Water quality indexes and multivariate statistic methods application: case study of Mirin Lagoon

Abstract

The irrational use of water bodies has been a main concern worldwide, requiring the steady monitoring of water sources in order to track whether the water quality parameters are within the guidelines limits. Therefore, the current study aims to evaluate the water quality of Mirim Lagoon by using a water quality index and statistical methods. Also, this study seeks to analyze, by the correlation matrix, the influence of the parameters used in WQI on the results of the index for Mirin Lagoon, as well as compare and validate, via a significance test, the results of WQIValentini with the results of WQICETESB, widely applied in Brazil. The water samples analyzed in this study were collected over the years 2018, 2019, 2020, 2021, and 2022 from several monitoring points of Mirim Lagoon. The WQICETESB results of the monitoring points in Mirim Lagoon, ranged from 58.3 to 81, falling within acceptable and good water quality, respectively. Regarding the correlation-WQI parameter, phosphorus was the most representative parameter for the WQICETESB result in the monitoring points ML1, ML2, ML3, ML4, and ML5, while the thermotolerant coliforms was the most significant parameter for the points ML6 and ML7. On the other hand, the WQIValentini results ranged from 48.0 to 76.9, maintaining the absolute majority within acceptable and good ranges. Moreover, in order to compare the results of WQICETESB with the WQIValentini, and, hence, prove or disprove the equivalence of both, a paired t-test was used, demonstrating that there is no significant variation between both indexes. The results of this study enabled us to evaluate the Mirim Lagoon water quality by using WQICETESB and WQIValentini, as well as by using multivariate statistical methods. In addition, regarding the use of the WQIValentini to assess Mirin Lagoon water quality, there was evidence that this index generates results statistically indistinguishable from those generated by the WQICETESB, proving its applicability for water quality monitoring of this lagoon. Furthermore, it is worth mentioning that the use of an adequate equation with fewer variables provides a reduction in the costs of physicochemical and microbiological analysis, which may encourage environmental agencies to invest more in water quality monitoring.