Tarawa atoll, the main and most populated island in the Republic of Kiribati, is located just north of the equator in a region of the Pacific where rainfall is highly variable largely due to the influence of El Niño and La Niña episodes. Groundwater resources on this and other atolls occur in the form of ‘freshwater lenses’. In Kiribati, groundwater is the main source of water supply for domestic and other demands. Rainwater provides an additional but relatively minor source of water owing to the impacts of long drought periods associated with La Niña episodes.
Freshwater lenses are highly vulnerable to climate variability and to the impacts of human populations, particularly from over-pumping and from biological and chemical pollution. The shallow depths to groundwater table (typically 2 m) and the highly permeable coral sands allow water and pollutants to quickly infiltrate from the surface to the groundwater. In most of South (urban) Tarawa, where the population is estimated to be 60,000 on a land area of only 16 km2, the pollution from pit toilets, septic tanks, animals, fuel storages and other sources is high. The population density in some urban centres on South Tarawa exceeds 10,000 people per km2.
Groundwater is pumped from a network of infiltration galleries (or ‘skimming wells’) in two mainly unpopulated islands, Bonriki and Buota. It is treated and supplied via a pipe distribution system to meet part of South Tarawa’s present potable water demand.
Extensive groundwater studies have been undertaken over the past 30 or so years to investigate and monitor the groundwater resources, particularly of Bonriki island. These have included drilling of multi-level monitoring boreholes, geophysical studies, estimation ofgroundwater recharge from detailed measurements and water balance approaches and estimation of sustainable yields using groundwater models. Recommended pumping rates for the 22 Bonriki infiltration galleries have been based on these sustainable yield estimates. In recent years, pumping from Bonriki has been at higher than estimated sustainable rates and there are indications that this is having an adverse impact on the long-term sustainability of the groundwater.
As population increases, the contribution of the Bonriki & Buota groundwater resources to Tarawa’s future water demand is becoming less significant. There is a real need to not only properly manage the groundwater pumping from Bonriki and Buota but also to provide additional water resources.
Most global climate models project an increase in annual and seasonal rainfall during this century. While droughts are projected to become less frequent. A potential climate change impact, however, is longer droughts caused by stronger La Niña events. Mean sea level is projected to rise by 50 - 140 mm by 2030 and 160 – 580 mm by 2090. The potential impacts of sea level rise on land and hence on groundwater resources are not well known as coral growth can keep pace with sea level rise to some extent.
The presentation will cover the above items and raise important water resource management issues including threats to Tarawa’s overall water security from climatic and non-climatic factors and possible options to cope with these. These threats are relevant not only to Tarawa but also other similar small Pacific islands with high and ever-increasing populations.
Tony Falkland is a highly respected Australian water engineer, working for Island Hydrology Services, Canberra.