Spatiotemporal Contaminant Occurrence and Risk Assessment in the Arroio Dourado River Under the Influence of Agriculture and a Deactivated Dumpsite

This study evaluated the impacts of agricultural activities and a decommissioned landfill on the Arroio Dourado River and identified critical sampling periods and strategic monitoring locations. In total, 492 parameters were analyzed in surface water, 392 in sediment, and 350 in fish. Noncarcinogenic health risks (HI) and ecological risk (RQ) were assessed. Multivariate analyses (principal component analysis [PCA] and hierarchical cluster analysis [HCA]), supported by statistical tests, revealed differences among the six surface water campaigns (C1-C6, p < 0.05). C1, collected in April (autumn), showed higher levels of metals, heavy metals, and physicochemical parameters. Phenanthrene and naphthalene, classified as PAHs, were detected in C1. Summer campaigns (C5-C6) showed higher coliforms, residual chlorine, turbidity, color, phosphorus, and metals (Fe, Cr, and Mn). During the mid-season campaign (C3), pesticides such as atrazine, simazine, buprofezin, cyproconazole, flutriafol, metalaxyl-M, paclobutrazol, and naphthalene were detected. Sediment heterogeneity was driven by spatial variation (p < 0.05), with P1 likely influenced by agricultural activity and P3 showing higher metal levels. Heavy metals were also detected in fish samples. Although HI and RQ values remained below safety thresholds, the presence of toxic substances highlights the need for continued monitoring. P4 was identified as a key site for capturing cumulative impacts of agricultural and urban activities.