Microplastic Contamination in Water and Sediments of the Middle Tocantins River, Brazilian Amazon Region: Abundance, Characterization, and Spatial Distribution

Microplastic (MP) contamination is a growing environmental concern in freshwater ecosystems, yet large Neotropical rivers remain understudied. This study investigated the occurrence, abundance, and spatial distribution of MPs in water and sediment samples from the middle Tocantins River, near Imperatriz, Maranhão, Brazil. Sampling was conducted monthly from February to May 2025 at four sites with distinct land-use characteristics. Microplastics were extracted using density separation and characterized by size, color, and morphology. The results revealed widespread MP contamination in both environmental compartments, with median water abundances ranging from 2000 to 20,000 items m−3 and sediment concentrations up to 333.3 items kg−1 dry weight. No statistically significant spatial differences were observed, suggesting diffuse and continuous pollution sources rather than localized inputs. Particles < 0.5 mm dominated both matrices, with fibers and blue/black particles being the most frequent types, indicating contributions from textile fibers, urban runoff, and tire wear. These findings confirm chronic MP pollution in the Tocantins River with a homogeneous distribution along the studied stretch, highlighting the need for integrated watershed management to mitigate plastic inputs into this vital river system. It is important to note that particle identification was performed exclusively by visual stereomicroscopy; no Fourier-transform infrared (FTIR), Raman, or other spectroscopic confirmation was performed. Therefore, the particles reported here are visually identified microplastics, and both polymer types and source inferences are indicative and must be interpreted with caution. Water concentrations were derived from small-volume (500 mL) grab samples extrapolated to items m−3 and should be regarded as semi-quantitative, order-of-magnitude estimates rather than absolute values.