Trasporto di microplastiche durante piene fluviali: esempi dal fiume Arno (Toscana, Italia)

Microplastics are ubiquitous pollutants which pose a huge threat to ecosystems and human health. Plastic production has grown exponentially over time, and, consequently, also the plastic waste. Rivers are seen as the major vectors to deliver microplastics in marine environments, but they also are efficient temporary reservoirs. However, there is a lack of information on the dispersal pattern and transport of microplastics in river sediments, as previous researches mainly have focused on diversification and quantification of microplastics and their impacts on freshwater environments. This study aims at providing new insights on this topic by analysing river sediments deposited after flood events and trying to identify any relationship between the depositional setting/process and the amount of microplastics accumulated in fluvial sediments. To achieve this goal, a total of 37 sediment samples form five alternate bars of the Arno River (Tuscany, Italy) were analysed to provide a complete spectrum for microplastic abundances and distributions in sediments accumulated in different conditions. Specifically study depositional conditions were: i) under dominant tractional processes during floods (samples T), ii) during brief episodes of standing water during the overall waning phase (samples S), iii) during the latest stages of the flood in standing to slowly moving water (samples M) and iv) during the highest flood water levels, associated with floating vegetation debris (samples F). The microplastics were extracted from the sediment by using the sodium polytungstate diluted to 1.6 𝑔/𝑐𝑚3 and the organic rich samples were previously oxidated with the Wet Peroxide Oxidation technique to eliminate the organic matter. The mechanical sieving was used to obtain the grain size distribution of the sampled sediments. This approach led to the identification of some important features involving microplastic deposition in river sediments. As expected, the presence of microplastics was confirmed in all samples analysed, with variable concentrations ranging from 0.44 MPs/g up to 6.67 MPs/g, and a predominance of microfibres. Microplastic concentrations not only vary from a bar to another, but also within a single bar, suggesting there are different processes governing microplastics transport and depositions. Settling during the standing water, associated with the waning phase, seems to be the easier way for microplastics to deposit in river sediments, both within mud but also within interstitial space between gravelly clasts. Nevertheless, it was unexpected to see how many microplastics can be trapped in sandy deposits when they are moved under tractional conditions.