Mechanisms of Residual Saltwater Desalination Behind an Impervious Cut-Off Wall Under Seasonal Fluctuations and Permeability Anisotropy

Seawater intrusion remains a critical threat to coastal groundwater, where subsurface cut-off walls are commonly used for mitigation. This study employs 2D variable-density numerical modeling to investigate the impacts of hydraulic conductivity anisotropy (rk = 0.02–50) and seasonal inland groundwater fluctuations on residual saltwater desalination, quantified by means of RRSM and RRSL. Our results revealed that rk is inversely correlated with final desalination efficiency. Lower rk values (0.02–0.1) achieve exhaustive salt removal despite requiring longer flushing durations. Conversely, higher rk values significantly suppress efficiency and induce violent oscillations in desalination rates under seasonal forcing. A critical failure mechanism was identified: intensified vertical flow dynamics allow saltwater to “overtop” the barrier during low inland groundwater stages, triggering severe secondary intrusion. These findings underscore that conventional cut-off wall designs may be inadequate under dynamic boundaries, necessitating taller barrier configurations and precise anisotropy assessments to ensure long-term functional resilience in coastal aquifer management.