Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for
in Small Island Developing States>
PART B - ALTERNATIVE TECHNOLOGIES
2. TECHNOLOGIES APPLICABLE TO VERY SMALL, LOW CORAL ISLANDS
2.1 Freshwater Augmentation Technologies
2.1.1 Dug Wells
Dug wells are made simply by digging a hole in the ground. Experience
shows that the diameter of a dug well should be at least 1.2 m if two men
are to work together at the bottom of the well during the digging. On
average, depending on the particular soil, the ratio between diameter and
depth should be about 2:1, although cohesive soils may require a lower
ratio and extremely uncohesive soils may require the use of some
structural support to prevent the collapse of the well walls.
The depth to which a well can and should be dug depends largely upon the
type of ground and the fluctuation in the depth of the groundwater table
below the land surface. On small, low-lying islands, such as coral
islands, wells should be deep enough to ensure that they do not run dry at
low tide but not too deep so as to allow the entry of saline water into
the wells at high tide. The depth of water in the well is dependent on the
tidal response of the groundwater at the well location, but typically
should be about one-half to one metre below water table to allow bailing
to occur (Hofkes, 1981).
Most dug wells need an inner lining. Materials such as brick, stone,
masonry, concrete (cast within shuttering inside the hole), or precast
concrete rings may be used. If the well is dug in solid material (e.g.,
rock), the well may stand unlined, but lining the upper part is always
recommended in order to avoid contamination of the well by surface runoff
and debris. In loose sediments, the well should be lined over its entire
depth (Figure 19).
Figure 19. Dug well in fire granular aquifer (Hofkes,
The section of the well wall that penetrates the aquifer, if lined,
requires a lining with openings or perforations to allow groundwater to
flow into the well. In fine sand aquifers, it is impossible to provide a
lining with openings or perforations small enough to retain the fine sand
and prevent it from passing into the well. In such cases, the lining is
frequently extended over the entire depth of the well without any openings
or perforations (IRC, 1981). In such cases, the groundwater enters the
well from the unlined floor of the hole.
As noted above, the well should also be protected from surface pollution
by extending the well lining at least 0.5 m above the ground to form a
head wall around the outer rim of the well. A concrete slab should be
constructed on the ground surface, extending for 2 m around the well. The
concrete slab also seals any fissures between the well lining and the
walls of the excavated hole, preventing polluted surface water from
seeping into the well along the outer casing of the well. Similarly, it is
desirable for the top of the well to be covered, and a handpump fitted, to
further prevent contaminants from entering the well. The different types
of handpumps are discussed later in this chapter.
Extent of Use
Dug wells are used extensively on many low-lying islands throughout
SIDS. Dug wells are often used as a supplement to rainwater harvesting
Operation and Maintenance
The operation and maintenance of a dug well is simple. Water is taken
from a well by either lowering a container into the well and withdrawing
it, or by a pump (hand- or motor-powered). The operation and maintenance
of pump systems is most important. While the detailed maintenance of pumps
is beyond the scope of this book, given the wide range of pumping systems
available, all pumps need periodic inspection and repair, including
replacement of gaskets and moving parts. Handpumps are generally preferred
from a water conservation point of view as they cut down on overpumping
and minimise the risk of saltwater intrusion. The pump selected should be
easily operated and maintained by women and children, who, traditionally,
are the most frequent users. Spare parts must be readily available and
maintenance training provided immediately following installation of the
pump. For community wells, there should be a representative water
committee who will assume responsibility for keeping the well surrounds
clean and the pump maintained. Collection of a fee may be necessary to pay
for maintenance. Open wells should be inspected every day to ensure that
no debris enters the well, while closed wells should be inspected
periodically for the same reason. The areas surrounding the top of the
well should be kept clean to protect against contaminants polluting the
Level of Involvement
Most SIDS have village-level experience in the construction of dug
wells. This experience ranges from simply providing a hole in the ground
to constructing a properly lined and sealed well, fitted with a pump.
Skilled technicians may be required to install the pump, and care should
be taken when selecting a motor driven pump to ensure that the pumping
rate does not exceed the yield of the well. Overpumping could lead to
saltwater intrusion and loss of the well for freshwater uses.
Construction, operation, and maintenance costs depend on the type of
well and pumping system. Typical costs include labour for construction; a
liner for the well; a concrete slab; a cover; and a hand pump (if
applicable). Costs will vary considerably depending on the materials used
and their availability. Transportation, especially to the remote islands,
may add substantially to the cost. The price of handpumps is generally in
the range of $80 to $2 000.
Effectiveness of the Technology
Dug wells are one of the oldest water supply technologies available. Dug
wells are generally effective in providing water. With proper location
(away from potential sources of pollution) and construction, they are a
good method of obtaining freshwater.
Dug wells are especially suitable for use on very small, low-lying
islands where they can supplement the rainwater harvesting systems.
Dug wells are relatively simple to construct and use, and provide for
the availability of water during extended dry periods. Dug wells can be
used to supplement rainwater systems.
Dug wells are sometimes subject to water quality problems due to
subsurface and surface pollution.
Technology is well within the capability of communities on small islands
to construct and operate.
Further Development of the Technology
Emphasis should be placed on minimising saltwater intrusion through good
construction methods, appropriate well siting, and avoidance of
overpumping. Further research is needed on groundwater contamination on
small islands in order to optimise the siting of wells and sanitary
facilities. Further analysis of groundwater recharge in island
environments is also needed.
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Ayers, J.F. and H.L. Vacher 1983. A Numerical Model Describing Unsteady
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1984. Hydrogeology of Deke Island, Pingelap Atoll, Eastern Caroline
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of Rooftop Rainwater Catchment Systems in Micronesia. Water and Energy
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Fleming, S. 1987. Basic Water Supply and Sanitation. Women's
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Pumping Freshwater Overlying Saltwater. Groundwater,
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Intermediate Technology Publications, London.
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of the WERI Well. Water and Energy Research Institute Technical Report
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