Consideration of non-revenue water in urban water distribution networks is of special importance in planning urban water distribution network and in water consumption. Water loss, especially in the distribution networks, forms a large part of the unaccounted for water (UFW). Considering the serious crisis in water resources, and taking into account the existing limitations of supplying hygienic drinking water, increasing attempts must be made at reducing the volume of water loss. The present study evaluated the environmental effects of pollution leakage from broken pipes into the water distribution network in the Shokuhieh. General principles and concepts of UFW, apparent loss was reviewed. Based on these concepts, probable pipe break points, and the resulting contaminants entering the network were considered. A software model of the water distribution network in the study area was then used to investigate qualitative and quantitative results related to intrusion of contaminants into the water distribution network and diffusion of pollutants into the network. Results of the present study and physicochemical tests performed on water revealed that BOD and COD concentrations increased from 5-10 to 10-20 mg/L and the coliform count rose from zero to 20 MPN as a result of pipe breaks in the distribution network.