Netherlands

Floating macroplastic and macrolitter (>0.5 cm) were measured using the visual counting method, for which all items floating at the surface are counted from bridges. Only bridges that are safe and legally accessible, e.g. presence of pedestrian or bicycle paths, were selected. At each location, three to twelve observation points were selected, depending on the river width. The majority of the locations had five or six points (23 out of 26), two locations had three points, and only the downstream Meuse location had twelve points. For a measurement, all visible floating items were counted within a predefined observation track. The minimum observable item size depends on the bridge height (8-20 m), but was estimated to be at least 2.5 cm for all locations. Note that the width of the observation tracks depends on the field of view and the height above the water, and there varied between bridges and between points on the same bridge (12-34 m). The observation track width was quantified by selecting a reference object (e.g. bridge column, buoy, orange peels) and measuring the distance to the observation point. The sum of the observation track widths per bridge covered between 25% and 85% of the total river width. On each measurement day each point was measured four times for a five-minute period.