|Sourcebook of Alternative Technologies for Freshwater Augmentation in Small Island Developing States (UNEP-IETC, 1998, 230 p.)|
|Part A - Introduction|
|5. Methodology for the identification and classification of small islands|
The geology and hydrogeology of small islands greatly influences the type and distribution of their water resources. This influence is manifested mainly through the spatial distribution of rocks and soils with varying permeability and porosity. Surface water resources occur only on islands with soils of relatively low permeability. Groundwater resources are most abundant on small islands with soils and rocks of moderate to high permeability and porosity. Where the permeability or porosity is low, the availability of exploitable groundwater is generally low - although the converse is not necessarily true. Where permeability is very high, the mixing of freshwater and seawater is likely, resulting in the occurrence of brackish groundwater, which limits the availability of exploitable, fresh groundwater resources.
Small islands can be geologically classified in a number of ways. A convenient classification, outlined by UNESCO (1991) and adopted herein, is volcanic islands; limestone islands; coral atolls; bedrock islands; unconsolidated (or sand) islands; and, islands of mixed geology, although other classifications are equally valid (e.g., Hehanussa, 1993, presented a classification system for Indonesian islands based upon both geological and topographical characteristics). The UNESCO system is elaborated below.
Volcanic Islands: Common in the tropical regions of the Pacific Ocean (e.g., Hawaiian Islands, many islands of the Federated States of Micronesia, the Cook Islands, and French Polynesia), volcanic islands also occur in the Indian Ocean (e.g., Mauritius) and in the archipelagos of the seas between the Indian and Pacific Oceans, as well as in the Caribbean where the West Indies, with the exception of the Bahamas and Cayman Islands, consist of volcanic and related intrusive igneous rock islands. There are at least two sub-types of volcanic islands; namely, the andesitic sub-type which normally forms as island arcs on the continental sides of deep trenches, and the basaltic or oceanic sub-type which rises from the ocean floor in the middle of tectonic plates. Volcanic islands of the andesitic sub-type generally have low permeability and water-bearing properties (Peterson, 1985). Groundwater yields are generally low (less than 1 l/sec). The basaltic sub-type of volcanic islands, where lava rather than pyroclastic rock predominates, varies in permeability and, hence, in exploitable groundwater. Where the lava flows are young, as in the Hawaiian Islands, Western Samoa and French Polynesia, permeability and groundwater potential are high. In older, basaltic islands with a higher degree of pyroclastic material, as in many of the islands of the Federated States of Micronesia, permeability is low and exploitable groundwater limited.
Limestone Islands: Common in the oceans and seas within the humid tropics, examples of limestone islands include raised coral atolls such as Nauru, Niue and many of the islands in Tonga in the Pacific Ocean. Raised atolls are uplifted coral atolls that have undergone subsequent erosion and karstification. Some limestone islands and coral atolls have been subsequently tilted and may be covered by other geologic deposits (e.g., the volcanic ash layers on the limestone islands in Tonga, and the phosphate deposits on Nauru and Christmas Island, Indian Ocean). Limestone islands are generally karstic and weathered as a result of alternate periods of submergence and exposure due to fluctuating sea levels. Caves and solution cavities are often found along the shoreline and within the interior of the islands. The permeability of the limestone is often very high (generally greater than 1 000 m/day) and, consequently, due to saltwater intrusion, freshwater lenses are generally no more than about 10 cm to 20 cm thick even though the islands may be quite wide (e.g., Tongatapu where the island ranges from 3 km to 5 km in width).
Coral Atolls: Common in the Pacific Ocean (e.g., the islands of Kiribati and Tuvalu, and the Marshall Islands) and in the Indian Ocean (e.g., the Maldives, some of the islands in Seychelles, and the Cocos (Keeling) Islands), there are many variants of the coral atoll-type of island but typically they consist of a chain of low coral islands surrounding a shallow lagoon. Coral atolls generally consist of a layer of recent (Holocene) sediments, comprising mainly coral sands and fragments of coral, on top of older limestone similar to that described above. An unconformity separates these two layers at typical depths of 10 m to 20 m below mean sea level. Several, deeper unconformities may exist due to fluctuations in sea level which result in alternate periods of emergence and submergence of the atoll. During periods of emergence, solution and erosion of the reef platform can occur, while further deposition of coral limestone can occur during periods of submergence. The upper sediments are of primary importance from a hydrogeological viewpoint as freshwater lenses occur solely or mainly within this layer (Figure 6). The occurrence of such lenses within this layer is due to its moderate permeability (typically 5 to 10 m/day) compared with the higher permeability of the older limestone (typically 50 to 100 m/day). Permeabilities greater than 1 000 m/day occur in solution cavities within the limestone. These extremely high permeabilities allow almost unrestricted mixing of freshwater and seawater which is less likely to occur in the upper sediments. The upper unconformity, therefore, is one of the main controlling features of the depth of the freshwater lens.
Bedrock Islands: Formed by igneous or metamorphic rocks such as granite, diorite, gneiss and schists, bedrock islands are mainly found on continental shelves or adjacent to large islands of similar geology. Many of the islands of the Seychelles are of this type.
Figure 6. Small island fresh-water lens (exaggerated vertical scale).
Unconsolidated Islands: Generally consisting of sand, silt and/or mud, unconsolidated islands are generally found in the deltas of major rivers (e.g., in the Bay of Bengal).
Mixed Geology Islands: Common amongst the oceanic islands are those islands with a mixture of volcanic and limestone rocks. Over long periods of time, the geologic nature of these islands can change. For instance, volcanic islands can subside, forming fringing reefs, and, eventually, erode further, leaving coral limestone as the only rock type visible above sea level.