Assessment of surface water quality based on physical and chemical parameters in a GIS, for three rivers in southern Brazil

Land use and land cover changes significantly impact water quality worldwide. In Brazil, the expansion of agricultural activities and the introduction of pollutants severely compromise surface water quality, posing risks to aquatic life. This study evaluated, for the first time, the water quality of the São Camilo, Santa Fé, and Pioneiro Rivers in the western region of Paraná, a key agricultural area in southern Brazil. Using a Water Quality Index (WQI) integrated with Geographic Information Systems (GIS), water samples were collected in winter 2022 and summer 2023 at three points along each river: upstream, midstream, and downstream. Eleven physical and chemical parameters were analyzed. Spatial interpolation via the Inverse Distance Weighting (IDW) method was used to generate WQI maps. Results showed medium water quality in the São Camilo River in both seasons. In the Santa Fé River, quality ranged from medium to bad, particularly at the urban-impacted midstream point. In the Pioneiro River, water quality was medium, with improved conditions at the downstream site. Findings indicate that untreated domestic effluents, agricultural runoff, and aquaculture waste significantly contribute to water degradation. The integration of WQI and GIS proved effective in assessing spatial patterns of water quality and can support environmental monitoring and public policy planning.