Microplastics enter river systems from a range of sources – both primary and secondary.
Microplastics are now thought to be ubiquitous in marine environments with recent studies finding traces in 67% of sampled sharks (Parton et al. 2020), travelling their way up the food chain through zooplankton and even in newly discovered deep sea amphipods (Eurythenes plasticus).
These microplastics can be either primary or secondary in nature – plastics that are already smaller than <5mm in length such as nurdles, synthetic fibres and microbeads or plastics that have fragmented from larger objects. Most plastics entering aquatic ecosystems do so from terrestrial sources – either through coastal or riverine vectors. Rivers have long been used by industry and to carry our waste away. Plastic producing or plastic using industries located near water bodies can lose nurdles or plastic pellets used in manufacturing processes directly into aquatic environments while synthetic fibres and microbeads (from laundry and cosmetics) are more likely to be discharged in effluent from Wastewater Treatment Plants (though these can be effective at removing microplastics the efficacy does vary between WWTPs). Meanwhile rivers act as catchments for surface runoff and watersheds which can carry secondary microplastics from a wide range of sources including plastic litter, landfills and vehicle tires (which can be washed off roads into water systems as well as wind-borne).
While there is little doubt as to the severity of the problem of microplastics the dispersal and transportation of microplastics in river sediments has been poorly understood – until now. In a new study He et al. (2021) have modelled the movement of the common microplastics; polyethylene (PE), polypropylene (PP), polyamide (PA), and polyethylene terephthalate (PET) in river sediments. The results match common expectations that low density sedimental microplastics (such as PE and PP) are more mobile and therefore more likely to be carried further than the higher density sedimental microplastics (such as PA and PET). While the flow rate of water, particularly the flow velocity of the water layer in contact with the sediment, can affect the dispersal and transportation range of sedimental microplastics it seems that high density sedimental microplastics settle close to their source points while low density sedimental microplastics are carried further. As such river sediments could act as a sink for microplastics instead of transporting them to the oceans, these findings also have implications for tracing the sources of microplastics in river sediments which would aid investigations and enforcements.
It does, however, raise the question that, if river sediments are acting as a source for microplastics then what impact does navigational dredging have when disturbing, resuspending, transporting and dumping these sediments?
The paper “Dispersal and transport of microplastics in river sediments” by Beibei He, Mitchell Smith, Prasanna Egodawatta, GodwinA.Ayoko, Llew Rintoul and Ashantha Goonetilleke, 15 June 2021 is published in Environmental Pollution available through ELSEVIER.
by Andrew Pollard