All we can do is to catch what we need of it, "on the run," somewhere in the earthy part of its circuit.
Why our Drinking Water is Likely to be Impure.

Every drop of water that we drink or use, fell somewhere on the surface of the earth, in the form of rain or snow; and if we wish to find out whether it is pure and safe, we must trace its course through the soil, or the streams, from the point where it fell.

Our drinking water has literally washed "all outdoors" before it reaches us, and what it may have picked up in that washing makes the possibilities of its danger.
As it falls from the skies, it is perfectly pure--except in large cities or manufacturing centres, where rain water contains small amounts of soot, smoke-acids, and dust, but even these are in such small amounts as to be practically harmless.

But the moment it reaches the ground, it begins to soak up something out of everything that it touches; and here our dangers begin.
Risks from Leaf Mould.

Practically the whole surface of the earth is covered with some form of vegetation--grass, trees, or other green plants.