Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for Freshwater Augumentation
2.1.8 Well-tank Borehole Well
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
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. 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
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.
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.
Use of this technology allows the use of simple abstraction methods, and can
provide a reliable source of water.
The technology has an high capital investment cost, and is dependent upon a
reliable groundwater source.
There are no known cultural inhibitions relating to the use of this
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