Planetary Boundaries were defined to aid in understanding the impacts of human activities on the planet. The newly quantified boundary of New Entities has already drastically exceeded its limit, with plastics being one of the major contributors. Full Multi is a multi-medium model based on mass balance that calculates the fate and accumulation of microplastics and nanoplastics along a water body. However, some of the processes in the model have simplified equations. This study reviewed the sedimentation and ascent models, incorporating Reynolds number (Re), drag coefficient (Cd), and sphericity, and compared the results of both models under various scenarios to identify ways to optimize this type of study. The results showed a lower sedimentation/ascent velocity in the proposed model compared to the original. Among the simulated scenarios, the greatest difference was observed with changes in particle type. Variations in the medium only influenced cases where the polymer density was between the densities of the two different mediums. Variations in Re did not show significant impact, likely due to the large difference between the values used in the simulations (0.1 and 10,000) and those calculated by the model itself (10e-19).