Small Island Developing States (SIDS) have special physical, demographic and economic features. Their very reduced areas, shortage of natural resources (arable land, freshwater, minerals and conventional energy sources), geological and orogenic complexity, isolation and widespread nature of their territories, and exposure to natural disasters (typhoons, hurricanes, cyclones, earthquakes, volcanic eruptions and tsunamis) complicate the assessment, development and management of the available freshwater resources. The geophysical settings of many small islands leave them vulnerable not only to extreme climatological and seismic events but more critically to periods of low recharge and adverse environmental impacts, including pollution, saline intrusion, soil erosion and mass wasting.
SIDS have limited options when developing their freshwater resources. The relatively short length of surface water circulation in small islands limits the methods available to utilize them. Groundwater occurrences are highly dependent on regular recharge events. The dynamic nature of these occurrences is reflected by the rapid base flow recession and groundwater flow in both volcanic piles and atoll islands.
Groundwater occurs in two main aquifer types: perched and basal. Perched aquifers commonly occur over horizontal confining layers but may also be retained in compartments by series of vertical volcanic dikes. Some perched aquifers are under artesian pressure. Basal lenses can occur on both high and low islands in the form of coastal aquifers or freshwater lenses overlying sea water. Some of these fresh water lenses may be 20 or more meters thick; in small low lying coralline islands, freshwater lenses are generally much thinner than this. Between the freshwater lens and the underlying seawater a saline water transition zone exists. The transition zone thickness depends on both natural conditions (permeability of the geological formation, tidal and recharge fluctuations) and artificial influences (man-induced extraction). Neither the freshwater nor transition zones are of constant thickness; they vary spatially across islands and temporally, according to climatic influences on rainfall patterns.