DRASTIC model has been used to map groundwater vulnerability to pollution in many areas. Since this method is used in different places without any changes, it cannot consider the effects of pollution type and characteristics. Therefore, the method needs to be calibrated and corrected for a specific aquifer and pollution. In the present research, the rates of DRASTIC parameters have been corrected so that the vulnerability potential to pollution can be assessed more accurately. The new rates were computed using the relationships between each parameter and the nitrate concentration in the groundwater. The proposed methodology was applied to Astaneh aquifer located in north of Iran. Samples from groundwater wells were analyzed for nitrate content in thirteen locations. The measured nitrate concentration values were used to correlate the pollution potential in the aquifer to DRASTIC index. Pearson correlation was used to find the relationship between the index and the measured pollution in each point and, therefore, to modify the rates. The results showed that the modified DRASTIC is better than the original method for nonpoint source pollutions in agricultural areas. For the modified model, the correlation coefficient between vulnerability index and nitrate concentration was 68 percent that was substantially higher than 23 percent obtained for the original model.