Difference between revisions of "Aquifer properties"

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==Classification of aquifer properties==
 
==Classification of aquifer properties==
 
 
To produce the aquifer properties maps values of aquifer yield, storage type and saturated
 
thickness were divided into significant ranges.
 
 
  
 
===Storage and Flow Type===
 
===Storage and Flow Type===
  
A semi-quantitative assessment of aquifer storage type can be made based on geology and inferred porosity. 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. Storage and flow in unconsolidated sedimentary aquifers is always dominantly intergranular. Younger sedimentary rocks (e.g. Cenozoic and Mesozoic in age) tend to be more loosely consolidated, and groundwater storage and flow is often dominantly intergranular. Older sedimentary rocks tend to be well consolidated, and groundwater storage and flow is typically dominated by fracture flow. Karstic rocks are entirely dominated by rapid fracture flow. Volcanic and intrusive igneous rocks are typically dominated by fracture flow, although some intergranular flow can occur in weathered zones. In Precambrian basement, fracture flow dominates in unweathered rocks, and a mixture of fracture and intergranular flow and storage in weathered basins (MacDonald et al. 2010).  
+
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"
 
{| class = "wikitable"
 
|+ Aquifer Flow and Storage type
 
|+ Aquifer Flow and Storage type
|Description of Flow and Storage type||Main aquifers
+
|Flow and Storage Type||Description||Main aquifer groups
 
|-
 
|-
 
|Intergranular  
 
|Intergranular  
 
||Intergranular storage is highly significant. Rock porosity is generally >0.25. Intergranular flow is dominant.
 
||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  
 
|Intergranular and fracture  
 
||Significant intergranular storage, with mixed intergranular and fracture flow. The average porosity of rocks is approximately 0.1 – 0.25.
 
||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  
 
|Fracture  
 
||Predominantly fracture flow and storage, with only a minor component of intergranular storage. Average rock porosity is < 0.1.
 
||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).
 
|-
 
|-
|+ Special cases
 
 
|Fracture (karst)
 
|Fracture (karst)
||Predominantly fracture flow and storage within karst rocks.
+
||A special case in karst aquifers. Predominantly fracture flow and storage.
 +
||Calcareous (limestone and dolomite) aquifers in which karstic features have developed.  
 
|-
 
|-
 
|Fracture (weathered)
 
|Fracture (weathered)
||Significant fracture flow, with some intergranular storage in weathered zones of otherwise very low porosity <0.01) rocks. All crystalline basement rocks belong to this category - i.e., most Precambrian rocks, with the exception of metasedimentary rocks that show little deformation.
+
||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.
 
|}
 
|}
 
 
Special cases
 
 
   
 
   
  
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|}
 
|}
  
 
 
In general non-weathered
 
Precambrian basement is mapped to be of the lowest productivity. The more productive
 
aquifers in Africa are the Cretaceous-Tertiary sedimentary basins (Figure 4).
 
  
 
===Saturated thickness===
 
===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
  
  
 Saturated thickness – four significant ranges of saturated thickness were mapped –
+
Aquifer saturated thickness
Table 3. Greatest saturated thickness is mapped for the major sedimentary basins of
+
(m)
Africa (e.g. the Sirte and Kufra Basins in North Africa), whilst shallow weathered
+
Description
basement aquifers have been mapped as the thinnest aquifers (Figure 6).
+
<25
+
25-100
 +
100-250
 +
>250
 +
Thin
 +
Moderate
 +
Thick
 +
Very thick
  
Typical borehole yields for each [[Hydrogeological environments of Africa | aquifer group] were
+
{| class = "wikitable"
 +
|+
 +
|Aquifer saturated thickness (m)||Description
 +
|-
 +
|< 25
 +
||Thin
 +
|-
 +
|25 - 100
 +
||Moderate
 +
|-
 +
|100 - 250
 +
||Thick
 +
|> 250
 +
||Very thick
 +
|}
  
  
namely, typical borehole
 
yields, aquifer storage type (e.g. fracture or intergranular), and saturated aquifer thickness.
 
  
  

Revision as of 10:54, 29 September 2015

Africa Groundwater Atlas >> Additional resources >> 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.

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.

Classification of aquifer properties

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.


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 [[Hydrogeological environments of Africa | hydrogeological environments of Africa].

Yield

To develop the hydrogeology map used in 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
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


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 25-100 100-250 >250 Thin Moderate Thick Very thick

Aquifer saturated thickness (m) Description
< 25 Thin
25 - 100 Moderate
100 - 250 Thick > 250 Very thick



Citations and Links


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 BE and Taylor RG. 2012. Quantitative maps of groundwater resources in Africa. Environmental Research Letters, 7 (2), 024009. 10.1088/1748-9326/7/2/024009

MacDonald AM, Ó Dochartaigh BE, Bonsor HC, Davies J and Key R. 2010. Developing quantitative aquifer maps for Africa. British Geological Survey Internal Report IR/10/103, Nottingham, UK.


Africa Groundwater Atlas >> Additional resources >> Aquifer properties