The city of Salalah is located on the southern coast of the Sultanate of Oman. The city is in the center of a coastal plain, which extends from the foot of Jabal Al Qara to the Arab Sea and from the coastal town of Taqah in the east to Raysut in the west. Groundwater is the only source of water supply for the domestic and agricultural communities in the area. The abstraction rate from the aquifer has gone beyond the annual recharge rate thus reducing freshwater storage and allowing the saline water intrusion in the coastal area. Several studies targeted this problem in an attempt to find feasible solutions. One of those proposed solutions was to inject treated wastewater collected from the city of Salalah into the coastal aquifer to create an elevated head to avoid further deterioration of water quality by salinity. The implementation of this solution is currently in progress. A series of recharge wells were constructed in the agriculture strip near the coast and Garziz farm to the upstream near the Salalah wellfield. These wells will receive treated wastewater from the Salalah treatment plant through a pipeline. The present project aims at the study of the impact of treated wastewater injection into the coastal aquifer of Salalah on the upstream groundwater quality, and the effectiveness of the treated wastewater recharge in controlling the advancement of saline water into the fresh water in the Salalah coastal aquifer. In the present study MODFLOW, a three-dimensional groundwater model for flow through porous medium, is used. The model is a well-known finite difference package developed by McDonald and Harbaugh (1982). In addition to that a Mass Transport Three-dimensional for Multi-Species (MT3DMS), is a three-dimensional.