Development and testing of a new flexible, easily and widely 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. Among the available WQIs in the literature, several criticalities were recognized including: (a) mathematical complexity of the computation, (b) lack of inclusivity, (c) arbitrary weight assignment method, and (d) site-specificity of most of the indexes. The proposed index, the Chemical Water Quality Index (CWQI), aims to overcome these flaws and provides a computation based on simple mathematic equations that are easily manageable on spreadsheet software. The computation is divided into two steps: (i) parametrization of the variables and (ii) index determination. The parametrization consists of assigning a score (s) from circa1 to 10 to each chemical variable based on (i) measured concentrations and (ii) quality targets (e.g., the limits provided by the European 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 CWQI ranges from circa1 (very good quality) to 10 (extremely poor quality). The reliability and accuracy of the CWQI were assessed by (i) applying the computation to 1,810 waters and (ii) comparing our results with another available WQI. The CWQI outputs showed an optimal response with the number of variables exceeding the quality target with high correlation coefficients (r = 0.94; R2 = 0.89). Due to the simplicity of its computation, the absence of arbitrariness in the weightage of selected variables, and the independence of the proposed approach regarding the choice of the chemical parameters, CWQI can be easily and universally applied.