The oscillatory nature of the coastal longshore currents is one of the major factors that play a significant role in the spreading of discharged effluents in marine environments. Most previous investigations of the dispersion of discharged effluents through an outfall into the sea have been based on a simple sinusoidal tidal. In reality, tidal currents are more complicated and often contain oscillations of two different frequencies. An analytical solution of a two-dimensional advection-diffusion equation containing a developed spring-neap tidal current has been applied in the present paper. A parametric examination has been performed to study the effects of the spring-neap tidal currents on the concentration at the shoreline and along the centerline of discharge effluent plume from a single outfall. A comparison study of a constant discharge and a time-varying discharge strategy which depends on the spring-neap tidal current is also conducted. Finally, the interaction and merging of effluent discharge plumes from a system of two outfalls, including a single outfall installed with two-port diffuser, are investigated.