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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 textual information on this page was taken from a number of reports, which are referenced at the bottom of this page. Some of this information is outdated. If you have more recent information on the hydrogeology of Chad, please get in touch.'''
  
  
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|Urban population with access to safe drinking water (%) || || ||  || 71.8
 
|Urban population with access to safe drinking water (%) || || ||  || 71.8
 
|-
 
|-
|Population affected by water related disease ||No data||No data||No data||No data
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|Population affected by water related disease (per 1000 inhabitants) ||No data||No data||No data||No data
 
|-
 
|-
 
|Total internal renewable water resources (cubic metres/inhabitant/year) || || ||1069||  
 
|Total internal renewable water resources (cubic metres/inhabitant/year) || || ||1069||  
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|Freshwater withdrawal as % of total renewable water resources || ||1.925|| ||  
 
|Freshwater withdrawal as % of total renewable water resources || ||1.925|| ||  
 
|-
 
|-
|Total renewable groundwater (Million cubic metres/year) || || ||11,500 ||
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|Renewable groundwater resources (Million cubic metres/year) || || ||  ||  
|-
 
|Exploitable: Regular renewable groundwater (Million cubic metres/year) ||  || || ||  
 
 
|-
 
|-
 
|Groundwater produced internally (Million cubic metres/year) || || ||11,500  ||  
 
|Groundwater produced internally (Million cubic metres/year) || || ||11,500  ||  
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Source and more statistics at: [http://www.fao.org/nr/water/aquastat/data/query/index.html?lang=en FAO Aquastat].
 
Source and more statistics at: [http://www.fao.org/nr/water/aquastat/data/query/index.html?lang=en FAO Aquastat].
 +
  
 
==Geology==
 
==Geology==
  
This section provides a summary of the geology of Chad. More information is available in the report [https://www.bgs.ac.uk/africaGroundwaterAtlas/atlas.cfc?method=ViewDetails&id=AGLA060033 ‘Groundwater in North and West Africa: Chad’] (1988) (see References section, below).
+
This section provides a summary of the geology of Chad. More information is available in the report [http://www.bgs.ac.uk/africagroundwateratlas/fulldetails.cfm?id=AGLA060033 ‘Groundwater in North and West Africa: Chad’] (1988) (see References section, below).
  
 
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'''].
+
[[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'''
+
The geology of Chad is dominated by the Lake Chad Basin, a sedimentary basin formed in the Mesozoic era and infilled with older Tertiary Continental Terminal sandstones and argillaceous sands, and younger Pliocene-Quaternary sandy/clayey deltaic and lacustrine deposits (Ngatcha et al. 2008, UNICEF 2010).
  
The geology of Chad is dominated by the Lake Chad Basin, a sedimentary basin that started to form in the Mesozoic era. It is infilled with, at depth, Tertiary age sandstones and argillaceous sands of the Continental Terminal; and above these, Pliocene-Quaternary age sandy/clayey deltaic and lacustrine sediments (Ngatcha et al. 2008, UNICEF 2010).
+
Smaller outcrops of unconsolidated Quaternary deposits, largely alluvium in river valleys, also occur in other parts of the country overlying bedrock.
  
Other, smaller outcrops of unconsolidated Quaternary sediments - mostly alluvium in river valleys - occur in other parts of the country, overlying bedrock.
+
To the north and east of the Lake Chad Basin are older rocks. These include small outcrops of Cretaceous continental and marine sedimentary rocks; Cretaceous rocks are also present at depth below the Lake Chad Basin and other parts of the country.  
  
Across the northern part of the country, to the north and east of the Lake Chad Basin, are older rocks, mostly Palaeozoic sedimentary rocks, largely of continental origin and dating from the Cambrian to the Jurassic. There are also small outcrops of Cretaceous age continental and marine sedimentary rocks. These Cretaceous and Palaezoic rocks are also present at depth in parts of central Chad, buried beneath Quaternary and Tertiary sediments in the Lake Chad Basin.
+
Palaeozoic sedimentary rocks crop out over a large part of northern Chad, largely of continental origin and dating from the Cambrian to the Jurassic. They are also present at depth in parts of central Chad.  
  
There are some outcrops of volcanic rocks, particularly in the northwest, forming the Tibesti masasif.
+
There are some areas of volcanic rocks, particularly in the northwest
  
Precambrian metamorphic and crystalline basement rocks crop out in two main areas: one in eastern Chad and one in the northwest.  
+
Precambrian metamorphic and crystalline basement rocks crop out in eastern Chad and in the northwestern part.  
  
  
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|Key Formations||Period||Lithology
 
|Key Formations||Period||Lithology
 
|-
 
|-
!colspan="4"|Quaternary-Pliocene unconsolidated sedimentary
+
!colspan="4"|Quaternary
 
|-
 
|-
|Undifferentiated
+
|Chad Formation
 
||Quaternary
 
||Quaternary
||Relatively small outcrops of undifferentiated unconsolidated Quaternary sediments occur across Chad, including alluvium in river valleys, and areas of aeolian sand dunes in the arid north.
+
||Quaternary and Pliocene-Quaternary deposits, known as the Chad Formation, occur across most of the Chad basin. They consist of lake-margin, alluvial fan and deltaic deposits, with lacustrine clay layers, that are present at depths of up to 500 m. The clays are most extensive near to the present day lake shore (Goni 2008). 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.  
|-
 
|Chad Formation
 
||Quaternary-Pliocene
 
||Quaternary and Pliocene-Quaternary sediments, known as the Chad Formation, occur across most of the Chad basin. They consist of unconsolidated sediments of lake-margin, alluvial fan and deltaic origin, with some interbedded lacustrine clays. The Chad Formation is up to 500 m thick in Chad. Lacustrine clays are most extensive near to the present day lake shore (Goni 2008).  
 
  
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.  
+
Smaller outcrops of Quaternary deposits occur in other parts of the country, including alluvium in river valleys, and areas of aeolian sand dunes in the arid north.  
 
|-
 
|-
!colspan="4"|Upper Mesozoic sedimentary
+
!colspan="4"|Tertiary
 
|-
 
|-
 
|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
||Tertiary: Eocene, Oligo-Miocene, Neogene  
+
||Eocene, Oligo-Miocene, Neogene  
||The Oligo-Miocene to Neogene Continental Terminal Formation occurs across large parts of southern Chad, in the Lake Chad Basin. It consists of alternating sandstone, argillaceous sand and clay layers. They are generally at depths of 400 to 700 m, below the Quaternary-Pliocene Chad Formation. In some parts of the south and southeast of the Lake Chad Basin, the Continental Terminal rocks crop out at the ground surface, shown on the geology map.  
+
||The Oligo-Miocene to Neogene Continental Terminal formation occurs across large parts of southern Chad, in the Lake Chad Basin, and consists of alternating sandstone, argillaceous sand and clay layers. They are generally at depths of 400 to 700 m, covered by Quaternary and Pliocene deposits. In some parts of the south and southeast of the Lake Chad Basin, they crop out at the ground surface. The upper part of the series is a lacustrine sequence of Neogene age called the Chad Formation.
 
|-
 
|-
 
|
 
|
||Cretaceous
+
||Mesozoic (Permo-Triassic to Cretaceous)
||There are few outcrops of Cretaceous rocks at the ground surface in Chad, but they occur at depth in the Lake Chad Basin.
+
||There are few outcrops of Cretaceous rocks in Chad, but they occur at depth in the Lake Chad Basin.
  
The Lower Cretaceous is characterised by continental fluviatile and lacustrine clastic rocks, part of the Permo-Triassic to Lower Cretaceous age Continental Intercalaire Formation.  
+
The Lower Cretaceous is characterised by continental fluviatile and lacustrine clastic rocks, part of the Permo-Triassic to Lower Cretaceous Continental Intercalaire formation. The Upper Cretaceous includes marine to continental clastic rocks, including fossiliferous shales.  
 
 
The Upper Cretaceous includes marine to continental clastic rocks, including fossiliferous shales.
 
|-
 
!colspan="4"|Igneous Volcanic
 
|-
 
|Mesozoic to Quaternary
 
||
 
||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
+
!colspan="4"|Palaeozoic sedimentary
 
|-
 
|-
 
|
 
|
||Cambro-Ordovician - Jurassic
+
||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.  
+
||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. Thick continental sandstones within the Kurfa basin form the Nubian formations, which are at least 1000 m and possibly up to 4000 m thick.
 +
 
 +
  
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.
 
 
|-
 
|-
 
!colspan="4"|Precambrian Basement
 
!colspan="4"|Precambrian Basement
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|-
 
|-
 
|}
 
|}
 +
  
 
==Hydrogeology==
 
==Hydrogeology==
  
This section provides a summary of the hydrogeology of the main aquifers in Chad. More information at a country-scale is available in the report [https://www.bgs.ac.uk/africaGroundwaterAtlas/atlas.cfc?method=ViewDetails&id=AGLA060033 ‘Groundwater in North and West Africa: Chad’] (1988) (see References section, below).  
+
This section provides a summary of the hydrogeology of the main aquifers in Chad. More information for the whole country is available in the report [http://www.bgs.ac.uk/africagroundwateratlas/fulldetails.cfm?id=AGLA060033 ‘Groundwater in North and West Africa: Chad’] (1988) (see References section, below). For areas where the ResEau program has produced hydrogeological maps and descriptions, these provide the most detailed available information on groundwater and hydrogeology (see below).
 
 
For areas where the [http://www.unitar.org/pillars/2030-agenda-support/reseau-project '''ResEau'''] program has produced hydrogeological maps and descriptions, these provide the most detailed available information on groundwater and hydrogeology (see below).
 
  
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 [[Hydrogeology Map | 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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* A hydrogeological map at 1:500,000 scale was published in 1969 by BRGM and a scanned copy can be viewed on the [http://www.bgr.de/app/fishy/whymis/index.php?&type=country&id=TCD WHYMAP] site.  
 
* A hydrogeological map at 1:500,000 scale was published in 1969 by BRGM and a scanned copy can be viewed on the [http://www.bgr.de/app/fishy/whymis/index.php?&type=country&id=TCD WHYMAP] site.  
  
* [http://geoportal.reseau-tchad.org '''ResEau''']
+
* '''ResEau'''
  
 
The ongoing [https://reseau-tchad.org/ ResEau] program is collecting detailed new hydrogeological information and producing new hydrogeological maps for Chad. Eventually, the whole country will be covered at 1:500,000 scale, and already these scale maps are available for the north and east of the country. More detailed hydrogeological maps at 1:200,000 scale are being produced for specific important areas.  
 
The ongoing [https://reseau-tchad.org/ ResEau] program is collecting detailed new hydrogeological information and producing new hydrogeological maps for Chad. Eventually, the whole country will be covered at 1:500,000 scale, and already these scale maps are available for the north and east of the country. More detailed hydrogeological maps at 1:200,000 scale are being produced for specific important areas.  
  
Digital versions of these maps can be viewed in the [http://geoportal.reseau-tchad.org '''ResEau geoportal''']. Pdf versions of the maps, with accompanying hydrogeological descriptions, can be freely downloaded from [https://reseau-tchad.org ResEau].  
+
Digital versions of these maps can be viewed in ResEau [https://reseau-tchad.org/index.php?pId=8 geoportal]. Pdf versions of the maps, with accompanying hydrogeological descriptions, can be freely downloaded from [https://reseau-tchad.org/index.php?pId=10 ResEau]. A detailed [https://reseau-tchad.org/upload/Documents/Publication/ResEau1-Synthese-hydrogeologique-Nord&Est-Tchad.pdf synthesis of the hydrogeology of the north and east of Chad] is available as a report (in French).
 
 
A detailed [https://reseau-tchad.org/upload/Documents/Publication/ResEau1-Synthese-hydrogeologique-Nord&Est-Tchad.pdf synthesis of the hydrogeology of the north and east of Chad] is available as a report (in French).
 
  
 
These maps and descriptions are the key source for information on the hydrogeology of Chad.
 
These maps and descriptions are the key source for information on the hydrogeology of Chad.
  
  
  [[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].]]
+
  [[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 [[Hydrogeology Map | hydrogeology map]] resource page]]
  
  
 
The information below provides a summary of the hydrogeology of Chad at a national scale.  
 
The information below provides a summary of the hydrogeology of Chad at a national scale.  
  
The Lake Chad Basin in southern Chad is one of the largest sedimentary groundwater basins in Africa. It includes three main aquifers: an upper, unconsolidated Quaternary/Pliocene aquifer of lacustrine and deltaic deposits (the Chad Formation); a Tertiary sequence underlying this (the Continental Terminal formation); and a basal aquifer of Cretaceous age, which is less well understood.  
+
The Lake Chad Basin in southern Chad is one of the largest sedimentary groundwater basins in Africa. It includes three main aquifers: an upper, unconsolidated Quaternary/Pliocene aquifer of lacustrine and deltaic deposits; the underlying Tertiary Continental Terminal sequence; and a lower aquifer of Cretaceous age, which is less well understood.
 +
 
 +
Other aquifers in Chad are less well studied. They include unconsolidated Quaternary valley alluvium and other deposits; Palaeozoic sedimentary rocks in the north, part of the Nubian Sandstone Aquifer System; Precambrian basement rocks; and smaller areas of volcanic rocks.  
  
Away from the Lake Chad Basin, other aquifers in Chad are not as well studied or understood. Other aquifers include Quaternary valley alluvium and other unconsolidated sediments; Palaeozoic sedimentary rocks in the north, part of the Nubian Sandstone Aquifer System; Precambrian basement rocks; and smaller areas of volcanic rocks.
 
  
 
====Unconsolidated====
 
====Unconsolidated====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Named Aquifers||Aquifer Productivity||General Description||Water quality  
+
|Named Aquifers||Period||General Description||Water quality  
 
|-
 
|-
 
|Alluvium in river valleys; aeolian sand in the north
 
|Alluvium in river valleys; aeolian sand in the north
||Usually Moderate to High
+
||Quaternary
 
||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.
 
||
 
||
 +
|}
 +
 +
====Sedimentary - Intergranular Flow====
 +
{| class = "wikitable"
 +
|Named Aquifers||Period||General Description||Water quality
 
|-
 
|-
|Chad Formation: Quaternary (to Pliocene) - upper part of the '''Lake Chad Basin Aquifer'''
+
|Chad Formation: upper part of the '''Lake Chad Basin Aquifer'''
||High
+
||Pliocene-Quaternary
||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 are 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.  
  
 
Isotopic studies have highlighted recharge zones to the shallow aquifer in areas of higher elevation and in exposed sand dune areas; as well as infiltration from rivers Lake Chad and wetlands (GWP 2013). In the northern part, in Kanem region, studies have also show high recharge from the slopes of the mountain areas to the north.
 
Isotopic studies have highlighted recharge zones to the shallow aquifer in areas of higher elevation and in exposed sand dune areas; as well as infiltration from rivers Lake Chad and wetlands (GWP 2013). In the northern part, in Kanem region, studies have also show high recharge from the slopes of the mountain areas to the north.
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|}
 
|}
  
====Sedimentary - Intergranular Flow====
+
==== Sedimentary - Intergranular & Fracture Flow====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Named Aquifers||Aquifer Productivity||General Description||Water quality  
+
|Named Aquifers||Period||General Description||Water quality  
 
|-
 
|-
|Continental Terminal: Tertiary - lower part of the '''Lake Chad Basin Aquifer'''
+
|Continental Terminal: lower part of the '''Lake Chad Basin Aquifer'''
||Moderate to High
+
||Tertiary
 
||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
+
|Continental Intercalaire
||Unknown
+
||Permo-Triassic to lower Cretaceous
 
||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
+
|Nubian Sandstone  
||Moderate to High
+
||Mesozoic - Palaeozoic
 
||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.  
 
||Groundwaters vary from fresh to slightly brackish, with salinity ranges from 240 to 1300 ppm. High concentrations of ions including sodium, chloride, and sulphate reflect the leaching and dissolution processes of gypsiferous shales and clay, in addition to long groundwater residence times.
 
||Groundwaters vary from fresh to slightly brackish, with salinity ranges from 240 to 1300 ppm. High concentrations of ions including sodium, chloride, and sulphate reflect the leaching and dissolution processes of gypsiferous shales and clay, in addition to long groundwater residence times.
|}
 
 
====Igneous (mostly Volcanic)====
 
{| class = "wikitable"
 
|Named Aquifers||Aquifer Productivity||General Description||Water quality
 
|-
 
|
 
||Probably Low to Moderate
 
||Little is known about the aquifer properties of volcanic or other igneous rock aquifers in Chad.
 
||
 
 
|}
 
|}
  
 
====Basement====
 
====Basement====
 
{| class = "wikitable"
 
{| class = "wikitable"
|Named Aquifers||Aquifer Productivity||General Description||Water quality  
+
|Named Aquifers||Period||General Description||Water quality  
 
|-
 
|-
 
|Weathered basement, often in hydraulic continuity with overlying Quaternary alluvial sands
 
|Weathered basement, often in hydraulic continuity with overlying Quaternary alluvial sands
||Low
+
||Precambrian
 
||In deep valleys in the granitic massifs, the upper parts of the Precambrian rocks can be heavily weathered. These weathered basins form low to moderate productivity aquifers, especially where they are in hydraulic continuity with overlying permeable Quaternary sands, which supply additional groundwater storage.  
 
||In deep valleys in the granitic massifs, the upper parts of the Precambrian rocks can be heavily weathered. These weathered basins form low to moderate productivity aquifers, especially where they are in hydraulic continuity with overlying permeable Quaternary sands, which supply additional groundwater storage.  
 
||  
 
||  
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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]
+
These, and others, can be accessed through the [http://www.bgs.ac.uk/africagroundwateratlas/searchResults.cfm?title_search=&author_search=&category_search=&country_search=TD&placeboolean=AND&singlecountry=1 Africa Groundwater Literature Archive]
  
  
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GWP (Global Water Partnership). 2013. [http://www.gwp.org/Global/ToolBox/References/Lake_Chad_fact_sheet.pdf The Lake Chad Basin Aquifer System]. Global Water Partnership Transboundary Groundwater Fact Sheet, compiled by F Bontemps.
 
GWP (Global Water Partnership). 2013. [http://www.gwp.org/Global/ToolBox/References/Lake_Chad_fact_sheet.pdf The Lake Chad Basin Aquifer System]. Global Water Partnership Transboundary Groundwater Fact Sheet, compiled by F Bontemps.
  
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