Longitudinal gradients modulate the distribution, composition, and ecological risk of microplastics in a neotropical public water supply reservoir

Abstract

Abstract Microplastics (MPs) contamination was assessed in the water and surface sediment in the upstream, dam, and downstream zones of the Tapacurá Reservoir, a tropical public water supply system in northeastern Brazil, over the course of four sampling campaigns. MPs were detected in all samples, with abundances ranging from 13,880 to 27,780 MPs m −3 in the water and 10,940 to 50,760 MPs kg −1 in the sediment. Higher concentrations in the water occurred in the dam area, while accumulation in the sediment was greater downstream, reflecting contrasting transport and deposition dynamics along the longitudinal gradient. The composition of MPs differed significantly between compartments (PERMANOVA, p = 0.001), with fibers predominating in the water (73%) and fragments in the sediment (87%), and particles of 100–500 μm being the most common size class in both. Polypropylene and polyethylene were the dominant polymers, reflecting contributions from urban and agricultural sources in the watershed. Low degradation MPs prevailed in both compartments, while moderately to highly degraded particles were more frequent downstream, indicating progressive weathering throughout the system. Ecological risk indices indicated low to moderate risk in the water, but very high risk in the sediment, especially downstream (PLRI > 2500; Class IV), associated with the accumulation of highly hazardous polymers, with implications for benthic organisms and the food chain. These results demonstrate that tropical reservoirs function simultaneously as conduits and sinks for MPs, reinforcing the need for systematic monitoring in public water supply systems under anthropogenic pressure.