CAPTURE OF TIRE ROAD WEAR PARTICLES AND HEAVY METALS BY MANUFACTURED TREATMENT DEVICES IN STORMWATER SEWER SYSTEMS IN MOUNT PLEASANT, SOUTH CAROLINA
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Paterson, Kayli
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Tire road wear particles (TRWPs), a type of microplastic emitted due to friction while driving, vary in density, size, shape, and composition and are a concern due to material toxicity, particularly leachates. TRWPs are abundant in roadside soils and stormwater runoff; however, research is limited to their fate in grey-stormwater infrastructure. Stormwater is recognized as a significant vector of microplastic environmental pollution, but currently, no regulations exist to control this source pathway. This study reviewed existing policies and recent legal precedents that stormwater managers can use to mitigate the issue of microplastics in stormwater using best management practices (BMP) and evaluated the effectiveness of manufactured treatment devices (MTDs) as a structural BMPs to capture TRWPs. The MTDs studied were either multi-chambered, baffle-box separators or single-chambered hydrodynamic vortex separators. Sediments were taken curbside from roadways or parking lots, within MTDs, and outfalls in tidal creeks at six sites in Mt. Pleasant, South Carolina. Due to multiple issues of microplastic identification complexity, this study developed or refined TRWP identification methods for sediment matrices. Using NaCl solution (1.2 g/mL) and sodium polytungstate (1.9 g/mL), low- and high-density TRWP were separated, then digested (1 M NaOH) from the sediment matrix. Optical microscopy and a hot prober were used to enumerate and identify TRWPs. The concentration of TRWPs on the road, in the MTDs, and outfall was 6.3x105 TRWP/kg dw, 2.5x106 TRWP/kg dw, and 1.8x106 TRWP/kg dw, respectively. Particle length and widths were measured at one site to obtain TRWP morphology. TRWP length shrank significantly between the roadway source and outfall endpoint. ICP-MS analysis showed an accumulation of heavy metals within all MTDs and a strong correlation between concentrations of TRWPs and zinc (R2 = 0.42). The estimated TRWP accumulation in the MTDs over nine months between clean-outs was between 1.2x108 to 1.3x109 TRWPs. Further evaluation of the in-field performance of MTDs to capture regulated and emerging contaminants can help city planners and policymakers to make the most informed decisions to update policies regarding stormwater best management practices.