Experiments and modeling of flow processes in freshwater lenses in layered island aquifers: Analysis of age stratification, travel times and interface propagation

•Physical models reveal flow dynamics within layered freshwater lens aquifers.•Analytical and numerical models are used for verification and budgeting.•Travel times and age stratification show strong deviations from homogeneous models.•A layered aquifer shows a bimodal distribution of travel times and age repetition.•The results have practical implications for the delineation of protection zones.

SummaryThe management of fresh groundwater resources and the delineation of protection zones on islands require a thorough understanding of flow processes within freshwater lenses. Previous studies of freshwater lenses have mainly focused on interface geometries, disregarding flow patterns within the lens. In this study we use physical, analytical and numerical modeling to investigate the influence of vertically and horizontally layered aquifers and variations in recharge on the age stratification, travel times and transient development of freshwater lenses. Four generalized settings were examined in detail on a laboratory-scale. The experiments show significant deviations from homogeneous models. The case of a high permeability layer underlying a layer of lower permeability shows the strongest deviations in the processes investigated here. Water in the more permeable lower layer overtakes water flowing only through the upper layer, causing a bimodal distribution of travel times and a vertical repeating of the age stratification near the coast. The effects of heterogeneities revealed by the physical model experiments are expected to also occur on real islands and have thus to be considered when developing conceptual models for the management of such freshwater lenses.