Developing An Africa Hydrogeology Map

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Africa Groundwater Atlas >> Resource pages >> Developing the Africa Groundwater Atlas Hydrogeology Map

Hydrogeological Environments or Aquifer Groups

The aquifers of Africa are divided into the following five major aquifer groups, or hydrogeological environments. 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. More information on these hydrogeological environments can be found on the Overview of Groundwater in Africa page.

Basement aquifers

These incorporate all crystalline basement complex rocks, most of which are Precambrian in age. These develop distinctive local weathered (also called regolith) and fractured aquifers.

Igneous aquifers (largely volcanic)

These are dominantly volcanic rocks, but in some areas incorporate intrusive igneous rocks, such as granites. These form fractured, weathered aquifers that are often strongly controlled by the geometry and weathering of former lava flows.

Consolidated sedimentary aquifers

These have been subdivided according to the dominant groundwater flow type – fracture, intergranular, or a combination of fracture and intergranular.

Unconsolidated sedimentary aquifers

These are largely Quaternary but can also include 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.

The main hydrogeological environments in Africa

Aquifer flow and storage type

To develop the hydrogeology map for this Atlas, a semi-quantitative assessment of aquifer flow and storage 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.

Aquifer flow and storage type
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.


Aquifer flow and storage type across Africa


Aquifer yield

For the hydrogeology map for this Atlas, a synthesis of borehole yield data is used as a proxy for aquifer productivity. Borehole yield data were collated for aquifers across Africa, and their distribution was analysed to assess the typical yield of a borehole in an aquifer.

Six borehole yield categories were distinguished: they are assumed to relate to the average yield that would obtained from a single, effectively sited and developed borehole in the relevant aquifer (MacDonald et al. 2010). They were then interpreted to give a relative aquifer productivity, from very low to very high.

Aquifer productivity
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
Aquifer productivity across Africa


Sources

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). https://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.

Africa Groundwater Atlas >> Additional resources >> Hydrogeology Map