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Sewage sludge used as agricultural fertilizer has been identified as an important source of microplastics (MPs) to the environment. However, the fate of MPs added to agricultural soils is largely unknown. This study investigated the fate of MPs in agricultural soils amended with sewage sludge and the role of surface water runoff as a mechanism driving their transfer to aquatic ecosystems. This was assessed using three experimental plots located in a semi-arid area of Central Spain, which were planted with barley. The experimental plots received the following treatments: (1) control or no sludge application; (2) historical sludge application, five years prior to the experiment; and (3) sludge application at the beginning of the experiment. MPs were analyzed in surface water runoff and in different soil layers to investigate transport and infiltration for one year. The sewage sludge used in our experiment contained 5972–7771 MPs/kg dw. Based on this, we estimated that about 16,000 MPs were added to the agricultural plot amended with sludge. As expected, the sludge application significantly increased the MP concentration in soils. The control plot contained low MP concentrations (31–120 MPs kg−1 dw), potentially originating from atmospheric deposition. The plot treated five years prior to the experiment contained 226–412 and 177–235 MPs kg−1 dw at the start and end of the experiment, respectively; while the recently treated plot contained 182–231 and 138–288 MPs kg−1 dw. Our study shows that MP concentrations remain relatively constant in agricultural soils and that the MP infiltration capacity is very low. Surface water runoff had a negligible influence on the export of MPs from agricultural soils, mobilizing only 0.2–0.4% of the MPs added with sludge. We conclude that, in semi-arid regions, agricultural soils can be considered as long-term accumulators of MPs. [Display omitted] • Sewage sludge increases the concentration of microplastics in agricultural soils. • Microplastic concentrations remain relatively constant over time. • Microplastic infiltration to deeper soil horizons is very low. • Only about 0.2–0.4% of microplastics are mobilized by water runoff in one year. • The impacts of microplastic build-up on soil ecosystems should be further investigated. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • We assessed wastewater as a pathway for MP contamination in a Mediterranean catchment. • WWTPs remove 93% of MPs but still contribute up to 50% of catchment discharge. • MP river concentrations increase strongly downstream of urban and industrial areas. • High temporal variations are observed in relation to rain events. • Additional sources are likely to contribute to the observed MP concentrations. [ABSTRACT FROM AUTHOR]