Gas stations are one of the most important sources of soil and groundwater contamination in urban areas around the world. In this work, a mathematical model was applied to assess the risk of the physical media contamination. The model is based on the chemical properties of fuels and site-specific hydrological properties. The group of hydrocarbons selected were the monoaromatic: benzene, toluene, ethylbenzene and xylene (BTEX). The mathematical model was used to evaluate the behaviour of each compound in each partitioning phase (dissolved, volatile, solid and non-aqueous liquid phase). Furthermore, mass flux of BTEX reaching groundwater was calculated according to steady flow under two different conditions: (1) considering only sorption and volatilization, and (2) adding biodegradation. In all cases, the total mass of BTEX leachate was greater when biodegradation was not considered. In areas with unsaturated zone thickness greater than two meters, sorption is the main process affecting BTEX transport. Otherwise, when water table is placed at shallower depths, contaminants are predominately dissolved. In all cases, the lowest proportion of BTEX were in the volatile phase. The model presented in this work proved to be an interesting tool to evaluate the behaviour of BTEX in soil and groundwater and its applicability being directly related to environmental and urban management of pollution from gas stations. The methodology applied in this work could be extrapolated to any site of the world with the same problematic. Particularly, the theoretical model allowed to defined zones with high and low risk of contamination in Bahia Blanca, Argentina.