Marketing campaigns for commodity bottled waters try to make us think water must be "pure" to be good. Being clean and healthy isn’t enough - water has to be pure, and the purer the better. These marketers tell us that nothing but hydrogen and oxygen should be in our water. Ironically, this misconception means people often drink distilled water when taking mineral supplements, which contain the same minerals that have been removed from the water.
Even distillation, which removes more contaminants than any other purification method, does not produce completely "pure" water. Rising steam is supposed to leave all impurities behind in the distilling process, but in fact gases, some chemicals, and some organic compounds can be taken along with the steam. Active carbon filters are used to eliminate those remaining contaminants from distilled water, but some impurity remains with this process, too.
In reverse osmosis (RO), water molecules are forced through a rubber membrane, leaving impurities behind. But gases, some chemicals (including chloramine and arsenic), and some bacteria can beat this technique, too.
So there is no such thing as pure water. It’s a myth. Natural water has mineral content. By removing minerals, water becomes acidic and aggressive, meaning it will seek to replace the minerals removed. Water treated by either distillation or RO will become acidic upon contact with air airborne carbon dioxide reacts with the water, taking the place of the removed minerals or contaminants.
Because water is a universal solvent, rainwater collects particles and chemicals even as it’s falling. Geological strata only add more to the composition the minerals and trace elements of the local area give each water its distinct terroir. Underground geology may filter water for decades or even millennia; when the water finally emerges at the source, it may not be "pure," but it is nevertheless clean and healthy. Clean, healthy water does not have to be pure. In fact, the waters with the most epicurean interest contain minerals and trace elements.