Influence of sea-level rise on freshwater lenses of different atoll island sizes and lens resilience to storm-induced salinization

Abstract

Human populations inhabiting remote coral atoll islands in the tropical Pacific Ocean rely heavily on thin unconfined fresh groundwater lenses (FWLs) as their main natural source of potable water. Potential threats to the continuing viability of atoll FWLs may be associated with eustatic sea-level rise (SLR), but little is known about the influence of SLR on the FWLs of different atoll island sizes and the resilience of FWLs to salinization caused by storm-induced wave washover of low-lying land. Here, numerical modelling is carried out to generate steady-state FWL configurations at three chosen island sizes (400, 600 and 800 m widths), upon which a 40 cm SLR scenario is imposed, followed by transient simulations to assess saline damage and recovery over a 1-year period after a storm washover event on a 600 m wide island. Two different approaches are tested for taking into account the existence of a freshwater swamp, formed where a topographic depression exposes the central region of a FWL above the ground surface. These ‘groundwater window’ and ‘specified flux’ approaches yield similar results, implying that complex integrated surface and groundwater modelling may be unnecessary to develop realistic models of FWL configuration where central freshwater swamps exist. Thereafter, steady-state solutions indicate how FWL dimensions are not related in a linear manner to atoll island size, such that smaller islands develop much more restricted lenses than larger islands. Partly in consequence, smaller FWLs display greater potential for disturbance by SLR, while larger and more robust FWLs may show more resilience. Results from simulation of saline damage caused by storm washover produce a somewhat counterintuitive finding: in a post-SLR situation it is apparent that FWL vulnerability to washover salinization may actually be reduced, owing to the thinner layer of unsaturated substrate lying above the water table (i.e. FWL surface), into which saline water can infiltrate and accumulate during a storm washover event.