A Manchester river has been found to have the worst level of microplastic pollution in the world apparently. I think I know why this is, so many houses have bathroom/ kitchen waste pipes dumping effluent into surface water drains. Extensions are particularly bad in this respect. They just route the pipes into the nearest drain.
https://www.thetimes.co.uk/article/bed-of-river-tame-in-manchester-has-world-s-worst-microplastic-pollution-xkpc7x989