COS 115-2 - Co-accumulation of plastic and nitrate in urban stormwater detention ponds

Abstract: An estimated 9.5 billion metric tons of plastic have been produced since 1950, with most discarded to landfills or as litter. Despite rising awareness of plastic as a pollutant, much remains unknown about plastic movement through terrestrial and freshwater systems. Understanding plastic movement via stormwater is particularly salient because urban runoff delivers pollution to receiving water bodies. Stormwater flow networks in cities typically include a ubiquity of detention basins that mitigate flooding, while aggregating pollution from identifiable watersheds. Urban runoff is well known to carry large load of nitrogen (N) from myriad non-point sources, yet N, unlike plastics, undergoes a host of biogeochemical reactions, so it remains unclear how coincident N and plastics are in stormwater runoff. Accordingly, this study aims to answer these questions: (1.) What are the size, shape, and polymer characteristics of plastic pollution found in urban stormwater detention basins? (2.) Are N and plastic pollution co-delivered to stormwater basins? And (3.) What watershed development and socioeconomics features are associated with plastic pollution in stormwater ponds? Ponds were selected along a gradient of urban land use types and sampled during the dry season. Macroplastics were collected through visual surveys, while microplastics were isolated from soil samples through density extraction. Polymer type was identified through Raman spectroscopy.

Plastics were found in the soils of 20 out of the 22 sampled stormwater detention basins. The total surface area of plastic particles in the soil was 1.86 mm2 per cm2 of soil. Of the total 510 particles collected, 377 were fragments, 64 were films, 52 were fibers, 16 were foams, and only 1 was a bead. Dominant polymer types included polyethylene (52%), polypropylene (21%), polyethylene terephthalate (16%), and polystyrene (4%). Soil plastics were significantly correlated to soil organic matter (SOM) (rho = 0.52) and soil nitrate + nitrite concentrations (rho = 0.46). Principle components analysis revealed strong positive associations of plastic concentrations with watershed impervious surface cover, urban land use, and soil nitrate concentrations, and a negative associate with large watershed area:detention basin area ratio. Model selection using information criteria from a host of candidate general linear models (comprised of varying combinations of predictor variables) identified best model that predicted soil plastic concentration as a function of watershed population density and watershed:basin area ratio. Plastic waste enters the environment through human actions, and as these results indicate watershed properties are key to understanding of plastic fluxes in the environment.