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Most urban areas received piped water supplies derived from treated surface water, but in urban areas not served by municipal water supplies, groundwater is often used, generally from shallow hand dug wells. IN rural areas, many areas also use surface water supplies, but shallow groundwater from hand dug wells and boreholes is also used.  
Most urban areas received piped water supplies derived from treated surface water, but in urban areas not served by municipal water supplies, groundwater is often used, generally from shallow hand dug wells. IN rural areas, many areas also use surface water supplies, but shallow groundwater from hand dug wells and boreholes is also used.  
A database of approximately 2600 groundwater points (boreholes and hand dug wells) was compiled by UNICEF (2010). These are mostly in the west of the country, in the regions of Ouham, Nana Gribizi, Lobaye, Ombella Mpoko, Kemo and Lower Kotto. Boreholes were drilled as part of several projects, mainly by international donor organisations. The average depth of boreholes drilled in dominantly sandy lithologies is about 25 m; and for those drilled in mixed clay-sandy deposits is about 60 m (UNICEF 2010).


=== Groundwater management===
=== Groundwater management===
Line 257: Line 254:
In the 1980s, the National Water Corporation (SNE) was reponsible for water supplies in urban areas, in partnership with the Ministry of Public Works and Urban Development; and in rural areas, the Department of Agricultural Engineering of the Ministry of Agriculture and Livestock was responsible for developing and maintaining water points.  
In the 1980s, the National Water Corporation (SNE) was reponsible for water supplies in urban areas, in partnership with the Ministry of Public Works and Urban Development; and in rural areas, the Department of Agricultural Engineering of the Ministry of Agriculture and Livestock was responsible for developing and maintaining water points.  


There are limited data on the existence, locations and characteristics of groundwater sources, including boreholes and hand dug wells. UNICEF (2010) compiled a database of approximately 2600 groundwater points (boreholes and hand dug wells) from available information from government and NGO sources; however, there is limited information on the water points. No geological log information is available, and little information on groundwater quality. Some had information on rest water level. These water points are clustered in the west and south of the country; there is little information for the east and north.  
There are limited data on the existence, locations and characteristics of groundwater sources, including boreholes and hand dug wells. UNICEF (2010) compiled a database of approximately 2600 groundwater points (thought to be largely boreholes and hand dug wells, but information on water point type is often not available) from available information from government and NGO sources; however, there is limited information on the water points. No geological log information is available, and little information on groundwater quality. Some had information on rest (static) groundwater level. These water points are mostly clustered in the west and south of the country, in the regions of Ouham, Nana Gribizi, Lobaye, Ombella Mpoko, Kemo and Lower Kotto.  Boreholes were drilled as part of several projects, mainly by international donor organisations. The average depth of boreholes drilled in dominantly sandy lithologies is about 25 m; and for those drilled in mixed clay-sandy deposits is about 60 m (UNICEF 2010).
 


==References==
==References==

Revision as of 14:12, 28 September 2015

Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of the Central African Republic

The textual information on this page was taken from the publications listed in the reference section at the bottom of this page. If you have updated information on the hydrogeology of the Central African Republic, please get in touch.


Compilers

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

Geographical Setting

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

General

The Central African Republic has a low relief formed by erosion, with occasional inselbergs forming isolated higher areas of ground. In the northeast and northwest of the country are small two mountain ranges which rise to over 1,300 m. A ridge of high ground between the two mountain ranges forms a boundary between the Lake Chad basin to the north, and the Congo basin to the south.

Estimated Population in 2013* 4,616,417
Rural Population (% of total) (2013)* 60.5%
Total Surface Area* 622,980 sq km
Agricultural Land (% of total area) (2012)* 8.2%
Capital City Bangui
Region Central Africa
Border Countries Chad, Sudan, South Sudan, the Democratic Republic of the Congo, the Republic of the Congo, Cameroon
Annual Freshwater Withdrawal (2013)* 72.5 Million cubic metres
Annual Freshwater Withdrawal for Agriculture (2013)* 0.6%
Annual Freshwater Withdrawal for Domestic Use (2013)* 82.9%
Annual Freshwater Withdrawal for Industry (2013)* 16.6%
Rural Population with Access to Improved Water Source (2012)* 54.4%
Urban Population with Access to Improved Water Source (2012)* 89.6%

* Source: World Bank


Climate

The Central African Republic has a tropical to semi-arid climate, with average annual rainfall ranging from 1,700 mm in the south to 700 mm in the north. There are two wet seasons, the main one in August-September and the smaller one in May-June.

Average temperatures range from about 23 degrees in the south to 26 degrees in the north, with a larger seasonal variation in temperatures in the north. Calculated evaporation potential ranges from 1,500 mm/year in the south to over 1,900 mm/year in the north.


Average monthly precipitation for the Central African Republic showing minimum and maximum (light blue), 25th and 75th percentile (blue), and median (dark blue) rainfall Average monthly temperature for the Central African Republic 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

The north of the country is part of the Lake Chad basin, with sometimes ephemeral rivers draining to the Chari and the Logone, both of which flow into Chad and to Lake Chad. The south is part of the Congo basin, and the main rivers are the Obangui and the Sangha, and their tributaries, which are largely perennial.

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

Soil

Much of the country is covered by lateritic soils. Mapped soil types include plinthosols, ferrasols and arenosols, with some acrisols.

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

Land cover

The south of the country is dominated by evergreen tropical forest. Much of the central part of the country is dominated by decidous forest and grassland. In the north, the dominant land cover is mostly shrubland.

Land Cover Map of the Central African Republic (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 the Central African Republic. More information is available in the report ‘Groundwater in North and West Africa’ (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).

Another geology map is available in the report by Chirico et al.. 2010. Much of the information on the geology below was derived from this report.


Summary

Most of the country is underlain by crystalline metamorphic Precambrian basement rocks of the African shield.

Younger rocks cover only about a quarter of the country, mainly in the west and north, and smaller areas in the far south. They include small outcrops of Palaeozoic sandstones; Mesozoic sandstones formations; Tertiary sandstone formations; and Quaternary lacustrine and alluvial deposits.


Geological Environments
Key Formations Period Lithology
Quaternary
Lacustrine deposits, alluvium, laterite/ferricrete Quaternary Quaternary deposits are widespread in the Lake Chad basin in the north, comprising Neo-Chadian or Recent alluvium and lacustrine deposits with thick layers of recent clays (Chirico et al. 2010, United Nations 1988). Alluvium is also seen in river valleys across the country, and there are unconsolidated Quaternary deposits infilling depressions and swampy areas of the Sangha basin (United Nations 1988).

Tropical chemical weathering has created extensive lateritic and ferricretic crusts across the country, which in places may be up to 40 m thick (Chirico et al. 2010).

Phanerozoic sedimentary
Tertiary Bambio Sandstone (in the south); Continental Terminal formations (north) In the north are Continental Terminal type formations in the Lake Chad basin, which form small distinct plateaus, and comprise ferruginous sandstones, sands and cuirasses forming the edge of the Lake Chad basin (Chirico et al. 2010, United Nations 1988).

The Bambio Sandstone formation, comprising silicified sandstones, crops out in a small area in the southwest, overlying the Carnot Sandstone (Chirico et al. 2010, United Nations 1988).

probably Cretaceous Carnot and Mouka Ouadda Sandstone formations; small outcrops in north Mesozoic sedimentary rocks, probably mostly or all Cretaceous in age, form thick continental sequences resting in angular disconformity on the basement complex, in two outcrops that form distinct plateaus.

In the west are the Carnot Sandstones, which are dominantly fluvial in origin, with some lacustrine deposits. They comprise a succession of conglomerates, sandstones and mudstones which can reach 300 to 400 m in thickness (Censier 1990, Chirico et al. 2010).

In the east are the Mouka Ouddda Sandstones, also fluvial in origin, and generally less than 500 m thick. They comprise layers of sandstone and conglomerate (Chirico et al. 2010).

Mambéré and Kombélé formations Palaeozoic The two major Paleozoic formations are the Mambéré Formation in the west of the country, and the Kombélé Formation in the east, both of glacial origin (Censier et Lang 1992, Chirico et al. 2010). The Mambéré Formation is a tillite composed of both basal and flow tills as well as reworked glacial deposits derived from sandstone, conglomeratic sandstone, and siltstone that occur in continuous beds, lenses, and isolated blocks. The Kombélé Formation is a conglomeratic-sandstone-tillite composed of glacial outwash sediments. Both the Mambéré and the Kombélé formations generally range in thickness from 30 to 50 m and are unconformably overlain by Cretaceous sandstone formations. The total extent of the Palaezoic formations in unknown, because it is overlain by more recent formations (Chirico et al. 2010).
Crystalline Basement
Igneous intrusive Proterozoic-Cambrian Discontinuous basic intrusions, including large masses of batholithic granite (e.g. the Bouar plateau and Mt Yade) and doloritic veins (e.g. upstream of Bangui and Nola region) (United Nations 1988).
Proterozoic Not extensively metamorphosed. These rocks overlie the base complex and are subdivided into three units: an upper unit with a series of schists, quartzites, limestones and sandstones; a middle unit consisting mainly of carbonate formations with a glacial conglomerate at the base; and a lower unit represented by quartzites alternating with sericitic schists (United Nations 1988). The limsetones and other carbonate formations, which include dolomitic limestones and dolomites (UNICEF 2010) are significant for groundwater potential.
Granitic-gneissic basement Archean A base complex subdivided into two groups: an upper group formed of quartzites and schists; and a lower group formed of gneisses, micaschists, amphibolites, granulites, migmatites and anatectic granites (United Nations 1988).


Hydrogeology

This section provides a summary of the hydrogeology of the main aquifers in the Central African Republic. More information is available in the report ‘Groundwater in North and West Africa’ (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).


Unconsolidated (Intergranular Flow)

Named Aquifers Period General Description Water quality
Upper Lake Chad basin Quaternary - Tertiary The sedimentary sequence in the Lake Chad basin, in the north of the country, forms a series of aquifers at different depths. The upper layers are unconsolidated alluvial and lacustrine deposits, formed of sands and gravels, interbedded with low permeability silts and clays. The upper part is Quaternary; the lower Tertiary in age.

Aquifer productivity is controlled largely by lithology, with the sands, and particularly the gravels, usually highly permeable with high groundwater storage potential.

The uppermost aquifer layer is unconfined, but the clay and silt layers can act as confining layers, creating perched aquifers and artesian water levels.

Alluvial and lacustrine deposits Quaternary (possible Tertiary) In river valleys and depressions in the rest of the country are alluvial and lacustrine deposits, sometimes many tens of metres thick. Where they are dominated by sand, they can form local shallow aquifers. In other areas, such as in Alindao Bangui, although they exceed 150 m in thickness, they are dominated by clay, and have low permeability and do not form a productive aquifer (UNICEF 2010).


Sedimentary - Intergranular & Fracture Flow

Named Aquifers Period General Description Water quality
Lower Lake Chad basin - Continental Terminal Tertiary (possibly Cretaceous) Below the unconsolidated Quaternary and Tertiary deposits of the Lake Chad basin are consolidated Tertiary sandstones at depth, in which groundwater flow is largely intergranular. Below these, there may be Cretaceous sandstones, in which fracture flow may dominate, but little or no exploration at this depth has been done in the Central African Republic (UNICEF 2010).
Carnot and Mouka Ouadda Sandstone formations Cretaceous These formations are dominated by sandstones and are likely to have moderate to high permeability. This, combined with their thickness and areal extent, and the probability that they receive large volumes of recharge through direct rainfall infiltration, means they are likely to form a moderately to highly productive aquifer.

There are clay-rich layers interbedded with the sandstone, but information from drilling indicates that the aquifer is likely to be largely unconfined (UNICEF 2010).

Groundwater levels are generally 30 to 40 m below ground level (UNICEF 2010).


Sedimentary - Fracture Flow (Karst)

Named Aquifers Period General Description Water quality
Carbonate aquifers in basement Precambrian The Proterozoic (upper Precambrian) includes a series of not extensively metamorphosed sedimentary rocks, including limestones and other carbonate rocks. The extent of these formations is not well known: they are known or thought to occur at Bangui and along the Oubangui river south of Bangui; at and east of Bakouma; south of Kembé; in the Zémio-Djema area in the east of the country; and along the southern border (UNICEF 2010).

They are known to have developed karstic features - enlarged fractures - by chemical and mechanical weathering, which can lead to very high permeabilities. Karstic aquifers can be highly productive, although they can also be vulnerable to contamination, and to seasonal and longer term variability in rainfall.


Basement

Named Aquifers Period General Description Water quality
Crystalline metamorphic and meta-sedimentary rocks; igneous intrusive rocks Precambrian These generally form low productivity, local and discontinous aquifers, controlled by the presence of fracturing at depth, and the degree and nature of near-surface weathering, both of which increase the permeability of the largely impermeable un-altered basement rocks. Weathering is usually a few tens of metres thick. Typical sustainable yields from boreholes in basement aquifers are on the order of 300 to 1000 litres/hour. However, there are also reports of boreholes yielding more than 10 m³/hour (UNICEF 2010). The depth to groundwater level in the aquifers is typically 5 to 20 m (UNICEF 2010).

The aquifers are generally unconfined, and receive recharge mainly by direct rainfall infiltration. Where the basement aquifers are overlain by permeable Quaternary alluvium, groundwater in both aquifers may be in hydraulic continuity, and the basement aquifer may receive additional recharge from enhanced storage in the overlying alluvium.


Groundwater Status

Relatively little is known about the current status of groundwater resources of the Central African Republic. UNICEF (2010) collated some information on groundwater levels in different aquifers, but more information is needed in order to better assess groundwater resources, development potential and any negative impacts on groundwater. In particular, information from the drilling, testing and operation of water boreholes, in order to better understand the distribution and nature of the aquifers (their thickness - e.g. the thickness of the lateritic/weathered layer in basement areas; depth to groundwater level and groundwater flows in productive layers; and aquifer productivity).


Groundwater use and management

Groundwater use

Most urban areas received piped water supplies derived from treated surface water, but in urban areas not served by municipal water supplies, groundwater is often used, generally from shallow hand dug wells. IN rural areas, many areas also use surface water supplies, but shallow groundwater from hand dug wells and boreholes is also used.

Groundwater management

In the 1980s, the National Water Corporation (SNE) was reponsible for water supplies in urban areas, in partnership with the Ministry of Public Works and Urban Development; and in rural areas, the Department of Agricultural Engineering of the Ministry of Agriculture and Livestock was responsible for developing and maintaining water points.

There are limited data on the existence, locations and characteristics of groundwater sources, including boreholes and hand dug wells. UNICEF (2010) compiled a database of approximately 2600 groundwater points (thought to be largely boreholes and hand dug wells, but information on water point type is often not available) from available information from government and NGO sources; however, there is limited information on the water points. No geological log information is available, and little information on groundwater quality. Some had information on rest (static) groundwater level. These water points are mostly clustered in the west and south of the country, in the regions of Ouham, Nana Gribizi, Lobaye, Ombella Mpoko, Kemo and Lower Kotto. Boreholes were drilled as part of several projects, mainly by international donor organisations. The average depth of boreholes drilled in dominantly sandy lithologies is about 25 m; and for those drilled in mixed clay-sandy deposits is about 60 m (UNICEF 2010).

References

The following references provide more information on the geology and hydrogeology of the Central African Republic.

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

Censier C. 1990. Characteristics of Mesozoic fluvio-lacustrine formations of the western Central African Republic (Carnot Sandstones) by means of mineralogical and exoscopic analyses of detrital material. Journal of African Earth Sciences Vol.10, Issues 1-2, pp 385-398.

Censier C et Lang J. 1992. La formation glaciaire de la Mambéré (République Centrafricaine), Reconstitution paléogéographique et implications à l’échelle du Paléozoïque africain. Geologische Rundschau, Vol.81, No. 3, pp 769-789

Chirico PG, Barthélémy F and Ngbokoto FA. 2010. Alluvial Diamond Resource Potential and Production Capacity Assessment of the Central African Republic. Prepared in cooperation with the Bureau de Recherches Géologiques et Minières and the Direction Générale des Mines under the auspices of the U.S. Department of State; Scientific Investigations Report 2010–5043.

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

UNICEF. 2010. République Centraficaine: etude de faisabilite des forages manuels: identifaction des zones potentiellement favorables. Practica/Enterprise Works/VITA/UNICEF

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