Why don't you drink mountain spring water?

Answer 1

Just because the spring is in the mountain does not mean it has not been contaminated. Animals fall in and die or die and fall in. Nasty debris can fall in - rotting leaves, animal feces and urine, etc. The water can pick up nasty stuff as it percolates through the ground including heavy metals. Springs are also not immune to growing bacteria or harboring parasites.

That said, I usually will still drink from a mountain spring.

Note that just because bottled water says it came from a mountain spring, it does not mean it isn't contaminated.