Strata where water is are likely to be segregated. Yes, contamination does happen, but not often, and if it does it is because —as in the Gulf—some bookkeeper was trying to cut corners. Better get used to relatively clean water. Human beings, really, have never known anything else.
Darn - we may have to do what the forefathers did, drink beer.
In point of fact, in much of the world, like the Third World and the T*rd World areas, water is usually not safe to drink.
Only in developed nations where pollution can be controlled, and water purified, does man have safe water.
In most of America, do NOT drink directly from surface water. Parasites and diseases are common in surface waters.
Tap water, on the other hand is treated and IS safe. Technology, and Western societies - science based, linear logic, patriarchal culture - gotta love ‘em.