Subsurface freshwater in oceanic islands is typically shaped like a thin lens due to limited land area and recharge, often the primary freshwater source for local communities and highly vulnerable to seawater intrusion (SWI). Freshwater injection (FI) is considered as a feasible strategy for mitigating SWI in coastal aquifers. However, its transient effectiveness for freshwater lens (FWL) development and SWI mitigation in island aquifers and how the design parameters like FI depth, intensity, duration and injectant concentration affect its performance remain poorly understood. To address this, this study employs a two-dimensional, variable-density island groundwater model to simulate the transient responses of FWL development and SWI mitigation to various FI patterns. Five indicators are developed for comprehensive evaluation, including (1) freshwater recovery efficiency (FRE), and the relative changes in (2) average water table elevation (WTE), (3) FWL depth, (4) FWL volume, and (5) total aquifer salt mass. Results reveal FI universally raises average WTE, expands FWL dimensions, and promotes aquifer desalinization. Injection intensity is the primary driver of WTE rises and salt mass reduction, with higher intensities consistently yielding greater WTE rises and salt mass reductions. Deeper injection within the mixing zone increases FWL depth, but reduces the net gain in FWL volume. Moreover, early-stage FI is highly efficient for expanding FWL volume, often yielding FRE values above 100%, but FRE converges toward zero over time as the system moves toward a new hydrodynamic equilibrium, returning diminishing marginal benefits for long-term FI.
Yu et al. (Thu,) studied this question.