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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

2.1.1 Dug Wells

Technical Description

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

Figure 19. Dug well in fire granular aquifer (Hofkes, 1981).

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 systems.

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 well.

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.

Costs

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.

Suitability

Dug wells are especially suitable for use on very small, low-lying islands where they can supplement the rainwater harvesting systems.

Advantages

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.

Disadvantages

Dug wells are sometimes subject to water quality problems due to subsurface and surface pollution.

Cultural Acceptability

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.

Information Sources

Ayers, J.F. and R.N. Clayshulte 1983. Reconnaissance Level Investigation of Salt-water Intrusion on Kuttu Island, Satawan Atoll, Truk State. Water and Energy Research Institute Technical Report No. 48, University of Guam, Guam.

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., H.L. Vacher, R.N. Clayshulte, D. Strout, and R. Stebinsky 1984. Hydrogeology of Deke Island, Pingelap Atoll, Eastern Caroline Islands. Water and Energy Research Institute Technical Report No. 52, University of Guam, Guam.

Boschi, I. 1982. Construction of Hand Dug Wells and Protection Gainst Pollution:interregional Project for the Implementation and Evaluation of Special Public Works Programmes. International Labour Organization, Geneva.

Chandler, R.L. and D.B. McWhorter 1975. Upconing of the Saltwater-Freshwater Interface Beneath a Pumping Well. Groundwater, 13(4):354-359.

Cooper, H.H. 1959. A Hypothesis Concerning the Dynamic Balance of Fresh Water and Salt Water in a Coastal Aquifer. Journal of Geophysical Research, 64 (4):461-467.

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. Memoires of the 18th Congress. International Associations of Hydrogeologists, Cambridge. Part 1, pp. 85-90.

Diersch, H.J., D. Prochnow, and M. Thiele 1984. Finite-element Analysis of Dispersion-affected Saltwater Upconing Below a Pumping Well. Applied Mathematical Modeling, 8(5):305-312.

Dillaha, T. A. and W. J. Zolan 1987. An Investigation of the Quality of Rooftop Rainwater Catchment Systems in Micronesia. Water and Energy Research Institute Technical Report No. 45, University of Guam, Guam.

Fleming, S. 1987. Basic Water Supply and Sanitation. Women's Development Training Programme, University of South Pacific, Tonga.

Gampher, C. and S. Gates 1989. Well Construction Using Curved Hollow Block. Haribon Foundation, Manila.

Haubold, R.G. 1975. Approximation for Steady Interface Beneath a Well Pumping Freshwater Overlying Saltwater. Groundwater, 13(3):254-259.

Hofkes, E.H. (Ed.) 1981. 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.

Kerr, C. 1988. Community Water Development. Intermediate Technology Publications, London. 192 pp.

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: Proceedings of the Fifth Sea Water Intrusion Meeting, UNESCO International Hydrological Programme, Medmenham, England. pp. 127-135.

Stark, R. 1988. Well Construction Manual. Sarvodaya Rural Technical Services, Moratuwa.

Vanderberg, A. 1975. Simultaneous Pumping of Fresh and Saltwater from a Coastal Aquifer. Journal of Hydrology, 24:37-43.

Watt, S.B. and W.E. Wood 1977. Hand Dug Wells and Their Construction. Intermediate Technology Publications, London.

Winter, S.J. 1987. A Ferrocement Well for Micronesia. Waterlines, 6(2):20-22.

Winter, S.J. 1988. Construction Manual for a Ferrocement Well. United Nations Development Programme, New York.

Winter, S.J. and L.D. McCleary 1984. Some Improvements in the Design of the WERI Well. Water and Energy Research Institute Technical Report No. 54, University of Guam, Guam .

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