Contaminant spills are frequently encountered in coastal sites where many industrial plants are located. Refineries and petrochemical plants are often built close to the sea for easy transport of crude oil and final by-products. The migration of organic compounds spilled in the subsurface of coastal sites can be influenced by the effects of seawater intrusion in the aquifers discharging to the sea. An improved version of TMVOC, belonging to the TOUGH2 family of numerical reservoir simulators, can model the migration of multicomponent organic mixtures under multiphase conditions accounting for the effects of sodium chloride dissolved in the aqueous phase. The thermophysical properties of groundwater as a function of temperature, pressure, and salinity are evaluated following the basic approach used for saline brines in EWASG, a specialized thermodynamic module of TOUGH2 originally developed to model geothermal reservoirs. Simulations of a multicomponent organic spill using a two-dimensional vertical numerical model show the effects of seawater intrusion on the distribution of contaminants within the aquifer. The effects of the construction of an impervious wall on the nonaqueous phase liquid (NAPL) plume migration, as a means to contain the spreading of plume toward the sea, are also investigated.