Particle size and hydrologic influence on microplastic accumulation in streambed sediments downstream of a municipal point source

, Margenat Henar, Drummond Jennifer D., Nel Holly A., Krause Stefan, Stonedahl Susa H., Sabater Francesc.

Wastewater treatment plants (WWTP) act as a point source of microplastics (MP, defined as 1 – 1000 μm) to freshwater ecosystems, releasing over 5 million MPs/day/WWTP to streams. Previous studies have shown a correlation between MP abundance and high-density population areas, but less is known on how river hydrodynamics play a role in MP accumulation in streams. We investigated the spatial distribution of microplastics within sediments every 15 m downstream of a WWTP effluent in Cànoves stream (Montseny, Catalonia) during Spring 2019. We compared measurements to an upstream-control site and another site near to the WWTP bypass that added untreated wastewater when discharge exceeded capacity. The 450 m reach consisted of three geomorphically altered reaches interspersed between three unaltered buffer reaches, each 75 m. During baseflow conditions, we measured per site MP abundance, length, area, particulate organic matter quantity, grain size distribution, and local hydrologic conditions (velocity, depth, and width). MPs were quantified following the Nile Red fluorescence method and characterized as large (¿ 64 µm) or small (10 to 64 µm). MPs in sediment samples were mainly fragments with a higher abundance of small MPs vs. large MPs, 150 particles/g vs. 22 particles/g, respectively. We found that both small MP abundance and size was negatively correlated with distance from the point source, suggesting a combination of fragmentation of larger particles and/or preferential filtration of larger particles along the stream. A negative correlation of large MPs with distance from the WWTP was not found unless the bypass was included, suggesting these infrequent inputs are an important source of MPs to the stream, especially the larger MPs. Greater MP abundance was observed in the areas with increased organic matter and smaller sediment grain sizes. These results demonstrate the complex size-dependent microplastic transport processes that determine accumulation patterns in streambed sediments.

View online