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24th Salt Water Intrusion Meeting and the 4th Asia-Pacific Coastal Management Meeting, 4 –8 July 2016, Cairns, Australia

Assessment of climate variability and abstraction impacts on the Bonriki freshwater , Tarawa, Kiribati

Amandine L. Bosserelle1, Sandra C. Galvis1,2, Vincent E.A. Post2,3, Adrian D. Werner2,3, and Peter Sinclair1 1Geoscience Division, Pacific Community, Suva, Fiji 2School of the Environment, Flinders University, Adelaide, Australia 3National Centre for Research and Training, Adelaide, Australia

ABSTRACT Availability of freshwater is a major issue on atoll islands, because besides rainfall, which can be irregular, fresh groundwater lenses are the only other source of natural freshwater. This study focusses on the atoll of Tarawa in the Republic of Kiribati, located in the Central Pacific. The south part of the atoll, South Tarawa, is the urban, political, administrative and commercial centre of the Republic of Kiribati. In this area, over 50,000 people, about half of the population of the country, reside in less than 16 km2. The growing population of South Tarawa relies on the freshwater lens found under the 0.7 km2 Bonriki water reserve as their main source of freshwater. Salinisation, driven by increased abstraction to meet the rising demand, is a key concern.

Analysis of hydrogeological information and historical records of salinity and water level measurements were used to develop a conceptual model of the Bonriki freshwater lens. This was used to underpin a 3D variable-density numerical groundwater model of the groundwater flow system was built using SEAWAT. The model was used to assess the impacts from groundwater abstraction and climate variability on the freshwater lens.

The model results indicated that rainfall variability and pumping have a strong control on the temporal dynamics of the freshwater lens that are apparent in field observations, as expected. The numerical model scenarios further showed that the rate and distribution of abstraction is a key factor in the historical variability of salinity, which is recorded at the island’s water treatment plants. A range of sustainable abstractions, i.e. abstractions volumes that ensure that salinities at the groundwater galleries do not exceed a threshold value of 1500 μS/cm, were identified under different rainfall scenarios. The future use of these modelling results are expected to provide the population of South Tarawa with more reliable management strategies for protecting their long-term extraction of freshwater from the Bonriki lens.

KEYWORDS: atoll hydrogeology; groundwater modelling; climate variability; groundwater abstractions

Contact Information: Sandra C. Galvis, Pacific Community, Geoscience Division, private Mail Bag, 241 Mead Road, Nabua, Suva, Fiji, Phone: +679 865 39 55, Email: [email protected]

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