can you be more specific as to why you say this?
Most water supply treatment processes begin with relatively clean water,, then filter it, add flocculation for further treatment, and either chlorinate and fluoridate for further killing of any germs in the supply.
Contaminated water doesn’t specify the type of contamination,...bacteriological, chemical, odor, appearance, turbidity, color,...etc.
A century ago, most rural ranches and farms in the mid west and western states had well water which looked like iced tea from corrosion of the well pipes. It was plenty healthy, but nowhere near the appearance of today’s tapwater.
Most municipalities will have a 1 day to 1 week reservoir of water supply available at water usage rates of 250-300gal/capita/day. It is quite feasible to survive on less than 5gal/capita/day.
Building codes place the highest priority on fire protection, with most potable water main also used for fire fighting.
Read up on the topic from those who work at it for a living over several generations. AWWA and ASCE are professional sources to study this low tech solution.