Microplastic contamination in coastal environments: Assessing the role of rainfall and stormwater inputs

Stormwater is increasingly recognised as a contributor to microplastic contamination in aquatic environments. Yet, the contribution of rainfall in flushing microplastics through urban drainage systems into coastal environments remains less quantified. In Australia and other urbanised areas worldwide, stormwater is discharged into coastal areas via river mouths, and directly onto beaches via concrete piped outlets. This study examined microplastic concentrations in stormwater discharged from engineered concrete outlets and natural river mouths before, during and after rain events along the Adelaide coast, South Australia. Water and sediment samples were analysed after density separation and filtration and using Fourier-Transform Infrared (μ-FTIR) spectroscopy for polymer verification. 54% of water samples and 70% of sediment samples contained microplastics. Concentrations in concrete drain waters were almost double those in rivers (1.71 ± 0.29 vs 0.91 ± 0.20 microplastics L^-1), with sediment concentrations also significantly higher in concrete drains (0.88 ± 0.11 vs 0.72 ± 0.11 microplastics g^-1 dry weight). Rainfall events showed no significant difference on microplastic concentrations, though concrete outlets showed elevated microplastic concentrations in water after heavy rainfall. Overall, fibres dominated (>80% of particles), primarily polyethylene and polyester, with tyre-derived particles also detected. This study highlights that concrete stormwater drains serve as concentrated pathways for the discharge of microplastics into coastal systems, underscoring the need to consider targeted management and mitigation strategies.