Hydrogeology of Togo

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

Authors

Masamaéya Dadja-Toyou Gnazou, Lome University, Togo

Edem Bawoubadi Sabi, Lome University, Togo

Sani M. Tairou, Lome University, Togo

Wohou Akakpo, Department of Water Resources, Togo

Kpadja Agouda, Department of Water Resources, Togo

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

Geographical Setting

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

General

Estimated Population in 2013* 6,816,982
Rural Population (% of total)* 61%
Total Surface Area* 54.390 sq km
Agricultural Land (% of total area)* 71%
Capital City Lome
Region West Africa
Border Countries Benin, Burkina Faso, Ghana
Annual Freshwater Withdrawal (2013)* 169 Million cubic metres
Annual Freshwater Withdrawal for Agriculture* 45%
Annual Freshwater Withdrawal for Domestic Use* 53%
Annual Freshwater Withdrawal for Industry* 2%
Rural Population with Access to Improved Water Source* 40%
Urban Population with Access to Improved Water Source* 91%

* Source: World Bank


Climate

Togo is a narrow country extending from the border with Burkina Faso in the north to the Atlantic Ocean (Gulf of Guinea) in the south. The north of the country is dominated by savannah while the coast is generally a low lying plain with numerous lagoons and marshes. The Togo Mountains extend from the south west border with Ghana to the north east border with Benin, reaching a maximum elevation of over 900 m.


The climate of Togo is classified as Tropical Savannah. Annual average temperatures are slightly higher in the north and lower in the south. They also decrease with altitude in the mountainous regions. Precipitation is generally lower in the north and higher in the south but also increases slightly over the mountainous regions.



Precipitation and temperature vary throughout the year. Average temperature across the country shows two peaks throughout year in March and November. When averaged spatially across the country, precipitation shows a relatively wet period between April and October; however this is split into two distinct wet seasons in the south of the country, which occur during April-July and September-October.

Average monthly precipitation for Togo showing minimum and maximum (light blue), 25th and 75th percentile (blue), and median (dark blue) rainfall Average monthly temperature for Togo 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 River Oti flows from Burkina Faso in the north, through Togo and into Ghana where it enters Lake Volta.


The River Mono drains south from the Togo Mountains, marking the border with Benin, before entering the Gulf of Guinea in the south east of the country.

The largest lagoon along the coastal region is Lake Togo, which is fed by the Zio River draining the south western part of the Togo Mountains.

The Department of Water Resources, which sits within the Ministry of Water, is responsible for river flow gauging. All observed data is held within the Department of Water Resources and will be made available online through the Systèmes Intégrés d’information sur l’eau project.


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


Soil

Soil Map of Togo (For more information on the datasets used in the map see the soil resources section)
Clay-rich Lixosols, which are common in the northern part of Togo, generally reflect stable geological conditions and natural savannah vegetation.


Leptosols are common along the Togo Mountain chain. These are generally shallow soils that form over hard rock. To the south of the mountainous region, iron-rich Plinthosols are dominant. These typically form over gently undulating landscapes.


Nitosols, which form on alluvium and are generally very productive, are common along the low-lying coastal plain in the south of Togo.


Fluvisols have developed along the valley of the River Oti in the north and the River Zio in the south.


Land cover

Land Cover Map of Togo (For 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 Togo. 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). The map is available to download as a shapefile (.shp) for use in GIS packages.


Togo Geology2.png
Geological Environments
Key Formations Period Lithology Structure
Sedimentary – Coastal Basin
Coastal Basin Cretaceous – Pleistocene Basal unconsolidated sands, limestone, marl, phospharenite, continental sediments and Quaternary sands. Sequence deposited in a compartment of the Adina Fault in the south of Togo.
Precambrian Mobile/Orogenic Belt
Dahomeyide Belt (External, Suture, and Internal Zones) Late Proterozoic – Early Paleozoic The Dahomeyide Belt is located on the eastern boundary of the West Africa Craton. It consists of a series of nappes and thrust sheets representing the external, suture and internal zone of the Pan-African Orogeny.

The external zone comprises two metasedimentary structural units (Buem and Atacora), which are tectono-metamorphic equivalents of the lower and middle sequences of the Volta Basin. The eastern margin of the external zone is dominated by reworked Eburnean granitoids.

The suture zone forms a chain of small mafic to ultramafic massifs (Akuse, Agou, Ahito, Djabatouré, Kabye and Derouvarou). These rocks represent the roots of an arc-type crust, containing granulitic and eclogitic metasediments (kyanite and garnet-bearing quartzite and gneiss), pyroxene meta-cumulates, carbonatites, amphibolites, serpentinites and talc-schists.

The internal zone in the East, which corresponds to the Benin-Nigerian Plain, comprises ortho- and migmatitic-gneisses, volcanic-sedimentary complexes and several granitoid intrusions.

The Dahomeyide Belt was deformed during five phases of the Pan-African event.
Precambrian Metasedimentary
Bombouaka and Oti Supergroups (Volta Basin mega-sequences) Neo-Proterozoic Continental rift deposits consisting of megasequences of sandstone (Bombouaka Supergroup) and mudstone (Oti Supergroup).
Precambrian Craton
Dorsale de Leo ou de Man (West African Craton) Neo-Archean (2064±90 Ma) – Lower Proterozoic (2300-1600Ma) Acidic to basic ortho-metamorphic rocks (gneiss, migmatite, amphibolites, pyroxenite) and granitic plutonic rocks. Polycyclic basement that outcrops in the north of Togo.



Hydrogeology

This section provides a summary of the hydrogeology of the main aquifers in Togo. 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). The map is available to download as a shapefile (.shp) for use in GIS packages.

There are three main hydrogeological environments in Togo:

  • Basement aquifers, including the West African Craton and Dahomeyides Chain
  • The Volta Basin aquifer
  • Coastal Sedimentary aquifers

The basement aquifers and Volta Basin aquifer represent 94% of the area of Togo. The basement is predominantly composed of low permeability granite, gneiss and migmatite and groundwater occurs in the weathered horizon or fractures. The Volta Basin comprises sandstone and quartzite while the Coastal Basin is a layered sedimentary sequence that dips gently from north to south.


Togo Hydrogeology2.png


Sedimentary - Intergranular//Fracture

The coastal basin sedimentary aquifer (Keta Basin) is a multi-layered aquifer. Intergranular flow dominates in the shallower systems, but fracture flow is important at depth. The coastal basin contains the following aquifers in a layered system (see cross section):

  • Quaternary Sand Aquifer
  • Continental Terminal Aquifer
  • Paleocene Limestone Aquifer
  • Maestrichtian Aquifer

These aquifers are separated by thick aquicludes.


Named Aquifers General Description Water quantity issues Water quality issues Recharge
Quaternary Sand Aquifer This forms a 2-3km wide aquifer along the coast and is typically exploited by wells for domestic use. Flow is predominantly intergranular and the aquifer is unconfined.

The properties of this aquifer are largely unknown, however the aquifer thickness varies between 10 and 30 m and the water table depth varies between 0.5 and 3 m.

Continental Terminal Aquifer This is the most heavily exploited aquifer in the basin, providing the drinking water supply for the city of Lome. Flow is predominantly intergranular and the aquifer is unconfined. Transmissivity is typically on the order of 10-³ – 10-2 m²/s. Storage is typically 1-8%. Borehole yields generally vary between 10 and 200 m³/h.

The aquifer thickness varies between 20 and 80 m, the water table depth varies between 1.5 and 50 m, and boreholes are generally drilled to depths of 10-60 m.

This aquifer is overexploited for drinking water and industrial use in the Lome Region.
  • Affected by sea water intrusion due to overexploitation in the Lome Region
  • Conductivity varies from 100 to 22,000 microSiemens/cm
  • Chloride varies from 5 to 8800 mg/l
  • High nitrate (up to 700 mg/l) is also observed in the Lome Region
  • pH values vary between 4 and 7.3
Recharge occurs directly from rainfall.
Paleocene Limestone Aquifer This is a confined aquifer in which fracture flow is dominant. Transmissivity is typically on the order of 10-³ to 10-² m²/s. Borehole yields generally vary between 20 and 150 m³/h. The aquifer thickness varies between 15 and 40 m.
  • Conductivity varies from 400 to 1600 microSiemens/cm
  • Nitrate is not an issue but ammonium and hydrogen disulphide can be elevated
  • pH values vary between 6.7 and 7.6
Recharge occurs in the north of the basin by infiltration through the Continental Terminal aquifer.
Maestrichtian Aquifer The Maestrichtian Aquifer comprises sandstone (and sometimes sandy limestone) and flow is predominantly intergranular. It is mainly exploited in the northern part of the basin where it is more easily accessible, although it remains confined across the basin. The lateral extent of this aquifer towards the south of the basin is currently unknown due to the significant depth of the sediments.

Transmissivity is typically on the order of 10-3 – 10-2 m²/s. Storage is typically 1-3%. Borehole yields generally vary between 15 and 140 m³/h. The aquifer thickness varies between 5 and 25 m and boreholes are generally drilled to depths of 80-150 m (these would need to be significantly deeper in the southern part of the basin).

  • Conductivity varies from 60 to 520 microSiemens/cm
  • High iron contents are often observed (2-2.5 mg/l)
  • pH values vary between 4.6 and 7.9
Recharge occurs in the north of the basin by infiltration through the Continental Terminal aquifer.

Consolidated Sedimentary - Fracture Flow

Named Aquifers General Description Water quantity issues Water quality issues Recharge
Volta Basin Aquifers Discontinuous aquifers occur in the sandstone, quartzite and silexite of the Volta Basin. These are generally unconfined in the north of the basin, but confined by the Mango Clay Formation in the south.

Borehole yields are reported between 10 and 250 m3/day (0.1-3 l/s).

Typical aquifer thicknesses are unknown however boreholes are generally drilled to depths of 20-100 m. In the unconfined regions, the depth of the water table is usually 3-39 m.

The aquifers of the Volta Basin are generally exploited by hand pumps so there are few issues of groundwater availability. Groundwater from the Volta Basin aquifers have low mineralization. The Volta Basin aquifers are recharged by rainfall and surface water in the northern unconfined region.


Basement

Named Aquifers General Description Water quantity issues Water quality issues Recharge
West African Craton and Dahomeyides Chain The West African Craton (north of Dapaong) and the Dahomeyides Chain are the principal groundwater-bearing formations of the basement. However, these are generally discontinuous aquifers, either related to fracturing or alteration/weathering of the bedrock.

The properties of the basement aquifers are controlled by the frequency of fracturing, which varies depending on the nature of the rock, bedding, structural position and tectonic history. Harder rocks are generally more fractured, while schistose rocks are more deformable and therefore less fractured.

Alteration or weathering of the bedrock, along cracks and towards the surface, creates aquifers of limited volume that are unevenly distributed in space. These aquifers generally only have thicknesses of 3 to 15 m and are usually exploited by large diameter wells.

Aquifers related to alteration or weathering typically have porosities of 2-5%, with hydraulic conductivity on the order of 10-³ m/d. Fractured basement aquifers have lower porosity (1%) and may produce yields of up to 120 m³/day (1 l/s).

The weathered basement aquifers are generally unconfined while the fractured basement aquifers may be confined by the altered overburden. The thickness of the water-bearing fractured zone is largely unknown.

In the basement aquifers of the West African Craton the depth of the water table typically varies between 0.5 and 20 m, and boreholes are generally drilled to depths of 14-60 m. In the Dahomeyides Chain the water table may extend to a depth of 30 m and boreholes are generally drilled to depths of 35-70 m.

The basement aquifers do not always provide sufficient borehole yields to be considered a viable source. Groundwaters from the West African Craton aquifers typically have low mineralisation, while those from the Dahomeyides Chain can be highly mineralised (up to 1700 microSiemens/cm).

High nitrate concentrations (50-620 mg/l) have been reported in parts of the plateau region.

Recharge to the basement aquifers is typically from rainfall and surface water.


Groundwater Status

The basement aquifers are laterally discontinuous and thus relatively unproductive. However, exploitation is predominantly by hand pump and the aquifers are generally capable of sustaining low rates of abstraction. Nitrate concentrations in the basement aquifers are often elevated to >50-620 mg/l.

The Continental Terminal aquifer in the coastal basin supplies 70% of the public water supply in Lome, through the Togolese Company of Water (TdE). In addition to TdE boreholes, there are many industrial and private boreholes (estimated around 2500 in the Lome Region) that also exploit this aquifer in the Agoe Plateau region. As a result, groundwater levels in the Continental Terminal aquifer have fallen by between 0.5 and 12 m. Saline intrusion is also an issue as a result of overexploitation in the coastal region.

Groundwater use and management

Groundwater use

Around 85% of the total public national water supply in Togo comes from groundwater (DGEA, 2009).

The main groundwater dependent cities are listed in the table below, along with the volume of groundwater abstracted per year by the Togolese Company of Water (TdE, 2010).

City Volume of groundwater abstracted (m3/yr) City Volume of groundwater abstracted (m3/yr)
Lome 13,873,129 Tabligbo 893,454
Tsevie 656,564 Aneho 160,541
Vogan 100,310 Bassar 207,636
Tchamba 111,290 Kante 106,037
Sotouboua 63,648 Guerin Kouka 54,106

The main groundwater dependent industries are the phosphates industry (with an estimated groundwater abstraction of 4 M m³/yr) and the brewing industry (with an estimated groundwater abstraction of 0.35 M m³/yr).

The amount of groundwater removed by aquifer, both by the TdE and through private boreholes (where known), is given in the table below (MEAHV/DGEA 2013, TdE).

Aquifer Volume abstracted by TdE (M m³/yr) Volume abstracted by industry and private boreholes (M m3/yr) Volume abstracted rurally (M m³/yr) Total (M m³/yr)
Continental Terminal 10.6 4.3 1.7 16.6
Paleocene Limestone 3.3 4.7 1.0 9
Maestrichtian 4.2 2.0 0.4 6.6
Basement Aquifers 0.22 ? ? ?
Volta Basin 0.26 ? ? ?

It is estimated that there are around 6442 boreholes with hand pumps and 108 improved springs in Togo (DGEA 2009). Electric pumps are used in 80 boreholes by the TdE and in thousands more privately owned boreholes.

Groundwater management

The key groundwater legislation in Togo is Loi No 2010-004 Portant Code de l'Eau (Groundwater Code).

The main institutions responsible for groundwater supply and management in Togo are:

Le Ministère de l’Eau, de l’Assainissement et de l’Hydraulique Villageoise (MEAHV)
Ministry of Water, sanitation and village hydraulics
Responsible for the implementation of national policy related to water and sanitation, and for monitoring in collaboration with other ministries or institutions
La Société Togolaise des Eaux (TdE)
Togolese Company of Water
Responsible for supplying the greatest possible number of urban households at the lowest possible cost, and for ensuring the collection and disposal of waste water in urban centres where the equipment exists
La Société de patrimoine Eau, Assainissement Urbain (SP-EAU)
Heritage Society of Water and Urban Sanitation
Ensures the management and development of state-controlled drinking water supply and sanitation in urban areas

Transboundary aquifers

Togo shares the sedimentary coastal aquifer with Ghana, Benin and Nigeria but to date there are no significant transboundary issues.

For further information about transboundary aquifers, please see the Transboundary aquifers resources page

Groundwater monitoring

The Department of Water Resources, which sits within the Ministry of Water, is responsible for groundwater monitoring. However, there are currently no national groundwater level or groundwater quality monitoring programmes.