Responses of atoll freshwater lenses to storm-surge overwash in the Northern Cook Islands

Terry, James P.; Falkland, Anthony C.

Responses of atoll freshwater lenses to storm-surge overwash in the Northern Cook Islands | Réponse des lentilles d’eau douce à une lame de submersion cyclonique dans les Iles Cook septentrionales Respuestas de las lentes de agua dulce de un atolón sobrelavado de una onda de tormenta en las Northern Cook Islands 北库克群岛环礁淡水透镜体对风暴潮越浪的响应 Respostas das lentes de água doce em atóis a submersões provocadas por marés ciclónicas nas Ilhas Cook do Norte

2010

Terry, James P. | Falkland, Anthony C.

A category 5 tropical cyclone swept a storm surge across remote Pukapuka Atoll in the Northern Cook Islands (South Pacific Ocean) in late February 2005. Groundwater salinity (specific conductance) observations are reported for the 2-year post-storm period, with the aim of investigating the effects of saltwater intrusion on thin freshwater lenses within the atoll islets. This is the first article to present field observations of such an event. Specific conductance at shallow depths increased dramatically from potable conditions (approximately 1,000 μS/cm) to brackish levels unsuitable for drinking (up to 10,000 μS/cm) shortly after the cyclone. Subsequently, the freshwater lenses required 11 months to recover. Within the thickest aquifer, a well-defined saline plume formed at 6 m depth, sandwiching a freshwater layer beneath it and the base of the lens. Plume dispersal proceeded only gradually, owing to its formation at the start of the SW Pacific regional dry season and the low tidal range on Pukapuka. Consequently, the remnant of the plume was still present 26 months after the saltwater incursion. An important finding was that the freshwater horizon preserved at depth maintained salinity levels below 1,800 μS/cm (i.e. within usable limits) for at least 5 months after surface overwash.

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