Sea level rise due to anthropogenic climate change is an existential threat to low lying Pacific islands and their inhabitants. The Island of South Tawara is the capital and main hub for the Republic of Kiribati and home to around 50,000 people, half of the Kiribati population. The island is low lying, with a maximum elevation 3 m above mean sea level, meaning most of the 15 km2 island and its freshwater lens water supply are at risk of ocean overtopping and inundation. A climate change adaptation proposal has recently been funded by the New Zealand government looking to provide around 3 km2 of reclaimed land at a higher elevation by dredging and pumping lagoon sand onto a tidal flat area. The land reclamation offers secure additional land for habitation and in the long term the possibility to expand the area of freshwater resource. However, during construction pumping of seawater and sand threatens existing freshwater.
An existing variable density groundwater model of the freshwater lens system was adapted to investigate the period during and after construction. Transient 3D simulations of the existing lens and reclamation area were completed using the variable density code Modflow USG-Transport. Risks to existing freshwater resources during construction can be mitigated by phased placement of dredged material and appropriate drainage. The time taken for freshwater resources to develop under reclaimed land depends heavily on future rainfall and groundwater recharge. It is estimated to take multiple decades such that other freshwater sources will be required for new housing in the short and medium term.