Environmental risk assessment is a step towards identification, analysis, and classification of risk factors and thus reduction of the possibility of adverse consequences. In this research, a novel approach for environmental risk assessment on groundwater pollution is applied. By combination of aquifer vulnerability DRASTIC map, pollution severity and prioritizing of the plain regions by the TOPSIS method, more sensitive regions of Qazvin aquifer in Iran are identified. In the first step, seven hydro-geological characteristics of the aquifer are overlaid to produce the potential vulnerability map. Nitrate is used as the pollution parameter and its value in monitoring wells is measured by sampling. Spatial distribution of nitrate concentration is investigated using the ordinary kriging method. The TOPSIS ranking method is also applied to estimate the probability of occurrence of pollution based on five affecting criteria defined and quantified in regions of the aquifer. By production of these three layers, the risk map of the aquifer is generated. Results indicate that 9% of the area of the aquifer is categorized in the high risk level which needs an emergency recovery action plan. Also, sensitivity analysis on the parameters of the aquifer vulnerability shows the effect of the soil media more than other parameters.