Africa Groundwater Atlas Hydrogeology Maps
The 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. The maps will be available to download as digital, GIS-enabled shapefiles with two themes:
- geology, with geological interpretations that are particularly relevant to hydrogeology; and
- a combined classification of aquifer type and aquifer productivity.
More detail on these classifications is below.
Benefits of the Atlas country hydrogeology maps
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.
Limitations of the Atlas country hydrogeology maps
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.
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 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 (hydrogeological) 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 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).
Aquifer type is defined by combining the hydrogeological environment (the basic rock type) with the dominant method of groundwater flow and storage in the aquifer.
Each aquifer is defined as belonging to one of four main 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.
|Basement aquifers||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)||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.|
|Consolidated sedimentary aquifers||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||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.|
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.
|Aquifer flow/storage type||Summary||Main aquifer groups|
|Intergranular||Intergranular storage is highly significant. Rock porosity is generally >0.25. Intergranular flow is dominant.||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.|
|Intergranular and fracture||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.|
|Fracture||Predominantly fracture flow and storage, with insignificant intergranular permeability and porosity (average porosity is < 0.1).||Volcanic and intrusive igneous rocks (although minor intergranular flow can occur in weathered zones in these rocks).|
|Fracture (karst)||A special case in karst aquifers. Predominantly fracture flow and storage.||Calcareous (limestone and dolomite) aquifers in which karstic features have developed.|
|Fracture (weathered)||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.||All crystalline basement rocks belong to this category - i.e., most Precambrian rocks, with the exception of metasedimentary rocks that show little deformation.|
Borehole yield is used as a proxy for aquifer productivity in the Atlas country hydrogeology maps. Estimation of typical boreholes yields in the main aquifer types was based on an analysis of collated borehole yield data for aquifers across Africa.
Six main 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 (see table below). In some cases, combined classes are used to describe aquifers which are very heterogeneous and have a large range in observed yields, or where there isn't enough information to be confident about assigning only one class.
|Aquifer productivity||Yield range (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|
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 digitised geology map includes some linework defining coasts that was created using Esri© data, used with permission of Esri©.
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 the current understanding of national geology and hydrogeology, given the scale of the geological base map linework and 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 on this page http://earthwise.bgs.ac.uk/index.php/List_of_Authors).
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). http://nora.nerc.ac.uk/17892/ 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.
See also the Quantitative groundwater maps for Africa project webpage.