Hydrogeology of Cote d'Ivoire

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Africa Groundwater Atlas >> Hydrogeology by country >> Hydrogeology of Côte d'Ivoire

Most of the textual information on this page was taken from the chapter on Côte d'Ivoire in the report‘Groundwater in North and West Africa’ (UN 1988). Some of this information is outdated. If you have more recent information on the hydrogeology of Cote d'Ivoire, please get in touch.


Compilers

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

Geographical Setting

Côte d'Ivoire is in west Africa with a southern coast on the Gulf of Guinea. The country is largely flat, less than 500 m, with local relief. The west of the country is more hilly, with parts rising above 500 m, to a maximum of about 1200 m elevation. The south has many low lying areas and a few isolated or groups of low hills rising to around 650 m elevation.

Map of Côte d'Ivoire (For more information on the datasets used in the map see the geography resources section)

General

Estimated Population in 2013* 20,316,086
Rural Population (% of total) (2013)* 47.2%
Total Surface Area* 318,000 sq km
Agricultural Land (% of total area) (2012)* 64.8%
Capital City Yamoussoukro
Region Western Africa
Border Countries Liberia, Guinea, Mali, Burkina Faso, Ghana
Annual Freshwater Withdrawal (2013)* 1549 Million cubic metres
Annual Freshwater Withdrawal for Agriculture (2013)* 38.4%
Annual Freshwater Withdrawal for Domestic Use (2013)* 41.1%
Annual Freshwater Withdrawal for Industry (2013)* 20.5%
Rural Population with Access to Improved Water Source (2012)* 67.8%
Urban Population with Access to Improved Water Source (2012)* 91.5%

* Source: World Bank


Climate

The southern coastal zone has an equatorial climate with the highest rainfall, at around 2,400 mm/year, and rainfall is fairly evenly distributed throughout the year. The centre of the country has a tropical climate, with average annual rainfall of around 1,000 to 1,400 mm, and two wet seasons: the main one starting in December and the smaller one from August to October. The north is drier, with average annual rainfall betweewn 400 and 1,000 mm, concentrated in a short rainy season from July to September.

Average monthly precipitation for Côte d'Ivoire showing minimum and maximum (light blue), 25th and 75th percentile (blue), and median (dark blue) rainfall Average monthly temperature for Côte d'Ivoire 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

There are four main rivers in Côte d'Ivoire: the Cavally, Sassandra, Bandama and Comoé, all of which flow from north to south, into the Atlantic Ocean. There are also several smaller coastal rivers, which also generally flow from north to south into the Atlantic, but some discharge into lagoons before they reach the coast. In the north, there are several tributaries of the Niger and the Black Volta rivers. The Niger tributaries flow northwards towards Mali.

The highest river flows occur from April to October, with lowest flows in January and February.

There are a number of dams on some of the major rivers, related to hydroelectric schemes.


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

Soil

Soil Map of Côte d'Ivoire (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 equatorial with forest; the centre is dominated by tropical forest; and the north is dominated by savannah vegetation.

File:Côte d'Ivoire LandCover.png
Land Cover Map of Côte d'Ivoire (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 Côte d'Ivoire. More information is available in the report ‘Groundwater in North and West Africa: Côte d'Ivoire’ (UN 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).

CotedIvoire Geology2.png


Summary

A narrow strip along the southern coast forms the coastal basin. Here, sedimentary rocks of Cretaceous-Tertiary age and unconsolidated Quaternary sediments form a wedge which narrows towards the north, where it terminates at the edge of the basement rock, less than 35 km from the coast. The basin is crossed from west to east by a fault with a displacement of several thousand metres, which separates two distinct zones:

- to the north is a shallower basin where the sedimentary infill is not usually more than 300 m thick

- to the south is a deep basin, where the sedimentary infill reaches 4,000 to 5,000 m thick.

There is a series of lagoons, part of an ancient hydrographic system, which have very thick mud deposits and are now submerged.

Across the rest of the country are basement rocks.


Geological Environments
Key Formations Period Lithology
Coastal sedimentary basin
Unconsolidated coastal sediments Quaternary The upper part of the sedimentary basin infill, especially in the 'low plateau' areas south of the lagoons, close to the shore, are Quaternary formations with a sand-clay continental formation about 20 m thick overlying a series of marine clays interbedded with sands, together about 10 m thick.
Continental Terminal Cretaceous-Tertiary The lower part of the sedimentary infill in the coastal basin comprises detrital continental Tertiary (Miocene-Pliocene) formations - mostly coarse sands - and, in the deepest areas, Upper and Middle Cretaceous marine formations, mostly sandstone, including ferruginous sandstone, and clays. Together, these form the Continental Terminal formation (sometimes, the Cretaceous is treated as a separate aquifer).
Basement (West African Shield)
Birrimian Upper and Middle Precambrian Metamorphic rocks, mainly of sedimentary origin. These are generally schists, with meta-sandstones and meta-conglomerates in places. In places there are granitic intrusions, accompanied by volcano-sedimentary formations of tuffs, breccias, and green rocks. These form bands running from north-northeast to south-southwest, and are heavily folded, sometimes vertical.
Prebirrimian Lower Precambrian The oldest basement formations include crystalline gneiss, granites and migmatites. They are most common in the west of the country (although migmatites are found everywhere).


Hydrogeology

This section provides a summary of the hydrogeology of the main aquifers in Côte d'Ivoire. More information is available in the report [ADD LINK UN] (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).

CotedIvoire Hydrogeology2.png

Note: the term Continental Terminal is sometimes used to describe the whole of the coastal basin aquifer, from Cretaceous to Quaternary, and sometimes to describe only the Quaternary, or only the Tertiary, or both the Cretaceous and Tertiary formations. Here, it is used to describe the Cretaceous-Tertiary aquifer in the coastal basin that underlies the Quaternary aquifer. The Quaternary aquifer is described separately, highlighting its unconsolidated nature and high vulnerability.


Unconsolidated

Named Aquifers Period General Description Water quality
Coastal basin Quaternary The southern part of the coastal basin consists of Quaternary formations, including coastal and marine sands. In their upper part, these form a moderately to highly productive aquifer, capable of sustaining borehole yields of at least 5 to 50 m³/hour. In some places with very permeable coarse sands, a borehole yield of at least 210 m³/hour for a drawdown of 3.1 m was recorded (a specific capacity of at least 1600 m³/day/m) (an underlying clay layer protects the aquifer from intrusion by the underlying salt water).

The aquifer is recharged by direct infiltration of rainwater, and recharge is high, with effective infiltration of 50 percent of rainfall.

Fresh water lies in density equilibrium on a layer (or wedge) of salt water, or on impermeable strata.

Where there is no seawater intrusion, groundwater in the upper layers is fresh. The salt water at depth has salinity ranging from 100 to 2500 mg/l.


Sedimentary - Intergranular & Fracture Flow

Named Aquifers Period General Description Water quality
Continental Terminal - coastal basin Cretaceous-Tertiary This aquifer is buried below the shallow Quaternary aquifer in the coastal basin, and they may be in hydraulic continuity. Groundwater flow is dominantly intergranular, and the aquifer is likely to be largely unconfined. Transmissivity is high, with values of up to and possibly more than 10,000 m²/day, and storage coefficients of 10 to 15 percent. Sustainable borehole yields may be on the order of 10 to 200 m³/h. Groundwater generally has low mineral content, with dry residue usually between 40 and 150 mg/l; calcium and magnesium of 1 to 4 mg/l; bicarbonate of 3 to 6 mg/l; and chloride of 7 to 14 mg/l. It is almost always acidic, with pH close to 4.5.

Basement

Named Aquifers Period General Description Water quality
Birrimian aquifers Upper and Middle Precambrian Most evidence on the groundwater potential of these aquifers is from the 'cocoa belt'. Near-surface weathering of these rocks typically produces argillaceous, clayey sediments, which have low permeability and do not contain much groundwater. The thickness of the weathered zone depends on the lithology of the rocks: in schists it is 45 m thick on average, but where schists are interbedded with sandstone or quartz, it does not exceed 30 m. Where the weathered zone is more than 40 m thick, there is typically very little groundwater; and where it is more than 60 m thick, groundwater is virtually non-existent.

Groundwater is preferentially found in fractures in quartz seams, or in volcanic layers, and are more common where sandstone or quartz strata are present. In most cases, boreholes drilled in sands are between 15 to 20 and 35 to 40 m deep, and groundwater is usually found at a depths of 10 to 20 m below soil level. There is little groundwater at depths of below about 30 m. Specific capacity values are usually between 24 and 72 m³/day/m (a yield of 1 to 3 m³/hour per metre of drawdown).

The argillaceous near-surface material must be cased out in boreholes to prevent fine particles blocking well screens.

Groundwater has moderate mineral content, with dry residues of between 250 and 500 mg/l, sometimes up to 650 mg/l, and a pH close to 7 (neutral).
Granitogneiss aquifers Lower Precambrian In the quartz grained granitogneiss rocks, groundwater occurs in shallow weathered zones, where the rock has weathered to sand and gravel; and in deeper fractures in unweathered bedrock.

The weathered zone is 20 to 30 m thick on average, but sometimes as much as 60 m. Transmissivity values for this zone are in the order of 8 to 85 m²/day. Generally speaking, the most weathered (the most sandy and gravelly) zones, which have the highest permeability, are found at the base of the weathered zone.

At the surface, there are usually sand-clay layers. Unweathered bedrock at the ground surface is rare.

Annual fluctuations in groundwater level vary from a few metres to 6-10 m, with the smallest fluctuations in areas of low elevation.

Groundwater has moderate mineral content, with dry residues of between 250 and 500 mg/l, sometimes up to 650 mg/l, and a pH close to 7 (neutral).


Groundwater Status

Groundwater in the coastal Quaternary and Continental Terminal aquifers is highly vulnerable to contamination, both by seawater intrusion and by pollution from urban industry and domestic waste. Many studies have been done of the risks to and effects on groundwater, particularly in the Abidjan area (e.g. Douagui et al. 2012, Kouassi et al. 2013 and Issiaka et al. 2006).

Seawater intrusion may be caused by falling groundwater levels, which themselves may be linked to intensive groundwater pumping and/or to rainfall variability (Oga et al. 2006).

Groundwater use and management

Groundwater from the Quaternary aquifer is the main source of drinking water supply in Abidjan and most of the rest of the coastal zone. Groundwater from shallow boreholes and hand dug wells in the basement aquifer is the main source of water in rural areas across the rest of the country.

At least 13,000 groundwater sources had been recorded in the country in 2008, mostly boreholes, with hand dug wells the next numerous.


References

The following references provide more information on the geology and hydrogeology of Cote d'Ivoire.

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

Douagui et al. 2012. Assessment of the bacteriological quality and nitrate pollution risk of Quaternary groundwater in the southern part of Abidjan District (Côte d’Ivoire). Journal of Hydro-environment Research, Vol.6, Issue 3, pp 227-238

Issiaka et al. 2006. Vulnerability assessment of the Abidjan Quaternary aquifer using the DRASTIC method. In: Groundwater quality in Africa, ed. Y Xu and B Usher, Taylor and Francis/Balkema, Rotterdam, pp 115-124

Kouassi et al. 2013. Modeling of Quaternary Groundwater Pollution Risk by GIS and Multicriteria Analysis in the Southern Part of Abidjan District (Côte d’Ivoire). Journal of Hydro-environment Research, Vol.6, Issue 3, pp 227-238.

Oga MS, Marlin C, Dever L, Filly A and Njitchoua R. 2008 . Hydrochemical and isotopic characteristics of coastal Groundwater near Abidjan. In: Adelana and MacDonald (eds) Applied Groundwater Studies in Africa, pp 371-389.

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


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