Dynamic responses of freshwater-seawater mixing zones to seasonal freshwater-level fluctuations and the presence of kinetic mass transfer between mobile and immobile domains have been analyzed using numerical models. Mixing enhancement is mainly controlled by the unsynchronized behavior of concentration distributions in the mobile and immobile domain. Such effect is maximized at the aquifer bottom when the retention timescale (the reciprocal of mass transfer rate) in the immobile domain is comparable to the period of freshwater-level fluctuations. Kinetic mass transfer may alter the time lag between periodic freshwater-level fluctuations and the movement of the mixing zone, causing the expansion and contraction of the mixing zone. Mixing enhancement by kinetic mass transfer is nonuniform in the mixing zone, and the mixing zone thickness may vary significantly within a period. By contrast, large dispersion coefficients may create thicker mixing zones but may not cause such unsynchronized behavior and alter the time lags of different concentration contour lines; that is, the mixing enhancement is rather uniform in the mixing zone. The dynamics of mixing zone development is sensitive to the flow velocity, which is influenced by the hydraulic conductivity, amplitude of the freshwater-level fluctuations, and the capacity ratio of kinetic mass transfer.