Abstract: To effectively evaluate the diffusion risk of contaminants in groundwater and to establish a systematic risk assessment methodology, this study investigates a case of an oily wastewater dumping site. A three-dimensional unsteady flow numerical model was developed using FEFLOW software, based on a generalized characterization of the site's hydrogeological conditions and pollution characteristics. After calibrating the model through inverse parameter estimation, a solute transport model was constructed upon the groundwater flow framework. This model simulated the migration of total petroleum hydrocarbons(TPH), a typical pollutant in oily wastewater, by incorporating advection, dispersion, and adsorption-desorption processes. Based on the simulated pollution levels for sensitive receptors during the assessment period, the diffusion risk was classified into three grades: low, medium, and high. The simulation results indicate that under natural attenuation conditions, it would take 6 800 days for TPH concentrations to decrease below the regulatory standard value. A diffusion risk of TPH to both the lower confined aquifer and the downstream residential areas was identified, with risk levels classified as high and medium, respectively.