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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
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

Technical Description

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
(larger image)

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) Islands.

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 limestone islands.

Figure 22

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 activities.

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
with 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
At time 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
readings obtained during dry periods between 1985 and 1988, it would be expected that the driest
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
that the new gallaries are an effective means of extracting groundwater from a freshwater lens,
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.

Costs

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.

Island Cost of Water ($/m3)
Home Island, Cocos (Keeling) Islands $1.22
Kiritimati (Christmas Island), Kiribati $0.67
Aitutaki, Cook Islands $0.24

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.

Suitability

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.

Advantages

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 gallery construction.

Disadvantages

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.

Cultural Acceptability

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 owned.

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.

Information Sources

AGDHC [Australian Government Department of Housing and Construction] 1975. Operation and Maintenance Manual for South Tarawa Water Supply System (Gilbert and Ellice Islands Colony). Australian Government Department of Housing and Construction, Canberra.

AGDHC [Australian Government Department of Housing and Construction] 1982. Kiribati: Tarawa Water Resources Pre-design Study. Australian Government, Department of Housing and Construction, Canberra. (unpublished report).

AGDHC [Australian Government Department of Housing and Construction] 1986. Review of Tarawa Water Supply Project. Australian Government, Department of Housing and Construction, Canberra. (unpublished report).

Anderson, M.P. 1976. Unsteady Groundwater Flow Beneath Strip Oceanic Islands. Water Resources Research, 12(4):640-644.

Ayers, J.F. and H.L. Vacher 1983. A Numerical Model Describing Unsteady Flow in a Freshwater Lens. Water Resources Bulletin, 19(5):785-792.

Ayers, J.F. and H.L. Vacher 1986. Hydrogeology of an Atoll Island: A Conceptual Model from Detailed Study of a Micronesian Example. Groundwater, 24(2):185-198.

Barber, B. 1994. Republic of the Marshall Islands, Rural and Urban Water Supply and Sanitation Review. United Nations Development Programme Water Supply and Sanitation Program and Republic of the Marshall Islands Environmental Protection Authority Report, UNDP, New York.

Bear, J. and G. Dagan 1964a. Moving Interface in Coastal Aquifers. American Society of Civil Engineers, Journal of the Hydrological Division, 90 (HY4):193-216.

Bear, J. and G. Dagan 1964b. Intercepting Freshwater above the Interface in a Coastal Aquifer. International Association of the Hydrological Sciences, Berkeley.

Chidley, T.R.E. and J.W. Lloyd 1977. A Mathematical Model Study of Freshwater Lenses. Groundwater, 15(3):215-222.

CSC [Commonwealth Science Council] 1984. Workshop on Water Resources of Small Islands. Commonwealth Science Council Technical Publications Series No. 143, Part 1; Commonwealth Science Council Technical Publications Series No. 154, Part 2; Commonwealth Science Council Technical Publications Series No. 182, Part 3.

Custodio, E. 1985. Saline Intrusion. In: Hydrogeology in the Service of Man. Memoire of the 18th Congress, International Association of Hydrogeologists, Cambridge. Part 1, 85-90.

El-Kadi, A.I. 1983. Modeling Infiltration for Water Systems. International Ground Water Modeling Center, Delft, The Netherlands.

Engmann, C.A. 1983. Report on Surface Water Infiltration Systems: Handbook Development. Department of Civil Engineering, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne.

Falkland, A.C. 1983a. Christmas Island (Kiritimati) Water Resources Study. Australian Department of Housing and Construction Report No. HWR83/03, Australian Development Assistance Bureau, Canberra.

Falkland, A.C. 1983b. Groundwater Resources Study of Christmas Island, Republic of Kiribati. In: Proceedings of the International Conference on Groundwater and Man, Australian Water Resources Council Conference Series 8, Part 3, 47-56.

Falkland, A.C. 1984. Development of Groundwater Resources on Coral Atolls: Experiences from Tarawa and Christmas Island [Now Called Kiritimati], Republic of Kiribati. In: Proceedings of the Regional Workshop on Water Resources of Small Islands, Commonwealth Science Council Technical Publications Series No. 154, Part 2, 436-452.

Falkland, A.C. 1988a. Cocos (Keeling) Island Water Resources and Management Study. ACT Electricity and Water Report No. HWR88/12, Canberra.

Falkland, A.C. 1988b. Practical Experiences with the Assessment of Groundwater Resources on Coral Atolls. In: Proceedings of the Southeast Asia and the Pacific Regional Workshop on Hydrology and Water Balance of Small Islands. UNESCO-ROSTSEA, Nanjing. pp. 204-212.

Falkland, A.C. 1992a. Review of Tarawa Freshwater Lenses, Republic of Kiribati. ACT Electricity and Water Report No. HWR92/681, Australian International Development Assistance Bureau, Canberra.

Falkland, A.C. 1992b. Review of Groundwater Resources on Home and West Islands, Cocos (Keeling) Islands. ACT Electricity and Water Report No. HWR92/1, Australian Construction Services, Department of Administrative Services, Canberra.

Falkland, A.C. 1994a. Climate, Hydrology and Water Resources of the Cocos (Keeling) Islands. Atoll Research Bulletin No. 400.

Falkland, A.C. 1995a. Guidelines for Water Supply Galleries. ACT Electricity and Water Report No. HWR94/09, Canberra

Falkland, A.C. 1995b. Water Monitoring Annual Report. ACT Electricity and Water Report No. HWR95/04, Canberra.

Falkland, A.C. 1995d. Water Monitoring Report, July-september 1995. ACT Electricity and Water Report No. HWR95/11, Canberra.

Hofkes, E.H. (Ed.) 1983. Small Community Water Supplies, Technology of Small Community Water Supply Systems in Developing Countries. International Reference Centre for Community Water Supply and Sanitation Technical Paper Series No. 18, John Wiley and Sons, New York.

Hunt, B. 1985. Seepage to Collection Gallery near Sea-coast. Water Resources Research, 21(3):311-316.

Hunt, C.D. and F.L. Peterson 1980. Groundwater Resources of Kwajalein Island, Marshall Islands. Water Resources Research Center Technical Report No. 126, University of Hawaii, Hawaii. 91 pp.

Lawlor, J.P. 1946. Skimming Trench Solves a Coral Island Water Supply Problem. Engineering News-Record, 993:83-85.

Marjoram, T. 1983. Pipes and Pits under the Palms: Water Supply and Sanitation in the South Pacific. Waterlines, 2(1):14-17.

Mather, J.D. 1977. Saline Intrusion and Groundwater Development on a Pacific Atoll. In: Proceeding of the Fifth Sea Water Intrusion Meeting, UNESCO-International Hydrological Programme, Medmenham, England. pp. 127-135.

Metutera, T. 1989. Water Resources Assessment, Planning, Development and Management in Kiribati. In: Interregional Seminar on Water Resources Management Techniques for Small Island Countries. UNDTCD Report No. ISWSI/SEM/15, United Nations Development Programme, New York.

Mink, J.F. 1976. Groundwater Resources of Guam, Occurrence and Development. Water Resources Research Center Technical Report No. 1, University of Guam, Guam.

Mink, J.F. 1986. Groundwater Resources and Development, Trust Territory of the Pacific Islands. United States Environment Protection Agency, Region 9, Washington.

Olsen, J.P. 1984. Marshall Islands (Kwajalein Island) - Island Water System. In: Proceedings of the Regional Workshop on Water Resources of Small Islands, Commonwealth Science Council Technical Publication No. 182, Part 3, 104-113.

Peterson, F.L. 1984. Groundwater Recharge, Storage and Development on Small Atoll Islands. In: Proceedings of the Regional Workshop on Water Resources of Small Islands, Commonwealth Science Council Technical Publications Series No. 154, Part 2, 422-430.

UNESCO [United Nations Education Scientific and Cultural organization] 1991. Hydrology and Water Resources of Small Islands, A Practical Guide. Studies and Reports on Hydrology No. 49, UNESCO, Paris.

Wirojanagud, P. and R.J. Charberneau 1985. Saltwater Upconing in Unconfined Aquifers. American Society of Civil Engineers, Journal of Hydrological Engineering, 111(3):417- 434.

Xu, W. and F. Lu 1988. Development of Crevice Water Resources by Gallery Well in Zhoushan Island. In: Proceedings of the Southeast Asia and the Pacific Regional Workshop on Hydrology and Water Balance of Small Islands, UNESCO-ROSTSEA, Nanjing, China. pp. 47-51

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