Microplastics in gully pot sediment in urban areas: Presence, quantities and characteristics

Stormwater is widely recognized as a pathway for transporting pollutants, including microplastics, from sources in urban environments to receiving waters. Gully pots are often where urban runoff drains into the piped network; they typically include a trap where sediments accumulate. The aim of this work was to contribute to a better understanding of the fate of microplastics as they enter into the urban drainage system, and the role of gully pots in trapping microplastics. Sediment samples collected from 29 gully pots were analysed for non-carbon-black and carbon-black (e.g. tire wear particles) microplastics larger than 40 μm using μ-FTIR and ATR-FTIR, respectively. Commonly found polymers in descending order were PP>EPDM>EVA>PS>SBR, PP was most common both by mass and by number of microplastics. The total concentration of carbon black and non-carbon black microplastics ranged from 709 to 10 600 items/100 g dry matter (DM), (median: 2960 items/100 g). Estimated mass of non-carbon black microplastics ranged from 0.19 to 490 mg/100 g, (median: 3.66 mg/100 g). In total 21 different types of microplastics were detected, the majority of these (13) were carbon black and eight non-carbon black polymer types. By number and the carbon black particles accounted for up to 68% of the microplastics (average 30%), this stress the importance of using analytical methods enabling the detection of both carbon-black and non-carbon black microplastics. Furthermore, the results indicate that gully pots can act as temporary sinks for microplastics, mainly for microplastics larger than 125 μm. The amount of microplastics found in gully pots, together with the very large number of gully pots sited in urban areas, indicates that gully pots can potentially trap large amounts of microplastics, and thus if gully pots are fitted and maintained properly they could significantly contribute to reducing the amount of microplastics reaching receiving waters.