Spatial and temporal patterns of microplastic accumulation in stormwater pond sediments: Implications for urban stormwater management

Stormwater ponds are widely used to manage urban runoff and retain sediment-associated pollutants, but their role in capturing microplastics (MPs) remains unclear. This study evaluated MP accumulation in sediments from six stormwater ponds representing residential, commercial, industrial, and highway land uses in northwestern Pennsylvania. Sediment samples were collected over two years from inlet, middle, and outlet zones and classified by proximity to the observed flow path and analyzed for MPs between 250 μm and 5 mm. MPs were detected in all ponds, with concentrations ranging from 0 to 1103 MP/kg dry sediment. Across all samples, the mean concentration was 141.2 ± 182.8 MP/kg and the median was 84.4 MP/kg. Highway and commercial ponds showed higher descriptive concentrations, but pond- and land-use-level patterns were interpreted descriptively because samples were nested within ponds and some land-use categories were represented by one pond. Pond zone alone did not explain MP patterns across the dataset. Higher concentrations were observed in Year 2, although this pattern is interpreted cautiously because sampling year was partially confounded with sampling approach. Outlet Year 2 samples had the highest median concentration, suggesting a possible temporal shift in outlet sediments. Flow-path proximity was associated with MP accumulation, with the clearest within-year support observed in Year 1; the Year 2 flow-path comparison was limited by only two off-flow-path samples. These findings indicate that stormwater ponds function as heterogeneous reservoirs for MPs and suggest that within pond hydrologic pathways may influence accumulation patterns. Monitoring and maintenance programs should incorporate flow-path-based sampling and inspection of outlet sediment accumulation areas rather than relying solely on inlet-focused assessments.