Development and testing of a new flexible, rapid and easily applicable Chemical Water Quality Index (CWQI)

Water Quality Indices (WQIs) are numeric parameters that summarize the overall quality status of freshwaters compared to quality standards by aggregating multiple physicochemical data into a single value. The first WQI was elaborated by Horton (1965) and since then several new and updated WQIs were computed and developed. The formulation of a WQI is divided in four steps: (i) selection of the variables, (ii) transformation of the variables to a common scale, (iii) weightage of the variables and (iv) computation of the index. Among the available WQIs in the literature several criticalities were recognized, including: (a) mathematical complexity of the computation, (b) lack of inclusivity, as several WQIs are formulated for specific sets of variables, thus preventing the inclusion of other parameters such as emerging chemical pollutants, (c) weight assignment, often based on arbitrary criteria, and (d) site-specificity of most of the indexes. The proposed Chemical Water Quality Index (CWQI) aims to overcome these flaws and provides a computation based on simple mathematic equations that are easily manageable on Excel software. The computation is divided into two steps: (i) parametrization of the variables and (ii) index determination. The parametrization consists in assigning a score (s) from 1 to 10 to each chemical variable based on (i) measured concentrations and (ii) quality targets (e.g., the limits provided by the Italian legislation for drinking waters). In the second step, a weight (w), directly proportional to the score (s), is assigned to each parameter, allowing to overcome any bias related to subjective assignments from the user. The resulting CQWI ranges from 1 (excellent quality) to 10 (extremely poor quality). The reliability of the CQWI was assessed by (i) applying the computation to more than 500 waters with a known “chemical quality status” and (ii) comparing our results with already published and available WQIs. Due to the simplicity of its computation, the absence of arbitrariness in the weightage of selected variables, and the independency of the proposed approach regarding the choice of the chemical parameters, CWQI can be easily and universally applied.