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 This information is outdated.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

Please cite this page as: Upton & Ó Dochartaigh, 2016.

Bibliographic reference: Upton, K. & Ó Dochartaigh, B.É. 2016. Africa Groundwater Atlas: Hydrogeology of the Republic of the Congo. British Geological Survey. Accessed [date you accessed the information]. http://earthwise.bgs.ac.uk/index.php/Hydrogeology_of_Republic_of_Congo

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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.

The Republic of the Congo. Map developed from USGS GTOPOPO30; GADM global administrative areas; and UN Revision of World Urbanization Prospects. For more information on the map development and datasets see the geography resource page.

General

Capital city	Brazzaville
Region	Central Africa
Border countries	Gabon, Cameroon, Central African Republic, Democratic Republic of the Congo
Total surface area*	342,000 km2 (34,200,000 ha)
Total population (2015)*	4,620,000
Rural population (2015)*	1,566,000 (34%)
Urban population (2015)*	3,054,000 (%)
UN Human Development Index (HDI) [highest = 1] (2014)*	0.5906

* Source: FAO Aquastat

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.

More information on average rainfall and temperature for each of the climate zones in Republic of Congo can be seen at the Republic of Congo climate page.

These maps and graphs were developed from the CRU TS 3.21 dataset produced by the Climatic Research Unit at the University of East Anglia, UK. For more information see the climate resource page.

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.

Major surface water features of the Republic of the Congo. Map developed from World Wildlife Fund HydroSHEDS; Digital Chart of the World drainage; and FAO Inland Water Bodies. For more information on the map development and datasets see the surface water resource page.

Soil

Soil Map of the Republic of the Congo, from the European Commission Joint Research Centre: European Soil Portal. For more information on the map see the soil resource page.

Land cover

Land Cover Map of the Republic of the Congo, from the European Space Agency GlobCover 2.3, 2009. For more information on the map see the land cover resource page. 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 overview of the geology at a national scale (see the Geology resources page for more details). Geology of the Republic of the Congo 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 resource page. 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. Basement: granite, 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 overview of the type and productivity of the main aquifers at a national scale (see the Hydrogeology map resource page for more details). Another map of water resources in the Republic of Congo and Gabon at 1:1,000,000 scale was published by BRGM/CIEH (1982), with an accompanying report. Hydrogeology of the Republic of the Congo at 1:5 million scale. For more information on how the map was developed see the Hydrogeology map resource page . 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. Return to the index pages Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of the Republic of the Congo