Yeah I wouldn't worry at all if I was in an unpolluted area, but where I live there are towns and factories etc near by...
It is not only towns and factories that pollute rivers. Some really beautiful crystal clear rivers in beautiful countryside have been polluted through mining. The river Ystwyth in Wales is just one example due to the Cwmystwyth Mines, the river looks idyllic and crystal clear in beautiful scenery but still carries elevated levels of metal pollutants even though the mines have been closed since 1940. If you have any concerns then contact the Environment Agency
https://www.gov.uk/government/organisations/environment-agency#org-contacts