Modeling polymer-specific exposure to micro- and macroplastics in freshwaters at high spatial resolution at the country-scale

Starting from plastic pollution in oceans as a widely recognized environmental problem, the research focus has also shifted towards rivers which were found to be a major inflow of plastics into the oceans. However, the source of plastic pollution itself is mostly upstream and for instance related to waste water treatment plants, littering of on-the-go consumer packages or agricultural films. For Switzerland a recent study modeled the release of the seven most used plastic polymers (LDPE, HDPE, PP, PS, EPS, PVC, PET) as micro- und macroplastic to the environment. Release maps of plastic emissions were obtained at high spatial resolution of 100 m by 100 m for soil grid cells and for each single river section in Switzerland. The aim of the current work was to couple this release model with its high spatial resolution to a model for the transport, accumulation and removal of the plastic polymers.

The model for the Swiss river and lake network allows to follow plastic pollution through every stream in Switzerland from the sources towards the outflows to the neighboring countries of Switzerland. We differentiate between the different plastic polymers and micro- and macroplastic. Furthermore, we consider physical properties (e. g. density, size) to establish parameters for accumulation and removal or cleaning rates of plastics in each river section and lakes based on parameters such as slope, urbanization or volume. In detail, we model the movement of plastic mass along the rivers based on average flow velocities while in lakes we consider sedimentation rates (accumulation) based on literature data. Input for the model is the yearly release of the seven polymers into about 2000 river sections. The transport model considers a network of over 600,000 river segments and 210 lakes. Our model provide contamination data on the scale of each river section which compared with so far available catchment scale model will provide an even closer look at rivers and local sources and sinks of plastics. However, many parameters regarding micro- or macroplastic transport in natural rivers at a large scale are still unknown or hydrological parameters on a country scale are not available. Therefore, a compromise between data availability and implementation of physical processes in the model had to be found. First results show the strength of our model to trace plastic particles regardless the size or polymer type. We are able to detect lakes as major sinks which substantially reduce plastic transport in rivers.

Our work can help to better understand the sources of the global plastic pollution but rises the need for experimental data on plastic transport in the environment. The large-scale understanding of plastic transport processes will provide policy makers with options were to tackle the spread of plastic pollution in the most efficient way.