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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa>


2.1.8 Well-tank Borehole Well

Technical Description

This is a type of borehole in combination with a well. The latter serves as storage. This technology is used in cases where hydrogeological conditions are such that neither structure alone can meet the operational needs of the beneficiaries.

The borehole is drilled to groundwater level, and the well-tank, which is 0.5 m to 1 m away from the borehole, is drilled to a depth such that the static level of the borehole is at least 6 m higher. This difference in elevation provides a sufficient water level in the well to permit easy abstraction of the stored water. This water depth in the well-tank is maintained by the construction of a junction between the well-tank and the borehole at the bottom of the well-tank at the static water level (Figure 35).

Extent of Use

This technology has been used in projects in Mauritania. Similar systems, with slightly different arrangements of the components, exist in Egypt, Libya and Sudan.

Operation and Maintenance

Since the well-tank is below the static water level in the borehole, water enters the well-tank by gravity through the perforated junction between the well and the borehole. The depth of water in the well-tank becomes that of the borehole. Abstracting of water is carried out in the traditional way from the well-tank. Maintenance is carried out periodically by cleaning the well-tank as and when necessary. The well-tank should be protected against contamination as described elsewhere in this volume.

Figure 35

Figure 35. Well tank - borehole well.


 

Level of Involvement

The capital investment in the borehole may be prohibitive for individuals, but the technique can be established jointly by individuals within communities. The technology can be implemented with limited technical support from extension agencies.

Costs

The costs include the drilling costs of the borehole and well-tank, on the one hand, and the cost of providing the superstructure of the well, on the other. The cost per linear metre of establishing the borehole, which can be up to between 100 m and 400 m deep, is over $300. The cost of establishing and protecting the well is between $260 and $408/linear metre.

Effectiveness of the Technology

The borehole well is as efficient as the simple well, and, generally, is more reliable. The well-tank system experiences fewer seasonal fluctuations in water level compared to other wells in the regions in which the technology has been applied, and, as a result, rarely dries up. Suitability Well-tanks are adaptable for use in all terrains. However, in areas with discontinuous aquifers, where groundwater is captured in very hard, fractured geologic structures, and/or where wells cannot readily penetrate the substrate, storage can be provided by surface cisterns. Similarly, in sedimentary basins, shallow wells may be more appropriate storage structures.

Environmental Benefits

Use of this technology makes groundwater readily available at the surface. If used within conservation areas, this technology can provide water for environmental rehabilitation programmes, especially in areas where this would otherwise be impossible. However, care should be taken to avoid over-abstraction of groundwater, and prevent contamination of groundwater from surface sources.

Advantages

Use of this technology allows the use of simple abstraction methods, and can provide a reliable source of water.

Disadvantages

The technology has an high capital investment cost, and is dependent upon a reliable groundwater source.

Cultural Acceptability

There are no known cultural inhibitions relating to the use of this technology.

Information Sources

CIEH, 01 BP 369 Ouagadagou 01, Burkina Faso, Tel. 307112, Fax: 362446.

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