Microplastic emission characteristics of stormwater runoff in an urban area: Intra-event variability and influencing factors

Stormwater runoff is considered a major pathway for land-based microplastic transportation to aquatic environments. By applying time-weighted stormwater sampling at stormwater outlets from industrial and residential catchments, we investigated the emission characteristics and loads (number- and mass-based) of microplastics to aquatic environments through urban stormwater runoff during rainfall events. Microplastics were detected in stormwater runoff from industrial and residential catchments in the concentration range of 68-568 n/L and 54-639 n/L, respectively. Polypropylene and polyethylene were found as major polymers accounting for around 60% of total microplastics. The fragment was the dominant shape of microplastics, and the most common size class were 20-100 μm or 100-200 μm. The microplastic load emitted from industrial and residential catchments were estimated to be 1.54 - 46.1 x 10⁸ and 0.63 - 28.5 x 10⁸ particles, respectively. The discharge characteristics of microplastics inter– and intra–event were affected by the land-use pattern and rainfall characteristics. The concentration of microplastics did not significantly differ between industrial and residential catchments, but the composition of polymer types reflected the land-use pattern. The microplastics in stormwater were more concentrated when the number of antecedent dry days (ADDs) was higher; the concentration of microplastics was generally peaked in the early stage of runoff and varied according to rainfall intensity during a rainfall event. The contamination level and load of microplastics were heavily affected by the total rainfall depth. Most microplastics were transported in the early stage of runoff (19–37% of total runoff time), but the proportion of larger and heavier particles increased in the later period of runoff. The microplastic emission via stormwater runoff was significantly higher than that through the discharge of wastewater treatment plant effluent in the same area, implying that stormwater runoff is the dominant pathway for transporting microplastics to aquatic environments.