Bismillahi Al-Rahmani Al-Rahim

 Uncertainty Regarding The Purity Of Dirt
Q
 What if you have dirt and you are unsure whether it is pure or not (i.e. you're in the middle of a park and all the water fountains are out, and all there is is dirt), does one presume its pure?

 A
 al-salamu `alaykum wa rahmatullahi wa barakatuhu

As for the issue of the dirt, the first section of Zad Al-Mustaqni` includes
If one has doubts about the filthiness or purity of water or something else, he acts according to certainty. (m: If you knew that it was pure and doubt about it now being filthy, then it is pure until proven otherwise. If you knew that it was filthy and doubt about it not being pure, then it is filthy until proven otherwise.)
a nd so the dirt is assumed to be pure when there is nothing to indicate otherwise.

And Allah knows best.

wa al-salamu `alaykum
--musa