Solution Exercise 3

a) There are a number of different surface and underground environments one may encounter in Africa. The natural recharge of groundwater is related to climatic conditions such as average temperatures and evapotranspiration, and geological factors such as porosity and infiltration rates. The potential for natural recharge is relatively low across much of Southern Africa, improving to the north, due primarily to increased precipitation.

Approximately 55 percent of the region is covered by low permeability formations. These are largely basement rocks with aquifer systems developed in the weathered overburden and in the fractured bedrock. The aquifers developed in these areas are unconfined, not spatially extensive and locally developed. In general, only modest groundwater supplies can be abstracted sustainably from these aquifers and large-scale groundwater well fields are not feasible. Fissured aquifer systems are associated particularly with the Karoo formations (interlayered shales and sandstones) found extensively throughout southern Africa. The formations are normally low yielding, but where the rocks have been subjected to deformation and intrusion of dolerites, a secondary permeability resulting in good aquifers may be found.

The unconsolidated intergranular aquifer systems occur as large inland basins, such as the Kalahari and Congo basins, in coastal aquifers and in alluvial aquifers in river channels, banks and flood plains.

Karst aquifers are not wide-spread in the region, but constitute some of the most productive aquifers in Namibia, Botswana, Zimbabwe and South Africa. They occur in highly soluble rock, most notably limestone and dolomite. Groundwater flow is concentrated along secondarily enlarged fractures and fissures and other connected openings, where the chemical dissolving action of slightly acidic (rain) water can take place.

b) Similar to the variations in the hydrogeological conditions, the artificial recharge techniques also vary widely. The artificial recharge techniques can be broadly categorized as follows:

Table Exercise 3-1  Categorized artificial recharge techniques.

Direct surface techniques
  • Flooding
  • Basins or percolation tanks
  • Stream augmentation
  • Ditch and furrow system
  • Over-irrigation
Direct subsurface techniques
  • Injection wells or recharge wells
  • Recharge pits and shafts
  • Dug well recharge
  • Bore hole flooding
  • Natural openings, cavity fillings
Combination surface – subsurface techniques
  • Basin or percolation tanks with pit shaft or wells.
Indirect Techniques
  • Induced recharge from surface water source
  • Aquifer modification

In the Southern Africa region, MAR started in the coastal aquifers, therefore the emphasis was on direct surface techniques and this is one possibility for the potential project. So, (1) the traditional ‘sand dams on alluvial aquifers’ are a possible approach. For the more challenging hard-rock environments, (2) injection wells are a more likely solution. For either environment, if agriculture is a major water user, (3) over irrigation is a possible option. In the less likely scenario in which the aquifer has natural openings, (4) treated wastewater could be introduced.

Return to Exercise 3


Managed Aquifer Recharge: Southern Africa Copyright © 2021 by Eberhard Braune and Sumaya Israel. All Rights Reserved.