<p>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 <i>U</i> 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<InlineEquation ID="IEq1"><EquationSource Format="TEX">\(^{-1}\)</EquationSource></InlineEquation>, 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 (<InlineEquation ID="IEq2"><EquationSource Format="TEX">\(\beta =+0.072\)</EquationSource></InlineEquation> mg L<InlineEquation ID="IEq3"><EquationSource Format="TEX">\(^{-1}\)</EquationSource></InlineEquation> yr<InlineEquation ID="IEq4"><EquationSource Format="TEX">\(^{-1}\)</EquationSource></InlineEquation>; <InlineEquation ID="IEq5"><EquationSource Format="TEX">\(R^{2}=0.398\)</EquationSource></InlineEquation>; <InlineEquation ID="IEq6"><EquationSource Format="TEX">\(p=0.0007\)</EquationSource></InlineEquation>), 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 (<InlineEquation ID="IEq7"><EquationSource Format="TEX">\(\beta =+0.027\)</EquationSource></InlineEquation> mg L<InlineEquation ID="IEq8"><EquationSource Format="TEX">\(^{-1}\)</EquationSource></InlineEquation> yr<InlineEquation ID="IEq9"><EquationSource Format="TEX">\(^{-1}\)</EquationSource></InlineEquation>; <InlineEquation ID="IEq10"><EquationSource Format="TEX">\(R^{2}=0.594\)</EquationSource></InlineEquation>; <InlineEquation ID="IEq11"><EquationSource Format="TEX">\(p<0.0001\)</EquationSource></InlineEquation>), reflecting a near-fourfold concentration increase since baseline. Total phosphorus compliance declined from 88.2% (2000–2009) to 61.2% (2020–2025), signalling emerging eutrophication risk. Rainy-season FC concentrations were 45% higher than dry-season values (4800 vs. 3300 NMP 100 mL<InlineEquation ID="IEq12"><EquationSource Format="TEX">\(^{-1}\)</EquationSource></InlineEquation>; <InlineEquation ID="IEq13"><EquationSource Format="TEX">\(p=0.0008\)</EquationSource></InlineEquation>). These findings demonstrate that achieving SDG 6 Target 6.3 in the Paraíba do Sul basin requires urgent wastewater infrastructure expansion, adoption of nutrient load management frameworks, climate-adaptive basin governance, and creation of a unified inter-institutional monitoring platform.</p>

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

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2026年7月2日

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Hugo Pimentel Tavares,
Nilo Antônio de Souza Sampaio,
João Ercio Miranda Junior,
Luiz Guilherme de Andrade Aguiar,
Maria da Glória Diniz de Almeida,
Ronald Palandi Cardoso,
Dayana Elizabeth Werderits Silva

摘要

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\(^{-1}\); \(R^{2}=0.594\); \(p<0.0001\)), reflecting a near-fourfold concentration increase since baseline. Total phosphorus compliance declined from 88.2% (2000–2009) to 61.2% (2020–2025), signalling emerging eutrophication risk. Rainy-season FC concentrations were 45% higher than dry-season values (4800 vs. 3300 NMP 100 mL\(^{-1}\); \(p=0.0008\)). These findings demonstrate that achieving SDG 6 Target 6.3 in the Paraíba do Sul basin requires urgent wastewater infrastructure expansion, adoption of nutrient load management frameworks, climate-adaptive basin governance, and creation of a unified inter-institutional monitoring platform.