The evaluation of the mixing zone for a proposed expansion to a liquefied natural gas facility on the northeast coast of the State of Qatar in support of an environmental impact assessment was performed using the Generalized Environmental Modeling System for Surface Waters (GEMSS). The circulating seawater at the facility absorbs heat during the liquefaction of natural gas, resulting in a maximum temperature rise of 10degreesC. The heated seawater is discharged through a long canal into the Arabian Gulf with minimum jet momentum. GEMSS was calibrated, verified, and then used to model the thermal plume for a proposed expansion of the facility. A probabilistic approach for defining the mixing zone using World Bank standards was developed. Model results show that the thermal plume is vertically stratified, with a relatively large surface area and a relatively small bottom contact area. The advantage of this design is that it maximizes heat exchange with the atmosphere by increasing the driving force and isolating the temperature increase from benthic organisms.