Testing the pH of water throughout Canada, in Northern climes, during geochemical surveys, WHILE EXPLORING FOR GOLD OCCURRENCES, the figure I most often arrived at was about 6.0 pH, or slightly acid, for lake and stream water. A pH of 7.0 was "high" and rare, although this is neutral pH, neither acid nor base by definition. At the time, we were warned to look for acid rain run off which would be a pH of 5.5 (or about that of lemon juice) - in the spring time. I never tested that low, ever. Highest I ever got was about 7.5. I never tested in southern Ontario or Sask-Alberta where the water is guaranteed to be hard and contain lots of salts.
Our bodies are buffered (contain mineral salt) for acids and will absorb a lot of that sort of thing. Stomach acid is 10% HCl, which is about 3 moles per litre or far lower than zero pH. That is why it burns your throat when you throw up. (Everybody now, gag). We could stand water in the low pH ranges if it did not have dangerous heavy metals in it. The real danger of acid rain is that it de-oxygenates water, kills fish and promotes the uptake of heavy metals such as mercury and lead and other poisons in the ecosystem and food chain.
What we are not buffered for and cannot stand much of is alkali salts. High pH therefore is much more dangerous to us. Gypsum in water (Calcium sulphate CaSO4*H2O) will make you very ill fast. If you get a strong hydroxide in your system you have to flush thoroughly and for a long time. Draino is not food.
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