nobby said:Is this as in an ideal world where I could be sure that I was leaching out tannins and not replacing them with various chemicals washed off the fields into the water supply?
Tannins are water soluble, organic phenolic compounds and found in some water naturally but at low concentrations, so I think that osmosis will be working in your favour.
Of course if the river is polluted then Id not put anything "in it" I was going to later put in my mouth.