Influence of layered heterogeneity on freshwater lens development and seawater upconing in island aquifers: insights from integrated physical and numerical modelling.

Sustaining groundwater resources in coastal and island aquifers is increasingly challenged by seawater intrusion driven by groundwater abstraction, land-use change, and climate-related shifts in recharge. Freshwater lenses are particularly vulnerable, and their response is strongly conditioned by subsurface heterogeneity, although many conceptual and analytical approaches still assume homogeneous conditions. The role of layered heterogeneity in controlling freshwater-lens development and saltwater upconing is investigated through an integrated framework combining controlled laboratory experiments, density-dependent numerical modelling with FEFLOW, and comparisons with classical analytical solutions.

Sandbox experiments are conducted under homogeneous and stratified configurations to examine lens evolution under steady recharge and during pumping. The heterogeneous setting, characterized by contrasts in vertical hydraulic conductivity, markedly altered lens geometry by reducing its maximum thickness, laterally extending the mixing zone, and promoting preferential flow pathways. Numerical simulations successfully reproduced the observed system behavior and enabled further exploration of pumping scenarios beyond the limitations of the physical model.

Results indicate that stratification accelerates and intensifies saltwater upconing, effectively lowering sustainable pumping rates and increasing vulnerability to salinization under human impacts. Analytical solutions are shown to overestimate lens stability and delay the predicted onset of upconing in layered systems, highlighting limitations when applied to heterogeneous coastal aquifers.

The findings provide quantitative evidence that layered heterogeneity exerts a first-order control on seawater-intrusion dynamics relevant to integrated water resources management. The combined physical–numerical approach supports improved assessment of pumping sustainability, monitoring design, and adaptation strategies to enhance resilience to salinization under changing climate and extraction pressures.