You're trying to tell me that water that falls from the sky has no impurities?
I believe he's referring to the fact that rain water starts as ground water, goes through an evaporative process and then precipitates from air from condensation, a process similar to distillation.
Which came first though?
The groundwater?
or the rainwater?
Chicken and egg stuff going on here.
The desert is a true treasure for him who seeks refuge from men and the evil of men. In it is contentment In it is death and all you seek (Quoted from "The Bleeding of the Stone" Ibrahim Al-Koni)