN 1988). The aquifer productivity declines in the direction of the Silurian rocks, because they become shallower and change from poorly consolidated sand to a more compact, well consolidated sandstone that has lower permeability (UN 1988).  
 
||The main Cretaceous aquifer is the thick sandstone bed of Maastrichtian age, at the top of the Cretaceous sequence, which is an aquifer of major importance in Guinea Bissau. It has been explored and exploited most intensively and at shallowest depths close to its outcrop zone in central and southern regions, and by a number of deeper boreholes 200-260 m deep on the island of Bissau and at Farim (UN 1988). The aquifer productivity declines in the direction of the Silurian rocks, because they become shallower and change from poorly consolidated sand to a more compact, well consolidated sandstone that has lower permeability (UN 1988).  
  
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The groundwater is mainly of bicarbonate and calcium sulphate type, with relatively low sulphate and relatively soft. In the centre of the country it tends to have a low mineral content (<500 ppm) but up to 1500 ppm in the south (Bolama) (UN 1988). Occasional instances of saline intrusion were identified, such as as Bedanda (4000 ppm of chlorides) (UN 1988).
 
The groundwater is mainly of bicarbonate and calcium sulphate type, with relatively low sulphate and relatively soft. In the centre of the country it tends to have a low mineral content (<500 ppm) but up to 1500 ppm in the south (Bolama) (UN 1988). Occasional instances of saline intrusion were identified, such as as Bedanda (4000 ppm of chlorides) (UN 1988).
|}
 
 
====Igneous====
 
{| class = "wikitable"
 
|Aquifer||Aquifer Productivity||Description
 
|-
 
|Mesozoic Igneous Intrusive rocks
 
||Unknown
 
||Very little is known of the aquifer characteristics of these rocks. They are likely to be crystalline with very low intergranular porosity and permeability, so that groundwater potential will depend largely on the degree and type of  weathering and/or fracturing in the rocks. Groundwater is likely to be present mainly in the uppermost few tens of metres. Overall aquifer productivity is likely to be low.
 
 
|}
 
|}
  
 
====Sedimentary - Fracture flow (Palaeozoic aquifers)====
 
====Sedimentary - Fracture flow (Palaeozoic aquifers)====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||Description
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|Aquifer||Description
 
|-
 
|-
 
|Devonian
 
|Devonian
||Low Productivity
 
 
||Drilling into these rocks at shallow depths to about 20 m in the Nhabijocs plain (Bombadinca) showed them to be well consolidated with low permeability and to form a poor aquifer.  
 
||Drilling into these rocks at shallow depths to about 20 m in the Nhabijocs plain (Bombadinca) showed them to be well consolidated with low permeability and to form a poor aquifer.  
 
|-
 
|-
 
|Silurian  
 
|Silurian  
||Low Productivity
 
 
||The Buba Group has been explored by drilling water boreholes in a number of places at Buba, Guilege, Gadamael and Sangonha in the northeast and southeast of the country. The dominantly fine grained, sometimes clay-rich sandstones, black carbonaceous shales and intercalations of dolerite, sometimes metamorphosed, all have low permeability and formed very poor aquifers.  
 
||The Buba Group has been explored by drilling water boreholes in a number of places at Buba, Guilege, Gadamael and Sangonha in the northeast and southeast of the country. The dominantly fine grained, sometimes clay-rich sandstones, black carbonaceous shales and intercalations of dolerite, sometimes metamorphosed, all have low permeability and formed very poor aquifers.  
 
|-
 
|-
 
|Cambro-Ordovician
 
|Cambro-Ordovician
||Generally Low, occasionally Moderate Productivity
 
 
||The Cambrian rocks, dominated by fine grained, well consolidated sandstones and shales, generally form very poor aquifers. Small local aquifers can be found in shallow weathered zones, and particularly in sandstones, which typically have slightly higher permeability.
 
||The Cambrian rocks, dominated by fine grained, well consolidated sandstones and shales, generally form very poor aquifers. Small local aquifers can be found in shallow weathered zones, and particularly in sandstones, which typically have slightly higher permeability.
 
Little is known of the groundwater potential of the Ordovician sandstones, although drilling in the Canjadude region showed the sandstone to be compacted, with low permeability, and unproductive.
 
Little is known of the groundwater potential of the Ordovician sandstones, although drilling in the Canjadude region showed the sandstone to be compacted, with low permeability, and unproductive.
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====Basement====
 
====Basement====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||Description
+
|Aquifer||Description
 
|-
 
|-
 
|Precambrian (Neoproterozoic)
 
|Precambrian (Neoproterozoic)
||Low to Moderate Productivity
 
 
||Crystalline basement rocks have virtually no intergranular porosity and permeability, and groundwater flow and storage is entirely dependent on the nature and degree of weathering and/or fracturing of the rock. A typical pattern in basement rocks is 'pockets' of weathering forming weathered basins, typically a few tens of metres deep and a few tens or hundreds metres across, in which there is enhanced permeability and groundwater storage potential. Typically, these kind of weathered basins have enough permeability and groundwater storage capacity to supply a borehole hand pump supply.
 
||Crystalline basement rocks have virtually no intergranular porosity and permeability, and groundwater flow and storage is entirely dependent on the nature and degree of weathering and/or fracturing of the rock. A typical pattern in basement rocks is 'pockets' of weathering forming weathered basins, typically a few tens of metres deep and a few tens or hundreds metres across, in which there is enhanced permeability and groundwater storage potential. Typically, these kind of weathered basins have enough permeability and groundwater storage capacity to supply a borehole hand pump supply.
 
|}
 
|}

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