It does not matter how high the source of water is when it comes to safety. Water can be contaminated by acid rain at the top of a mountain, just as well as it can be polluted in the city .
The bottom line is this: Always purify any water that does not come from a water treatment plant. There is enough information in this discussion to help you access clean water, even when disaster strikes.
Think through the various suggestions and then make a plan that will work for you. Advance preparation is always the best plan for survival.