Storm events influence the transport of macroplastics in urban streams

Abstract

Plastic litter is a globally pervasive pollutant. Storms are likely key drivers of plastic transport to oceans, but plastic transport during rising and falling limbs of storm hydrographs is rarely measured. Measurements of plastic movement throughout individual storms will improve watershed models of plastic dynamics. We used cameras to quantify macroplastic movement (i.e., particles > 5 mm) in rivers before, during, and after individual storms (N = 18) at 10 sites within three North American watersheds. Most storms showed no difference in macroplastic transport between rising and falling hydrograph limbs or evidence of hysteresis (transport rate range = 0-236 items/30 min). Total macroplastic exported during storm events was positively related to storm magnitude and was greatest at more urban sites. Thus, macroplastic transport during storms was driven by storm size and land use. The quantitative relationships between macroplastic movement and hydrology will improve discharge-weighted calculations of macroplastic transport which can benefit modeling, monitoring, and mitigation efforts. PRACTITIONER POINTS: Macroplastic particles (i.e, > 5 mm) are both retained in urban streams (e.g., in debris dams), and move downstream during baseflow and stormflow conditions Storm flows are key periods of macroplastic transport: transport rates are higher on both rising and falling limbs of storm hydrographs relative to baseflow. The amount of macroplastics moving during storm flows is positively related to storm intensity. The predictive relationships generated between storm flow and macroplastic transport will improve estimates of annual export, and policies for macroplastic pollution reduction.

Keywords: camera; macroplastic; storms; stormwater; surface water; urban rivers; water pollution.

FIGURE 1

This study was conducted in three watersheds in North America (top panel): North branch Chicago River (A), Don River (B), and Ipswich River (C). Each watershed has a site downstream (green), one site mid‐watershed (blue), and two upstream sites (yellow and purple).

FIGURE 2

Hydrographs for 18 storm events used in this analysis. Time was recorded 15‐minute increments. Events with one peak are in the first 2 rows (A‐K), events with 2 peaks are in the (plots L‐P), those that did not fit the previous 2 categories are in the fourth row (plots Q‐R). Storm phase is depicted by color. Black bars represent video data collection.

FIGURE 3

Macroplastic transport rates by storm phase (points = mean). The total number of 30‐minute segments of video footage for each phase is shown below the plots. Small letters above show differences among phases (Dunn's multiple comparison test).

FIGURE 4

Macroplastic transport rate by stream discharge for two events which suggested hysteresis. The color of the points represents the storm phase. The line connects the points in order of occurrence.

FIGURE 5

Storm magnitude (storm flow ratio) relative to total macroplastics transported A) in Chicago and Don Rivers individually, and B) all data combined. Storm speed (rising slope) in C) Chicago and Don Rivers individually, and D) all data combined.