Editing Hydrogeology of Guinea Bissau

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====Unconsolidated====
 
====Unconsolidated====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||Description
+
|Aquifer||Description
 
|-
 
|-
|Quaternary to Tertiary
+
|Quaternary to Tertiary - Low to Moderate Productivity
||Low to Moderate Productivity
 
 
||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.
 
||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.
  
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|}
 
|}
  
====Sedimentary - Mixed Intergranular and Fracture flow====
+
====Sedimentary - Mixed Intergranular and Fracture flow - Low to High (Variable) Productivity====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||Description
+
|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
+
|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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====Igneous====
 
====Igneous====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||Description
+
|Aquifer||Description
 
|-
 
|-
|Mesozoic Igneous Intrusive rocks
+
|Mesozoic Igneous Intrusive rocks - Unknown aquifer potential
||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.
 
||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) - Generally Low; occasionally Moderate Productivity ====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Aquifer||Aquifer Productivity||Description
+
|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.
 
|}
 
|}
  
====Basement====
+
====Basement - Low to Moderate Productivity====
 
{| 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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