The role of urban streams in the microplastics contamination scenario: the case study of the Mugnone Creek (Florence, Italy)

Due to their spread, abundance and potential impact on food security and human health, microplastics (MPs) are emerging global pollutants. Metropolitan areas are among the main sources of MPs (1 μm - 5 mm); indeed, about 80% of the MPs found in the oceans come from freshwaters. In particular, impervious surfaces runoff in urban areas results in the transport of large quantities of solid wastes, comprising MPs, to the superficial water bodies. Thus, the ecological state of urban streams represents a reliable indicator to evaluate the environmental impact of a city. In this study, we report data about MPs in stream sediments and waters of a minor urban stream, the Mugnone Creek (MC), which flows across the highly urbanized city of Florence (Italy) and discharges to the Arno River.

Several sites along the 17 km-long MC were chosen, including “greenfield” sites upstream of the Florence urban area, urban-impacted sites located along congested roads, and the MC outlet. The stream sediments were collected in June 2019, while stream waters were recovered via glass bottles twice a year (June and December) in 2019 and 2020, to account for seasonal variability. Stream discharge was simultaneously determined during water sampling to allow mass flow calculations of contaminants.

Water samples were filtered onto glass microfiber filters (ø 47 mm) and observed by HD digital stereomicroscope; a similar method was followed for sediments after a density separation step (NaCl saturated solution) and H₂O₂ digestion. Fourier Transform Infrared Spectroscopy (FT-IR) was used for identification and characterization of MPs. Microparticles classification was based on polymer type, shape and colour.

MPs concentration in sediments showed an increasing trend from the pre-urban site to the outlet. A maximum value (1.540 MPs/kg) was reached immediately after the Terzolle Creek confluence, which drains the large University Hospital District of Careggi. Fibers were the dominant shape class of polymers observed and blue/black items stand out among the colour classes. The highest concentrations of MPs in water samples were recorded during winter seasons (up to 16.000 items/m³), with a predominance of fibers and blue/black colours. Polymer classification by FTIR indicated the presence of (in order of abundance): PA (polyamide), PET (Polyethylene Terephthalate), SBR (butadiene-styrene rubber), PP (Polypropylene), blend PP+PE (PP+Polyethylene), PTFE (Polytetrafluoroethylene) and PU (Polyurethane). The black-SBR polymers likely related to tyre abrasion occurring during vehicles driving, since they were especially found on a site close to traffic-congested roads. In addition to synthetic particles, high concentrations of natural fibers (mainly cellulose) were found in waters at all sites. Up to 10⁹ synthetic particles are estimated to be discharged daily by MC to the Arno River during the winter season, a load much higher than creeks with similar urbanization context worldwide. Mass loads of natural fibers were of the same order of magnitude of MPs in every season.

Studies are in progress to elucidate the impact on local biota and to characterize the anthropic pressure on the Arno River, aiming to improve the knowledge about the environmental status of one of the main Italian river basins.