Microplastics Concentrations in the Upper Blackstone River Watershed, Massachusetts, Usa

Microplastics, particles of plastic < 5 mm in size, have been discovered in freshwater water bodies across the globe. Negative impacts on freshwater organisms from microplastics potentially include toxicity and starvation due to ingestion of plastic particles. We collected 36 grab samples of river water during dry weather from five locations on the Blackstone River and its tributaries in the fall of 2021 and analyzed them for microplastics concentrations. Much of the watershed is located within Worcester, Massachusetts (pop. ~205,000) or adjacent towns and all sample sites receive street runoff from stormwater sewers. The five sample sites included two tributaries, Kettle Brook and Beaver Brook, with land use characteristics ranging from suburban (56% forest and 10% developed) to highly urbanized (13% forest and 68% developed). A site downstream of the confluence of these tributaries on the Middle River as well as two more sites on the mainstem Blackstone River, located directly up- and downstream of the regional wastewater treatment plant outflow channel, were also sampled. All stream samples and four procedural blanks were oxidized to remove organic matter, and a density separation was performed to remove mineral sediment before samples were filtered onto gridded filter paper. Plastic particles were counted and categorized under 10x magnification. Care was taken at all steps to avoid contamination. Microplastics were found in every stream sample, > 90% of which were fibers. The average microplastics concentration at Beaver Brook (115 ± 57 mp/L, n = 8) was significantly higher than Kettle Brook (49 ± 14 mp/L, n = 8, p < 0.05). Samples collected upstream from the wastewater treatment plant outflow had significantly lower concentrations than those collected downstream (37 ± 16 and 58 ± 19 mp/L respectively, n = 8, p < 0.05). The microplastics concentration in the Middle River (68 ± 26 mp/L, n = 4) was not significantly different from any of the other sites. All sites had significantly higher concentrations than the blanks (p < 0.05). Our results provide the first confirmation of microplastics in the Blackstone River and three of its tributaries and suggest that land use and treated wastewater contribute to differences in concentrations across the watershed.