Glossary of Terms
Chapter 5: Review of case study site
5.2 Case study historical review
5.2.1 Issues and concerns
5.2.1.1 Freshwater lenses
The groundwater on which Tarawa relies occurs as a thin lens of freshwater floating over seawater in coral sand and limestone aquifers (White et al. 2007). Traditionally, communities have relied on these freshwater lenses for their water supply:
Fresh water was obtained from wells sunk to the level of the fresh-water lens. Potable water was generally available, even in times of drought, on all sections of the islets more than three hundred meters in width. There were exceptions to this, and on Onotoa, at Tebuarorae for instance, the well water was highly saline and odorous. The general availability nevertheless indicates that settlement pattern was not influenced by considerations of fresh water supply (Hockings 1989: 66).
170 In modern times, this situation has changed with increasing population densities, and the size of the freshwater lenses is becoming increasingly important. The critical factors determining the size of the freshwater lens and the sustainable pumping rates are the width of the island, the ease of transmission of freshwater through aquifers and the amount of rainfall (White et al. 2005).
The freshwater in the groundwater is recharged through rain, and lost through a combination of (Falkland 2003):
• Trees and bushes that tap into the freshwater lens;
• Domestic wells and infiltration gallery pumping schemes;
• Discharge to the ocean at the lens and island margins; and
• Tidal mixing generating a thick brackish transition zones between freshwater and seawater.
This means that some islands are better suited for pumping groundwater than others, with the wider islands, Bonriki and Buota, being the primary sources of water (Falkland 2003; Metutera 2003). Groundwater is extracted in Bonriki and Buota from infiltration galleries that are shown in Figure 5-5 and Figure 5-6. It is also worth noting that during the 1960s and 1970s, freshwater lenses in Betio, Bikenibeu and Bairiki were explored and subsequently pilot infiltration galleries were installed (Falkland 2003). It was later concluded that the freshwater lenses on these islands were inadequate and reserves and infiltration galleries were abandoned (Falkland 2003).
To allow sustainable extraction of groundwater, establishing sustainable yields for different islands is of critical importance, and considerable efforts have been invested over the years, each time adjusting conservative estimates upwards. For instance, the estimate for Bonriki, the island with highest sustainable yield, was estimated in 1982 as 750 m3/day, and subsequent investigations by Falkland, based on improved information, in 1992 adjusted the estimate upwards to 1,000 m3 / day, and in the most recent investigations in 2002, under the Asian Development Bank funded SAPHE project (described in section 5.2.4.1), an estimate was given for Bonriki at 1,350 m3/day (Falkland 2003). The sustainable yield estimates for the islands in
171 North Tarawa in 1992 was 3,850 m3 /day, but based on improved information and analysis this was adjusted upwardly in 2002 to 4,620 m3 /day (Falkland 2003).
Figure 5-5: Spatial layout of infiltration galleries in Bonriki
Figure 5-6: Spatial layout of infiltration galleries in Buota Source: Metutera 2003
Notes: Figure 5-5 and Figure 5-6 show the layouts of infiltration galleries on the islands of Bonriki and Buota. Thick blue lines indicate infiltration galleries and red lines indicate roads. Black dots indicate houses. The blue outlines indicate the edges of the islands. Figure 5--5 also shows an outline of the airport runway.
172 5.2.1.2 Droughts
Being an island, the only inflow of water into Tarawa is via rainfall, with the exception of smaller volumes of bottled water. Rainfall in the central Pacific, where Kiribati is located, is dependent on variations in sea surface temperatures in the central and eastern Pacific regions, and therefore dependent on El-Nino Southern Oscillation (ENSO) events (White et al. 1999b).
Rainfall in Tarawa is strongly correlated with the Southern Oscillation Index (SOI) which is used to indicate ENSO events (White et al. 1999c). This in turn leads to high variability in rainfall, and high coefficients of variation (Falkland 1999). Drought is a relative term and simply indicates below average soil moisture and rainfall to which society and ecosystems have adapted (White et al. 1999b). With high variability of rainfall in the central Pacific, this indicates relatively frequent occurrences of drought. Falkland indeed states for the Gilbert Group that:
[…] very high rainfall is associated with El Niño episodes and long droughts can be associated with La Niña episodes. (Falkland 2003: 4)
A major drought event occurred in 1998-1999 in South Tarawa (Falkland 2003; White et al.
1999b) and this led to definite increases in shallow groundwater salinity in the freshwater lenses on the reserves (White et al. 2008) as shown in Figure 5-7.
It has been shown that droughts in combination with over-extraction of water from groundwater resources significantly impacts on the thickness (i.e. size) of the freshwater lens (White et al.
1999c). For the case of South Tarawa, it is however difficult to assess what this means in terms of impacts on the community, as little is currently known about the volumes of alternative sources that are used, nor the losses that occur throughout the system (White et al. 1999c).
173 Figure 5-7: Impact of the 1998-2001 droughts on groundwater salinity
Notes: In this figure salinity is indicated by Electrical Conductivity (EC) and measured in S/cm and the graphs show the salinity for the combined freshwater lenses in Bonriki and Buota. The source of the figure is White et al. 2005.