Microplastic contamination of the oceans is one of the world’s most pressing environmental concerns. The terrestrial component of the global microplastic budget is not well understood because sources, stores and fluxes are poorly quantified. We report catchment-wide patterns of microplastic contamination, classified by type, size and density, in channel bed sediments at 40 sites across urban, suburban and rural river catchments in northwest England. Microplastic contamination was pervasive on all river channel beds. We found multiple urban contamination hotspots with a maximum microplastic concentration of approximately 517,000 particles m⁻². After a period of severe flooding in winter 2015/16, all sites were resampled. Microplastic concentrations had fallen at 28 sites and 18 saw a decrease of one order of magnitude. The flooding exported approximately 70% of the microplastic load stored on these river beds (equivalent to 0.85 ± 0.27 tonnes or 43 ± 14 billion particles) and eradicated microbead contamination at 7 sites. We conclude that microplastic contamination is efficiently flushed from river catchments during flooding.

海洋中的微塑料污染是世界上最紧迫的环境问题之一。由于来源，储藏和通量的量化很差，因此人们对全球微塑料预算中的地面部分还不太了解。我们报告了英格兰西北部城市，郊区和农村河流集水区40个地点的河床底泥中，按类型，大小和密度分类的集水区范围内的微塑性污染模式。在所有河床河床上普遍存在着微量的塑料污染。我们发现了多个城市污染热点，其最大微塑性浓度约为517,000颗粒m ^-2。在2015/16年冬季经历了一段时间的严重洪灾之后，对所有站点进行了重新采样。在28个地点的微尘浓度下降了，而18个地点的微尘浓度下降了一个数量级。洪水将这些河床中存储的大约70％的微塑料负荷（相当于0.85±0.27吨或43±140亿个颗粒）输出，并消除了7个地点的微珠污染。我们得出的结论是，洪水期间有效地从河流流域冲洗掉了微塑性污染物。