Water quality index (WQI) calibration in the Paute River hydrographical basin, south inter-Andean region of Ecuador, based on the environmental agreement nº 097-A

This research aimed to calibrate, for the first time in Ecuador, a Water Quality Index (WQI) to measure the enrichment of nutrients in the Paute River hydrographic basin, south region of Ecuador. It was based on the environmental agreement nº 097-A, which determines water uses considering physical, chemical, and microbiological parameters. We used and compared the two standards to assess the WQI (the one developed for the United States and the one calibrated for Brazil) to identify which one best fits the studied region. Water records generated by the Hidropaute Business Unit during the 2012–2017 time series were used as samples. Eleven sampling sites (P1–P11) were distributed along the Paute River. For data comparison, the Kruskal–Wallis non-parametric statistical test was applied with a significance level of 5%. The Principal Components Analysis (PCA) was used to order the data. The PCA results indicated that the eutrophication gradient was positively correlated with phosphate (r = 0.83) and BOD (r = 0.83), where the average total phosphate concentration (± standard deviation) was 0.73 ± 1.51 mgL⁻¹, classifying the basin water as hypereutrophic. Significant differences were also found between the average values of the original WQI developed in 1970 for the United States and the WQI calibrated for Brazil in 2012, being lower for Brazil (p < 0.05), characterizing this index as more restrictive. We conclude that the last WQI is an efficient tool for monitoring aquatic ecosystems in the south region of Ecuador, as it allows a more accurate estimate of water quality related to eutrophication.