Developing An Africa Hydrogeology Map: Difference between revisions

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[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Additional resources | Resource pages]] >> Developing the Africa Groundwater Atlas Hydrogeology Map
[[Africa Groundwater Atlas Home | Africa Groundwater Atlas]] >> [[Additional resources | Resource pages]] >> Developing the Africa Groundwater Atlas Country Hydrogeology Maps


==Hydrogeological Environments or Aquifer Groups==
==What the Atlas country hydrogeology maps show==


The aquifers of Africa are divided into the following four 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 | '''Overview of Groundwater in Africa''']] page.
The maps provide an overview of the hydrogeology and geology of each of 48 countries in Africa, at 1:5 million scale. They show:


; Basement aquifers
* '''geology''', with geological interpretations that are particularly relevant to hydrogeology; and
* a combined classification of '''aquifer type''' and '''aquifer productivity'''.


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.
More detail on these classifications used in the Atlas country hydrogeology maps is below.  


; Igneous aquifers (largely volcanic)
===Geology===


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.
The Atlas geology maps show the geology of each country at a 1:5 million scale. Their main aim is to show key geological differences that are relevant to aquifer characteristics and hydrogeology. The geological classifications are broadly lithostratigraphical (based on the age and lithology (type) of the rocks), but geological units with similar aquifer (hydrogelogical) characteristics may be grouped together in the Atlas maps, whereas in other geological maps they are separated. However, where possible, the geological unit names used in the maps are those used in the relevant country.  


; Consolidated sedimentary aquifers
===Aquifer Type===


These have been subdivided according to the dominant groundwater flow type – fracture, intergranular, or a combination of fracture and intergranular.  
Aquifer type is defined as one of four main hydrogeological environments, listed below. 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 | '''Overview of Groundwater in Africa''']] page.  


; Unconsolidated sedimentary aquifers
*'''Basement aquifers''': crystalline basement rocks (metamorphic and igneous) most of which are Precambrian in age. These rocks develop distinctive local weathered (also called regolith) and fractured 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.
*'''Igneous aquifers''': mainly volcanic rocks, but in some areas also including intrusive igneous rocks, such as granites, that are younger than the Precambrian. These form fractured, weathered aquifers that are often strongly controlled by the geometry and weathering of former lava flows.
 
*'''Consolidated sedimentary aquifers''': all consolidated (solid) sedimentary rocks. These are usually from Tertiary to Cambrian in age, but also include some older Precambrian sedimentary rocks that have not been highly metamorphosed. They have been subdivided according to the dominant groundwater flow and storage type – fracture, intergranular, or a combination of fracture and intergranular.
 
*'''Unconsolidated sedimentary aquifers''': largely Quaternary but also include some 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.


[[File:Africa_Hgcl_Envs.png|thumb| 400px|center| The main hydrogeological environments in Africa]]
[[File:Africa_Hgcl_Envs.png|thumb| 400px|center| The main hydrogeological environments in Africa]]


==Aquifer flow and storage type==
====Groundwater flow/storage type====


To develop the [[Hydrogeology Map | '''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 way that groundwater flows through aquifers, and is stored, is a key hydrogeological characteristic, and is a fundamental part of the classifications used in the Atlas hydrogeology maps. The maps use a semi-quantitative assessment of aquifer flow and storage type, 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.  
 
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
|Flow and storage type||Description||Main aquifer groups
|Groundwater flow/storage type||Description||Main aquifer groups
|-
|-
|Intergranular  
|Intergranular  
Line 54: Line 57:
|}
|}
   
   
[[File:AfricaFlowStorage.png | center | border |thumb| Aquifer flow and storage type across Africa| 300px]]


[[File:AfricaFlowStorage.png | center | border |thumb| Aquifer flow and storage type across Africa| 400px]]
==Aquifer yield==


For the [[Hydrogeology Map | '''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.
===Aquifer productivity===


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.
Borehole yield is used as a proxy for aquifer productivity in the Atlas hydrogeology maps. Borehole yield data, collated for aquifers across Africa, were analysed to assess the typical yield of a borehole in the main aquifer types. Six borehole yield classes are distinguished. These classes relate to the estimated average sustainable yield that could obtained from a single, effectively sited and developed borehole (MacDonald et al. 2010). The classes are described in terms of relative aquifer productivity, from Very Low to Very High:


{| class = "wikitable"
{| class = "wikitable"
Line 87: Line 87:
|}
|}


[[File:AfricaAqProd.png | center | border |thumb| Aquifer productivity across Africa| 400px]]
[[File:AfricaAqProd.png | center | border |thumb| Aquifer productivity across Africa| 300px]]





Revision as of 10:38, 8 January 2019

Africa Groundwater Atlas >> Resource pages >> Developing the Africa Groundwater Atlas Country Hydrogeology Maps

What the Atlas country hydrogeology maps show

The maps provide an overview of the hydrogeology and geology of each of 48 countries in Africa, at 1:5 million scale. They show:

  • geology, with geological interpretations that are particularly relevant to hydrogeology; and
  • a combined classification of aquifer type and aquifer productivity.

More detail on these classifications used in the Atlas country hydrogeology maps is below.

Geology

The Atlas geology maps show the geology of each country at a 1:5 million scale. Their main aim is to show key geological differences that are relevant to aquifer characteristics and hydrogeology. The geological classifications are broadly lithostratigraphical (based on the age and lithology (type) of the rocks), but geological units with similar aquifer (hydrogelogical) characteristics may be grouped together in the Atlas maps, whereas in other geological maps they are separated. However, where possible, the geological unit names used in the maps are those used in the relevant country.

Aquifer Type

Aquifer type is defined as one of four main hydrogeological environments, listed below. 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: crystalline basement rocks (metamorphic and igneous) most of which are Precambrian in age. These rocks develop distinctive local weathered (also called regolith) and fractured aquifers.
  • Igneous aquifers: mainly volcanic rocks, but in some areas also including intrusive igneous rocks, such as granites, that are younger than the Precambrian. These form fractured, weathered aquifers that are often strongly controlled by the geometry and weathering of former lava flows.
  • Consolidated sedimentary aquifers: all consolidated (solid) sedimentary rocks. These are usually from Tertiary to Cambrian in age, but also include some older Precambrian sedimentary rocks that have not been highly metamorphosed. They have been subdivided according to the dominant groundwater flow and storage type – fracture, intergranular, or a combination of fracture and intergranular.
  • Unconsolidated sedimentary aquifers: largely Quaternary but also include some 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

Groundwater flow/storage type

The way that groundwater flows through aquifers, and is stored, is a key hydrogeological characteristic, and is a fundamental part of the classifications used in the Atlas hydrogeology maps. The maps use a semi-quantitative assessment of aquifer flow and storage type, 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
Groundwater flow/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 productivity

Borehole yield is used as a proxy for aquifer productivity in the Atlas hydrogeology maps. Borehole yield data, collated for aquifers across Africa, were analysed to assess the typical yield of a borehole in the main aquifer types. Six borehole yield classes are distinguished. These classes relate to the estimated average sustainable yield that could obtained from a single, effectively sited and developed borehole (MacDonald et al. 2010). The classes are described in terms of 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 >> Resource pages >> Developing the Africa Groundwater Atlas Hydrogeology Map