Editing Hydrogeology of Chad

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[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Hydrogeology by country | Hydrogeology by country]] >> Hydrogeology of Chad
 
[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Hydrogeology by country | Hydrogeology by country]] >> Hydrogeology of Chad
 
[[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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The geology map shows a simplified version of the geology at a national scale (see [[Geology | the Geology resources page]] for more details).
 
The geology map shows a simplified version of the geology at a national scale (see [[Geology | the Geology resources page]] for more details).
  
[https://www.bgs.ac.uk/africagroundwateratlas/downloadGIS.html '''Download a GIS shapefile of the Chad geology and hydrogeology map'''].
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  [[File:Chad_Geology2.png | center | thumb| 500px | Geology of Chad at 1:5 million scale. Developed from USGS map (Persits et al. 2002). For more information on the map development and datasets see the [[Geology | geology resource page]]]]
 
 
  [[File:Chad_Geology3.png | center | thumb| 500px | Geology of Chad at 1:5 million scale. Developed from USGS map (Persits et al. 2002). For more information on the map development and datasets see the [[Geology | geology resource page]]. [https://www.bgs.ac.uk/africagroundwateratlas/downloadGIS.html Download a GIS shapefile of the Chad geology and hydrogeology map].]]
 
  
 
'''Summary'''  
 
'''Summary'''  
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The sediment lithology was controlled by the climate during deposition, with wet periods favouring the deposition of clays and diatomites; periods of alternating wet and dry seasons tending to produce sandy deltaic deposits; and dry periods favouring the formation of aeolian sand dunes. The deposits therefore range from pure sand to clayey sand, and include some kaolinitic clays. Abrupt changes in clay and sand content are common, and the sediments usually occur as overlapping lenses.  
 
The sediment lithology was controlled by the climate during deposition, with wet periods favouring the deposition of clays and diatomites; periods of alternating wet and dry seasons tending to produce sandy deltaic deposits; and dry periods favouring the formation of aeolian sand dunes. The deposits therefore range from pure sand to clayey sand, and include some kaolinitic clays. Abrupt changes in clay and sand content are common, and the sediments usually occur as overlapping lenses.  
 
|-
 
|-
!colspan="4"|Upper Mesozoic sedimentary  
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!colspan="4"|Mesozoic sedimentary  
 
|-
 
|-
 
|Continental Terminal; also a small area of Eocene marine deposits in northern Chad
 
|Continental Terminal; also a small area of Eocene marine deposits in northern Chad
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||There are a few isolated small outcrops of igneous rockos in the east and south of the country, of which little is known. The largest outcrop of igneous rocks is in the Tibesti massif, which are thought to be dominantly Tertiary to Quaternary volcanic rocks, mainly lavas but also including some tephra and other volcaniclastic rocks (Permenter and Oppenheimer 2007).  
 
||There are a few isolated small outcrops of igneous rockos in the east and south of the country, of which little is known. The largest outcrop of igneous rocks is in the Tibesti massif, which are thought to be dominantly Tertiary to Quaternary volcanic rocks, mainly lavas but also including some tephra and other volcaniclastic rocks (Permenter and Oppenheimer 2007).  
 
|-
 
|-
!colspan="4"|Palaeozoic to Lower Mesozoic sedimentary
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!colspan="4"|Palaeozoic sedimentary
 
|-
 
|-
 
|
 
|
||Cambro-Ordovician - Jurassic
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||Cambro-Ordovician, Silurian, Devonian, Carboniferous
||Palaeozoic and Lower Mesozoic continental sandstone sequences crop out over a large part of northern Chad, and are known to be present at depth below younger deposits in the Lake Chad Basin in the centre of the country.  
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||Palaeozoic continental sandstone sequences crop out over a large part of northern Chad, and are known to be present at depth below younger deposits in the Lake Chad Basin in the centre of the country.  
  
In the north, this includes the Kufra basin, with largely Cambrian to Carboniferous rocks, which is shared with Libya and the Sudan. Thick continental sandstones within the Kufra basin form the Nubian formations, which are at least 1000 m and possibly up to 4000 m thick.  
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In the north, this includes the Kufra basin, with largely Cambrian to Carboniferous rocks, which is shared with Libya. Thick continental sandstones within the Kurfa basin form the Nubian formations, which are at least 1000 m and possibly up to 4000 m thick.  
 
|-
 
|-
 
!colspan="4"|Precambrian Basement
 
!colspan="4"|Precambrian Basement
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The hydrogeology map shown on this page shows a simplified version of the type and productivity of the main aquifers at a national scale (see the [[Africa Groundwater Atlas Hydrogeology Maps | hydrogeology Map]] resource page for more details).  
 
The hydrogeology map shown on this page shows a simplified version of the type and productivity of the main aquifers at a national scale (see the [[Africa Groundwater Atlas Hydrogeology Maps | hydrogeology Map]] resource page for more details).  
 
[https://www.bgs.ac.uk/africagroundwateratlas/downloadGIS.html '''Download a GIS shapefile of the Chad geology and hydrogeology map'''].
 
  
 
===Other hydrogeological maps ===
 
===Other hydrogeological maps ===
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  [[File:Chad_Hydrogeology3.png| center | thumb| 500px | Hydrogeology of Chad at 1:5million scale. For more information on how the map was developed see the [[Africa Groundwater Atlas Hydrogeology Maps | hydrogeology map]] resource page. [https://www.bgs.ac.uk/africagroundwateratlas/downloadGIS.html Download a GIS shapefile of the Chad geology and hydrogeology map].]]
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  [[File:Chad_Hydrogeology2.png| center | thumb| 500px | Hydrogeology of Chad at 1:5million scale. For more information on how the map was developed see the [[Africa Groundwater Atlas Hydrogeology Maps | hydrogeology map]] resource page]]
  
  
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|-
 
|-
 
|Alluvium in river valleys; aeolian sand in the north
 
|Alluvium in river valleys; aeolian sand in the north
||Usually Moderate to High
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||Generally High
 
||Outcrops of Quaternary alluvium in major river valleys are likely to be permeable and to have high groundwater storage potential, and shallow water tables. Where they are hydraulic connection with groundwater in underlying bedrock, such as weathered Precambrian basement, they can contribute significantly to local groundwater potential.
 
||Outcrops of Quaternary alluvium in major river valleys are likely to be permeable and to have high groundwater storage potential, and shallow water tables. Where they are hydraulic connection with groundwater in underlying bedrock, such as weathered Precambrian basement, they can contribute significantly to local groundwater potential.
 
||
 
||
 +
|}
 +
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====Sedimentary - Intergranular Flow====
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{| class = "wikitable"
 +
|Named Aquifers||Aquifer Productivity||General Description||Water quality
 
|-
 
|-
|Chad Formation: Quaternary (to Pliocene) - upper part of the '''Lake Chad Basin Aquifer'''
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|Chad Formation: upper part of the '''Lake Chad Basin Aquifer'''
 
||High  
 
||High  
 
||The Pliocene-Quaternary Chad Formation aquifer within the Lake Chad Basin is dominated by permeable sands that are present at depths of up to 500 m; the top surface of the aquifer varies from 250 to 350 m. Clays are most extensive near to the present day lake shore (Goni 2008). The upper part of the aquifer is often unconfined but in some places semi-confined to confined by clay layers (Goni 2008), and in its shallowest zones is in direct hydraulic contact with the Logone-Chari river system and with Lake Chad. Parts of the lower aquifer are confined by clay layers, and under artesian pressure. Sub-aquifers are identified within the overall basin, including the Kanem, Koros and Chari-Baguirmi areas. Borehole yields of 2.5 litres/second (l/s) have been recorded (United Nations 1988), but it is likely that much higher potential yields are possible.  
 
||The Pliocene-Quaternary Chad Formation aquifer within the Lake Chad Basin is dominated by permeable sands that are present at depths of up to 500 m; the top surface of the aquifer varies from 250 to 350 m. Clays are most extensive near to the present day lake shore (Goni 2008). The upper part of the aquifer is often unconfined but in some places semi-confined to confined by clay layers (Goni 2008), and in its shallowest zones is in direct hydraulic contact with the Logone-Chari river system and with Lake Chad. Parts of the lower aquifer are confined by clay layers, and under artesian pressure. Sub-aquifers are identified within the overall basin, including the Kanem, Koros and Chari-Baguirmi areas. Borehole yields of 2.5 litres/second (l/s) have been recorded (United Nations 1988), but it is likely that much higher potential yields are possible.  
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|}
 
|}
  
====Sedimentary - Intergranular Flow====
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==== Sedimentary - Intergranular & Fracture Flow====
 
{| class = "wikitable"
 
{| class = "wikitable"
 
|Named Aquifers||Aquifer Productivity||General Description||Water quality  
 
|Named Aquifers||Aquifer Productivity||General Description||Water quality  
 
|-
 
|-
|Continental Terminal: Tertiary - lower part of the '''Lake Chad Basin Aquifer'''
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|Continental Terminal: lower part of the '''Lake Chad Basin Aquifer'''
 
||Moderate to High
 
||Moderate to High
 
||The alternating sandstones and clay rich bands of the Continental Terminal underlie the Pliocene-Quaternary Chad Formation; together they form the Lake Chad Basin Aquifer. The Continental Terminal lies at depths of between approximately 100 to 200 m up to 400 to 600 m. It is artesian, confined by overlying clay bands. Average borehole yields are high, given as 27.5 l/s (United Nations 1988).  
 
||The alternating sandstones and clay rich bands of the Continental Terminal underlie the Pliocene-Quaternary Chad Formation; together they form the Lake Chad Basin Aquifer. The Continental Terminal lies at depths of between approximately 100 to 200 m up to 400 to 600 m. It is artesian, confined by overlying clay bands. Average borehole yields are high, given as 27.5 l/s (United Nations 1988).  
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Groundwater is abstracted for drinking water in rural and urban areas, but extraction costs are considerable.  
 
Groundwater is abstracted for drinking water in rural and urban areas, but extraction costs are considerable.  
 
||High fluoride concentrations in groundwater are seen.
 
||High fluoride concentrations in groundwater are seen.
|}
 
 
==== Sedimentary - Intergranular & Fracture Flow====
 
{| class = "wikitable"
 
|Named Aquifers||Aquifer Productivity||General Description||Water quality
 
 
|-
 
|-
|Continental Intercalaire: Cretaceous
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|Continental Intercalaire
 
||Unknown
 
||Unknown
 
||This aquifer occurs at depth in the Lake Chad Basin in southern and central Chad. Relatively little is known of its properties in Chad.  
 
||This aquifer occurs at depth in the Lake Chad Basin in southern and central Chad. Relatively little is known of its properties in Chad.  
 
||
 
||
 
|-
 
|-
|Nubian Sandstone: Palaeozoic to Mesozoic
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|Nubian Sandstone  
 
||Moderate to High
 
||Moderate to High
 
||The Nubian Sandstone aquifer in the Kufra basin is at least 1000 m and possibly up to 4000 m thick, and is dominated by well-cemented sandstones, with intercalations of shales and clays. Groundwater flow is largely through fractures but also by intergranular flow. The Nubian Sandstone aquifer system as a whole, which extends across much of north Africa, forms a productive aquifer which contains huge volumes of largely fossil groundwater. Little specific investigation has been done on the aquifer in Chad.  
 
||The Nubian Sandstone aquifer in the Kufra basin is at least 1000 m and possibly up to 4000 m thick, and is dominated by well-cemented sandstones, with intercalations of shales and clays. Groundwater flow is largely through fractures but also by intergranular flow. The Nubian Sandstone aquifer system as a whole, which extends across much of north Africa, forms a productive aquifer which contains huge volumes of largely fossil groundwater. Little specific investigation has been done on the aquifer in Chad.  
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|}
 
|}
  
====Igneous (mostly Volcanic)====
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====Volcanic====
 
{| class = "wikitable"
 
{| class = "wikitable"
 
|Named Aquifers||Aquifer Productivity||General Description||Water quality  
 
|Named Aquifers||Aquifer Productivity||General Description||Water quality  
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|
 
|
 
||Probably Low to Moderate
 
||Probably Low to Moderate
||Little is known about the aquifer properties of volcanic or other igneous rock aquifers in Chad.
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||Little is known about the aquifer properties of volcanic rock aquifers in Chad.
 
||  
 
||  
 
|}
 
|}
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The following references provide more information on the geology and hydrogeology of Chad.
 
The following references provide more information on the geology and hydrogeology of Chad.
  
Many of these, and others, can be accessed through the [https://www.bgs.ac.uk/africaGroundwaterAtlas/atlas.cfc?method=listResults&title_search=&author_search=&category_search=&country_search=TD&placeboolean=AND&singlecountry=1 Africa Groundwater Literature Archive]
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These, and others, can be accessed through the [https://www.bgs.ac.uk/africaGroundwaterAtlas/atlas.cfc?method=listResults&title_search=&author_search=&category_search=&country_search=TD&placeboolean=AND&singlecountry=1 Africa Groundwater Literature Archive]
  
  

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