Hydrogeology of Republic of Congo

Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of the Republic of the Congo

Most of the textual information on this page was taken from the chapter on the Congo in the report ‘Groundwater in Eastern, Central and Southern Africa’ (UN 1989). This information is outdated. If you have more recent information on the hydrogeology of the Republic of the Congo, please get in touch.

Compilers

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

Geographical Setting

The Republic of the Congo lies on the equator. Parts of the country are relatively low lying plain areas, including the Congo Basin, the Niari valley and the coastal plain. Hills and plateaus surround the Congo Basin, and occur in other areas including the northwest and the central part of the country (including the Bateké plateaus), rising to over 800 m. The Mayombe range of mountains and the Chaillu massif also rise to over 800 m.

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

General

Estimated Population in 2013*	4,447,632
Rural Population (% of total) (2013)*	35.5%
Total Surface Area*	341,500 sq km
Agricultural Land (% of total area) (2012)*	31.0%
Capital City	Brazzaville
Region	Central Africa
Border Countries	Gabon, Cameroon, Central African Republic, Democratic Republic of the Congo
Annual Freshwater Withdrawal (2013)*	46 Million cubic metres
Annual Freshwater Withdrawal for Agriculture (2013)*	8.7%
Annual Freshwater Withdrawal for Domestic Use (2013)*	69.6%
Annual Freshwater Withdrawal for Industry (2013)*	21.7%
Rural Population with Access to Improved Water Source (2012)*	38.8%
Urban Population with Access to Improved Water Source (2012)*	95.7%

* Source: World Bank

Climate

In the north of the country the climate is equatorial and rain occurs throughout the year, with two slightly less rainy periods from December to February and in July. The centre of the country is a sub-equatorial zone, with two distinct wetter seasons in March/April and October/November. The southwest is a wet tropical zone, with a rainy season that lasts from about October to May, and a dry season from June to September. Total rainfall everywhere is relatively high, at least 1000 mm/year and up to 3000 mm/year in the wettest zones.

Effective precipitation is also high, estimated for the years 1955 to 1970 at between 750 and 1250 mm/year in the coastal basin; 200 to 750 mm/year in the Niari valley and the Congo Basin; and 1000 to 1250 mm/year over the central plateaus.

• 
  Koppen Geiger Climate Zones
• 
  Average Annual Precipitation
• 
  Average Temperature

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

Surface water

There are many major rivers in the Republic of the Congo, in two main basins: the Congo River in the centre and north (which extends over several countries), and the Kouilou-Niari in the southwest. There are also small coastal basins, and the upper part of the Nyanga and Ogouué basins. Most are perennial, owing to the high rainfall and groundwater baseflow.

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

Soil

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

Land cover

Land Cover Map of the Republic of the Congo (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 Republic of the Congo. More information is available in the report Groundwater in Eastern, Central and Southern Africa: Congo (1989) (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).

Geological Environments

Key Formations	Period	Lithology
Quaternary unconsolidated
River alluvium and coastal deposits	Quaternary	Alluvium sands and silts cover immense areas of swamp in the Congo Basin and are up to 400 m thick. Smaller areas of coastal sands occur along the coastal plateau.
Cenozoic sedimentary
	Cenozoic	Found in the coastal plain and the central Bateké plateaus. In the coastal plain, a series of sedimentary rocks rests on Lower Precambrian Mayombe series. From youngest to oldest, they include sandy to sandy-argillaceous rocks (50 to 200 m); sandy clays and argillaceous sands (up to 150 m); coarge grained ferruginous sands (up to 100 m); dolomitic sandstones and clays; and dolomitic limestones (60 to 400 m). The Bateké plateaus series comprises around 300 m of sandstones overlain by 40 to 90 m of sandy silts.
Mesozoic sedimentary
	Mesozoic	Seen in very small areas along the banks of the Congo River and its tributaries in Pool and in northern Likouala.
Precambrian metasedimentary, metamorphic and granitic
Inkisi series; Mpioka series; schist-limestone series in Djoué valley; Bouenza and Louila series	Upper Precambrian	Meta-sedimentary formations, including: - the Inkisi series southwest of Brazzaville (600 m thick), including conglomerates, arkoses and sandstones; - the Mpioka series in the Chaillu massif, including conglomerates and sandstones, often calcareous; - a schist-limestone series in the Djoué valley in the southeast (700 to 1000 m thick), comprising mainly carbonate rocks - dolomites, marly limestones, marls and oolitic and stromatalitic limestones, with some sandstones at the top; - a tillite series in the Lower Congo basin; - the Bouenza series, cropping out on the northeast slope of the Niari syncline and resting directly on the Chaillu granitic complex, which comprises argillites, feldspathic sandstones, limestones, marls and calcareous sandstones; and - the Louila series, the counterpart of the Bouenza series in the southwest of the Niari syncline, cropping out only in the Mayombe mountains between the upper and lower tillite series. Between 650 and 1000 m thick, it includes clays, argilites, sandstones and marls, and marly limestones.
Granite and gneiss, schists, quartzites	Middle and Lower Precambrian	Granitic and gneissic basement rocks are seen in the Chaillu massif and in western Sangha. Metamorphic, deformed formations seen in the Mayombe mountains and the Niari valley. The metamorphic rocks include schists and metamorphosed sandstones (quartzites) and dolomites.

Hydrogeology

This section provides a summary of the hydrogeology of the main aquifers in the Republic of the Congo. More information is available in the report Groundwater in Eastern, Central and Southern Africa: Congo (1989) (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 map of water resources in the Republic of Congo and Gabon at 1:1,000,000 scale was published by BRGM/CIEH (1982).

Unconsolidated

Named Aquifers	Period	General Description	Water quality
Congo Basin Alluvium	Quaternary	These deposits cover an immense area and are thought to be up to 400 m thick. They are likely to be relatively highly permeable and to store a large volume of groundwater.	Thought to generally have a low mineral content and a pH of less than 6.5.

Sedimentary - Intergranular Flow

Named Aquifers	Period	General Description	Water quality
Coastal basin	Cenozoic	A series of sands (variously clay-rich or argillaceous), clays, dolomitic sandstones and dolomitic limestones. Many potash, oil and water exploration boreholes have been drilled to depths of between 250 and 400 m. Recorded yields from borehole tests were generally between 10 and 20 m³/hour.	Overall the groundwater has a low mineral content, below 250 mg/l, with a relatively low hardness and a pH of close to 7. However, chloride and sulphate can be high, varying between 2 and 80 mg/l, and there is relatively high salinity in tested boreholes at depths of 250 to 400 m, generally between 0.6 and 3.5 g/l (NaCl equivalent).
Stanley Pool series	Cenozoic	Located at Brazzaville, this is a dominantly sandstone series, sometimes marly or kaolinitic.	Thought to generally a low mineral content and a pH of less than 6.
Bateké plateau series	Cenozoic	A few shallow (e.g. 20 to 30 m) boreholes are used for water supply in some areas. One deep (300 m) borehole drilled in 1960 provided a yield of abotu 6 m³/hour for 20 to 25 m of drawdown (a specific capacity of about 6 to 7 m³/day/m). The highest permeabilities are thought to be in sandy series, with values of between about 5 and 60 m²/day. Low productivity perched aquifers are known, from 3 to 70 m depth, which sometimes show small spring flows.	Generally a low mineral content and a pH of between 5.3 and 6.

Sedimentary - Fracture Flow

Named Aquifers	Period	General Description	Water quality
Inkisi series; Mpioka series; schist-limestone series in Djoué valley; Bouenza and Louila series	Upper Precambrian	Drilling tests indicated moderate to high permeability. Borehole yields in similar formations in Gabon are between 0.5 and 24 m³/hour. A borehole at Loudima yielded 16 m³/hour with hardly any drawdown. Other boreholes between 20 and 70 m deep had rest water levels of between 6 and 35 m below ground level, and specific capacity values of 0.2 to 432 m³/day/m.	Groundwater in the schist-limestone formations has an average mineral content of 350 to 500 mg/l, and is of average hardness, with a pH of 7 or higher. Other formations, where tested, generally have a low mineral content and a pH of 6 to 6.5.

Basement

Named Aquifers	Period	General Description	Water quality
Granite and gneiss, schists, quartzites	Middle and Lower Precambrian	Generally low permeability and groundwater potential.	Typically low mineral content, with a pH of around 6.

Groundwater Status

Recharge to aquifers in the Republic of the Congo is likely to be high, because of high rainfall and the widespread distribution of relatively highly permeable aquifers.

Groundwater use and management

Groundwater use

The abundance of surface water in the country means that groundwater is not likely to be very heavily exploited. The town of Pointe-Noire is supplied by groundwater from at least three boreholes. Private industrial groundwater supplies are also used in Pointe-Noire. Programmes of borehole drilling for village water supplies were carried out in the 1980s, including the a FRG-KFW project which constructed 80 boreholes with hand pumps in the Niari region.

Groundwater management

In the 1980s, several government agencies were involved in water supply, including the Ministry of Planning (responsible for coordinating drinking water supply); the Department of Agricultural Engineering and Farm Machinery (DGRMA) (responsible for hydro-agricultural development, and also involved in village water supply drilling projects); the National Water and Energy Corporation (SNDE) (responsible for urban water supply); and the Ministry of Energy and Water Supply (established in 1984 and responsible for coordinating the study, exploitation and management of the country's water resources).

Until the late 1980s at least, there had been only a few groundwater studies, including the development of a groundwater supply for the town of Pointe-Noire in the 1950s; a number of local drinking and industrial water supply projects; some studies related to dam construction; and a water resources planning map, with accompanying report of Gabon and Congo published by BRGM and the Comite Interafricain d'Etudes Hydrauliques (CIEH).

References

The following references provide more information on the geology and hydrogeology of the Republic of the Congo. They can also be accessed through the Africa Groundwater Literature Archive

CIEH/BRGM. 1982. Carte de Planification des Ressources en Eau du Gabon et du Congo; scale: 1 : 1 000 000. Comité Interafricain d'Etudes Hydrauliques, Ouagadougou & Bureau de Recherches Géologiques et Minières (BRGM), Orléans

CIEH/BRGM. 1982. Notice explicative de la carte de planification des ressources en eau du Gabon et du Congo. Serie hydrogeologie de Comite Interafricain d'Etudes Hydrauliques (CIEH), 116 pp, Ouagadougou, Burkina Faso.

United Nations. 1989. Groundwater in Eastern, Central and Southern Africa: Congo. United Nations Department of Technical Cooperation for Development, Natural Resources/Water Series No.19

The reference list in the UN report provides other sources of information, although none are more recent than 1982.

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