Tarawa atoll is within the Gilbert group of islands in Western Kiribati
South Tarawa
Main urban area in Kiribati, with the 2010 census recording 50,182 people of the more than 103,058 total population
Bonriki Water Reserve
Primary raw water supply for the Public Utilities Board reticulated water system used by at least 67% of the population
Bonriki Freshwater Lens
The groundwater system under the atoll island is in a delicate balance between rainfall recharge and sea water instrusions
Atoll Hydrology
Groundwater water levels are highly variable due to their close relationship with water levels in the lagoon and ocean

Climate and Abstraction Impacts on Atoll environments

The CAIA project aims to improve the understanding of climate variations and human impacts on atoll groundwater resources and identify technical and practical management tools that can be used by Government and communities to strengthen water security.

Small Island Resources

Small island atoll states have limited and vulnerable groundwater resources. With the CAIA project, groundwater resources will be assessed in two different atoll environments, Bonriki in Kiribati and Vaitupu in Tuvalu, using hydrogeological investigations techniques and groundwater modelling.



The assessment of these resources coupled with the development of a sustainable yield concept for a freshwater lens under predicted climate and abstraction pressures will improve the resilience of Pacific Islands Communities in the future.


Freshwater Lens

The groundwater on atolls is often described as a ‘lens’ of freshwater ‘floating’ on more dense brackish water. This very thin and fragile freshwater resource relies on being regularly recharged by rainfall. Concerns over the salinisation of these fragile water sources due to rises in sea level, changes in climate variability and extremes, and human activities are increasingly raised by atoll communities and governments.

Threats to Groundwater 

Whilst rises in sea level pose a longer term threat to freshwater lenses, the more immediate threats are from over abstraction and inappropriate land use activities, including poor sanitation practices, intensive cropping or animal husbandry in unsuitable locations. It is expected that population pressure and climate impacts will place the limited groundwater resources of atoll countries under an ever increasing threat.



Atolls islands are low-lying reef carbonate islands. They usually consist of a series of narrow islands or motus surrounding a relatively shallow seawater lagoon. There are 261 atolls among the south-west Pacific Ocean. The largest number of atolls (77) are found in French Polynesia (Tuamotu Archipelagos)

Analytical Groundwater Model

An analytical model solves highly simplified representations of aquifers (e.g. properties of the aquifer are considered to be constant in space and time, aquifer geometry is uniform) to enable a rapid solution to a given problem.

Numerical Groundwater Model

A numerical model divides space and/or time into discrete pieces, enabling more complex and potentially more realistic (relative to analytical models) representations of a groundwater system to be developed. Two dimensional cross-sectional numerical models consider only a vertical slice of the study area. Three dimensional numerical models consider areal and vertical extents of the study area.

Physical Groundwater Model

A common form of physical model is the sand-tank, which is used to visualise physical processes, albeit under highly simplified and smaller-scale conditions than encountered in the field, that are otherwise difficult to observe.