A GIS-integrated QUAL2 K modeling framework for simulating water quality dynamics under variable pollution scenarios in the Corumbataí River basin

Surface water is highly vulnerable to pollution in developed and densely urbanized catchments, where point and diffuse sources interact. In this study, an integrated approach was employed combining analysis of a long-time series of water quality data with mathematical modeling and the use of spatial information (GIS). The QUAL2K model was calibrated and validated in the Corumbataí River considering an extensive analysis of the basin's complex pollution matrix. Calibration performance ranged from satisfactory to very good, with R² values of 0.80–0.99, NSE values of 0.48–0.98, and PBIAS values of −8.83% to 0.05%. Phosphorus, Escherichia coli (E. coli), and biochemical oxygen demand (BOD) were identified as the main pollutants. Historical median phosphorus concentrations were approximately twice the legal limit, while median E. coli concentrations reached nearly 30,000 CFU 100 mL−¹ downstream of the urbanized area. Pollution reduction scenarios considered improvements in wastewater treatment and collection coverage and reductions in diffuse loads. Under the most optimistic scenario, average BOD, total phosphorus, and E. coli concentrations could be reduced by 52.8%, 66.1%, and 92.7%, respectively. Nevertheless, downstream reaches would remain non-compliant with water quality standards, suggesting that additional improvements in wastewater treatment and diffuse pollution control exceeding 70% are necessary.