Monitoring the pollution degree of water treatment plants (WTPs) or any water sources requires routine and continuous water quality measurements, which is considered a challenging task.  For this reason, applying a statistical tool to deal with a single value instead of a huge number of water quality values is very useful for a decision maker in this field. This work aimed to evaluate the characteristics of the purified water that is supplied from the most important water purification stations in Basra governorate for drinking use. Additionally, the study aimed at using the water quality index (WQI) for examination of the water quality, and obtaining a new equation of the WQI for water treatment plants based on the weighted arithmetic index. In this consent, 96 treated water samples were collected, regularly, from eight water purification plants in Basra city over a period of 12 months in 2021, for measuring the main physicochemical properties. Results showed that the studied units supplied treated water with the WQI varying between 50 and 72, and only the water supplied from Al-Asmaee unit could be classified as good water for drinking. The new developed formula of the WQI was valuable and applicable to examine the water quality of any water treatment unit in Basra city. It is recommended to use ridge linear regression (RLR) as the best statistic approach. The coefficient of R2 founded by RLR method was 96.1369. This method is helpful to be applied for a decision maker for monitoring different resources of water in case of the availability of enough data.