MICROPLASTIC CONCENTRATIONS IN THE URBAN BLACKSTONE RIVER AND ITS TRIBUTARIES, WORCESTER COUNTY, MASSACHUSETTS

Microplastics (MPs), particles of plastic < 5 mm in size, have been discovered in freshwater water bodies across the globe. Negative impacts on freshwater organisms from MPs potentially include toxicity and starvation due to ingestion of plastic particles. We analyzed 134 grab samples of river and pond water from 10 locations on the highly urbanized Blackstone River and its tributaries from September 2021 through April 2023 for MP concentrations. Much of the upper watershed is located within the city limits of Worcester, Massachusetts (pop. ~205,000). The Blackstone ultimately meets the Atlantic Ocean at Narragansett Bay. The 10 sample sites included the tributaries Kettle Brook, Tatnuck Brook, Beaver Brook, and the Middle River in Worcester and the mainstem Blackstone in Millbury and Uxbridge (MA). Upstream areas ranged from 15 to 380 km² and land use characteristics ranged from rural (80% forest and 8% developed) to highly urbanized (12% forest and 86% developed). All but one of our sites receive street runoff from stormwater sewer outfalls. Samples and procedural blanks were oxidized to remove organic matter and filtered onto gridded filter paper. Plastic particles were counted and categorized manually under 10x magnification. Though we took thorough measures to minimize contamination, contamination of blanks was an issue and complicated drawing firm conclusions from some of our data. That said, MPs were found in every stream sample, usually at concentrations that exceeded the procedural blanks run concurrently. Average MP concentrations at our sites (separated by sampling season) ranged from 7 to 115 particles per liter. There was a statistically significant difference in concentrations between the most highly urbanized tributary and those with less developed land, and samples collected downstream of the regional wastewater plant had more MPs than samples collected immediately upstream. Along the mainstem Blackstone, we may see a trend of increasing MP concentrations downstream. Finally, we noted seasonal variability at sites sampled in both fall and spring. Our results confirm the presence of microplastics in the Blackstone River and its tributaries and suggest that land use, treated wastewater, and seasonal hydrologic differences may contribute to variability in concentrations across the watershed.