Temporarily trapped: stormwater pond sediment is a key transient sink for microplastic debris.

Average microplastic (MP) abundances in each of 15 stormwater ponds in London, Canada ranged from 0.7 ± 0.7 particles per gram of dry weight sediment (g-1 dw) to greater than 2323 ± 4643 g-1 dw. Omitting one wetland cell and one hybrid pond produced a significantly higher median MP abundance for forebay versus main basin samples (p = 0.0058), and inlets produced greater mean abundances than outlets and open areas (20 ± 49, 10 ± 17, 4 ± 4 g-1 dw, respectively). Considering all 15 ponds, the industrial/commercial and construction ponds produced higher means (684 ± 2387 g-1 dw; 19 ± 22 g-1 dw) than residential and open counterparts (6 ± 10 g-1 dw; 6 ± 11 g-1 dw). Fragments were the most common particle type in all but one pond, and fibre concentrations relative to other particle types were greatest in residential ponds. Copious styrene butadiene rubber (SBR) particles in an industrial pond are considered tyre wear due to proximity to manufacturers' parking areas and loading docks, and an automotive technology training school. Paint/coating chips accounted for 25% of the fragments analysed. With greater MP abundances than in sediment from tributaries, lakes and beaches of the same watershed, stormwater ponds prove to be exceptional transient sinks for MP debris.This article is part of the Theo Murphy meeting issue 'Sedimentology of plastics: state of the art and future directions'.