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Stampriet Aquifer

Stampriet Aquifer

The area of the STAS The area of the STAS

The assessment of the Stampriet Transboundary Aquifer System (STAS) was undertaken based on a multi-disciplinary methodology developed by the UNESCO International Hydrological Programme (IHP) and the International Groundwater Resources Centre (IGRAC) that includes the collection and processing of national data (hydrogeological, socio-economic and environmental, gender, legal and institutional), and the harmonization of data across all three countries to enable a joint assessment of the transboundary resource.

Activities were done within the framework of the Governance of Groundwater Resources in Transboundary Aquifers (GGRETA) project, executed by UNESCO-IHP, in close cooperation with IGRAC, and funded by the Swiss Agency for Development and Cooperation (SDC). The assessment has been carried out by a team familiar to the area and composed of professionals of Botswana, Namibia and South Africa. Apart from collecting and studying relevant literature for assessment and diagnostics, the team has spent much attention to compiling basic data and to GIS mapping. In total, more than 40 thematic maps were prepared and have been uploaded to the Information Management System (IMS) developed by the project.

Location, delineation and type of aquifer

The Stampriet Transboundary Aquifer System (STAS) covers a large arid region stretching from Central Namibia into Western Botswana and South Africa’s Northern Cape Province. It contains two confined sandstone aquifers and the overlying unconfined Kalahari aquifer units. The delineation of the STAS area follows the outer boundary of the so-called Ecca Group of geological formations within the catchments of the Auob and Nossob rivers. The STAS is a very large transboundary aquifer system, receiving insignificant recharge, in a dry region without permanent surface water.

General features of the STAS area

The STAS area is 86,647 km2 in extent and has a generally flat topography, gently sloping from NW to SE, between 1,500 and 900 m above mean sea level. It has a hot and dry climate, with an annual mean temperature of 19-22 oC and mean rainfall ranging from 140 mm/yr in the SW to 300 mm/yr along the northern and north-eastern border. During the period May through September there is hardly any rainfall. The Namibian sector of the area covers approximately two-thirds of the area and is almost completely in use as agricultural land. The Botswana sector occupies 18% and includes from North to South three distinct land use zones: agricultural land (mainly in Ghanzi district), wildlife management area and national park. The South African sector (7%) is entirely used as national park. With an estimated population of nearly 50,000 – more than 90% of which in Namibia – the area is sparsely populated. Major settlements are Aranos and Koes, but their population is less than 5,000; all other settlements have less than 2,000 inhabitants. Commercial industrial and mining activities are absent.

Water and water use in the area

Given the climatic and other geographic features, there are no permanent rivers in the STAS area. Apart from the ephemeral rivers Auob and Nossob that provide some water during the rainy season, there are surface water pans scattered over the area that collect and store water for livestock watering; these reserves can last a few months after the rains. The only permanent and dependable water resource in the area is groundwater. Groundwater is withdrawn from the Kalahari, Auob and Nossob aquifers, by means of dug wells and boreholes (see below). It is estimated that at least 20 million cubic metre per year is abstracted; 65% of this volume comes from Kalahari aquifers, 33% from the Auob aquifer and 2% from the Nossob aquifer. The breakdown of overall water use is as follows: 52% for irrigation, 32% for stock watering and 16% for domestic use. In general, the urban centres and villages receive water from governmental and parastatal water supply corporations.  Private land owners usually have their own wells.

The Stampriet Transboundary Aquifer System

Location of boreholes in the STAS area Location of boreholes in the STAS area

From a geological point of view, the STAS area is part of a huge sedimentary basin in which a thick sequence of layers has been deposited. The layers of Carboniferous through Jurassic age are together known as Karoo Supergroup and contain mainly sandstones, shales, mudstones, siltstones and limestone. They are covered by a blanket of sediments of the Kalahari Supergroup, of Tertiary-Quaternary age and consisting predominantly of sand, calcrete (duricrust), gravel, clayey gravel, sandstone and marl.

The Kalahari sediments contain phreatic aquifers (free water table), easily within reach of the local inhabitants. Their lateral extent is limited, hence they form local aquifers, and the Kalahari sediments as a whole do not constitute a transboundary aquifer system. By drilling a deep borehole in 1912 near Stampriet, a deeper aquifer was struck – the Auob aquifer– that produced a free-flowing or artesian well. This aquifer is laterally continuous across the international borders, like the deeper located Nossob aquifer. Both aquifers are confined sandstone aquifers, producing truly artesian conditions in parts of the depressed valleys of the Auob and Nossob rivers, which prompted the name Stampriet Artesian Basin (SAB).

The confined Auob and Nossob aquifers, and the overlying discontinuous phreatic Kalahari aquifers, form the Stampriet Transboundary Aquifer System. Conceptually, the physical processes taking place in this system are reasonably understood, but quantification is still limited –in spite of many efforts made over a long period of time. What is known on the groundwater quantity processes of the confined aquifers is summarized in simplified form on the map of the conceptual model shown on the next page. Apart from diffuse recharge by downward seepage from the Kalahari aquifers, there are a few recharge zones in the Western part of the STAS area where sinkholes facilitate concentrated recharge during rare wet years. The mean annual recharge rate for these confined aquifer units is likely to be significantly less than that of the Kalahari aquifers, for which rates of around 1 mm/year, averaged over the area, have been estimated. The general direction of groundwater flow is from NW to SE. In the South-Eastern quadrant of the area, groundwater massively seeps upward from the confined aquifers and discharges into the Kalahari formations, from where it evaporates. Groundwater salinity in this zone – known under the name Salt Block – therefore is rather high.

Main groundwater management challenges in the area

Geological SW-NE cross-section through the STAS area Geological SW-NE cross-section through the STAS area

Lack of monitoring data (climate, groundwater abstraction, water levels, water quality) seriously hampers a systematic diagnostic analysis. Nevertheless, the findings and combined experiences of the assessment team have revealed a number of challenges.

As regards groundwater quantity, real stress is not yet being observed. Lack of monitoring may be an explanation, but the exceptionally low intensity of groundwater withdrawal certainly plays a major role. If for some reasons (population growth, economic development, etc) the demand for water in the region would increase significantly, then very soon the groundwater resources may run short of meeting these demands. Therefore it is very important to initiate effective control of groundwater quantity, e.g. by some initial practical steps such as solving the problem of water spillage by leaking boreholes in the Auob aquifer and preventing future problems by improved regulation of drilling.

Groundwater quality has its natural variations. Most notable are generally poor conditions in or near the Salt Block zone. Pollution, however, may also lead to groundwater quality degradation elsewhere in the area. The confined transboundary aquifers have very low vulnerability to pollution, but they will experience higher withdrawal pressures if overlying Kalahari aquifers become polluted. The more shallow and usually phreatic Kalahari aquifers are vulnerable to pollution; in particular in the Namibian sector the pollution risk is often medium to high due to irrigated agriculture (using fertilizers and pesticides) and environmentally unfriendly sanitation and waste disposal practices. Partly from the groundwater management point of view and partly for health reasons, there is scope for enhancing water supplies and even more for improving sanitation in the entire area.