Overview of Groundwater in Africa: Difference between revisions

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Crystalline basement rocks of Precambrian age underlie much of Africa. They form low productivity aquifers that provide small rural water supplies for tens, if not hundreds, of millions of people. Groundwater occurs where the rocks have been significantly weathered and/or in fracture zones, most of which are usually shallower than a few tens of metres depth. Borehole and well yields are generally low, but usually sufficient for rural demand.
Crystalline basement rocks of Precambrian age underlie much of Africa. They form low productivity aquifers that provide small rural water supplies for tens, if not hundreds, of millions of people. Groundwater occurs where the rocks have been significantly weathered and/or in fracture zones, most of which are usually shallower than a few tens of metres depth. Borehole and well yields are generally low, but usually sufficient for rural demand.
[[File:weathered basement.png | 400px | thumb| Weathered basement aquifer |left ]]


====Volcanic====
====Volcanic====


Volcanic rocks underlie a small but significant proportion of Africa's land area, and are an important water source for tens of millions of people, many of whom live in the drought stricken areas of the Horn of Africa. Groundwater in volcanic aquifers is found within palaeosoils and fractures between lava flows. Yields can be high, and springs are important sources in highland areas.
Volcanic rocks underlie a small but significant proportion of Africa's land area, and are an important water source for tens of millions of people, many of whom live in the drought stricken areas of the Horn of Africa. Groundwater in volcanic aquifers is found within palaeosoils and fractures between lava flows. Yields can be high, and springs are important sources in highland areas.
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<li style="display: inline-block;"> [[weathered basement.png| 300 px| left | Groundwater occurrence in a weathered basement aquifer]] </li>
<li style="display: inline-block;"> [[volcanic_aquifers.png| 300 px| right| Groundwater occurrence in a volcanic rock aquifer]] </li>
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====Consolidated sedimentary aquifers====
====Consolidated sedimentary aquifers====
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Unconsolidated sediments directly underlie much of Africa, and are extremely important for both rural and urban water supplies. Unconsolidated sands and gravels occur in most river valleys throughout Africa, and in many coastal areas. These deposits are often highly permeable and can store large volumes of groundwater at shallow depths, which is easy to exploit by traditional shallow wells and boreholes.  
Unconsolidated sediments directly underlie much of Africa, and are extremely important for both rural and urban water supplies. Unconsolidated sands and gravels occur in most river valleys throughout Africa, and in many coastal areas. These deposits are often highly permeable and can store large volumes of groundwater at shallow depths, which is easy to exploit by traditional shallow wells and boreholes.  
<center>
<div><ul>
<li style="display: inline-block;"> [[sedimentary_aquifers.png| 300 px| left | Groundwater occurrence in a consolidated sedimentary aquifer]] </li>
<li style="display: inline-block;"> [riverside_alluvium.png| 300 px| right| Groundwater occurence in unconsolidated valley alluvium]] </li>
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===Further Information===
===Further Information===

Revision as of 13:48, 6 April 2016

Africa Groundwater Atlas >> Additional resources >> Overview of Groundwater in Africa

Groundwater in Africa

Groundwater has many advantages as a water supply source, particularly where populations are still largely rural and demand is dispersed across large areas. The main advantages and limitations of groundwater as a water resource are summarised in the table below.


Advantages and limitations of groundwater as a water resource in Africa
Advantages Limitations
Found in most environments and usually available close to point of demand In some environments, considerable investment is needed to locate and develop suitable sites
Generally excellent natural water quality & suitable for potable use with no prior treatment There is increasing recognition of natural water quality constraints, such as sulphate, iron, arsenic and fluoride
Naturally more protected from contamination than surface waters As human development increases, the threat of pollution needs to be assessed in relation to the nature of the intrinsic aquifer protection
Large volumes of natural storage & seasonal variations in amount or quality aren't usually significant. More drought resistant than surface waters Groundwater can be vulnerable to over-abstraction, particularly in low productivity aquifers and/or as demand and the ability to abstract large volumes of water both grow. Long term changes in rainfall patterns can also impact on groundwater recharge and renewal
Lends itself well to principles of community management. Can be developed incrementally, often at relatively low cost/initial capital investment As overall water supply coverage increases, more hydrogeologically difficult areas can remain unserved, and they become more costly to develop

Groundwater Occurrence in Africa

How and where groundwater occurs depends primarily on geology; geomorphology/weathering; and rainfall (both current and historic). The interaction between these three factors gives rise to complex hydrogeological environments with countless variations in the quantity, quality, ease of access to and renewability of groundwater resources. Developing groundwater resources therefore depends on a good understanding on hydrogeology.

Africa has huge diversity in geology, climate and hydrology, and as a result, the hydrogeology of Africa is hugely variable. At a continental scale, there are four main types of hydrogeological environment (or aquifer type): basement aquifers; volcanic aquifers; consolidated sedimentary aquifers (which can be dominated by either fracture and/or intergranular flow); and unconsolidated sedimentary aquifers. The hydrogeology - how groundwater exists and behaves - is different in each hydrogeological environment, and so different methods are needed to find, abstract and manage groundwater in each group. A detailed description of these environments is in MacDonald and Davies (2001); and a summary is below.

The main hydrogeological environments in Africa

Basement

Crystalline basement rocks of Precambrian age underlie much of Africa. They form low productivity aquifers that provide small rural water supplies for tens, if not hundreds, of millions of people. Groundwater occurs where the rocks have been significantly weathered and/or in fracture zones, most of which are usually shallower than a few tens of metres depth. Borehole and well yields are generally low, but usually sufficient for rural demand.

Volcanic

Volcanic rocks underlie a small but significant proportion of Africa's land area, and are an important water source for tens of millions of people, many of whom live in the drought stricken areas of the Horn of Africa. Groundwater in volcanic aquifers is found within palaeosoils and fractures between lava flows. Yields can be high, and springs are important sources in highland areas.

Consolidated sedimentary aquifers

Consolidated sedimentary rocks underlie around one third of Africa's land area, and can form thick, highly productive aquifers. The most significant aquifers are sandstones and limestones, which can be exploited for large urban as well as rural supplies. Mudstones however, which account for about 65% of all sedimentary rocks in Africa, contain little groundwater, and careful study is required to develop groundwater supplies from mudstones.

Unconsolidated sedimentary aquifers

Unconsolidated sediments directly underlie much of Africa, and are extremely important for both rural and urban water supplies. Unconsolidated sands and gravels occur in most river valleys throughout Africa, and in many coastal areas. These deposits are often highly permeable and can store large volumes of groundwater at shallow depths, which is easy to exploit by traditional shallow wells and boreholes.

Further Information

More information on geology and aquifer characteristics across Africa can be found in these resource pages: geology; hydrogeology map; and aquifer properties. More detailed information on aquifers in each country can be found in the country pages.

More maps summarising the hydrogeology of Africa: Quantitative Groundwater Maps for Africa

MacDonald, A.M. & Davies, J. 2000. A brief review of groundwater for rural water supply in sub-Saharan Africa. British Geological Survey Report WC/00/033.

MacDonald, A.M., Bonsor, H.C., Ó Dochartaigh, B.É. & Taylor, R.G. 2012. Quantitative maps of groundwater resources in Africa. Environmental Research Letters 7(2).

MacDonald, A.M. & Calow, R.C. 2009. Developing groundwater for secure water supplies in Africa. Desalination 248, 546-556. doi: 10.1016/j.desal.2008.05.100


Africa Groundwater Atlas >> Overview of Groundwater in Africa