Aquifer properties are the hydraulic characteristics of aquifers, which we use to describe the aquifer, and understand how groundwater exists and behaves in that aquifer - in other words, the hydrogeology of that aquifer. Key aquifer properties are permeability (or transmissivity); storage; and thickness.
To get reliable information on aquifer properties, aquifer testing must be carried out. Drilling and carrying out controlled test pumping of boreholes allows estimates of aquifer thickness, permeability, transmissivity and storage to be made. Without controlled test pumping, it is not possible to accurately estimate these aquifer properties.
In many parts of Africa, quantitative aquifer properties data are scarce, and surrogate data and information must be used instead in order to characterise aquifers. The most commonly available hydrogeological data are geology; borehole depth; and borehole yield. These have been used to develop the hydrogeology map used in this Atlas.
Aquifer storage and flow type
For the hydrogeology map presented in this Atlas, a semi-quantitative assessment of aquifer storage and flow type was made 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.
|Flow and storage type||Description||Main aquifer groups|
|Intergranular||Intergranular storage is highly significant. Rock porosity is generally >0.25. Intergranular flow is dominant.||Unconsolidated sedimentary aquifers, and younger sedimentary rocks (e.g. Cenozoic and younger Mesozoic in age), which tend to be more loosely consolidated.|
|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.|
|Fracture||Predominantly fracture flow and storage, with only a minor component of intergranular storage. Average rock porosity is < 0.1.||Volcanic and intrusive igneous rocks (although minor intergranular flow can occur in weathered zones).|
|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.|
For more information see an Overview of Groundwater in Africa.
To develop the Africa-wide hydrogeology map that was then used to derive country maps for this Atlas, borehole yield data were collated for aquifers across Africa. Six yield categories were distinguished and applied to the mapped aquifers, as a proxy for aquifer productivity. The yield values refer to average yields from a single, effectively sited and developed borehole in the relevant aquifer (MacDonald et al., 2010).
|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|
See also the hydrogeology map of Africa.
Aquifer saturated thickness
A map estimating the saturated thickness of aquifers across Africa was developed by MacDonald et al. (2010). Four significant ranges of saturated thickness were distinguished. The greatest saturated thickness is seen in the major sedimentary basins of Africa (e.g. the Sirte and Kufra basins in North Africa), whilst shallow weathered basement aquifers are the thinnest.
|Aquifer saturated thickness (m)||Description|
|25 - 100||Moderate|
|100 - 250||Thick|
|> 250||Very thick|
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.