The mixing zone developed at freshwater-seawater interface is one of the most important features in complex coastal hydrogeologic systems, which controls subsurface flow and reactive transport dynamics. Freshwater-seawater mixing-zone development is influenced by many physical and chemical processes, such as characteristics of geologic formation, hydrodynamic fluctuations of groundwater and seawater levels, fluid-rock interactions, and others. Wide mixing zones have been found in many coastal aquifers all over the world. However, the mechanisms responsible for wide mixing zones are not well understood. In this thesis, two hypotheses were proposed to explain wide mixing zones in coastal aquifers: (1) kinetic mass transfer coupled with transient conditions, which create the movement of the mixing zone, may widen mixing zones; and (2) aquifer stratification may widen the mixing zone. The hypotheses were tested by both multiscale numerical simulations and laboratory experiments. Numerical simulations were based on a variable-density groundwater model by varying mass transfer parameters, including immobile porosity, mobile porosity, and mass transfer coefficient, and the hydraulic conductivity contrast between aquifer layers. Laboratory experiments were conducted in a quasi-two-dimensional tank, where real beach sands were installed and foodstuff dyes were used to visualize the development of freshwater-seawater mixing zone. Major conclusions included (1) the mixing zone can be significantly widened when the mass transfer timescale and the period of transient boundary is comparable due to the nonequilibrium mass transfer effects; and (2) a thick mixing zone occurs in low-permeability layer when it overlays upon a fast flow layer. These results not only improve the understanding of the dynamics of mixing-zone development and its associated geochemical processes in coastal aquifers, but also identify hydrogeologic conditions for the model of sharp-interface approximation to be valid. In addition to better understanding the mechanisms and dynamics of mixing zone, this thesis also investigates cost-effective management of coastal groundwater resources. To protect and conserve limited water recourses in coastal regions, interest in aquifer storage and recovery (ASR) has been growing in recent years. ASR is a promising strategy for water resources management and has been widely used in many contaminated and saline aquifers. However, its performance may be significantly constrained by mass transfer effects due to the mobilization of solutes initially residing in immobile domains. Better understanding of kinetic mass transfer effects on ASR is needed in order to aid the decision-making process. A numerical model is developed to simulate ASR performance by combining the convergent and divergent dispersion models with a first-order mass transfer model. By analyzing the concentration history at the pumping well, we obtain simple and effective relationships for investigating ASR efficiency under various mass transfer parameters, including capacity ratio and mass transfer timescales, and operational parameters. Based on such relationships, one can conveniently determine whether a site with mass transfer limitations is appropriate or not for ASR and how many ASR cycles are required for achieving a positive recovery efficiency (RE).