Hydrogeology of Chad

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Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of Chad

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 Some of this information is outdated.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.

Compilers

Dr Kirsty Upton and Brighid Ó Dochartaigh, British Geological Survey, UK

Geographical Setting

The low lying Lake Chad Basin covers much of Chad, with an elevation of about 250 m. There is also low lying land in the dry north of the country. To the north, east and south of the Lake Chad Basin, the land rises gradually to plateaus and mountains. The highest elevations are in the Tibesti massif in the north, at over 3000 m. In the east are plateaus lying at more than 900 m (United Nations 1988).

Map of Chad (For more information on the datasets used in the map see the geography resources section)

General

Estimated Population in 2013* 12,825,314
Rural Population (% of total) (2013)* 77.8%
Total Surface Area* 1,259,200 sq km
Agricultural Land (% of total area) (2012)* 39.7%
Capital City N'Djamena
Region Central Africa
Border Countries Niger, Libya, Egypt, Sudan, Central African Republic, Cameroon, Nigeria
Annual Freshwater Withdrawal (2013)* 879.6 Million cubic metres
Annual Freshwater Withdrawal for Agriculture (2013)* 76.4%
Annual Freshwater Withdrawal for Domestic Use (2013)* 11.8%
Annual Freshwater Withdrawal for Industry (2013)* 11.8%
Rural Population with Access to Improved Water Source (2012)* 44.8%
Urban Population with Access to Improved Water Source (2012)* 71.8%

* Source: World Bank


Climate

Chad has a varied climate. In the south, rainfall is high - an annual average of 1000 to 1400 mm. In the central zone, average annual rainfall is between 700 and 1000 mm. In the north, the climate is semi-arid, with average annual rainfall between 200 and 700 mm. The far north is arid, with total annual rainfall below 500 mm. The dry northern zones have high evaporation of around 2200 to 2300 mm (United Nations 1988).

Average monthly precipitation for Chad showing minimum and maximum (light blue), 25th and 75th percentile (blue), and median (dark blue) rainfall Average monthly temperature for Chad showing minimum and maximum (orange), 25th and 75th percentile (red), and median (black) temperature Quarterly precipitation over the period 1950-2012 Monthly precipitation (blue) over the period 2000-2012 compared with the long term monthly average (red)

For further detail on the climate datasets used see the climate resources section.

Surface water

Southern Chad is drained by two main perennial rivers: the Chari and its largest tributuary the Logone, both of which receive flow mainly from tributaries from outside the country. The Chari river drains into Lake Chad, which is the largest surface water resource in the country. The Lake Chad basin is one of the best studied in Africa, and is shared by a number of countries, including Chad, Niger, Nigeria and Cameroon. As well as the Chari river, it is fed by two other smaller rivers flowing from Nigeria and Cameroon.

In the central and northern parts of the country, all the rivers are ephemeral, generally flowing only for a few hours after heavy rain, or at most for a few months each year (United Nations 1988).

Surface Water Map of Chad (For more information on the datasets used in the map see the surface water resources section)

Soil

Soil Map of Chad (For map key and more information on the datasets used in the map see the soil resources section)

Land cover

Land cover in Chad is largely controlled by climate. In the wetter southern zone, arable crops are and forest are widespread. In the central zone, arable crops and forest become rarer and land cover becomes dominated by grassland and savanna. In the dry north, vegetation is scarce (United Nations 1988).
Land Cover Map of Chad(For map key and more information on the datasets used in the map see the land cover resources section)


Geology

This section provides a summary of the geology of Chad. More information is available in the report ‘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 the Geology resources page for more details).

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).

Smaller outcrops of unconsolidated Quaternary deposits, largely alluvium in river valleys, also 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.

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 areas of volcanic rocks, particularly in the northwest

Precambrian metamorphic and crystalline basement rocks crop out in eastern Chad and in the northwestern part.


Geological Environments
Key Formations Period Lithology
Quaternary
Quaternary Quaternary formations occur across most of the Chad basin and are dominated by lacustrine with some alluvial deposits. 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.

Tertiary
Continental Terminal; also a small area of Eocene marine deposits in northern Chad Eocene, Oligo-Miocene, Neogene 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.
Mesozoic (Permo-Triassic to Cretaceous) 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 Continental Intercalaire formation. The Upper Cretaceous includes marine to continental clastic rocks, including fossiliferous shales.

Palaeozoic sedimentary
Cambro-Ordovician, Silurian, Devonian, Carboniferous 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 up to 1000 m thick.


Precambrian Basement
Precambrian Granitic and crystalline rocks forming the higher mountain areas in the north, east and south.


Hydrogeology

This section provides a summary of the hydrogeology of the main aquifers in Chad. More information is available in the report ‘Groundwater in North and West Africa: Chad’ (1988) (see References section, below).

The hydrogeology map shows a simplified version of the type and productivity of the main aquifers at a national scale (see the Aquifer properties resource page for more details). A digital map of hydrogeological units is held by the Direction Nationale de l’Hydraulique (UNICEF 2010).

Direction Générale du Génie Rural et de l’Hydraulique Agricole(DGGRHA)

An older but higher resolution hydrogeological map at 1:500,000 scale was published in 1969 by BRGM and a copy can be viewed on the WHYMAP site.


 


Summary

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.

The


Unconsolidated

Named Aquifers Period General Description Water quality
Lake Chad aquifer Pliocene-Quaternary The Pliocene-Quaternary aquifer within the Lake Chad basin consists of sandy, deltaic and lacustrine deposits that are present at depths of up to 500 m; the top surface of the aquifer varies from 250 to 350 m. Much of the sequence comprises permeable sands. The upper part of the aquifer is unconfined and 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.

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.

In the centre of the basin, groundwater levels are typically deepest - often more than 25 m below ground level - although it is not clear whether these relate to the Quaternary or to the deeper Continental Terminal aquifer (see below). They tend to shallow towards the west of the basin, to between 10 and 25 m deep; and even more towards the southern edge of the basin, often to less than 5 m deep (UNICEF 2010).

Groundwater is abstracted for domestic use and livestock watering through hand dug wells (in the shallow aquifer) and boreholes.

In the Lake Chad basin Quaternary aquifer there are high nitrate concentrations linked to agriculture discharges and faecal contamination. High fluoride concentrations are also seen. Groundwater in parts of the northern Kanem region contains high concentrations of sodium carbonates and sodium sulphates.

Sedimentary - Intergranular Flow

Named Aquifers Period General Description Water quality
Continental Terminal of the Lake Chad Basin Tertiary The alternating sandstones and clay rich bands of the Continental Terminal underlie the Quaternary aquifer of the Lake Chad Basin, sometimes at depths of up to 400 to 600 m. It is artesian, confined by overlying clay bands. Isotopic studies suggest that groundwater in the Continental Terminal aquifer is at least 20,000 years old and is not replenished by modern recharge (GWP 2013).

Groundwater is abstracted for drinking water in rural and urban areas, but extraction costs are considerable.

High fluoride concentrations in groundwater are seen.


Sedimentary - Intergranular & Fracture Flow

Named Aquifers Period General Description Water quality
Continental Intercalaire 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.
Nubian Sandstone Mesozoic - Palaeozoic The Nubian Sandstone aquifer in the Kufra basin is up to 1000 m thick and dominated by sandstones.

Sedimentary - Fracture Flow

Named Aquifers Period General Description Water quality

Basement

Named Aquifers Period General Description Water quality
Weathered basement, often in hydraulic continuity with overlying Quaternary alluvial sands 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.


Groundwater Status

Groundwater quantity

Groundwater quality

Groundwater use and management

Groundwater use

Many rural areas, particularly in the drier north, depend on groundwater from shallow hand dug wells and boreholes for domestic and in some cases, also for livestock use in dry seasons. Groundwater is also used for small scale irrigation in the north.

Groundwater also provides a large proportion of water supply in many urban centres, including N'Djamena (United Nations 1988).

A small number of industrial users abstract groundwater, including for textile manufacture, sugar refining, cotton milling and brewing.

Groundwater management and monitoring

There is a national database of water points stored in the Direction de Nationale de l’Hydraulique, which has information on more than 10000 water points across Chad, with various amounts of information on each (UNICEF 2010).

Information from the water point database was used by UNICEF (2010) to produce a map indicating rest water level (depth to groundwater).

Transboundary aquifers

The Lake Chad Basin aquifer is shared between eight countries, including Chad, Niger, Nigeria and Cameroon. The Lake Chad Basin Commission (LCBC), based on the “Convention Relating to the Development of the Chad Basin” signed in 1964, was created in order to regulate the management of transboundary natural resources. Cameroon, Niger, Nigeria, Chad and the Central African Republic are members of the LCBC, and Sudan was granted membership in 2000 but has not yet ratified the convention. The commission is responsible for regulating and controlling water and natural resources uses in the basin, including groundwater management. To date, few groundwater programs have been implemented, but the LCBC has recognised groundwater importance and made efforts to fully include groundwater in its activities (GWP 2013).


The Nubian Sandstone Aquifer


For further information about transboundary aquifers, please see the Transboundary aquifers resources page


References

The following references provide more information on the geology and hydrogeology of Chad.

These, and others, can be accessed through the Africa Groundwater Literature Archive


Alker M. 2008. The Lake Chad Basin Aquifer System: a case study for the research project 'Transboundary groundwater management in Africa'. German Development Institute

GWP (Global Water Partnership). 2013. The Lake Chad Basin Aquifer System. Global Water Partnership Transboundary Groundwater Fact Sheet, compiled by F Bontemps.

Ngatcha BN, Mudry J and Leduc C. 2008. The state of understanding on groundwater recharge for the sustainable management of transboundary aquifer in the Lake Chad Basin. Presentation at 4th International Symposium on Transboundary Water Management, Thessaloniki, Greece, 15-18 October 2008

Scheider JL and Wolff JP. 1969. Carte Hydrogeologique de la Republique du Tchad; Scale 1:500 000. Bureau de Recherches Geologiques et Minieres (BRGM), Orleans.

United Nations. 1988. Groundwater in North and West Africa: Chad. United Nations Department of Technical Cooperation for Development and Economic Commission for Africa, Natural Resources/Water Series No. 18, ST/TCD/5.

UNICEF. 2010. Republic of Chad: feasibility study for manual drilling: mapping of favourable zones. Ministere de l'Environnement, de l'Eau et des Ressources Halieutiques. Practica/Enterprise Works/VITA/UNICEF


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