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[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Additional resources | Resource pages]] >> The Africa Groundwater Atlas country hydrogeology maps
[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Additional resources | Resource pages]] >> Africa Groundwater Atlas Country Hydrogeology maps


==The Atlas country hydrogeology maps==
Please cite page as: Africa Groundwater Atlas. 2019. Africa Groundwater Atlas Country Hydrogeology maps. British Geological Survey. Accessed [date you accessed the information]. ''Weblink''.


The country hydrogeology maps in this Atlas provide an overview of the hydrogeology and geology of each of 48 countries in Africa, at 1:5 million scale. The maps will be available to download as digital, GIS-enabled shapefiles with two themes:
 
==The Africa Groundwater Atlas Country Hydrogeology Maps==


* '''geology''', with geological interpretations that are particularly relevant to hydrogeology; and
The country hydrogeology  maps in this Atlas provide an overview of the hydrogeology and geology of each of 48 countries in Africa, at 1:5 million scale.  
* a combined classification of '''aquifer type''' and '''aquifer productivity'''.


More detail on these classifications is below.  
Most of these maps (currently for 38 countries) are available to [https://www.bgs.ac.uk/africagroundwateratlas/downloadGIS.html '''download as digital, GIS-enabled shapefiles'''].


====Benefits of the Atlas country hydrogeology maps====
The maps have two themes:


These maps provide an essential national overview of the groundwater resources of each country. For some countries in Africa, the Atlas hydrogeology map is the only available digital hydrogeology map.  
* '''geology''', with geological categories that reflect significant hydrogeological units (MacDonald et al. 2010); and
* '''hydrogeology''', with categories that combine '''aquifer type''' and '''aquifer productivity'''.


====Limitations of the Atlas country hydrogeology maps====
More detail on the maps is below.
 
{|
|-
|
[[File: 38CountryMapsLegend.jpg | 500px|center| thumb| Africa Groundwater Atlas hydrogeology maps for 38 countries in Africa, 1:5 million scale]]
|
|}
 
===Who might use the Atlas country hydrogeology maps===
 
These maps are likely to be useful to people interested in groundwater resources, hydrogeology or geology of any of the relevant African countries. They provide a national overview of the groundwater resources of each country. For some countries in Africa, the Atlas hydrogeology map is the only available digital hydrogeology map.
 
===Map Scale===
 
The maps provide a national-scale overview for each country, at a nominal scale of 1:5 million. This means that 1 cm on the maps is equivalent to 50 km on the ground. This makes them suitable for viewing at the following maximum sizes:


The large scale of the original map (1:5 million scale) means that the maps provide a country-scale overview - they do not show local scale detail. These maps are not based on field mapping and they have not been ground truthed. They may be different from other available hydrogeology maps.
* A2 – for the largest countries in Africa, such as Algeria and the Democratic Republic of Congo
* A3 – for countries of a size similar to Sudan and Mali
* A4 – for countries of a size similar to Kenya and Zambia
* A5 – for countries of a size similar to Ghana and Tunisia
* A6 – for the smallest countries in Africa, such as Burundi and Lesotho.  


Like all maps, these country hydrogeology maps are a two-dimensional representation of the complex three-dimensional hydrogeological reality. In most cases, these maps show the uppermost aquifer only. If there are other aquifers at depth, these are not shown. In some cases, where a major aquifer is overlain by relatively low permeability unconsolidated sediments (that don't form an important aquifer), the overlying unconsolidated sediments are not shown on the map.  
The maps are not suitable for providing detailed information on geology and hydrogeology at a sub-national (e.g. catchment) scale.


===Geology===
===Geology===


The Atlas geology maps show the geology of each country at a 1:5 million scale. Their main aim is to show key geological differences that are relevant to aquifer characteristics and hydrogeology. The geological classifications are broadly lithostratigraphical (based on the age and lithology (type) of the rocks), but geological units with similar aquifer (hydrogelogical) characteristics may be grouped together in the Atlas maps, whereas in other geological maps they are separated. However, where possible, the geological unit names used in the maps are those used in the relevant country.  
The Atlas geology maps show the geology of each country at a 1:5 million scale. The geological descriptions reflect significant hydrogeological units at a national scale for each country, and where possible reflect geological nomenclature used (at this scale) in each country. The geological classifications are broadly lithostratigraphical (based on the age and lithology (type) of the rocks), and the geological descriptions can include some or all of: geological age; lithological description; and whether sediment is consolidated or unconsolidated. Because of the focus on hydrogeology, geological units with similar aquifer (hydrogeological) characteristics may be grouped together in the Atlas maps, whereas in other geological maps they are separated.  
 
For some countries an additional geology map has been developed that shows where extensive unconsolidated sediments overlie bedrock. At the moment, this is only available for Botswana, Mauritania and Sudan. Some other countries in Africa also have extensive covers of unconsolidated sediments, but at present this has not been shown separately, primarily because of: the complexity of displaying three dimensional geology on a two dimensional map; the availability of geological information and understanding; and the need to adapt each country map to reflect national approaches to geological categorisation.


===Hydrogeology===
===Hydrogeology===


The hydrogeology theme shows a combination of these key hydrogeological parameters:  
The hydrogeology maps show a combination of these two key hydrogeological parameters:  


*'''aquifer type''' (based on the geological environment and groundwater flow/storage type); and
*'''aquifer type''' (based on the hydrogeological environment and the aquifer flow/storage type); and
*'''aquifer productivity''' (relative aquifer productivity, or groundwater potential).  
*'''aquifer productivity''' (relative aquifer productivity, or groundwater potential).  
The combination of aquifer type and aquifer productivity to classify the hydrogeology of each country provides a practical description that will be understood by hydrogeologists. Definitions of aquifer type and aquifer productivity are given below.


====Aquifer Type====
====Aquifer Type====


=====Hydrogeological Environment=====
Aquifer type is defined in terms of the hydrogeological environment, or the geological characteristics that largely control the nature of groundwater flow and storage in the aquifer. Each aquifer is defined as belonging to one of four main [[ Overview of Groundwater in Africa | '''hydrogeological environments''']] seen across Africa, with subdivisions of some of the categories (see table, below). These are a useful way of classifying aquifers, as each group has typical aquifer characteristics, and groundwater in each group behaves in similar ways.


Each aquifer is defined as belonging to one of the four main [[ Overview of Groundwater in Africa | hydrogeological environments]] seen across Africa, listed below. These are a useful way of classifying aquifers, as each group has typical aquifer characteristics, and groundwater in each group behaves more or less in similar ways.  
A key feature of the aquifer type classification is the dominant '''aquifer flow and storage type''' - the way that groundwater flows through, and is stored in, aquifers: e.g. in weathered zones; as intergranular (or matrix) flows through pores; or through fractures; or through karstic (solution-enhanced) features.


{| class = "wikitable"
{| class = "wikitable"
|+ Aquifer flow and storage type
|+Hydrogeological Environment
|Hydrogeological Environment||Summary
|Hydrogeological Environment||Summary||Main aquifer groups
|-
|-
|Basement aquifers
!colspan="4"|Unconsolidated
||Crystalline basement rocks (metamorphic and igneous) most of which are Precambrian in age. These rocks develop distinctive local weathered (also called regolith) and fractured aquifers.
|-
|-
|Igneous aquifers (mostly volcanic)
|Unconsolidated sedimentary aquifers  
||Mostly volcanic aquifers, but in some areas also including intrusive igneous rocks, such as granites, that are younger than the Precambrian. These form fractured, weathered aquifers that are often strongly controlled by the geometry and weathering of former lava flows.
||Unconsolidated sediments with dominantly intergranular flow.  Highly variable in distribution, thickness, geometry and lithology, and therefore in their hydrogeological characteristics. The hydrogeology map shows some of the most hydrogeologically significant outcrops of unconsolidated deposits in Africa, particularly where these overlie lower productivity bedrock aquifers. However, it does not show all unconsolidated deposits. Particularly, over some of the major consolidated sedimentary basins, unconsolidated deposits of hydrogeological significance are not always shown, because the underlying consolidated sedimentary rocks form more productive and significant aquifers.
||Aeolian (dune) sands, alluvium, coastal and marine sands and gravels. Mostly Quaternary but also include some Tertiary age sediments.
|-
|-
|Consolidated sedimentary aquifers
!colspan="4"|Consolidated sedimentary  
||Consolidated (solid) sedimentary rocks. These are usually from Tertiary to Cambrian in age, but also include some older Precambrian sedimentary rocks that have not been highly metamorphosed. They have been subdivided according to the dominant groundwater flow and storage type (see below for more detail) – fracture, intergranular, or a combination of fracture and intergranular.
|-
|-
|Unconsolidated sedimentary aquifers
|Consolidated Sedimentary Fracture
||Mostly Quaternary but also include some Tertiary age sediments. They are highly variable in their distribution, thickness, geometry and lithology, and therefore in their hydrogeological characteristics. The hydrogeology map shows some of the most hydrogeologically significant outcrops of unconsolidated deposits in Africa, particularly where these overlie lower productivity bedrock aquifers. However, it does not show all unconsolidated deposits. Particularly, over some of the major consolidated sedimentary basins, unconsolidated deposits of hydrogeological significance are not always shown, because the underlying consolidated sedimentary rocks form more productive and significant aquifers.
||Aquifers with predominantly fracture flow and storage, with insignificant intergranular permeability and porosity (average porosity is < 0.1).
|}
||
 
[[File:Africa_Hgcl_Envs.png|thumb| 400px|center| The main hydrogeological environments in Africa]]
 
=====Aquifer flow/storage type=====
 
The way that groundwater flows through aquifers, and is stored, is a key hydrogeological characteristic, and is a fundamental part of the classifications used in the Atlas hydrogeology maps. The maps use a semi-quantitative assessment of aquifer flow and storage type, based on geology and inferred porosity (MacDonald et al., 2010). The main distinction is whether groundwater is stored in, and flows through, '''fractures''' in a consolidated rock, or via '''intergranular''' flow in a porous rock or sediment matrix.
 
{| class = "wikitable"
|+ Aquifer flow and storage type
|Aquifer flow/storage type||Summary||Main aquifer groups
|-
|-
|Intergranular  
|Consolidated Sedimentary Intergranular
||Intergranular storage is highly significant. Rock porosity is generally >0.25. Intergranular flow is dominant.
||Aquifers in which intergranular flow and storage dominate. Rock porosity is generally >0.25.  
||Unconsolidated sedimentary aquifers, and younger consolidated sedimentary rocks (e.g. Cenozoic and younger Mesozoic in age), which tend to be loosely consolidated with significant intergranular permeability.
||Younger rocks (e.g. Cenozoic and younger Mesozoic in age), which tend to be loosely consolidated with significant intergranular permeability.
|-
|-
|Intergranular and fracture
|Consolidated Sedimentary Intergranular/Fracture
||Significant intergranular storage, with mixed intergranular and fracture flow. The average porosity of rocks is approximately 0.1 – 0.25.
||Aquifers with significant intergranular storage, with mixed intergranular and fracture flow. The average porosity of rocks is approximately 0.1 – 0.25.
||Older sedimentary rocks (e.g. older Mesozoic and Palaeozoic), which tend to be well consolidated with low intergrangular permeability.
||Older sedimentary rocks (e.g. older Mesozoic and Palaeozoic), which tend to be well consolidated with low intergrangular permeability.
|-
|-
|Fracture  
|Consolidated Sedimentary Karst (sometimes also Consolidated Sedimentary Fracture Karst or Consolidated Sedimentary Intergranular Fracture Karst)
||Predominantly fracture flow and storage, with insignificant intergranular permeability and porosity (average porosity is < 0.1).
||Aquifers with significant karstic features, in which karstic flow is significant. Fracture and/or intergranular flow and storage can also occur.
||Volcanic and intrusive igneous rocks (although minor intergranular flow can occur in weathered zones in these rocks).  
||Calcareous (limestone and dolomite) aquifers in which karstic features have developed.  
|-
!colspan="4"|Igneous
|-
|Igneous Intrusive
||Intrusive igneous rocks, often granitic.
||All intrusive igneous rocks younger than Precambrian.
|-
|Volcanic
||Volcanic fractured, weathered aquifers that are often strongly controlled by the geometry and weathering of former lava flows.
||All volcanic rocks younger than Precambrian.  
|-
|-
|Fracture (karst)
!colspan="4"|Basement
||A special case in karst aquifers. Predominantly fracture flow and storage.
||Calcareous (limestone and dolomite) aquifers in which karstic features have developed.
|-
|-
|Fracture (weathered)
|Precambrian Basement
||A special case in basement aquifers. Significant fracture flow in unweathered parts of very low porosity (<0.01) rocks, with some intergranular flow and storage in weathered zones.
||Crystalline basement aquifers with typical weathered/fractured aquifer properties
||All crystalline basement rocks belong to this category - i.e., most Precambrian rocks, with the exception of metasedimentary rocks that show little deformation.
||All crystalline basement rocks belong to this category - i.e., most Precambrian rocks, with the exception of metasedimentary rocks that show little deformation.
|}
|}
[[File:Africa_Hgcl_Envs.png|thumb| 400px|center| The main hydrogeological environments in Africa]]
   
   
[[File:AfricaFlowStorage.png | thumb| 40px|center| Aquifer flow and storage type across Africa]]
[[File:AfricaFlowStorage.png | thumb| 400px|center| Aquifer flow and storage type across Africa]]
 


====Aquifer productivity====
====Aquifer productivity====


Borehole yield is used as a proxy for aquifer productivity in the Atlas country hydrogeology maps. This is based on an analysis of collated borehole yield data for aquifers across Africa, to assess the typical yield of a borehole in the main aquifer types.  
Aquifer productivity is estimated by using borehole yield data as a proxy, which is the most widely available data for aquifers in Africa.  


Six borehole yield classes are distinguished. These classes relate to the estimated average sustainable yield that could obtained from a single, effectively sited and developed borehole (MacDonald et al. 2010). The classes are described in terms of relative aquifer productivity, from Very Low to Very High:
Available quantitative borehole yield data from across Africa were synthesised, and six main categories of borehole yield were distinguished, based on approximate ranges of yields (in litres/second or L/s) (MacDonald et al. 2010). The classes are described in terms of relative aquifer productivity, from Very Low to Very High (see the table below). These ranges are assumed to relate to the average yields that would be obtained from a single, effectively sited and developed borehole, to an appropriate depth (which will vary depending on the aquifer type), in the relevant aquifer. In reality in most cases, the aquifer productivity categories used in the Atlas maps are a largely qualitative assessment, because of the scale at which these maps have been developed; the heterogeneity of most of the aquifers; and the limited availability of aquifer properties data. The category codes applied, therefore, often span more than one of the ranges shown in the table below.


{| class = "wikitable"
{| class = "wikitable"
|+ Aquifer productivity  
|+ Aquifer productivity  
|Aquifer productivity||Yield range (litres/second or l/s)
|Aquifer productivity category||Approximate range in borehole yield (litres/second or l/s)
|-
|-
|Very high
|Very high
Line 118: Line 144:
[[File:AfricaAqProd.png | center | 400px|thumb| Aquifer productivity across Africa]]
[[File:AfricaAqProd.png | center | 400px|thumb| Aquifer productivity across Africa]]


==How the Atlas country hydrogeology maps were developed==


==How the Atlas country hydrogeology maps were developed==
The Atlas country hydrogeology maps were developed in ArcGIS by modifying and attributing a digital Africa-wide geology map at 1:5 million scale, using available hydrogeological and geological information.
 
The 1:5 million scale geology map was originally published by UNESCO (Furon and Lombard 1964), and was digitised by, and is made available through, the USGS (Persits et al., 2002). It can be accessed and downloaded in shapefile format via the USGS (details in References). 
 
The country maps are a further development of the Africa-wide [https://www.bgs.ac.uk/research/groundwater/international/africanGroundwater/maps.html '''quantitative groundwater maps'''] that were produced by BGS in 2012 ([https://nora.nerc.ac.uk/id/eprint/17892/ MacDonald et al. 2012]). The country maps use the hydrogeological understanding captured in the Africa-wide maps, and incorporate additional geological and hydrogeological information from individual countries. They reflect current understanding of national geology and hydrogeology, given the scale of the geological base map linework and the availability of geological and hydrogeological information and, where possible, they reflect different national approaches to geological and hydrogeological categorisation.
 
Where possible, the maps have been validated in collaboration with hydrogeologists from the relevant countries who are co-authors on the relevant country pages of the Africa Groundwater Atlas (co-authors are referenced on the relevant Atlas country page and in the [https://earthwise.bgs.ac.uk/index.php/List_of_Authors List of Authors]).
 
====Limitations of the Atlas country hydrogeology maps====
 
The Atlas country hydrogeology maps provide a national (country) scale overview of the hydrogeology and geology. They are not suitable for providing detailed information on geology and hydrogeology at a sub-national (e.g. catchment) scale. They are suitable for viewing at sizes from A2 down to A6, depending on the size of the country (see the section Map Scale, above).
 
Like all maps, the country hydrogeology maps are a two-dimensional representation of the complex three-dimensional hydrogeological reality. In most cases, the maps show the uppermost aquifer only. If there are other aquifers at depth, these are not shown. In some cases, where a major aquifer is overlain by relatively low permeability unconsolidated sediments (that don't form an important aquifer), the overlying unconsolidated sediments are not shown on the map.
 
The maps reflect the current understanding of national geology and hydrogeology, given the scale of the geological base map linework and availability of geological and hydrogeological information. 
 
Because the maps reflect hydrogeological and geological terminology used in individual countries, the names and descriptions of hydrogeological or geological units that cross country (national) borders may not agree on both sides of the border (resulting in some country border ‘edges’ that have different colours/attribution).


These country maps were developed from the Africa-wide [https://www.bgs.ac.uk/research/groundwater/international/africanGroundwater/maps.html quantitative groundwater maps] produced by BGS in 2010. For some countries, the maps have been developed in collaboration with Atlas co-authors to ensure that the maps represent the national scale hydrogeology of the country as well as possible. The maps were produced in ArcGIS.  
The maps do not necessarily show the same information as other hydrogeology or geology maps available for any country.  


==More Information==
==More Information==
Furon R and Lombard J. 1964. [https://unesdoc.unesco.org/images/0009/000920/092086mo.pdf Explanatory note: Geological Map of Africa (1/5,000,000) / Note explicative: carte geologique de l'Afrique (1/5,000,000)]. UNESCO and the Association for African Geological Surveys (ASGA). UNESCO, Paris, 1964.


MacDonald AM, and Davies J. 2000. [https://nora.nerc.ac.uk/501047/ A brief review of groundwater for rural water supply in sub-Saharan Africa]. British Geological Survey Technical Report, WC/00/033, 30pp.  
MacDonald AM, and Davies J. 2000. [https://nora.nerc.ac.uk/501047/ A brief review of groundwater for rural water supply in sub-Saharan Africa]. British Geological Survey Technical Report, WC/00/033, 30pp.  


MacDonald AM, Bonsor HC, Ó Dochartaigh BÉ and Taylor RG. 2012. Quantitative maps of groundwater resources in Africa. Environmental Research Letters, Vol. 7(2). https://nora.nerc.ac.uk/17892/ doi:10.1088/1748-9326/7/2/024009  
MacDonald AM, Bonsor HC, Ó Dochartaigh BÉ and Taylor RG. 2012. [https://nora.nerc.ac.uk/17892/ Quantitative maps of groundwater resources in Africa]. Environmental Research Letters, Vol. 7(2). doi:10.1088/1748-9326/7/2/024009  


MacDonald AM, Ó Dochartaigh BÉ, Bonsor HC, Davies J, and Key R. 2010. [https://nora.nerc.ac.uk/501779/ Developing quantitative aquifer maps for Africa]. British Geological Survey Internal Report, IR/10/103.
MacDonald AM, Ó Dochartaigh BÉ, Bonsor HC, Davies J, and Key R. 2010. [https://nora.nerc.ac.uk/501779/ Developing quantitative aquifer maps for Africa]. British Geological Survey Internal Report, IR/10/103.


See also the [https://www.bgs.ac.uk/research/groundwater/international/africangroundwater/maps.html Quantitative groundwater maps for Africa] project webpage.  
Persits F, Ahlbrandt T, Tuttle M, Charpentier R, Brownfield M, and Takahashi K. 2002. [https://pubs.usgs.gov/of/1997/ofr-97-470/OF97-470A/ Map showing geology, oil and gas fields and geologic provinces of Africa, Ver 2.0]. USGS Open File report 97-470 A.
 
USGS. 2002. [https://certmapper.cr.usgs.gov/geoportal/catalog/search/resource/details.page?uuid=%7BC7E34C48-9BD9-45C6-AD08-E549CFB3715A%7D Surficial geology of Africa (geo7_2ag)]. Shapefile for download. Accessed 11 March 2019.
 
See also the [https://www.bgs.ac.uk/research/groundwater/international/africangroundwater/maps.html '''BGS Quantitative groundwater maps for Africa'''] project webpage.  
 


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Revision as of 08:09, 18 September 2019

Africa Groundwater Atlas >> Resource pages >> Africa Groundwater Atlas Country Hydrogeology maps

Please cite page as: Africa Groundwater Atlas. 2019. Africa Groundwater Atlas Country Hydrogeology maps. British Geological Survey. Accessed [date you accessed the information]. Weblink.


The Africa Groundwater Atlas Country Hydrogeology Maps

The country hydrogeology maps in this Atlas provide an overview of the hydrogeology and geology of each of 48 countries in Africa, at 1:5 million scale.

Most of these maps (currently for 38 countries) are available to download as digital, GIS-enabled shapefiles.

The maps have two themes:

  • geology, with geological categories that reflect significant hydrogeological units (MacDonald et al. 2010); and
  • hydrogeology, with categories that combine aquifer type and aquifer productivity.

More detail on the maps is below.

Africa Groundwater Atlas hydrogeology maps for 38 countries in Africa, 1:5 million scale

Who might use the Atlas country hydrogeology maps

These maps are likely to be useful to people interested in groundwater resources, hydrogeology or geology of any of the relevant African countries. They provide a national overview of the groundwater resources of each country. For some countries in Africa, the Atlas hydrogeology map is the only available digital hydrogeology map.

Map Scale

The maps provide a national-scale overview for each country, at a nominal scale of 1:5 million. This means that 1 cm on the maps is equivalent to 50 km on the ground. This makes them suitable for viewing at the following maximum sizes:

  • A2 – for the largest countries in Africa, such as Algeria and the Democratic Republic of Congo
  • A3 – for countries of a size similar to Sudan and Mali
  • A4 – for countries of a size similar to Kenya and Zambia
  • A5 – for countries of a size similar to Ghana and Tunisia
  • A6 – for the smallest countries in Africa, such as Burundi and Lesotho.

The maps are not suitable for providing detailed information on geology and hydrogeology at a sub-national (e.g. catchment) scale.

Geology

The Atlas geology maps show the geology of each country at a 1:5 million scale. The geological descriptions reflect significant hydrogeological units at a national scale for each country, and where possible reflect geological nomenclature used (at this scale) in each country. The geological classifications are broadly lithostratigraphical (based on the age and lithology (type) of the rocks), and the geological descriptions can include some or all of: geological age; lithological description; and whether sediment is consolidated or unconsolidated. Because of the focus on hydrogeology, geological units with similar aquifer (hydrogeological) characteristics may be grouped together in the Atlas maps, whereas in other geological maps they are separated.

For some countries an additional geology map has been developed that shows where extensive unconsolidated sediments overlie bedrock. At the moment, this is only available for Botswana, Mauritania and Sudan. Some other countries in Africa also have extensive covers of unconsolidated sediments, but at present this has not been shown separately, primarily because of: the complexity of displaying three dimensional geology on a two dimensional map; the availability of geological information and understanding; and the need to adapt each country map to reflect national approaches to geological categorisation.

Hydrogeology

The hydrogeology maps show a combination of these two key hydrogeological parameters:

  • aquifer type (based on the hydrogeological environment and the aquifer flow/storage type); and
  • aquifer productivity (relative aquifer productivity, or groundwater potential).

The combination of aquifer type and aquifer productivity to classify the hydrogeology of each country provides a practical description that will be understood by hydrogeologists. Definitions of aquifer type and aquifer productivity are given below.

Aquifer Type

Aquifer type is defined in terms of the hydrogeological environment, or the geological characteristics that largely control the nature of groundwater flow and storage in the aquifer. Each aquifer is defined as belonging to one of four main hydrogeological environments seen across Africa, with subdivisions of some of the categories (see table, below). These are a useful way of classifying aquifers, as each group has typical aquifer characteristics, and groundwater in each group behaves in similar ways.

A key feature of the aquifer type classification is the dominant aquifer flow and storage type - the way that groundwater flows through, and is stored in, aquifers: e.g. in weathered zones; as intergranular (or matrix) flows through pores; or through fractures; or through karstic (solution-enhanced) features.

Hydrogeological Environment
Hydrogeological Environment Summary Main aquifer groups
Unconsolidated
Unconsolidated sedimentary aquifers Unconsolidated sediments with dominantly intergranular flow. Highly variable in distribution, thickness, geometry and lithology, and therefore in their hydrogeological characteristics. The hydrogeology map shows some of the most hydrogeologically significant outcrops of unconsolidated deposits in Africa, particularly where these overlie lower productivity bedrock aquifers. However, it does not show all unconsolidated deposits. Particularly, over some of the major consolidated sedimentary basins, unconsolidated deposits of hydrogeological significance are not always shown, because the underlying consolidated sedimentary rocks form more productive and significant aquifers. Aeolian (dune) sands, alluvium, coastal and marine sands and gravels. Mostly Quaternary but also include some Tertiary age sediments.
Consolidated sedimentary
Consolidated Sedimentary Fracture Aquifers with predominantly fracture flow and storage, with insignificant intergranular permeability and porosity (average porosity is < 0.1).
Consolidated Sedimentary Intergranular Aquifers in which intergranular flow and storage dominate. Rock porosity is generally >0.25. Younger rocks (e.g. Cenozoic and younger Mesozoic in age), which tend to be loosely consolidated with significant intergranular permeability.
Consolidated Sedimentary Intergranular/Fracture Aquifers with significant intergranular storage, with mixed intergranular and fracture flow. The average porosity of rocks is approximately 0.1 – 0.25. Older sedimentary rocks (e.g. older Mesozoic and Palaeozoic), which tend to be well consolidated with low intergrangular permeability.
Consolidated Sedimentary Karst (sometimes also Consolidated Sedimentary Fracture Karst or Consolidated Sedimentary Intergranular Fracture Karst) Aquifers with significant karstic features, in which karstic flow is significant. Fracture and/or intergranular flow and storage can also occur. Calcareous (limestone and dolomite) aquifers in which karstic features have developed.
Igneous
Igneous Intrusive Intrusive igneous rocks, often granitic. All intrusive igneous rocks younger than Precambrian.
Volcanic Volcanic fractured, weathered aquifers that are often strongly controlled by the geometry and weathering of former lava flows. All volcanic rocks younger than Precambrian.
Basement
Precambrian Basement Crystalline basement aquifers with typical weathered/fractured aquifer properties All crystalline basement rocks belong to this category - i.e., most Precambrian rocks, with the exception of metasedimentary rocks that show little deformation.


The main hydrogeological environments in Africa


Aquifer flow and storage type across Africa


Aquifer productivity

Aquifer productivity is estimated by using borehole yield data as a proxy, which is the most widely available data for aquifers in Africa.

Available quantitative borehole yield data from across Africa were synthesised, and six main categories of borehole yield were distinguished, based on approximate ranges of yields (in litres/second or L/s) (MacDonald et al. 2010). The classes are described in terms of relative aquifer productivity, from Very Low to Very High (see the table below). These ranges are assumed to relate to the average yields that would be obtained from a single, effectively sited and developed borehole, to an appropriate depth (which will vary depending on the aquifer type), in the relevant aquifer. In reality in most cases, the aquifer productivity categories used in the Atlas maps are a largely qualitative assessment, because of the scale at which these maps have been developed; the heterogeneity of most of the aquifers; and the limited availability of aquifer properties data. The category codes applied, therefore, often span more than one of the ranges shown in the table below.

Aquifer productivity
Aquifer productivity category Approximate range in borehole yield (litres/second or l/s)
Very high > 20
High 5 - 20
Moderate 2 - 5
Low to moderate 0.5 - 2
Low 0.1 - 0.5
Very low < 0.1
Aquifer productivity across Africa

How the Atlas country hydrogeology maps were developed

The Atlas country hydrogeology maps were developed in ArcGIS by modifying and attributing a digital Africa-wide geology map at 1:5 million scale, using available hydrogeological and geological information.

The 1:5 million scale geology map was originally published by UNESCO (Furon and Lombard 1964), and was digitised by, and is made available through, the USGS (Persits et al., 2002). It can be accessed and downloaded in shapefile format via the USGS (details in References).

The country maps are a further development of the Africa-wide quantitative groundwater maps that were produced by BGS in 2012 (MacDonald et al. 2012). The country maps use the hydrogeological understanding captured in the Africa-wide maps, and incorporate additional geological and hydrogeological information from individual countries. They reflect current understanding of national geology and hydrogeology, given the scale of the geological base map linework and the availability of geological and hydrogeological information and, where possible, they reflect different national approaches to geological and hydrogeological categorisation.

Where possible, the maps have been validated in collaboration with hydrogeologists from the relevant countries who are co-authors on the relevant country pages of the Africa Groundwater Atlas (co-authors are referenced on the relevant Atlas country page and in the List of Authors).

Limitations of the Atlas country hydrogeology maps

The Atlas country hydrogeology maps provide a national (country) scale overview of the hydrogeology and geology. They are not suitable for providing detailed information on geology and hydrogeology at a sub-national (e.g. catchment) scale. They are suitable for viewing at sizes from A2 down to A6, depending on the size of the country (see the section Map Scale, above).

Like all maps, the country hydrogeology maps are a two-dimensional representation of the complex three-dimensional hydrogeological reality. In most cases, the maps show the uppermost aquifer only. If there are other aquifers at depth, these are not shown. In some cases, where a major aquifer is overlain by relatively low permeability unconsolidated sediments (that don't form an important aquifer), the overlying unconsolidated sediments are not shown on the map.

The maps reflect the current understanding of national geology and hydrogeology, given the scale of the geological base map linework and availability of geological and hydrogeological information.

Because the maps reflect hydrogeological and geological terminology used in individual countries, the names and descriptions of hydrogeological or geological units that cross country (national) borders may not agree on both sides of the border (resulting in some country border ‘edges’ that have different colours/attribution).

The maps do not necessarily show the same information as other hydrogeology or geology maps available for any country.

More Information

Furon R and Lombard J. 1964. Explanatory note: Geological Map of Africa (1/5,000,000) / Note explicative: carte geologique de l'Afrique (1/5,000,000). UNESCO and the Association for African Geological Surveys (ASGA). UNESCO, Paris, 1964.

MacDonald AM, and Davies J. 2000. A brief review of groundwater for rural water supply in sub-Saharan Africa. British Geological Survey Technical Report, WC/00/033, 30pp.

MacDonald AM, Bonsor HC, Ó Dochartaigh BÉ and Taylor RG. 2012. Quantitative maps of groundwater resources in Africa. Environmental Research Letters, Vol. 7(2). doi:10.1088/1748-9326/7/2/024009

MacDonald AM, Ó Dochartaigh BÉ, Bonsor HC, Davies J, and Key R. 2010. Developing quantitative aquifer maps for Africa. British Geological Survey Internal Report, IR/10/103.

Persits F, Ahlbrandt T, Tuttle M, Charpentier R, Brownfield M, and Takahashi K. 2002. Map showing geology, oil and gas fields and geologic provinces of Africa, Ver 2.0. USGS Open File report 97-470 A.

USGS. 2002. Surficial geology of Africa (geo7_2ag). Shapefile for download. Accessed 11 March 2019.

See also the BGS Quantitative groundwater maps for Africa project webpage.


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