Urban Stormwater Runoff: A Major Pathway for Anthropogenic Particles, Black Rubbery Fragments, and Other Types of Microplastics to Urban Receiving Waters

Stormwater runoff
has been suggested to be a significant pathway
of microplastics to aquatic habitats; yet, few studies have quantified
microplastics in stormwater. Here, we quantify and characterize urban
stormwater runoff from 12 watersheds surrounding San Francisco Bay
for anthropogenic debris, including microplastics. Depth-integrated
samples were collected during wet weather events. All stormwater runoff
contained anthropogenic microparticles, including microplastics, with
concentrations ranging from 1.1 to 24.6 particles/L. These concentrations
are much higher than those in wastewater treatment plant effluent,
suggesting urban stormwater runoff is a major source of anthropogenic
debris, including microplastics, to aquatic habitats. Fibers and black
rubbery fragments (potentially tire and road wear particles) were
the most frequently occurring morphologies, comprising ∼85%
of all particles across all samples. This suggests that mitigation
strategies for stormwater should be prioritized. As a case study,
we sampled stormwater from the inlet and outlet of a rain garden during
three storm events to measure how effectively rain gardens capture
microplastics and prevent it from contaminating aquatic ecosystems.
We found that the rain garden successfully removed 96% of anthropogenic
debris on average and 100% of black rubbery fragments, suggesting
rain gardens should be further explored as a mitigation strategy for
microplastic pollution.