The City of Nablus relies on groundwater resources for water supply. Water is collected in central reservoirs and later distributed to the designated areas through a water distribution network. Due to the expansiveness of the network, the remoteness of the water sources, and the weariness in the network pipes and joints, chlorine concentrations were found to be way below the minimum allowable limits. This study investigated the disinfection problems that exist in the water network of Nablus. A composite database for the last 9 years (1996-2005) was created to facilitate the spatial and temporal assessment. Assessment of the current chlorination practice for a selected area within the Nablus drinking water network was carried out through a development of water network quality model using the GIS technology and EPANET. The model simulates the water flow in pipes, pressure at the supply nodes, and chlorine concentration distribution in pipes and nodes. Results of the analysis of the composite database showed that residual chlorine concentrations in many areas were low. This has led to the bacterial growth in these areas as indicated by total and fecal coliform readings. Model outcome showed that chlorine concentration varies from location to location in the network, which is in agreement with field observations. This model could then be used by the Municipality to determine the optimal chlorine dose such that chlorine concentration distribution across the network is within the acceptable limits.