Water Quality Indices (WQIs) refers to a set of water quality parameter data that aggregate to produce a single value to the water quality, reducing the huge amount of data into a simple and easy expression. Also enabling comparisons of water quality status for different locations and at different times, which eventually will help inform of water quality status to management and the public in a simple manner. Before generating and developing an appropriate mathematical model, there should be a process of evaluating, reviewing, and considering the Challenges and limitations of the existing developed models of similar nature. Therefore, this review paper intends to summarize the scientific literature papers from different countries related to the development of the water quality indices and practical applications. In general, five common steps are used in approaches of WQI calculation: (1) selection of parameter, (2) transform the data from a parametric system to a dimensionless system, (3) creation of subindices, and (4) compute the final WQI score by using the aggregation of subindices.

The Tigris River is a major source of Iraq’s drinking and agricultural water supply. An increase in pollution by heavy metals can be a great threat to human and aquatic life. In this study, the pollution index (PI) and metal index (MI) were used to evaluate the status of the Tigris River in Baghdad City. Five stations were chosen to conduct the study. Five heavy metals were analyzed: iron (Fe), lead (Pb), nickel (Ni), zinc (Zn), and chromium (Cr). The result of PI was ranked between “No effect to moderately affected for Fe; Slightly Affected to Seriously Affected for Pb; no effect to moderately affected for Ni, and no effect to strongly affected for Cr; only Zn was in the No effect class. While the Metal Index (MI) exceeded the warning threshold on all sites, its results ranged between 2.85 and 5.89. Pb was the most significant element that caused the rise of both indices where their values exceeded the allowable limits all the time, followed by Fe and Ni. increasing the concentration of heavy metals in the Tigris River, which returns to effluents of waste from many industrial facilities located on its bank, and the direct discharge of public sewage.