Water quality in the Rio de Janeiro section of the Paraíba do Sul River Basin (2012–2023)

Abstract

This study presents an analysis of water quality in the Rio de Janeiro section of the Paraíba do Sul River Basin for the period 2012–2023. The results showed that the parameters with the highest frequency of non-compliance were thermotolerant coliforms (66.5%) and total phosphorus (56.0%), indicating that the pollution pressure was predominantly domestic in origin. BOD and DO showed non-compliance in 39.3% and 38.2% of the samples, respectively, confirming a high organic load in the studied section. Turbidity (3.6%), pH (1.4%), and Nitrate (0.4%) remained within limits in the vast majority of samples. Turbidity was the only parameter with a statistically significant temporal trend (τ = 0.491; p = 0.043), with a Sen slope of 0.56 UNT/year, suggesting a progressive increase in sediment load during the evaluated period. The other parameters did not show a significant trend (p > 0.05), indicating stable concentrations throughout the analyzed time series. The findings indicate significant impairment of water quality in the Rio de Janeiro section of the Paraíba do Sul River basin, particularly regarding microbiological parameters and nutrients. Non-compliance rates of 66.5% for thermotolerant coliforms and 56.0% for total phosphorus highlight the inadequacy of wastewater treatment as the primary driver of water degradation—a pattern consistent with observations in other urban watersheds in southeastern Brazil.

Keywords: CONAMA 357/2005, Mann-Kendall, Paraíba do Sul River, principal component analysis, sanitation, water quality monitoring.