Vulnerability of the freshwater lens to groundwater abstraction and climate change on Muli Island, Maldives

Abstract

Abstract In small coral islands, groundwater is often the primary source of water supply. However, their aquifers are less resilient to climate and environmental changes because groundwater storage is limited by a small island area, flat terrain, and a thin freshwater lens. In the Maldives, groundwater has been historically abstracted from hand-dug wells. However, its quality has deteriorated owing to poor wastewater management and seawater intrusion. In this study, a three-dimensional, variable-density groundwater model (SEAWAT) was employed to examine the seasonal dynamics of a freshwater lens and its response to climate change and abstraction. Model results revealed that the baseline freshwater lens exhibits strong seasonal dynamics, resulting in up to 20% reduction in freshwater volume during prolonged droughts. Four scenarios were developed for 2025–2050: Two climate scenarios based on rainfall and temperature projections (S1: SSP2–4.5 and S2: SSP5–8.5), and two incremental scenarios relative to S1 that additionally include sea level rise (S3) and increased abstraction (S4). Scenarios S1 and S2 resulted in reductions in mean freshwater volume of 25.8 and 28%, respectively, compared to the baseline. In contrast, scenarios S3 and S4 caused smaller reductions than S1 and S2. Nevertheless, under S4, increased abstraction led to localised upconing and lens thinning, particularly during drought conditions. Collectively, prolonged drought, localised upconing, high submarine groundwater discharge, and increased paved surfaces indicate the principal vulnerabilities of the freshwater-lens system. These findings demonstrate the need for integrated environmental management, including the protection and enhancement of recharge, to safeguard fragile freshwater lenses.