Difference between revisions of "Africa Groundwater Atlas Hydrogeology Maps"
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[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Additional resources | Resource pages]] >> Africa Groundwater Atlas
[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Additional resources | Resource pages]] >> Africa Groundwater Atlas maps
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Revision as of 10:02, 2 July 2019
Please cite page as: Africa Groundwater Atlas. 2019. Africa Groundwater Atlas Country Hydrogeology maps. British Geological Survey. Accessed [date you accessed the information]. Weblink.
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- 1 The Africa Groundwater Atlas Country Hydrogeology Maps
- 2 How the Atlas country hydrogeology maps were developed
- 3 More Information
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. The maps will be available to download as digital, GIS-enabled shapefiles with 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 these classifications is below.
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.
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.
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.
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 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||Summary||Main aquifer groups|
|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 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 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.|
|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.|
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 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|
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.
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.