I and my dad and brother have always used filtered/treated well water mixed with coolant, without issue, in numerous cars that traveled over 100,000 miles on these type of mixed coolant. Two of our cars did another 200,000 miles beyond the first factory changes. Never had a spec of corrosion, extra wear or cooling issues. One is still on the original water pump at over 300,000 miles with Dex Cool/Well Water Mix since around 100k.
I would not use regular city tap water unless it was filtered or hose well water (unfiltered).
I would not use regular city tap water unless it was filtered or hose well water (unfiltered).