Perhaps this may help you. https://apps.who.int/iris/bitstream/handle/10665/44584/9789241548151_eng.pdf
Surface and river water quality can change rapidly, so reliable sampling and reliance on results difficult for drinking without reliable treatment. Water contact sports like swimming or other desired uses as irrigation may also have some limits. Knowledge of pollution sources, land uses, etc. can be helpful. Non point pollution sources likely in periods of storm runoff. Limits of indicators like fecal and total coliform do not guarantee quality, and there are other potential drinking health hazards like Guardia and Cryptosporidium that could be present.