In this study, a numerical model was developed to predict perfluorooctane sulfonate (PFOS) migration in a contaminated site due to precipitation and evapotranspiration over 100 years. The soil physicochemical properties including sorption isotherm characteristics were used to model PFOS transport (advection and dispersion), and attenuation (adsorption and decay) processes using the GeoStudio software. The model was calibrated using the results of a leachate test on a reconstituted contaminated soil specimen. The simulation of the contaminated site showed that in the current situation where the site is covered by a concrete slab, PFOS remains at the site beneath the concrete slab for a long time with decay being the dominant factor contributing to its reduction over time. It took several decades for PFOS to contaminate groundwater. The PFOS level in groundwater reached the threshold value for drinking water in 25 years and continued to increase in the long term, extending horizontally to an area up to five times the size of the contaminated site. This issue needs to be considered when digging boreholes for drinking water close to the contaminated site.