Atoll Groundwater Resources at Risk: Combining Field Observations and Model Simulations of Saline Intrusion Following Storm-Generated Sea Flooding

The restricted nature of naturally-occurring freshwater resources on atolls is one of the greatest impediments to human settlement on these small, dispersed and remote islands. Any anthropogenic or environmental pressures that deleteriously affect the quantity or quality of atoll water resources are therefore a matter of concern. This chapter focuses on such issues. It first presents an overview of the principal characteristics of atoll fresh groundwater aquifers, which exist in the form of thin lenses within the Holocene sands and gravels that comprise the sedimentary substrate of low-lying atoll islets. Factors that influence the vulnerability of these freshwater lenses are then considered. The chapter continues by summarising the findings of recent studies that investigated the effects of storm-wave washover across atoll islets on freshwater lens profiles, and the subsequent patterns of recovery over time. Both field and modelling approaches are used. Combined results suggest that following groundwater salinisation by seawater intrusion, at least a year is required for full aquifer recovery. Of particular interest, it is found that in spite of a strong saline plume forming at relatively shallow depths, a thin horizon of freshwater sometimes remains preserved deeper within the aquifer profile for several months after the initial disturbance. In the Pacific basin, shifting geographical patterns in severe tropical storm events related to climatic variability and change are a threat to the continuing viability of atoll fresh groundwater resources and the human populations dependent upon them