Large Seasonal Variation in Microplastic Concentration from the Truckee River, United States.

Microplastics (MPs) are ubiquitous in riverine environments worldwide, but arid river systems, characterized by large seasonal variations in discharge, remain understudied. Using a peristaltic pump, we collected samples for MP surface concentration analysis along the Truckee River in the western United States. Samples were collected using a 100 μm filter during fall 2022 and spring 2023 across 6 and 8 sites, respectively. The samples were analyzed with microscope-enabled Fourier Transform Infrared spectroscopy, and the resulting MP concentrations were compared to land use and river discharge. MP concentrations were higher in spring (mean 3.7 MP/L) than in fall (mean 0.4 MP/L), coinciding with an increase in river discharge. Statistical analyses revealed significant seasonal differences in polymer composition because of increased concentrations of poly-(ethylene terephthalate), polyethylene, and rubber in spring. These patterns likely reflect increased spring discharge mobilizing MPs from the surrounding landscape and riverbed sediments. These findings underscore the complexity of MP sources and transport and highlight the importance of seasonal sampling across discharge and land-use types. Additionally, polypropylene, poly-(ethylene terephthalate), and polyethylene were the dominant polymer types overall, demonstrating that reducing single-use plastic in the Truckee River Basin is important for protecting the river's water quality.