Microplastic Accumulation in Urban Stream Sediments: Vertical Distribution and Transport Dynamics

Microplastics (MPs) have emerged as persistent pollutants in urban freshwater ecosystems, yet their vertical distribution in stream sediments remains underexplored. This study investigated MPs at 5 cm and 10 cm depths across 17 sites in Branch Brook Park, Newark, NJ, during three sampling periods in 2022 and 2023. MPs were extracted through density separation and quantified using FTIR and Raman spectroscopy. The MP concentrations in stream sediments ranged from 560 to 3930 p/kg of dry sediment, with significantly higher abundances observed at 5 cm depth. The surface sediments consistently accumulated more MPs, especially during dry seasons, highlighting limited vertical infiltration under low-saturation conditions. The longitudinal spatial distribution did not show a notable trend along the urban stream course. Furthermore, there was a significant difference in MP accumulation between the three sampling periods, indicating a seasonal and temporal variation. The regression analyses showed weak correlations between MP concentrations and environmental parameters such as pH (R² = 0.02) and temperature (R² = 0.05), suggesting that physicochemical conditions alone exert limited control on MP accumulation compared to localized hydrological and land-use factors. These findings provide new insights and highlight the need for depth-integrated monitoring strategies and targeted pollution mitigation at stormwater entry points.