A comparative study on the performance of NSFWQIm and IRWQIsc in water quality assessment of Sefidroud River in northern Iran.

Abstract

Local water quality indices (WQIs) are usually developed by modifying internationally recognized indices, which are considered reference index. In previous works on WQIs, the performance of local and reference indices has not been compared with each other. Therefore, the question crosses the mind whether using local or international indicators individually can show all qualitative aspects of a water resource. In this study, it was aimed to make this comparison between NSFWQIm (National Sanitation Foundation Water Quality Index) as the reference index and IRWQIsc (Iranian Water Quality Index for Surface Water Resource-Conventional Parameters) as the local one through a case study (Sefidroud River in northern Iran). Based on the results, the indices' performance in representing the seasonal and spatial fluctuations of the river water quality was almost similar. However, the water quality scores obtained by the two indices were somewhat different. Based on NSFWQIm, the average water quality score of the river was about 49.0, by which the water was categorized in "bad" class, while IRWQIsc represented an average score of 56.1, which corresponds to "fairly good" class. Therefore, NSFWQIm identified the water quality with stricter criteria than IRWQIsc, regarding the public health. On the other hand, according to relative standard deviations (0.17 for NSFWQIm, 0.21 for IRWQIsc) of the obtained water qualities, IRWQIsc better distinguished between high and low water qualities of the river. Accordingly, in order to better interpret the quality conditions of surface water resources in Iran, it is necessary to use both indices since NSFWQIm (reference index) attracts attention to stricter international standards and requirements, while IRWQIsc (local index) causes more focus on seasons or locations with the worst water quality.