Groundwater

Groundwater in SADC

Groundwater is a critical source of water for people living in the SADC region, with approximately 37 % of the population of the region relying on formal or improved groundwater sources, 23 % from formal surface water supplies and 40 % relying on unimproved groundwater and surface water sources. This level of dependence is combined with the fact that approximately one third of the population of the region live in drought-prone conditions, making groundwater an even more precious resource (SADC 2011).

As part of the Regional Strategic Action Plan, SADC is leading a Groundwater and Drought Management Project. One of the primary outputs of this project will be the creation of a Regional Groundwater Management Institute of Southern Africa, to be located at the University of Bloemfontein, in South Africa.

In addition to the materials provided below, the Document library provides access to a series of Groundwater Briefing Notes created by the World Bank GWMate team. Also available in the Document library are the Cap-Net Groundwater Management in IWRM Training Manual and the UNESCO publication 'Transboundary Aquifers: Managing a Vital Resource'.

Groundwater Resources

Groundwater is used extensively throughout the southern African region, including the Limpopo River basin, supplying a large percentage of water for irrigation and rural water supply schemes (FAO 2004). This is especially true in rural areas, located away from surface water resources. Groundwater is also used extensively by the mining industry in the basin (CGIAR 2003).

The map below, generated by the WHYMAP initiative illustrates the distribution of groundwater resource types and recharge potential across the basin.

Due to limited surface water resources, particularly in the south of the country, Botswana's rural population is highly dependent on groundwater (FAO 2004). Approximately 65 % of water resources supplied in Botswana comes from groundwater (Els and Rowntree n.d.). Rural groundwater supplies are now augmented by surface water inputs from the North-South Water Carrier, which is due to be linked to Serowe in the Limpopo River basin during 2010.

While groundwater potential in Mozambique is generally quite high (Kundell 2007 and Barros 2009), Sogreah (1993) states that potential in the Mozambique portion of the Limpopo River basin is poor, with high mineralisation in many of the aquifers. The Dune area along the coast of Mozambique is thought to have good groundwater potential, with approximately 5 to 10 m³/h per km².

Groundwater plays an important role in water supply in rural areas of the South African portion of the Limpopo River basin, providing domestic and irrigation water in the order of approximately 850 Mm³/year. Many of the rural communities of this region are located on or near marginal aquifers with potential yields of 2 l/s (FAO 2004). Groundwater quality is relatively poor due to high salinity. Groundwater resources in the Dendron area of the Limpopo Province of South Africa have been severely overexploited. See the box below for details. WISA (2010) and DWAF (2004) make the following observations regarding the groundwater resources of the South African portion of the basin:

Marico sub-basin - high yields are obtained from dolomite aquifers and local groundwater sources (alluvial aquifers);
Crocodile sub-basin - widely available groundwater resources across the sub-basin, with approximately 125 Mm³/yr used for a combination of agricultural irrigation, domestic, industrial and municipal supplies.
Limpopo sub-basin - widely available groundwater resources with varying quality are the major source of water in the sub-basin, with intensive use focused North of Polokwane and around Dendron for domestic and irrigation uses.
Lvuvuhu/Letaba sub-basins - groundwater is used conjunctively (with surface water) through this sub-basin. Quantity and quality vary significantly depending on underlying aquifer characteristics.
Olifants sub-basin - high groundwater yields are associated with weathered hard rock and dolomites and used extensively for rural water supply and livestock watering.

Groundwater Resources in southern Zimbabwe are not very productive, with many of the domestic wells and boreholes supplying individual households and small communities drying up before the end of the dry season (FAO 2004). Land degradation resulting from poor land use management have meant that the remaining dambo wetlands of the Matabeleland south province have long dried up.

Groundwater Recharge

To date, no basin-wide groundwater recharge assessment has been performed, but localised data scattered across the basin does provide a certain insight into the recharge situation in the basin. In the absence of an abundance of groundwater recharge information, the map above can be used as a guide with respect to the distribution of recharge potential.

Groundwater recharge in the Limpopo river basin in Botswana is thought to be very low, with rates of approximately 1 to 3 mm/yr in the Kalahari and central regions, with 5 to 9 mm/yr in the eastern regions, excluding the Tuli block. Total water abstraction in Botswana in 1990 was 76 x more than annual recharge, with abstraction predicted to rise by almost 100 % by 2020.

Transboundary Aquifers

In addition to the local aquifer systems, the Limpopo River basin includes a series of transboundary aquifers. These aquifers are particularly important as they are shared by two or more countries, requiring cooperative management of water use/abstraction and sources of pollution that may affect them. The map below (left) 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)
Unconsolidated sediments (mainly Quaternary).

Exploitation of Groundwater

Groundwater is primarily abstracted through boreholes (groundwater wells), drilled from the surface by a groundwater drilling rig (possibly include picture). The map at the bottom of this page shows an estimated distribution of boreholes across the Limpopo River basin. The actual number of boreholes may vary significantly as many boreholes are not registered when drilled or were drilled prior to the creation of national inventories.

Overall the potential yields of boreholes in the Limpopo River basin is relatively low, limiting the extent to which groundwater can be used for large scale water supply. The exceptions to this are areas accessing the alluvial aquifers along the Limpopo River.

While potential yield is low, groundwater forms an essential resource in times of low stream/river flow (CGIAR 2003).

The images below show the drilling and construction of a very high yielding borehole in the Limpopo Province of South Africa. This borehole was drilled to a very broad diameter to handle the high rate of abstraction - 40 L/s. The borehole was drilled 180 m down, passing through basalts that yield poor quality water, down to a sandstone aquifer below that provides high yields of good quality groundwater. The borehole was constructed in such a way as to screen off the basalt layer, allowing only the good quality, deep groundwater to be abstracted.

The water from this borehole and the associated wellfield in Kromhoek provides water to the community of Alldays some 25 km away and Venetia Diamond Mine approximately 20 km away.

Over-Exploitation of Groundwater in Limpopo Province, South Africa

The Dendron area is one of the prime examples in South Africa where uncontrolled extraction of groundwater on private farms for irrigation purposes greatly exceeded recharge, leading to unsustainable development. In the 1970s and 1980s, on a cluster of farms on which boreholes supplied copious volumes of groundwater, a flourishing potato production industry developed in this semi-arid area. The area receives 440 mm mean annual summer rainfall, and the seasonal recharge varies between 3 and 35 mm (1-8 percent of the MAP). After a number of years and great expenditure, the granite aquifer became depleted and potato production ground to a permanent halt.

There were two issues in this case. The first was a lack of recognition of the fossil nature of the groundwater body, and the second was the way safe delivery was estimated. Borehole yield information was based on the initial drilling-rig blow test of the borehole. This test was later shown to be overgenerous. In recent years, DWAF has been recommending 30-50 percent of the blow yield for long-term use (Bang and Stimie, 1999).

Source: FAO 2004

Groundwater Information

The majority of groundwater information available in the region is at a country-level, with very little information aggregated at a basin or regional scale. SADC, supported by international financial and technical agencies, is currently leading an initiative to develop regional groundwater information resources to support transboundary water resources management and cooperation.

While much of the detailed information available on groundwater at a country-level such as borehole data use different protocols for naming and storing information, making integration problematic; general geological and hydrogeological information is to a large extent compatible (LBPTC 2010). As a result regional geology and hydrogeology maps are becoming available for the region.