A Combined Approach Using Water Quality Indexes and Statistical Analyses to Assess the Urban Surface Runoff: a Case Study in São Paulo Coastal Zone, Brazil

Abstract

Although the most conventional methods to determine the quality of an aquatic ecosystem are aimed to evaluate its physicochemical, microbiological and ecotoxicological characteristics, the monitoring of a large number of environmental variables can represent a high cost for developing countries like Brazil. However, a combined approach using water quality indexes and statistical analyses may help us to monitor water quality through a previous selection of a few key environmental variables. In this context, the objective of this study was to highlight the set of environmental variables most useful to assess the urban channels water quality of Guarujá, Brazil, whose diffuse loads flow continuously into four tourist beaches (Tombo, Enseada, Perequê and Iporanga). For this purpose, three different methods were applied: (i) an existent published large data set (30 environmental variables: 28 physicochemical/microbiological, plus 2 ecotoxicological) obtained during a previous monitoring of the urban drainage channels of these beaches; (ii) the potential risks for the aquatic local fauna and flora through established water quality indexes, such as IMPC, TSI and ALPI; and (iii) through statistical methods such as Cohen D test, Standardized Odds Ratio and Logistic Regression, the set of environmental variables with the greatest potential to cause acute and chronic toxicity in urban channels waters. This combined approach using water quality indexes and statistical analyses was effective to successfully reduce the number of environmental variables needed to assess the ecological status of this coastal area. Moreover, this combined approach was useful to gather intuitive and user-friendly environmental information that could help decision makers (i.e., public authorities and environmental agencies) to plan and perform low-cost and effective monitoring plans in different coastal zones worldwide, namely in developing countries such as Brazil.