Abstarct: Mashhad, with an area of over 200 square kilometers is located in the center of the Kashafrud Basin in Mashhad plain. Due to the limited water resources and the excessive exploitation from the aquifer, groundwater level is steadily declining in the plain and The average annual decline in the plain is about 1.03 m. The fundamental aim of this research is preparing the mathematical model of Mashhad. aquifer with the purpose of predicting the behaviour of the aquifer for different conditions (such as the effect of waste water collection project subsidence). For gaining to this aim, GMS computer code MODFLOW and with conceptual model approach were applied. After collecting the required data including pumping test data, driller’s logs, water level data and geophysical data, the conceptual model of the plain was constructed. The developed model was calibrated in steady state (October 2008) and transient state (November 2008 to October 2009) using water table data of 17 piezometric boreholes. Parameters K and S are the most important parameters that optimize in these stages baxsed on the obtained results, average values of K is 1 to 16 m/d and values of S varies betwean 0.25 and 0.00001. The sensitivity analysis was examined for the change of different parameters. The obtained results reveals that the most sensitivity of the model is related to the values of hydraulic conductivity. baxsed on the results obtained during the verification stage (November 2009 – October 2010) the accuracy of the model was confirmed. Finally, the verifed model was used for predicating the future condition of aquifer for scenario development and its impact on groundwater levels in the wastewater collection network used and the results were evaluated. The obtained results from applying the model indicate that waste water collection project may decrease the reacharge of the aquifer such that in the end of project the average annual decline will reach to 16 meter. Due to soil texture of mashhad city and the thickness of aquifer, the values of subsidence rates in supercritical condition varies from 50 cm to maximum 250 cm in Mashhad city. This project will expand our ability to manage for stabilizing the rate of exploitation and management for construction standards increase.