SURFACE WATER QUALITY PERFORMANCE
ASSESSMENT USING THE NSF WATER
QUALITY INDEX: A NINE-YEAR
BENCHMARKING STUDY IN RIO DE
JANEIRO STATE RIVERS, BRAZIL
(2015-2023)
Hugo Pimentel Tavares, Nilo Antônio de Souza Sampaio, Gilberto Santos, Carin von Mühlen
Abstract: This study presents a comprehensive performance assessment of surface water quality across 284 monitoring stations located in 150 rivers of Rio de Janeiro State, Brazil, covering the period 2015-2023. Using the National Sanitation Foundation Water Quality Index (NSF-WQI), adapted to Brazilian standards by the State Environmental Institute (INEA), a total of 6,481 valid IQA composite indices were computed from 135,708 physicochemical and microbiological records. The mean NSFWQI across the entire dataset was 54.33 (SD = 23.67), placing the overall classification at the lower portion of the 'Good' category. The distribution showed that 45.0% of measurements fell in the 'Good' (Bom) category, while 30.4% were classified as 'Poor' (Ruim) or 'Very Poor' (Muito Ruim), indicating persistent systemic contamination. Annual temporal analysis revealed statistically significant inter-annual variability (Kruskal-Wallis H = 155.15, p < 0.0001), with no linear improvement trend (R2 = 0.006, p = 0.839). Seasonality was a significant factor, with dry-season months yielding higher index values. The parameters most limiting water quality were thermotolerant coliforms (mean q = 41.09), dissolved oxygen (q = 44.40), and total phosphorus (q = 49.80), indicating predominantly domestic sewage pollution. Spatially, the worst conditions were identified in fully urbanised catchments of the Metropolitan Region of Rio de Janeiro.
Keywords: water quality, NSF WQI, surface water, environmental quality, Rio de Janeiro
DOI: 10.24874/IJQR20.03-12
Recieved: 23.02.2026 Accepted: 19.08.2026 UDC:
Reads: 31 
