Floating litter in the Dutch Rhine-Meuse delta 2021

We measured floating plastic and other anthropogenic litter at 26 measurement locations distributed across the Dutch reaches of the Rhine (IJssel, Waal, Nederrijn) and Meuse rivers (see Figure 1) between 28 January and 7 December, 2021. The Rhine enters the Netherlands from Germany at Spijk, and splits into the main Waal, IJssel and Nederrijn. The Waal is the main branch, and joins the Nederrijn-Lek branch at Rotterdam before flowing into the North Sea. The IJssel flows into Lake IJssel at Kampen. The Meuse enters the Netherlands from Belgium at Eijsden, and discharging into the tidal Hollands Diep estuary. Here, the Meuse is joined by a Rhine distributary before reaching the North Sea.<br>Floating macroplastic and macrolitter (&gt;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.

2021年1月28日至12月7日期间，研究团队在覆盖莱茵河(Rhine)（荷兰境内河段包含艾瑟尔河(IJssel)、瓦尔河(Waal)、下莱茵河(Nederrijn)）与默兹河(Meuse)荷兰流域的26个监测点位，开展了漂浮塑料及其他人为垃圾的调查测量工作（详见图1）。莱茵河于施皮克(Spijk)处从德国入境荷兰，随后分为三大支流：瓦尔河、艾瑟尔河与下莱茵河。其中瓦尔河为干流分支，在鹿特丹与下莱茵河-莱克河(Nederrijn-Lek)河段汇合后注入北海；艾瑟尔河则于坎彭(Kampen)处汇入艾瑟尔湖(Lake IJssel)。默兹河于艾赫登(Eijsden)处从比利时入境荷兰，最终排入潮汐型霍兰迪普(Hollands Diep)河口湾；在抵达北海前，该河段会接收一条莱茵河分流河道的来水。
本研究采用目视计数法(visual counting method)对粒径大于0.5厘米的漂浮大塑料与大颗粒垃圾(floating macroplastic and macrolitter)开展调查：即从桥梁上计数所有漂浮于水面的垃圾。仅选取安全且合法可进入的桥梁（如配备人行或自行车道的桥梁）作为监测点位。每个监测点位根据河道宽度选取3至12个观测点：26个点位中，23个点位设置5或6个观测点，2个点位设置3个观测点，仅下游默兹河点位设置了12个观测点。单次监测需统计预设观测带内所有可见漂浮垃圾。最小可观测垃圾尺寸受桥梁高度（8至20米）影响，经估算所有点位的最小可观测尺寸不小于2.5厘米。需注意，观测带宽度受视野范围与水面以上高度影响，不同桥梁及同一桥梁的不同观测点之间宽度存在差异（12至34米）。观测带宽度通过选取参考物体（如桥柱、浮标、橙皮等）并测量其至观测点的距离来量化。每座桥梁的观测带总宽度覆盖河道总宽度的25%至85%。每个观测点在每个监测日均进行4次、每次5分钟的观测。