Microplastic distribution and characteristics across a large river basin: Insights from the Neuse River in North Carolina, USA

While microplastics (MP) have been found in aquatic ecosystems around the world, the understanding of drivers and controls of their occurrence and distribution have yet to be determined. In particular, their fate and transport in river catchments and networks are still poorly understood. We identified MP concentrations in water and streambed sedi-ment at fifteen locations across the Neuse River Basin in North Carolina, USA. Water samples were collected with two different mesh sizes, a trawl net (>335 pm) and a 64 pm sieve used to filter bailing water samples. MPs >335 pm were found in all the water samples with concentrations ranging from 0.02 to 221 particles per m3 (p m-3) with a median of 0.44 p m-3. The highest concentrations were observed in urban streams and there was a significant correlation be-tween streamflow and MP concentration in the most urbanized locations. Fourier Transform Infrared (FTIR) analysis indicated that for MPs >335 pm the three most common polymer types were polyethylene, polypropylene, and poly-styrene. There were substantially more MP particles observed when samples were analyzed using a smaller mesh size (>64 pm), with concentrations ranging from 20 to 130 p m-3 and the most common polymer type being polyethylene terephthalate as identified by Raman spectroscopy. The ratio of MP concentrations (64 pm to 335 pm) ranged from 35 to 375, indicating the 335 pm mesh substantially underestimates MPs relative to the 64 pm mesh. MPs were detected in 14/15 sediment samples. Sediment and water column concentrations were not correlated. We estimate MP (>64 pm) loading from the Neuse River watershed to be 230 billion particles per year. The findings of this study help to better understand how MPs are spatially distributed and transported through a river basin and how MP con-centrations are impacted by land cover, hydrology, and sampling method.

Automated summary

While the presence of microplastics (MP) in aquatic ecosystems worldwide is known, there is still a lack of understanding regarding the factors and controls influencing their occurrence and distribution. This study examined MP concentrations in water and streambed sediment at various locations in the Neuse River Basin in North Carolina, USA. MP particles with a diameter greater than 335 µm were found in all water samples, with concentrations ranging from 0.02 to 221 particles per cubic meter. Urban streams had the highest concentrations, and a significant correlation between streamflow and MP concentration was observed in urbanized areas. The most common polymer types identified for MPs larger than 335 µm were polyethylene, polypropylene, and polystyrene. When smaller mesh sizes (>64 µm) were used for analysis, a higher concentration of MP particles, with the most common polymer type being polyethylene terephthalate, was observed. The ratio of MP concentrations between the two mesh sizes ranged from 35 to 375, indicating an underestimation of MPs by the larger mesh. MPs were also detected in 14 out of 15 sediment samples. The findings of this study provide valuable insights into the spatial distribution and transport of MPs in river basins, as well as the influences of land cover, hydrology, and sampling methods on MP concentrations.