An analytical model for chloride transport in coastal sediments under long-term sedimentation

Freshwater scarcity in coastal areas can be alleviated by exploiting groundwater in seabed sediments. Understanding the long-term distribution and transport of chloride in these sediments is essential for effective resource assessment. In this study, we develop an analytical model that captures the evolution of vertical chloride profiles in seafloor sediments under long-term sedimentation. The model simplifies the system by treating sediment burial as a moving boundary and allowing time-varying chloride concentrations at the seawater-sediment interface, while neglecting short-term hydrodynamic processes to focus on millennial-scale diffusion driven by long-term sedimentation. A semi-analytical solution is derived and validated against numerical simulations using COMSOL Multiphysics. Application to borehole data from the Pearl River Delta and offshore Hong Kong shows that the model captures key features of observed chloride profiles, with diffusion and sedimentation identified as dominant controls. While the model adopts simplified assumptions, it offers a computationally efficient framework that complements more detailed numerical approaches.