Project highlights

  • addresses the issue of plastic pollution of the environment, a problem of global significance;
  • will provide new insights into the behaviour of plastics in river systems and how plastic conveyance can be modelled;
  • will provide a novel means to estimate riverine plastic fluxes and yields at a variety of scales.

Overview

Plastic pollution of the environment is a major global concern, generating negative impacts on wildlife, their habitats and environmental aesthetics (Welden, 2020).  Rivers are widely recognised as major corridors for the transfer of discarded plastic waste within terrestrial landscapes. Estimates of the global riverine flux to oceans range from 1.15-2.41 (Lebreton et al., 2017) and 0.41-4 Mt yr-1 (Schmidt et al., 2017).  Relatively little, however, is known about how plastic moves through river systems.  Recent research suggests that rivers act as ‘jerky conveyor belts’ for plastic conveyance, just as they do for sediment (Ferguson, 1981). For example, macroplastics (> 5 mm) can be trapped by vegetation and other channel obstructions (Schreyer et al., 2021; Newbould et al., 2021) and micro- (1 mm – 5 mm) and nano-plastics (< 1 mm) can accumulate and be temporarily stored in channel bed sediments (Hurley et al., 2018).  This suggests that the transport of plastic debris in rivers can be conceptualised as a series of discrete ‘steps’ between sites of temporary storage, the dynamics of which will exhibit complex interdependencies between river geomorphology and hydraulics, catchment hydrology, riparian vegetation, and plastic characteristics (e.g. size, morphology, buoyancy, density, biofouling). Newbould et al., (2021) demonstrated that the transfer of buoyant macroplastic litter through a 1 km reach can be modelled probabilistically with stranding and release events at trapping points described by a stochastic process. The model was calibrated using field tracer data and offers a potential approach for modelling plastic conveyance through river channel networks that explicitly accounts for the transport behaviours of different types of plastic waste.

The aim of this study is to develop, calibrate and test a more general probabilistic catchment scale model of plastic conveyance.  This will increase our understanding of plastic fluxes and stores in river systems leading to improved predictions of plastic behaviour.  Ultimately, this improved understanding could be the key to closing the gap on the disparity between predicted emissions of plastics to the environment and the estimated stock of plastic in the world’s oceans (the so-called “missing plastic” question: Cózar et al., 2014; Weiss et al., 2021).

Host

University of Leicester

Theme

  • Climate and Environmental Sustainability

Supervisors

Project investigator

  • Dr. Mark Powell

Co-investigators

  • Prof Mick Whelan
  • Prof Sarah Gabbott

How to apply

Methodology

The conceptual model of Newbould et al., (2021) will be generalised and upscaled from reach to catchment scales.  Coding will be undertaken in Python.  The geomorphological, hydrological and vegetation characteristics of the channel network will be characterised using remote sensing (Tomsett and Leyland, 2019) and statistical (downstream and at-a-station hydraulic geometry) techniques.  This characterisation will be used to subdivide the network into discrete reaches (cells) with different trapping potentials.  The model will consider the cell-cell movement of plastic waste on the water surface, in suspension and along the channel bottom using estimates of the probability of storage within each cell.  Factors controlling plastic behaviour (travel distances, residence times in storage etc) will be identified from heuristic arguments supported by tracer experiments in the field and in a laboratory flume for plastic fragments of different sizes, shapes and densities.  These data will be used to calibrate and test the model.

Training and skills

The student will receive training in: computer modelling strategies and techniques with a focus on probabilistic modelling; coding in Python; Remote Sensing techniques for characterising the geomorphology and vegetation characteristics of channel networks and river corridors; field and laboratory techniques for conducting tracer experiments.

Partners and collaboration

We will establish a collaborative partnership with the Canal and River Trust who have a number of initiatives directed at tackling plastic pollution in UK waterways, many of which provide opportunities for engaging with the public on this important topic (Canal and River Trust, 2020).

Further details

Dr. Mark Powell, School of Geography, Geology and the Environment, University of Leicester, LE1 7RH. Tel. 0116 2523850; [email protected]

To apply to this project please visit: https://le.ac.uk/study/research-degrees/funded-opportunities/centa-phd-studentships

Possible timeline

Year 1

Research design phase; Training in modelling approaches, coding in Pythion and Remote Sensing techniques; Model development and channel network characterisation

Year 2

Model development; field and laboratory experiments

Year 3

Model calibration, testing and verification; Writing up

Further reading

Canal and River Trust (2020).  Find out about Plastic Pollution.  42624-plastic-pollution-pack-2020-find-out-about-plastic-pollution-brochure.pdf (canalrivertrust.org.uk)  Accessed 25/10/21.

Cózar, A., Echevarria, F., Gonzalez-Gordillo, J. I., Irigoien, X., Ubeda, B., Hernandez-Leon, S., et al., (2014).  Plastic debris in the open ocean.  Proceedings National Academy Science, 111, 10239–10244.  doi: 10.1073/pnas.1314705111.

Ferguson, R.I. (1981). Channel forms and channel changes. In Lewin J. (ed.) British Rivers, Allen and Unwin, London, 90–125.

Hurey, R., Woodward, J. and Rothwell, J. J. (2018).  Microplastic contamination of river beds significantly reduced by catchment-wide flooding. Nature Geoscience, 11, 251-257.  doi.org/10.1038/s41561-018-0080-1

Lebreton, L. C. M., van der Zwet, J., Damsteeg,. J. W., Slat, B., Andrady, A., and Reisser, J. (2017). River plastic emissions to the world’s oceans. Nature Communications, 8, 15611. doi: 10.1038/ncomms15611.

Newbould R.A., Powell D.M., Whelan M.J. (2021) Macroplastic debris transfer in rivers: A travel distance approach. Frontiers in Water, 3, 724596. doi: 10.3389/frwa.2021.724596

Schmidt, C., Krauth, T., and Wagner, S. (2017). Export of plastic debris by rivers into the sea. Environmental Science Technology, 51, 12246–12253. doi: 10.1021/acs.est.7b02368.

Schreyer, L., van Emmerik, T., Nguyen, T. L., Castrop, E., Phung, N. A., Kieu-Le, T. C., et al. (2021). Plastic plants: the role of water hyacinths in plastic transport in tropical rivers. Frontiers Environmental Science, 9, 686334. doi: 10.3389/fenvs.2021.686334

Tomsett, C. and Leyland, J. (2019) Remote sensing of river corridors: a review of current trends and future directions. River Research Applications, 35, 779–803. doi: 10.1002/rra.3479

Weiss, L., Ludwig, W., Heussner, S., Canals, M., Ghiglione, J.-F., Estournel, C., et al. (2021). The missing ocean plastic sink: gone with the rivers. Science, 373, 107–111. doi: 10.1126/science.abe0290.

Welden, N.A. (2020) The environmental impacts of plastic pollution.  In Letcher T.M. (ed.) Plastic Waste and Recyling, Academic Press, London, p. 195-222.

 

COVID-19

Potential impacts of the Coronvirus Pandemic can be managed as follows. The development of the probabilistic model can be supported remotely via online Teams meetings if needed.  Field work is UK based and can be conducted locally in nearby catchments.  Risks of infection are minimised by being outside and social distance between field assistants can be easily maintained.  Laboratory flume studies will be conducted at the University of Leicester and will not require travel to external sites.  The student will be mainly working on their own in the laboratory and social distance with technical staff and any assistants will maintained when required.  Having three methodological strands to the PhD provides considerable flexibility in rescheduling the different components of work should the situation require it.