We don't drink our creek water either!
It's polluted hard water and many of the artifacts from the creek can be thickly coated with lime. I am getting pretty good (if I say so myself) at guessing which lime and black creek scum coated hafted scrapers are Sioux Quartzite - they can all look like limestone coated in black. It's quite a guessing game.
I am giving vinegar a shot on a few of these lime coated scrapers to see what results. I understand it can take a few days to get it off. To cut the black creek scum, I was taught to use Shrimp Cocktail Sauce. It does great. Usually 1/2 a day gets the lions share of the black stuff.
I appreciate the information on vinegar use.
vfm