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SPECIAL SECTION: GROUNDWATER RESOURCES ASSESSMENT UNDER THE PRESSURES OF HUMANITY AND CLIMATE CHANGE

Climatic and Human Influences on Groundwater in Low Atolls

^a The Fenner School of Environment and Society, Australian National University, Canberra, ACT 0200 Australia
^b Ecowise Environmental, P.O. Box 1834, Fyshwick, ACT 2609, Australia
^c Public Utilities Board, P.O. Box 290, Betio, Tarawa, Republic of Kiribati
^d Ministry of Public Works and Utilities, P.O. Box 498, Betio, Tarawa, Republic of Kiribati
^e South Pacific Applied Geoscience Commission, Private Bag, GPO Suva, Fiji Islands
^f CIRAD Montpellier, France, and Research School of Pacific and Asian Studies, Australian National University, Canberra, ACT, 0200, Australia
^* Corresponding author (ian.white{at}anu.edu.au).

Received 3 July 2000.

Population centers in low, small islands have water supply problems that are among the most critical in the world. Limited land areas and extremely large soil hydraulic conductivities severely reduce surface runoff and surface storage, so that thin lenses of fresh groundwater floating over seawater comprise the major source of fresh water for people in many atolls. Atoll groundwater is extremely vulnerable to frequent El Niño Southern Oscillation (ENSO)–related droughts, salinization due to storm surges and sea-level rise, and to human activities with vadose zone transit times from surface to shallow groundwater being less than 1 h. We examine the relationship between groundwater, rainfall, and ENSO events in a low atoll, Tarawa, in the central and western Pacific Republic of Kiribati. Droughts can last as long as 43 months and occur with a current frequency of 6 to 7 years. The impact of droughts on the quality and quantity of a fresh groundwater lens is explored. The local drawdown of the water table due to pumping from long horizontal infiltration galleries is found to be less than diurnal tidal variations. The saturated hydraulic conductivity, K₀, of the Holocene unconsolidated coral sands was estimated from infiltration gallery drawdown in two islands. The geometric mean K₀ was 14.6 m d^–1 with a range from 0.9 to 111 m d^–1. These large K₀ values cause the rapid transmission of rainfall and surface pollutants through the unsaturated zone to groundwater. An example is given of Escherichia coli pollution due to traditional activities. Strategies for improving the adaptation of island communities and increasing resilience to climate change are discussed.

Abbreviations: EC, electrical conductivity • ENSO, El Niño Southern Oscillation • FDR, frequency domain reflectometry • PS, pumping station • SOI, Southern Oscillation Index

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