Green barriers to plastic transport in rivers: an indoor study

Every day, millions of tons of plastic debris are poured into rivers from industrial and civil waste or due to social carelessness and transported to the ocean. Here they decompose into small fragments, compromising the health and growth of fauna and flora that ingest or absorb them. In recent years the idea of using vegetation to trap and extract plastic waste has developed to limit this phenomenon. The aim of this work is to experimentally quantify the ability of aquatic vegetation in trap plastic and understand whether different biotic factors, hydraulic conditions or debris type influence it. Three of the most abundant macrophytes in European and Asian rivers are tested in this study, Myriophyllum spicatum, Potamogeton crispus and Phragmites australis. Natural samples of vegetation, taken along the Tiber, Ninfa-Sisto and Aniene rivers, are positioned into a recirculating flume, where the flow rate and the water depth can be varied. Once stationary flow conditions are reached, a known quantity of polystyrene fragments of different sizes (macroplastics, mesoplastics and microplastics) is added in the upstream part of the channel. The ratio between the fragments retained in the green barrier and the total added during the experiment defines the species' capacity to retain plastics. A change in seasonality, simulated by changing the water depth and the number of stolons inserted into the flume, is tested and its effects on the trapping efficiency is analysed. Three plant’s densities and two water depths are tested for each species. All three plant species show to effectively retain large and medium-sized plastic debris. Only the Myriophyllum spicatum, whose needle-like leaves form a denser network than the other two species, is also found to be efficient in retaining microplastics. The density of the area occupied by vegetation affects the number of trapped fragments, which increases for all species as the number of inserted stolons increases. The change in water depth has no significant impact on the results obtained. In conclusion, the three macrophyte species analyzed in this work can be used to create a barrier to the transport of plastics from rivers to oceans. A more complex structure of the vegetation allows the trapping of microplastics. A larger density of the area occupied by vegetation induces larger trapping efficiency, while hydraulic conditions appear to have no significant influence for the values tested in this study.