Rock layers on the surface of the earth that have water running through them that are drinkable have had rain water removing the elements and chemicals harmful to life for, generally, millions of years. The best water often comes from rock that was actually created by life (limestones, dolomites, etc), so that it had almost no harmful chemicals in it to begin with.

The poster you are disagreeing with is correct. The deep earth is full of really toxic stuff. Arsenic, lead, sulfur, radon, etc. Then there is the deep oil and gas, salt domes, asbestos. Huge volcanic eruptions called flood basalts are likely the cause of many mass extinctions due to the chemistry of the eruptions, not the ash. The mother Earth we all love is just the little skin on top of a cauldron of death.