Spatial hotspots of microplastic accumulation in sediment associated with stream outflows into lakes and estuaries

Microplastics are a major form of anthropogenic pollution, and over time, the sediment at the bottom of aquatic environments becomes the sink for the denser of these particles. By mapping and analyzing sediment from lake and estuary systems, this study aimed to find spatial relationships between water and sediment dynamics at stream-to-slack-water transitions and resulting microplastic sediment accumulation characteristics. Sediment was collected along transects extending from the stream mouth to open water depositional environments at four unique study sites. After a series of separations from collected sediment, microplastics were weighed to map longitudinal variations in plastic concentration. At all study sites, the highest concentrations of microplastics (up to 14% dry weight) in sediment were found to focus in spatial hotspots peaking 600–700 m down gradient from the transition to a low-energy environment in intertidal freshwater estuary systems, and 150 m downstream in a lake system, all being associated with environments of clay-dominated sediment deposition. The dominant types of plastics identified were cellophane and polydimethylsiloxane. We hypothesize these spatial hotspots of microplastic accumulation may result from the unique diversity of density ranges for microplastic sediment, ranging from just above 1 g/cm3, but below the 2.7 g/cm3 common for natural mineral sediment, thus creating plastic depositional locations that are spatially offset from those of common mineral grains.