Microplastics (MPs) are considered to be ubiquitous contaminants in freshwater ecosystems, yet their sources and pathways at the river catchment scale need to be better determined. This study assessed MP (55-5000 µm) pollution in a Mediterranean river catchment (central Spain) and aimed to identify the importance of wastewater as an environmental pathway. We sampled treated and untreated wastewaters, and raw and digested sludge from five WWTPs during two seasons. River water and sediments were sampled at three locations with different anthropogenic influences during three seasons. On average, 93% (47-99%) of MPs were retained by WWTPs. Concentrations in river water and sediment ranged between 1 and 227 MPs/m3 and 0-2630 MPs/kg dw, respectively. Concentrations strongly depended upon land-use, with pollution levels increasing significantly downstream of urban and industrial areas. Seasonality influenced the observed MP concentrations strongly. During high flow periods, higher water but lower sediment concentrations were observed compared to low flow periods. We estimate that 1 × 1010 MPs are discharged into the catchment via treated and untreated wastewater annually, which constitutes up to 50% of the total MP catchment discharge. Thus, we conclude that the wastewater system represents a major environmental pathway for MPs into Mediterranean rivers with low dilution capacity.

IMDEA Water Institute Science and Technology Campus of the University of Alcalá Av Punto Com 2 28805 Alcalá de Henares Madrid Spain

IMDEA Water Institute Science and Technology Campus of the University of Alcalá Av Punto Com 2 28805 Alcalá de Henares Madrid Spain; Cavanilles Institute of Biodiversity and Evolutionary Biology University of Valencia c Catedrático José Beltrán 2 46980 Paterna Valencia Spain

IMDEA Water Institute Science and Technology Campus of the University of Alcalá Av Punto Com 2 28805 Alcalá de Henares Madrid Spain; University of Alcalá Department of Analytical Chemistry Physical Chemistry and Chemical Engineering Ctra Madrid Barcelona KM 33 600 28871 Alcalá de Henares Madrid Spain

Norwegian Institute for Water Research Faculty of Science Masaryk University Kamenice 753 5 Brno 62500 Czech Republic

Norwegian Institute for Water Research Gaustadelléen 21 0349 Oslo Norway

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Physicochemical and biological ageing processes of (micro)plastics in the environment: a multi-tiered study on polyethylene