Regional and well-scale indicators for assessing the sustainability of small island fresh groundwater lenses under future climate conditions.

Salinization of the freshwater lens and well fields under future climate and groundwater demands threaten the sustainability of groundwater abstraction in small islands. In this paper, two simple indices are proposed for a comprehensive assessment of freshwater salinization in small islands. Values of these indices are computed based on a sharp-interface finite element numerical model with the interface matched to the lower limit of freshwater. The first index relates rainfall percentiles with the computed freshwater volume and provides regional assessment of changes in the volume stored in the freshwater lens over time. The second is a sustainability index, based on computed saltwater ratios in pumping wells as the performance indicator, which represents the well salinization risk at a spatial scale. The proposed methodology is illustrated with the example of Tongatapu Island. Freshwater lens dynamics and well salinization under various scenarios of dry, median, and wet general circulation model predictions-based unsteady recharge, sea level rise, and pumping demands were evaluated for the period 2010–2099. The freshwater lens of Tongatapu is dependent on rainfall, and the 60-month rainfall percentile is highly correlated with the total freshwater volume. Public well field indicates sustainability indexes of 45% and 100% under the current pumping conditions for the driest and the wettest scenarios, and with increased pumping and sea level rise, they reduce to 30% and 90%, respectively. Thus, management of pumping rates is essential for future freshwater sustainability. This methodology can be used for first-hand temporal and spatial estimations of small island freshwater lens salinization considering both the regional and well scales. [ABSTRACT FROM AUTHOR]