Difference between revisions of "Aquifer properties"

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[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Additional resources | Additional resources]] >> Aquifer Properties
 
[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Additional resources | Additional resources]] >> Aquifer Properties
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This page is in development. Please check back soon for updates.
  
 
==Aquifer properties==
 
==Aquifer properties==
  
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'''.  
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Aquifer properties are the hydraulic characteristics of aquifers, which are used to describe an aquifer and understand how groundwater in the aquifer exists and behaves. Aquifer properties are the most important way to describe the hydrogeology of an aquifer.  
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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.  
 
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.  
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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 | hydrogeology map]] used in this Atlas.  
 
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 | 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.
 
 
{| class = "wikitable"
 
|+ 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.
 
|}
 
 
  
For more information see an [[Overview of Groundwater in Africa| Overview of Groundwater in Africa]].
 
 
[[File:AfricaFlowStorage.png | center | border |thumb| Aquifer flow and storage type across Africa| 400px]]
 
 
===Aquifer yield===
 
 
To develop the Africa-wide [[Hydrogeology Map | 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).
 
 
{| class = "wikitable"
 
|+ 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
 
|}
 
 
[[File:AfricaAqProd.png | center | border |thumb| Aquifer productivity across Africa| 400px]]
 
 
See also the [[Hydrogeology Map | 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.
 
 
 
{| class = "wikitable"
 
|+ Aquifer saturated thickness
 
|Aquifer saturated thickness (m)||Description
 
|-
 
|< 25
 
||Thin
 
|-
 
|25 - 100
 
||Moderate
 
|-
 
|100 - 250
 
||Thick
 
|-
 
|> 250
 
||Very thick
 
|}
 
 
 
[[File:AfricaSatThick.png | center | border |thumb| Aquifer saturated thickness across Africa|  400px]]
 
 
===Citations===
 
 
MacDonald, A M, and Davies, J. 2000. [http://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, A M, Bonsor, H C, Ó Dochartaigh, B É, and Taylor, R G. 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, A M, Ó Dochartaigh, B É, Bonsor, H C, Davies, J, and Key, R. 2010. [http://nora.nerc.ac.uk/501779/ Developing quantitative aquifer maps for Africa]. British Geological Survey Internal Report, IR/10/103.
 
  
  
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[[Category:Additional resources]]
 
[[Category:Additional resources]]
 
[[Category: Overview of Groundwater in Africa]]
 
[[Category: Overview of Groundwater in Africa]]
 
 
[[Category:Africa Groundwater Atlas]]
 
[[Category:Africa Groundwater Atlas]]

Revision as of 15:15, 28 March 2018

Africa Groundwater Atlas >> Additional resources >> Aquifer Properties

This page is in development. Please check back soon for updates.

Aquifer properties

Aquifer properties are the hydraulic characteristics of aquifers, which are used to describe an aquifer and understand how groundwater in the aquifer exists and behaves. Aquifer properties are the most important way to describe the hydrogeology of an 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.



Africa Groundwater Atlas >> Additional resources >> Aquifer properties