where 𝐷 is the apparent diffusion coefficient in the pore water, 𝐷 is the effective diffusion coefficient (related to the flux), and 𝐷 is the diffusion coefficient in free water. The dimensionless quantities 𝜀 and Γ describe porosity and pore geometry, respectively, while 𝜌 is the bulk density of the porous medium and 𝐾 [m 3 /kg] the distribution coefficient describing reversible, instantaneous sorption of the species of interest (Br⁻ in this case) on the surface of the porous medium. Γ is usually defined as the ratio between the square of tortuosity (dimensionless parameter describing deviations of diffusion paths from a straight lines) and constrictivity (dimensionless parameter describing the variations in pore widths).