Groundwater in the Basin

Distribution of Aquifers

Groundwater is found in sub-surface formations called aquifers. In southern Africa groundwater is a critical resource, due to the limited availability and variable quality of surface water resources (UNEP 2009). An aquifer is a geological formation, or part of a formation below the surface that is capable of yielding a sufficient amount of water when tapped through boreholes, dug wells or springs (SADC 1991). There are three types of aquifers governing the occurrence and distribution of groundwater in the Kunene River basin (LNEC 1996):

• Pre-Cambrian crystalline units have low permeability with limited hydrogeological potential near fractures and in areas with disintegration of crystalline rocks. Such aquifers have commonly low productivity and limited potential. They occur mainly in the Upper Kunene.

• Shallow artesian aquifers are located between 10 and 50 meters deep, containing accumulated waters in the Kalahari basin or quaternary deposits in the Middle Kunene. These aquifers are very irregular in dimensions, depth, productivity and quality. Recharge is mainly through flooding in the Middle Kunene and precipitation in the Upper Kunene.

• Deep artesian aquifers (Cambrian and Infra-Cambrian) composed of various base materials. Depending on the parent material, the aquifer can have low permeability (shale, clay or silt) or behave like karst in areas with carbonate rich rock (such as limestone). These occur only in the western part of the Lower Kunene near the Atlantic Ocean.

A map of groundwater recharge in the Kunene River basin is provided below.

Transboundary Aquifers

A transboundary aquifer is an aquifer that crosses the boundary between two or more countries. Transboundary aquifers are often subject to “conflicts of interest” created by the resource partition with respect to different environmental, economic and social targets of the sharing countries. The management of transboundary aquifers is subject to multilateral agreements on water abstraction, prevention of pollution and any other relevant measure (Vasak and Kukuric 2006).

The map below is a regional overview of these transboundary aquifers, broken down into four key categories:

• PreCambrian 'Basement' aquifers;
• Volcanic Rocks;
• Consolidated sedimentary rocks (Cambrian and younger); and
• Unconsolidated sediments (mainly Quaternary).

There are two transboundary aquifers systems found in the Kunene basin (UNESCO 2008). The “Coastal Sedimentary Basin”, a small aquifer visible along the Atlantic Coast is composed of Precambrian basement, while the “Cuvelai-Etosha Basin” in the east of the basin is composed of Cambrian (and younger) consolidated sedimentary rocks.

Groundwater Quality

Groundwater in the basin is generally considered as being of good quality, although very little quantitative information is available, especially for the Angolan part of the basin.

In the Upper Kunene, the quality is good but with slightly elevated SO₄ (sulphate) concentrations, particularly in Karoo sandstones, shales and dolomites (DWA, 1986). In the Kalahari sands of the Middle Kunene, high salinity caused by the dissolution of mineral salts occurs in some sedimentary horizons. Also, the intrusion of saline water into coastal aquifers is a major threat to groundwater quality.  

Groundwater in Precambrian basement areas is generally fresh although it may be potentially corrosive with low pH values and it may contain a high iron concentration. Precambrian basement areas are usually also prone to high fluoride concentrations due to the weathering of fluorine bearing minerals (Chilton and Foster, 1995).

Groundwater Potential

The semi- to unconsolidated strata of the Kalahari Group are generally fine to very fine-grained and have low primary porosity. Boreholes drilled into these aquifers are generally low yielding and produce water of variable salinity, with a low productivity of 2 - 5 m³/h.  On deposits of sandy clay origin, water infiltration is low and the major part of precipitation is runoff with very little percolation to the groundwater. Where bedrock is intrinsically impermeable groundwater occurrence is limited to zones of secondary permeability formed by fracturing and faulting. In some cases folding of the strata causes movement along bedding planes which may result in the development of secondary porosity.

Alluvial or sand aquifers generally have limited storage but experience relatively rapid recharge after significant run-off events.

Despite low rainfall in the Lower Kunene, fracture aquifers can be found which provide adequate water for domestic consumption and livestock farming.
There are a large number of springs in this area as a result of the impermeable nature of the basement, whereby the water will flow slowly down the gradient until it encounters impermeable, massive bedrock and is forced to the surface (LNEC 1996).  In a few areas in the Lower Kunene, dolomite and limestone terrain fault structure or formation hosted aquifers undergo extensive solution weathering which enlarges the secondary porosity, giving rise to “karst” features. Solution weathering occurs along bedding planes, faults and joints, through the percolation of meteoric water containing carbonic acid from dissolution of atmospheric CO₂, which dissolves the carbonate rocks (MAWRD 2000)

Distribution of Boreholes

Boreholes are the most common form of groundwater abstraction in southern Africa (SADC 1991).  In the basin, boreholes are mostly drilled in the Lower Kunene due to the scarcity of surface water and low rainfall. Generally, the water table in the central ranges is between 10 and 30 meters deep, but in the Lower Kunene province, the water table is found up to 220 meters deep in areas away from rivers.

In the basin, there are more than 2 000 tube wells holes in operation, primarily for domestic use and agriculture but their yield is generally low, between 1 to 10 litres per second. Most of the boreholes do not exceed depths of 100 meters except in Lower Kunene. The last inventory with information on their spatial distribution (by province) dates from 1992 (Minader/FAO 2004).