Climate Adaptation in Atolls

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Development Objective 1 - Strengthen sustainable management of natural resources

Understanding the behaviour of a fresh groundwater lens in an atoll has resulted in improved water security for the community of South Tarawa, Kiribati, that relies upon it.

Groundwater in atolls is best understood as a thin freshwater lens that floats on denser underlying seawater. These freshwater lenses are often the only natural source of freshwater found in these island settings, and hence are critical to the communities that reside there. Groundwater in these settings is frequently accessed by shallow wells 3-5m deep, or through horizontal wells referred to as infiltration galleries, designed to skim the freshwater from the top of the water table.

The freshwater lens will naturally increase with rainfall recharge and become thinner and more brackish during periods of low rainfall such as drought. Groundwater in atolls is considered to be a very dynamic system, in contrast to the groundwater systems found in larger volcanic islands of the Pacific. In atoll systems the groundwater responds within periods of weeks or months to changes in recharge, while larger groundwater systems such as found in volcanic islands, may take years to tens of years to show a change. To date however groundwater systems in atolls have been treated the same as groundwater found in other geological settings, whereby a sustainable yield is determined and groundwater is pumped within that limit. While this approach may be appropriate over the longterm, it does not take into account the dynamic nature of the groundwater whereby in a period of low recharge, such as drought, the groundwater can become brackish making it unsuitable for its intended purpose.

An approach to groundwater management is therefore required for atoll freshwater lenses which takes into consideration this dynamic nature of the groundwater system and the need to maintain a suitable water quality at all times.

The EU funded ACP Secretariat supported SPC to undertake research of freshwater lenses in atolls, to investigate the impacts of climate and abstraction in atoll environments. The results from the numerical groundwater modelling of the Bonriki Water Reserve, Kiribati, gives insight into how the fresh water lens behaves under certain stresses and allowed the development of pragmatic options for future monitoring and management of these freshwater lenses.

The Bonriki Water reserve is the main source of freshwater for a population of more than 56,000 people living in South Tarawa. Since the 1980s, the Bonriki water reserve has been relied upon as the main potable water source for an ever increasing population. As the population increases so does its water needs and with a longterm sustainable yield estimated at 1.6 million litres of freshwater per day, or less than 25L/p/d with system losses taken into account, the demands placed upon the BWR makes this aquifer one of the most “worked” aquifer systems in the Pacific, where demand outstrips supply.

The Bonriki water reserve is, as elsewhere in the Pacific, subject to impacts from reduced recharge from drought and overtopping from storm surges, causing water quality impacts, which in some cases can make the water unusable for its intended purpose. Using historical and project collected monitoring data records SPC and its partners constructed a 3D numerical groundwater modelling for this small, 0.7 km2, but important aquifer. By projecting possible rainfall and storm surge scenarios, it has been possible to model the behaviour of the freshwater lens and the impact from severe droughts and wave overtopping on the underlying freshwater lens, including timing of impacts and as importantly recovery of the lens under different climate conditions. This increased knowledge has allowed us to rethink how we can best operate, and manage these unique and dynamic groundwater systems for improved water security.

Specifically for South Tarawa the results from the modelling have been able to guide the development of a pragmatic drought management approach for the aquifer. The approach to groundwater management focuses on the quality of the water rather than the quantity of water, ensuring the water supplied is suitable for its intended purpose. This is a fundamental shift in how freshwater lenses can be better managed in atoll environments.

Working alongside a range of government agencies in the South Tarawa drought committee, including resource managers, water supply operators, disaster managers, meteorologists, environmental and health officers it has been possible to clarify roles and responsibilities of each stakeholder agency, and develop an evidenced based framework for action, and mitigation response. The results from the project has resulted in a shift in attitude and behaviour from the “pump and forget” model to a more proactive approach to groundwater management and operation.

Supporting efforts, through bilateral funding within Kiribati by MFAT and DFAT, has seen the installation of variable speed pumps on the horizontal water supply wells or infiltration galleries. Being able to operate these pumps to either increase or decrease the rate and amount of water pumped at any one time allows the operators to actively manage the salinity of the groundwater abstracted and optimise the available water to an agreed water quality, resulting in increased efficiencies and ultimately improved sustainability.

The science employed in this project has been successful in demonstrating a new approach to groundwater management in atolls resulting in an improved and more robust drought management response plan. The learnings have been applied successfully elsewhere including the Laura lens in the Republic of Marshall Islands and has contributed to the operationalising of a more active and responsive approach to groundwater management in. While South Tarawa is still some way from meeting its future fresh water demands, the work from this project provides a sound basis for improved groundwater management contributing to water security and drought resilience in these unique groundwater environments in the Pacific.

Image: Bonriki Water Reserve, South Tarawa, Kiribati