Raman imaging spectroscopy approach for the microplastics detection in the Oka estuary in the Urdaibai biosphere reserve (Basque Country, Spain)

Microplastics (MPs) are contaminants of high concern due to their ubiquitous presence in the environment and their ability to adsorb and release other pollutants. Sediments, as final sinks of contaminants in coastal environments, are good indicators of the presence of MPs in estuaries. This study aims to investigate the abundance, geographical/ temporal trends and characteristics of MPs present in the Urdaibai estuary, located in an area protected by UNESCO. To our knowledge, this is the first time that the presence of MPs is reported in this area. Sediment samples were collected at five points in the estuary every three months for a year. The fraction between 250 μm and 2000 μm of the dry sediments was analysed by imaging-Raman microscopy, with tuned parameters and using a 785 nm laser, following an innovative methodology developed in our research group. The concentration of the MPs found ranges from 16 to 165 items per kilogram of dry sediment, being polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP) and polystyrene (PS) the most abundant. Only fragments and fibres were encountered, and PET was only found in the form of fibres. Although no general temporal trend was observed, one sampling site presented a significantly higher amount of MPs than the rest of sites, probably due to geomorphic and hydrodynamic reasons. This work highlights the importance of monitoring MPs in coastal environments, which will provide us with information to improve plastic waste management, develop clear regulations on the manufacture and use of plastic materials, and implement public awareness programmes. • Chemical imaging for monitoring MPs in complex matrix such as estuarine sediments • Raman spectral processing for quantitative analysis of MPs in environmental samples • The Oka river MPs contamination is comparable to the other studied European rivers • Sediments of Arteaga point in July 2022 is the main MPs hotspot (165 particles/kg) • PET fibres are the main MPs (34.6 %) followed by PE fragments (27.3 %) and PP (21.6 %)