Hydrogeology of Gabon

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

Authors

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

Most of the information on this page was taken from the chapter on Gabon in the report ‘Groundwater in Eastern, Central and Southern Africa’ (UN 1989).

Geographical Setting

Gabon lies on the equator. It has a coastline of about 800 km in the west, onto the Gulf of Guinea. Much of the country is quite flat, including plains and plateaus of the coastal basin, which are all less than 250 m elevation. Inland are some higher plateaus, including a chain of mountains running northwest-southeast that rise to nearly 1000 m; crystalline plateaus in the northeast; and the Bateke plateaus in the southeast.

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


General

Estimated Population in 2013* 1,671,700
Rural Population (% of total) (2013)* 13.3%
Total Surface Area* 257,670 sq km
Agricultural Land (% of total area) (2012)* 20.0%
Capital City Libreville
Region Central Africa
Border Countries Equatorial Guinea, Cameroon, the Republic of the Congo
Annual Freshwater Withdrawal (2013)* 139.1 Million cubic metres
Annual Freshwater Withdrawal for Agriculture (2013)* 29.0%
Annual Freshwater Withdrawal for Domestic Use (2013)* 60.9%
Annual Freshwater Withdrawal for Industry (2013)* 10.1%
Rural Population with Access to Improved Water Source (2012)* 63%
Urban Population with Access to Improved Water Source (2012)* 96.8%

* Source: World Bank


Climate

Gabon has a wet equatorial climate. Annual rainfall is highest in the northwest, at more than 3000 mm, falling towards the east and southeast to around 1500 mm. There are two rainy seasons, from September to mid December, and mid February to May, separated by a long dry season (June to September) and a short dry season (December to February).

Temperatures are high throughout the year, declining slightly from the north towards the east and south.


Average monthly precipitation for Gabon showing minimum and maximum (light blue), 25th and 75th percentile (blue), and median (dark blue) rainfall Average monthly temperature for Gabon 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 main river in Gabon is the Ogooué, which drains over 70% of the country, as well as parts of Cameroon and the Republic of the Congo. Its main tributaries are the Ivindo, in northeast Gabon, and the Ngounié. The second largest river is the Nyanga. There are also several coastal rivers, including the Mbéi and the Komo. There are a number of major dams on the larger rivers.

Most of the rivers are perennial. Monitoring of river flows shows relatively constant flow patterns throughout the year.

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

Soil

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

Land cover

Much of the country is covered by equatorial forest.
Land Cover Map of Gabon (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 Gabon. More detail can be found in the references listed at the bottom of this page. Many of these references can be accessed through the Africa Groundwater Literature Archive.

The geology map on this page shows a simplified version of the geology at a national scale (see the Geology resources page for more details).

Gabon Geology.png


Geological Environments
Key Formations Period Lithology
Proterozoic sedimentary sequence
Unconsolidated sediments Quaternary These sediments include coastal sand belts and fluvial alluvium in river valleys.
Coastal sedimentary basin Upper Palaeozoic to Mid Cretaceous (interior basin); Cretaceous to Tertiary (Atlantic/western basin) This basin is divided in two by the Lambarené crystalline horst structure.

The interior synclinal basin (200 x 60 km) contains up to 3500 m thickness of older fluvio-lacustrine and lagoonal continental formations: sandstone, marl, clay, shale and conglomerate.

The Atlantic, or western, basin (500 x 15 to 200 km) contains up to 8000 m of younger marine formations: carbonate-rich sandstone, argillite and detrital deposits.

Bateké plateau Tertiary This plateau in southeast Gabon consists of continental sandstones and sands that unconformably overlie Precambrian Francevillian formations.
Precambrian meta-sedimentary sequence
Mpioka, schist-limestone, Louila and Bouenza series Upper Precambrian These series occur within the Nyanga syncline.

The Mpioka series forms the core of the Nyanga syncline and includes schists, sandstone schists and argillites.

The schist-limestone series occurs in the plain within the circumfrence of the Nyanga syncline, and consists of limestones, dolomites and marls with frequent karstic features. There is a lateritic cover which can be up to 30 m thick.

The Louila and Bouenza series crop out at the edge of the schist-limestone series, the Louila on the west and the Bouenza on the east. They include sandstones and quartzitic sandstones, schists and argillites, and intermediate rocks.

Francivillian system (also Mayombian and Ogooué systems) Middle Precambrian The Francivillain system in southeast Gabon includes five formations, which contain deposits of detrital uranium (conglomerate, sandstone, pelite, jasper and pyroclastic deposits).

The Ogooué system in central Gabon contains volcano-sedimentary and metamorphic formations.

The Mayombian system in southwest Gabon contains conglomerate, arkose, quartzite and carbonate-rich schist. This middle series at the eastern edge of the Lower Precambrian Mayombe range (see below) also contains meta-sedimentary rocks.

Basement
Crystalline basement Lower Precambrian This system comprises the North Gabon massif, the Chaillu massif and the Mayombe range. They comprise largely granites.

The Mayombe range contains migmatites, granitogneiss, quartzites, mica schists and complex metamorphic rocks.


Hydrogeology

This section provides a summary of the hydrogeology of the main aquifers in Gabon. More information is available in the references listed at the bottom of this page. Many of these references can be accessed through the Africa Groundwater Literature Archive.

The hydrogeology map on this page 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).

Gabon Hydrogeology.png Hydrogeology Key.png


Summary

The Atlantic coastal basin, Bateké plateau and Upper Precambrian meta-sedimentary sequence form important aquifers in Gabon. Within the metamorphic and granitic Middle and Lower Precambrian basement rocks, there are only small, isolated aquifers.

The upper layers of the hard rocks are generally weathered and altered, mostly to laterite, to depths of up to 30 m (thickest in the limestone series), and possibly thicker in granite areas. These altered layers, in particular laterites, form low productivity shallow aquifers that feed perennial springs, but which have very low flows in the dry season. With a typically low mineral content, these shallow groundwater resources are widely used nationally for traditional supplies, but they typically have high sediment content and high vulnerability to contamination, and are not widely targeted for improved supplies.


Unconsolidated

Named Aquifers General Description
Coastal sedimentary basin Quaternary The coastal sand belts form an aquifer recharged by
Coastal sedimentary basin Tertiary: Upper Miocene to Pliocene

Sedimentary - Intergranular Flow

Named Aquifers General Description Water quantity issues Water quality issues Recharge

Sedimentary - Intergranular & Fracture Flow

Named Aquifers General Description Water quantity issues Water quality issues Recharge

Sedimentary - Fracture Flow

Named Aquifers General Description Water quantity issues Water quality issues Recharge

Basement

Named Aquifers General Description Water quantity issues Water quality issues Recharge


Groundwater Status

Groundwater quantity

Groundwater quality

Groundwater use and management

Groundwater use

Groundwater management

Groundwater monitoring

Transboundary aquifers

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

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

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