Microplastics in the sediments of the Tijuana River Basin, Mexico

The current study evaluates the abundance and characteristics (shape, color and chemical composition) of microplastics (MPs) of the Tijuana River Basin, a binational river system shared by Mexico and the USA and located in the northwest corner of Mexico. The investigation was conducted on microplastics in the riverbed and river shoreline of 11 sites along the main tributaries of the Tijuana River. The median abundance of riverbed and river shoreline sediments was 118 and 183 MPs kg⁻¹, respectively. The dominant shape found was fiber (65%) followed by fragments (19%) and films (10%); the predominant identified polymers were PP polypropylene (46%), PE polyethylene (34.3%) and PS polystyrene (11.4), with lower numbers of polyacrylic, polyamide (PA) and polyethylene terephthalate (PET). Eleven colors were recorded, the most abundant being black (43.7%) and transparent (22.6%). Wastewater treatment plant (WWTP) discharges, littering and inappropriate or insufficient solid waste management are the main sources of MPs. The results highlighted the widespread distribution of MPs in the Tijuana River Basin sediment.