Occurrence and Quantification of Natural and Microplastic Items in Urban Streams: The Case of Mugnone Creek (Florence, Italy)

Abstract

The terrestrial environment is an important contributor of microplastics (MPs) to the oceans. Urban streams, strictly interwoven in the city network and to the MPs' terrestrial source, have a relevant impact on the MP budget of large rivers and, in turn, marine areas. We investigated the fluxes (items/day) of MPs and natural fibers of Mugnone Creek, a small stream crossing the highly urbanized landscape of Florence (Italy) and ending in the Arno River (and eventually to the Tyrrhenian Sea). Measurements were done in dry and wet seasons for two years (2019-2020); stream sediments were also collected in 2019. The highest loads of anthropogenic particles were observed in the 2019 wet season (10⁹ items/day) at the creek outlet. The number of items in sediments increased from upstream (500 items/kg) to urban sites (1540 items/kg). Fibers were the dominant shape class; they were mostly cellulosic in composition. Among synthetic items, fragments of butadiene-styrene (SBR), indicative of tire wear, were observed. Domestic wastewater discharge and vehicular traffic are important sources of pollution for Mugnone Creek, especially during rain events. The study of small creeks is of pivotal importance to limit the availability of MPs in the environment.

Keywords: FTIR; Florence; fibers; microplastic; urban rivers.

Figure 1

The urban area of Florence (Italy) and the location of sampling sites along the MC.

Figure 2

Example of MP* (red and black fibers) identified during binocular observations in the sediment sample and its position in the quarter of filter investigated (yellow area).

Figure 3

(a) Visible light map of the filter substrate with an SBR microfiber lying on it; (b) 2D FTIR imaging maps where the intensity of the following bands was mapped: 3000–2900 (aromatic and aliphatic CH stretch), 1605 (aromatic ring stretch), and 1450 cm⁻¹ (CH₂ bend). The chromatic scale of each map qualitatively shows the absorbance intensity as follows: blue, green, yellow, red; (c) FTIR reflectance spectra of the SBR microfiber and diagnostic bands (* symbol), assigned according to reference standards reported in [49].

Figure 4

(a) Visible light map of the filter substrate with a cellulosic microfiber lying on it; (b) 2D FTIR imaging maps where the intensity of the following bands was mapped: 3500–3100 (O–H stretch, hydroxyl groups of the anhydroglucose unit), 3000–2900 (stretch of methyl and methylene C–H bonds). The chromatic scale of each map qualitatively shows the absorbance intensity, as follows: blue, green, yellow, red. Maps have dimensions of 700 × 700 μm²; (c) FTIR reflectance spectra of the cellulosic microfiber and diagnostic bands (* symbol), assigned according to reference standards reported in [50].

Figure 5

Contents of MP* (MP*/kg d.w.) in sediments of the MC at the AM1–AM10 sampling sites.

Figure 6

Fibers, fragments, and films (%) repartition of MP* in the MC sediments.

Figure 7

Items/m³ in the water of the MC at the four different sampling sites (AM0, AM1, AM3, AM9) and different sampling seasons. Classification based on (a) synthetic (MP) vs. natural (cellulosic) fibers and colors (b). In December 2019, no distinction between MPs and natural fibers was made. White dotted color refers to transparent particles.

Figure 8

Particle classification (%) in water samples based on the shape. Fibers were distinguished as synthetic or natural or indicated with * when this distinction was not made.

Figure 9

Classification of synthetic polymers in water samples. Natural polymers (cellulose) are also reported.

Figure 10

Fluxes (items/day) of total (MP*, black marker) and natural (red marker) particles at each sampling site along MC in the investigated seasons.