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. 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.

本研究于2021年1月28日至12月7日期间，在莱茵河（Rhine）与默兹河（Meuse）的荷兰境内河段（涵盖艾瑟尔河（IJssel）、瓦尔河（Waal）、下莱茵河（Nederrijn）三条支流）的26个监测点位开展漂浮塑料及其他人为源垃圾的调查，监测点位分布详见图1。
莱茵河于斯派克（Spijk）处从德国流入荷兰，随后分岔为主要支流瓦尔河、艾瑟尔河与下莱茵河。其中瓦尔河为主要分汊河道，在鹿特丹与下莱茵河-莱克河汊道汇合后注入北海；艾瑟尔河于坎彭（Kampen）处注入艾瑟尔湖（Lake IJssel）。默兹河于艾赫登（Eijsden）处从比利时流入荷兰，最终汇入潮汐型霍兰迪普河口湾（Hollands Diep estuary），在注入北海前，该河段还会接纳莱茵河的一条分汊河道。
本研究采用目视计数法对尺寸大于0.5厘米的漂浮大型塑料（macroplastic）及大型人为垃圾（macrolitter）进行调查：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（例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个点位中绝大多数（23个）设置了5或6个观测点，2个点位设置3个观测点，仅默兹河下游的1个点位设置了12个观测点。每次监测时，在预设的观测带内计数所有可见的漂浮垃圾。
可观测的最小垃圾尺寸取决于桥梁高度（8至20米），但所有点位的可观测最小尺寸均估算为至少2.5厘米。需注意，观测带宽度取决于视野范围与水面以上的桥梁高度，不同桥梁间及同一桥梁的不同观测点间的观测带宽度存在差异，范围为12至34米。观测带宽度通过选取参考物体（例如桥柱、浮标、橙皮）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占河道总宽度的比例介于25%至85%之间。
每个观测点在每日监测时段内进行4次时长为5分钟的计数。