Anthropogenic Land Use in Permanent Preservation Areas Within Urban Perimeters as a Determinant of Water Quality: A Case Study in the Peixe River Watershed

Surface water degradation has intensified due to anthropogenic pressures, especially in urban areas, where unplanned land use compromises the integrity of aquatic ecosystems. This study investigated the relationship between water quality and land use in a Permanent Preservation Area (PPA) within an urban perimeter in Caçador, Santa Catarina, Brazil. Monthly sampling was conducted throughout 2024 at 11 points distributed along urban and rural sections of the river and its tributaries. Physicochemical and microbiological parameters were evaluated, and the Water Quality Index (WQI) established by the National Sanitation Foundation (NSF) was calculated in order to associate the results with the sampling points, complemented by Principal Component Analysis (PCA) to identify multivariate patterns of spatial variability in water quality across the study area. In parallel, the PPA within the urban perimeter was delimited according to current environmental legislation, and land use was classified using ArcGIS and Google Earth Pro. The results revealed greater water quality degradation in urban stretches of the river, particularly at sampling point SP7, which recorded the lowest dissolved oxygen concentration (3.10 mg L−1), alongside elevated values of biochemical oxygen demand (5.23 mg L−1), total phosphorus (2.94 mg L−1), nitrate (18.75 mg L−1), and thermotolerant coliforms (2759.20 MPN 100 mL−1). The WQI ranged from 40.18 (SP7: bad category) to 73.57 (SP1: good category), reflecting a pronounced spatial gradient of water quality degradation associated with increasing urbanization along the river course. Mapping of the PPAs revealed that only 43.72% of the total area was covered by native vegetation, while the remaining 56.28% was occupied by anthropogenic land uses, including miscellaneous use (30.32%), agriculture (9.09%), buildings (6.09%), roads (4.64%), and railway infrastructure (5.81%). PCA accounted for 89.06% of the total data variance and indicated that greater interaction of sampling points with urbanized areas was consistently associated with reduced water quality, thereby demonstrating the direct influence of anthropogenic activities on the environmental parameters assessed throughout the study area. These findings demonstrate that land use patterns directly affect water quality and reinforce the need for riparian forest restoration, expanded sanitation infrastructure, and more sustainable urban planning.