Characteristics and fate of plastic pollution in urban stormwater ponds

Stormwater runoff is often assumed to be an important pathway for microplastics from the terrestrial to the marine environment, although few studies have attempted to quantify the significance of this pathway or the interactions between stormwater infrastructure and plastic pollution. The objective of this study was to deter-mine what factors influence the concentrations and behaviors of microplastics in stormwater ponds. Samples were taken from the water and bottom sediments of six stormwater ponds in Tampa (Florida, USA) using a neuston net and a sediment dredger. They were processed using a combination of density separations, visual sorting, and Raman spectroscopy. Concentrations ranged by several orders of magnitude between sites and rounds of sampling (0.0-55.5 items/m3 in water, 2.5-203.0 items/kg dry weight in sediment) but were com-parable to other studies. The sediments of fenced and residential sites had significantly lower plastic count concentrations, compared to unfenced sites with mixed land uses. The ratio of impervious drainage area to pond surface area was found to be positively correlated with sediment concentrations. Particle shapes in water were more variable than those found in sediments, suggesting that regular-shaped plastics tend to settle first. Circu-larity was identified as an important parameter in determining settling behaviors. Shape characteristics were similar to those observed in a downstream river, suggesting that degradation leading to the observed shapes occurred prior to entering the ponds. This study highlights the importance of stormwater infrastructure in un-derstanding plastic transport and how plastic shape characteristics can impact their behavior in the environment.