Five decades of integrated surface water quality monitoring in the Paraíba do Sul river basin reveal persistent regulatory failure and emerging nutrient risks for SDG 6 achievement

The Paraíba do Sul River Basin is a strategically critical freshwater resource for approximately 20 million people across the states of São Paulo, Rio de Janeiro, and Minas Gerais, Brazil, and has been subject to systematic water quality monitoring by multiple state and federal agencies since 1977. This study presents the first integrated spatiotemporal analysis of basin-wide water quality through a unified dataset of 345,942 measurements collected at 534 monitoring stations between 1977 and 2025, incorporating data from CETESB, INEA, ANA-HidroWeb, and IGAM across 524 physicochemical and biological parameters. Compliance with Class 2 freshwater standards defined by CONAMA Resolution 357/2005 was evaluated for nine priority parameters; temporal trends were quantified using linear regression on annual medians for 2000–2024; and seasonal dynamics were assessed through Mann-Whitney U tests. Results reveal a persistent regulatory failure in thermotolerant faecal coliform (FC) compliance: only 23.3% of all measurements met the legally permissible threshold of 1000 NMP 100 mL \(^{-1}\) , deteriorating to 16.5% in 2020–2025, reflecting widespread untreated domestic sewage discharge. Dissolved oxygen (DO) compliance improved significantly from 48.2% in 2000–2009 to 80.7% in 2010–2019 ( \(\beta =+0.072\) mg L \(^{-1}\) yr \(^{-1}\) ; \(R^{2}=0.398\) ; \(p=0.0007\) ), but this gain proved climatically fragile during the 2014–2015 drought (regression to 48.8% compliance). Nitrate exhibited the strongest temporal signal in the dataset ( \(\beta =+0.027\) mg L \(^{-1}\) yr \(^{-1}\) ; \(R^{2}=0.594\) ; \(p