Sourcebook of Alternative Technologies for Freshwater Augumentation in Small Island Developing States


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PART B - ALTERNATIVE TECHNOLOGIES

1. TECHNOLOGIES GENERALLY APPLICABLE TO ISLAND STATES

1.4 Water Conservation

1.4.3 Groundwater Assessment Technologies

In order that groundwater resources be used within their sustainable limits, it is essential that the sustainable yield be estimated. This process can be accomplished in a number of ways, using a variety of techniques, both simple and complex. The accuracy of the estimate will largely be a function of the availability of appropriate data and the extent of the analyses. Because of the importance of groundwater to the freshwater resources available on small islands, some of these methods are reviewed in this section.

Technical Description

Groundwater assessment is most important if the resource is available to be utilised for water supply purposes. All too often, saltwater intrusion becomes a major problem in SIDS because of over-utilisation of groundwater. While groundwater assessment technologies applicable to larger islands (greater than say 2 000 km2) are well documented in standard textbooks (e.g., Driscoll, 1987), assessment of freshwater lenses in SIDS are less well known. Detailed information on the assessment of freshwater lenses on small coral islands, which may also apply to raised atolls and small volcanic islands with freshwater lenses, has been included in Annex 4.

Some of the methods applicable to larger islands include hydrogeologic mapping and reports; geophysical surveying at the ground surface; borehole sampling; and, geophysical logging of borehole data. Methods applicable to SIDS include preliminary investigations involving ground and/or aerial surveying; measuring water levels in existing wells; and, using empirical relationships to forecast groundwater volumes and movements. More detailed investigations that can be undertaken in SIDS include the use of geophysical methods; drilling and well testing; and, development of permanent salinity monitoring systems. An important element in these methodologies is the estimation of groundwater recharge.

Extent of Use

Groundwater assessment technologies have been used in most SIDS. An example of a water balance study being done on Tarawa, Kiribati, is given in Annex 4.

Operation and Maintenance

Often water-level and water-quality stations are established during the assessment phase of a project and it is advisable to continue to monitor such stations to gain a greater understanding of the response and sustainability of the groundwater resource. These monitoring stations should be maintained for long-term monitoring.

Level of Involvement

Professional engineers and/or scientists are required to undertake most groundwater assessments. However, local staff should assist wherever possible to learn the necessary field skills from the professionals in order to maintain regular monitoring of groundwater levels and water quality.

Costs

Assessment costs may be high ,especially if drilling is involved. However, depending on the amount of data available, low-cost empirical methods may be used for preliminary assessments.

Effectiveness of the Technology

If correctly implemented, groundwater assessments are effective in determining the suitability and sustainability of fragile freshwater lenses for use as water supply sources. Without proper assessment, these freshwater resources may be over-used, resulting in saltwater intrusion. Suitability Since groundwater is a major water source for community/village water supplies in SIDS, it is most suitable to implement this technology at the local level. On low-lying, coral islands, it is essential that groundwater assessments be carried out due to their extreme vulnerability to contamination of the freshwater lens.

Advantages

Use of groundwater assessment techniques contributes to the sustainable development and use of a limited resource, enabling the optimisation of the location of galleries and wells with regard to water quantity and quality as well as of water abstraction rates and determination of operational conditions. Groundwater monitoring conducted as part of the assessment process identifies the need for potential water treatments and contributes to pollution prevention by allowing rapid response to declines in water quality. Groundwater assessment techniques also provide an avenue for the transfer of water resources management technologies to local staff.

Disadvantages

This technique may incur high investigation costs, especially if drilling is involved.

Cultural Acceptability

Access across private lands for the conduct of water assessment studies can be a problem on some islands.

Further Development of the Technology

Better methods for estimating evapotranspiration are required to more accurately assess the rate of recharge to freshwater lenses on small islands. Computer modelling may become more effective as more is known about island, freshwater lens aquifers.

Information Sources

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