Methodological proposal for the establishment of a water quality index using multivariate analysis based on Brazilian legislation

The present study aims to propose a methodology for establishing water quality indexes which is based on the application of multivariate statistics, to be used within the limitations of the qualitative variables established by the existing legislation in any country. Data sets used were obtained from two water quality monitoring points in a spring located in the Mirim-São Gonçalo watershed, class II, which is located in the state of Rio Grande do Sul, Brazil. Factorial Analysis was used to define the qualitative variables and their respective weights in each source, whereas the Principal Component Analysis was used to extract the factors. For the construction of the water quality boundaries of classes, CONAMA Resolution 357/2005 was used for class II freshwater sources, along with reference values obtained from the literature. The new proposed water quality index WQI was compared to the WQI suggested by the Environmental Company of the State of São Paulo—CETESB (WQI CETESB ) to verify the similarities or disparities between them. The results indicate that the index used by CETESB is not adequate for the reality of this watershed, considering that it does not represent the reality of the water quality of the water bodies. Furthermore, this index does not include all the important variables, besides presenting weights for the variables with distorted values. It is therefore concluded that the proposed methodology for constructing water quality indexes is of significant value for the qualitative management of water resources.