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
2.1.2 Infiltration Galleries
In freshwater lenses on small low islands, particularly coral atolls,
relatively large scale extraction systems have been successfully
implemented using infiltration galleries. Infiltration galleries skim
water off the surface of the lens, thus distributing the pumping over a
wide area. This distributed pumping can avoid the problems of excessive
drawdown and consequent up-coning of saline water caused by localised
pumping from individual wells. A number of different types of infiltration
galleries have been used on small islands. These can generally be divided
into two categories; namely, open trenches; and, buried conduits (Figure
20). Open trenches are reasonably simple to construct unless the depth to
water table is excessive (more than 2 m to 3 m).
Figure 20. Detailed cross section through an infiltration
gallery constructed from horizontal slotted
PVC pipes. Vertical
concrete cylinders have been used for the pump well and access manholes.
The design was used for six galleries on Home Island, Cocos (Keeling)
Planning for galleries should take account of many factors, including
water demand and water quality requirements; the sustainable yield of the
groundwater system (usually in the form of freshwater lenses which will
require a prior assessment of the groundwater potential to have been
conducted); available space for construction of galleries (the area
required for the galleries may also be needed for other purposes);
proximity to population centres and other potential pollution sources
(which will dictate the need for, and extent of, water treatment
required); depth to the water table (which will dictate the depth of
excavation - areas where the depth to water table is higher than normal
should be avoided to minimise excavation costs); nature of material in the
unsaturated zone (known areas of cemented calcareous material (= hardpan)
should be avoided); and, the permeability of upper aquifer sediments
(which will determine the zone of influence of a particular gallery on the
surrounding saturated sediments - and, hence, the length of gallery
required to sustain a given pumping rate). Guidelines for the design of
infiltration galleries are provided in a number of reports (see Falkland,
1988a; UNESCO, 1991), and are summarised in Annex 2.
Extent of Use
Open trenches, covered with simple roof structures, are used on
Kiritimati (Christmas Island), Kiribati. These were first constructed as
temporary water supplies in the 1950s. However, as the technology has
proven effective, open trenches have been were constructed using the same
basic design into the late 1980s. Although covered to some extent, they
are subject to surface contamination from sources such as crabs, birds and
humans (Falkland, 1983a, 1984a, 1990a). Seawater intrusion has also been
caused by the overpumping of some of these galleries, emphasising the need
for a proper assessment of pumping rates. Due to the potential
contamination from surface pollutant sources, open trenches are not
generally recommended as the preferred gallery construction method since
the buried conduit systems offer better pollution minimisation potentials.
Buried conduit systems have been installed and are successfully
operating on a number of atolls in the Pacific and Indian Oceans. A
typical cross section through an atoll with an infiltration gallery
utilising a buried conduit system is shown in Figure 21. Examples of this
type of infiltration gallery in the Pacific Ocean are found on Kwajalein,
Republic of the Marshall Islands (Hunt and Peterson, 1980; Olsen, 1984);
Tarawa, Republic of Kiribati (AGDHC, 1986; Falkland, 1992a); and,
Aitutaki, Cook Islands (Binnie and Partners, 1984; Falkland, 1994a,
1995a). One of the first examples of a buried conduit infiltration gallery
in the Pacific was installed on Tinian, Northern Mariana Islands, in 1945
(Lawlor, 1946). There, the conduits were constructed from two rows of
perforated steel cylinders laid at a depth of about one metre below water
level in an excavated trench. Graded crushed coral was used for bedding
and backfilling and a layer of clay was used to seal the trench. The more
recently constructed Kwajalein galleries consist of perforated PVC pipes
surrounded by graded crushed coral, feeding to small diameter pump wells
at approximately 60 m spacing (Olsen, 1984). On Tarawa, two types of
buried conduit galleries have been used. The earlier type, constructed on
the islands of Bonriki and Buota in the mid-1970s, consisted of two rows,
one above the other, of unjointed hollow concrete blocks, laid in a
cruciform pattern, leading to a central concrete block construction pump
pit (AGDHC, 1975). In the mid-1980s, 23 galleries were installed as part
of a major water supply infrastructure upgrade. These galleries consisted
of 100 mm diameter slotted PVC pipes laid in linear patterns leading to
cylindrical fibreglass or ferrocement pump pits (AGDHC, 1982, 1986). The
pipes were laid with their inverts slightly below mean sea level. Since
early 1992, groundwater has been extracted from six infiltration galleries
on the island of Laura on Majuro Atoll (Barber, 1994). On the main island
(Aitutaki) of the Aitutaki atoll group, Cook Islands, infiltration
galleries have been constructed at a number of locations around the
coastline within predominantly volcanic soils and sediments. This island
is predominantly volcanic, but is fringed with coral sediments and reefs.
Aitutaki can be described as a near-atoll due to the presence of volcanic
rock above sea level. The original galleries were constructed using porous
concrete pipes (approximately 900 mm in diameter), but have, more
recently, been extended using slotted PVC pipe (225 mm diameter).
In the Indian Ocean, examples are found in the Cocos (Keeling) Islands
(Falkland, 1992b, 1994a, 1994b, 1994c).
In Barbados, infiltration galleries are used close to the coastline
where the freshwater floats on saltwater. These infiltration galleries
differ from the ones used on low-lying coral islands, as the depth to the
freshwater lens is much higher. Figure 21 shows the layout of the type of
infiltration galleries used in Barbados. This type is typical for
Figure 22. Infiltration gallery used in Barbados.
Operation and Maintenance
If galleries are evenly distributed over a freshwater lens, the ideal
operational mode is an evenly balanced, continuous pumping rate from each
of the galleries. This operational mode ensures that the degree of
drawdown of the water table in each gallery due to pumping is minimised.
Consequently, the upward movement of the transition zone is also
minimised. In practice, there are often reasons why the ideal operational
mode cannot be achieved. The most common reason is that some galleries
within a given freshwater lens may be sited near the edge of the lens, or
in marginal zones where the lens may seasonally contract, causing an
elevation in salinity. Specific rules may be necessary for particular
galleries. An example is the need to reduce pumping from a gallery if the
salinity starts to rise. Such control can be exercised if there are
sufficient monitoring data to determine the effects of pumping on the
salinity, during both wet and dry cycles.
Routine inspections of galleries should be carried out in the normal
course of work, and flow rates monitored. Any unusual reduction or
increase in flow can be determined from meter readings. The condition of
gallery hatches, pump wells, ladders, float switches, suction pipes,
gallery pipe entries and concrete bases should also be inspected during
regular water salinity monitoring. Annual inspections of all galleries and
their associated equipment should be conducted. The inspections at each
gallery should include examination of the base of the gallery pump
stations (to check for structural integrity and amount of sediment on
floor); the concrete pump wells (to inspect walls for any signs of
structural failure, and to check the integrity of seal between well and
inflow pipes); the float switch (to check its general condition, to
measure level relative to base of pump well, and to check against previous
levels to ensure continued accurate level readings); the suction pipe,
strainer, and mounting brackets (to check their general condition); covers
and hatches (to check their general condition); and, the pumps, meters,
and valves (to check their general condition - meters and valves also
require internal inspection, and cleaning if necessary).
The main components of galleries that may require maintenance are the
gallery conduits. If these are constructed from PVC pipes, and there are
sufficient manholes to allow access to these pipes, maintenance should be
a relatively simple task. It is essential that galleries have an easy
means of access to permit periodic cleaning of sediment from the conduits.
While it is difficult to estimate the amount of sediment in the pipes by
inspection, as they are laid below the water table, the need for cleaning
will usually become evident by a larger drawdown than normal within the
pump well for a given pumping rate. Regardless of any sediment problems
that may have occurred in gallery pipes, it is recommended that all
gallery pipes be cleaned out after two years of operation. If no or very
little sediment is found, then the procedure will not need to be repeated
again unless problems arise in the future. However, if a reasonable amount
of sediment is found, then the procedure should be repeated regularly
every two years.
Operational experience with galleries on Tarawa (approximately 9 years),
Kiribati, and Home Island (approximately 5 years), Cocos (Keeling)
Islands, has shown that the amount of maintenance required on the
galleries is very small indeed. Operational requirements are no more
complicated than checking pump operation and carrying out monitoring
Figure 23. Reduction of salinity in water reticulated to
Home Island, Cocos (Keeling) Islands,
as a result of gallery
construction. At time TI, water was pumped from three pump wells fitted
very short lateral pipes. At times T2 and T3, fourth and fifth pump wells
of similar construction
were commissioned. During this period and
shortly after it, the highest chloride readings were obtained
T4 (March-Octorber 1991), five 300 m long galleries were constructed at
the sites of the
former pump wells. A sixth and similar gallery was
commissioned at time T5. Based on the chloride
during dry periods between 1985 and 1988, it would be expected that the
period on record (1991) would have resulted in higher chloride
readings than were obtained. The
fact that they were significantly
lower at the end of 1991 than in the period 1985-1988 is evidence
the new gallaries are an effective means of extracting groundwater from a
particularly a fragile one such that on Home Island.
Monitoring is an essential feature of water management. This principle
applies as much to the operation of a water supply system based on
infiltration galleries as to other methods. Monitoring provides relevant
information on water resources and water supply systems to managers and
operational personnel to facilitate informed decision-making about current
operational procedures and future planning for water supply. Monitoring is
essential where maximum groundwater usage (within sustainable limits) is
required. The following basic monitoring is recommended: rainfall
monitoring; flow monitoring; and, water salinity monitoring. Other
monitoring that may be useful, depending on circumstances, includes
bacteriological monitoring; and, monitoring of potential chemical
pollutants (nitrate/nitrite, hydrocarbons, agricultural chemicals, etc.).
Level of Involvement
If infiltration galleries are properly constructed, the level of skill
required to maintain them is low. For this reason, as well as the benefits
of improved or maintained water quality, infiltration galleries have a
significant advantage over drilled wells, which can suffer from problems
such as clogging, and often requiring redrilling.
Capital costs vary according to local conditions, but include the costs
of labour and local materials, imported materials and equipment, and
external technical and professional assistance. Operation and maintenance
costs are also variable according to the local cost of fuel and
electricity (if applicable), the cost of labour, and the level of
maintenance actually performed. Estimated unit costs (including capital
costs discounted at an interest rate of 6 percent over a 25 year lifetime,
and operation, maintenance ,and monitoring costs) of supplying groundwater
from galleries on three islands where the technology has been used are
shown in Table 5. Cost comparisons with other water supply technologies
indicate that, for communities which rely on groundwater supplies, the use
of infiltration galleries is the least expensive option (see Annex 3).
TABLE 5. Cost of Water from Infiltration Galleries.
||Cost of Water ($/m3)
|Home Island, Cocos (Keeling) Islands
|Kiritimati (Christmas Island), Kiribati
|Aitutaki, Cook Islands
Effectiveness of the Technology
The objective of using this technology is to improve or maintain the
quality (in terms of salinity) of the fresh groundwater. From detailed
data collected from galleries constructed on Home Island, it has been
shown that, if properly implemented, the technology is effective in
improving water quality. Monitoring of other galleries (e.g., on Bonriki
and Tarawa) has shown that the quality of the fresh groundwater resources
has been maintained. Figure 23 demonstrates the reduction of salinity in
the water pumped from the freshwater lens on Home Island on South Keeling
atoll in the Cocos (Keeling) islands over recent years.
Infiltration galleries are suitable for serving as community water
supplies on small coral sand islands. Galleries minimise the potential for
saltwater intrusion, compared to dug or drilled wells, by skimming water
from the top of the freshwater lens over a large area.
The main advantage of infiltration galleries over dug and drilled wells
is improved water quality (lower salinity) at a similar pumping rate.
Infiltration galleries can lead to the reductions in the salinity of
existing water supplies where other, less appropriate methods of
extraction have been used historically. Additional advantages include low
maintenance requirements and relatively simple construction techniques
(e.g,, there is no requirement for a drilling rig). Infiltration galleries
may also be less expensive to construct than drilled wells, depending on
local rates of labour and charges for mechanical equipment. In many SIDS,
the cost of labour is relatively low while the cost of mechanical
equipment can be relatively high. The relative cost of excavating
galleries compared with drilling holes will depend on the balance between
manual labour and mechanical equipment (such as excavators) used in the
Limitations of infiltration galleries compared with dug or drilled wells
include an higher initial cost of construction than dug wells or drilled
wells, depending on the depth and method of excavation; and, a greater
requirement for access to land than dug wells and drilled wells, which may
cause problems with land tenure.
Acceptance of the technology has generally been favourable. On Home
Island, for instance, the residents have commented on improvements in
water quality since the galleries were installed. The technology has also
been well-received by operators and consumers alike on Aitutaki. However,
the need to utilise large areas of land for large-scale implementation of
galleries can lead to problems with land owners. An example is Bonriki,
Tarawa, where the local residents are somewhat resentful of the fact that
their land is being used for the galleries. In contrast, land ownership
was not an issue on Aitutaki where galleries were built with the local
landowners' blessings. There was general willingness to allow the
government to develop the water on their land for the common good of all
the people on the island. The problem of land ownership will not exist on
Kiritimati (Christmas island), where a proposed water project (1996/1997)
will construct galleries (buried collector type) to replace and extend
existing rudimentary galleries (open trenches), as the land is government
Further Development of the Technology
The current slotting pattern used in the PVC slotted pipes that are used
to construct infiltration galleries tends to be an even array of slots.
This has the effect of drawing greater amounts of water from those
portions of the aquifer closest to the gallery (unlike the effects of the
more distant slots typically used in a pumped well, for example). The
slotting pattern could be modified to distribute the effects of pumping
more evenly over the surface of a freshwater lens. The exact arrangement
of slots, therefore, requires further research. This research should
involve a review of available literature and the possible construction of
a prototype with varied slot patterns in a number of arms. This could be
done as part of a project requiring the installation of new or additional
galleries. Monitoring of pumping and salinity along the gallery would be
required to assess the effectiveness of the various slot patterns, while
dye tracer studies could be used in tracking the source of water adjacent
to the gallery.
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