Seasonal variation, polymer hazard risk and controlling factors of microplastics in beach sediments along the southeast coast of India

Microplastics (MPs) and its associated organic and inorganic contaminants are one among the significant health hazards to almost all biota, including human. We investigated the polymer hazard risk and its adsorbed contaminants in MPs at six prominent beaches of Chennai on the southeast coast of India. The spatial variation of MPs during the northeast (NE) monsoon (range: 76-720 items/kg, mean: 247.4 items/kg) was higher than that during southwest (SW) monsoon (range: 84-498 items/kg, mean: 302.7 items/kg). In both the seasons, polyethylene (PE) and polypropylene (PP) were the dominant polymers and fibre was the predominant shape of MPs, likely to be derived from fishing, textile and urban activities in this region. Scanning electron microscope (SEM) images exhibited various surface weathering features including grooves, cracks, fractures, adhering particles, pits, vermiculate textures and fibre reinforcements. Energy dispersive X-ray spectrometer (EDS) results showed that MPs have adsorbed major (Si, Al, Na, Mg, Ca, Fe and Ti) and trace (Cu, Cr, Ni, Pb and Zn) metals. Though pollution load index (PLI) presented low degree of MP contamination in the beach sediments, hazardous polymers such as polyvinyl chloride (PVC), polyamide (PA) and polystyrene (PS) contributed to high polymer hazard index (PHI) and potential ecological risk index (PERI), posing very high risk to the biota. The trajectories obtained from particle-tracking coupled with hydrodynamic simulation clearly showed that 20% of MPs settled along the coast and the remaining moved towards north, alongshore and offshore (~50 km) within 30 days, and in NE monsoon due to current reversal, the floating debris and MPs have drifted towards south, ~40 km in 30 days, indicating the role of circulation in the fate and transport pathways of plastic debris.