Reducing uncertainty of floating plastic transport estimates in rivers using the visual counting method

Macroplastics have been found in many compartments of freshwater systems amongst which floating at the water surface. To quantify the floating macroplastic flux in rivers, the visual counting method was developed. This method is based on visual observations from bridges, and has already been applied in various river systems across the world, including the Rhine-Meuse delta. A two-year dataset of monthly field measurements on ten bridges across the Rhine, Meuse, and IJssel rivers has been collected. This dataset revealed the high variability of the floating macroplastic flux in both time and space. Except for extreme flooding events, the fluctuations are not always simply related to the discharge or season. This finding raises the questions of how to assure representative field observations. Representative field observations are important, as they are typically inter- and extrapolated in time and space. If the timing or location of the measurement is not representative of the ‘normal’ condition, then the extrapolation of that measurement will be associated with a large uncertainty range, resulting in over- or underestimations of the total floating macroplastic flux in the river. As field observations play a major role in calibrating and validating river plastic transport and emission models, it is essential to minimize the uncertainty of field-based floating plastic transport estimates. To optimize the visual counting method and explore its limits, we executed three experiments. The first experiment demonstrated that the temporal variability at bridge level is high, but can be attenuated by repeated measurements. The second experiment showed how many observation points on the bridge are sufficient to account for the spatial variability of the macroplastic flux across the river cross profile. The third experiment determined that the size limit of the visible macroplastics is 1 cm² on bridges that are up to 5 meter above water level and 4 cm² for bridges up to 15 meter above water level. The findings of these experiments endorse the effectiveness of the visual counting method and allow for a substantiated implementation of this method in floating macroplastic monitoring campaigns across river networks worldwide.