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.

The water resources of the 32 atolls of the Federated States of Micronesia (FSM) are under continual threat from El Niño-induced droughts and other natural hazards. With government policies emphasizing sustainable development of atoll-island communities, local managers are in need of tools for predicting changes in the availability of fresh groundwater, which communities depend upon during droughts that incapacitate rain-catchment systems. An application of a recently developed, readily portable algebraic model is demonstrated here, to estimate the freshwater-lens thickness of atoll islands in the FSM, a key component of FSM groundwater resource assessment. Specifically, the model provides estimates of the lens thickness of atoll islands in the FSM during normal and drought conditions. The model was tested for use in the FSM through comparison with available lens data under both average rainfall conditions and intense drought conditions, and then applied to major islands of each atoll within the FSM. Results indicate that out of 105 major islands on FSM atolls, only six would likely retain sufficient groundwater to sustain the local community during an intense drought.

Groundwater flow and the geochemical evolution of a freshwater lens in an aquifer on a Pacific atoll were investigated by hydrogeochemical surveys. Sulfur hexafluoride measurements showed that deeper groundwater and groundwater at the periphery of the lens are older, consistent with a downward and outward groundwater flow scheme. This is the typical flow scheme on Pacific atolls where a Holocene–Pleistocene unconformity restricts the shape of the freshwater lens. Enrichment of Mg/Ca in the groundwater is another indicator of a longer residence time, because contact between the groundwater and the carbonate sediments composing the aquifer leads to the release of Mg from high-Mg calcite and the precipitation of Ca as low-Mg calcite. Groundwater quality was also affected by anthropogenic nitrogen loading and aboveground organic matter, which were altered by denitrification and sulfate reduction in the aquifer, especially in the older groundwater. The chemical composition of the groundwater in the center of the island, where saline water is up-coning, implies that freshwater recharge dilutes the older saline water, which as time passes will eventually be replaced by newly recharged freshwater.

In order to evaluate the behavior of a freshwater lens in the uplifted atoll island of Minami-Daito, Japan, groundwater level and electrical conductivity were simultaneously measured in six monitoring wells. The monitoring allowed determination of the position and variability of the top and bottom positions of the freshwater lens. The freshwater lens periodically oscillates with phase lags (delay time) every 3 h following sea tides. Recharge from local rainfall temporarily increases the volume of the freshwater lens but is disturbed by the low-permeability muddy sediments deposited on the central lowland of the island. Changes in the groundwater levels correlate well with rainfall, after first removing semi-diurnal, diurnal, and long-term components of the sea tides from the groundwater level data using a multiple regression analysis. Changes in the water electrical conductivity provide information on the temporal and spatial fluctuation of the freshwater lens. The monitoring scheme for this freshwater lens could be applied to other uplifted atolls, contributing to better evaluation of the potable groundwater resources and to making freshwater use sustainable on other islands.