Purity: Purification of Changed Water by Mixing with Kurr, Flowing, or Rainwater

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 50

With regard to water into which an intrinsic impurity like blood or urine has fallen and there is a change in its smell, colour, or taste, in the event that it is connected to kurr or flowing water, or it rains on it, or wind makes the rain fall on it, or rainwater flows on it from a gutter while it is raining, in all of these cases, if the change disappears, it becomes pure. However, the rainwater, kurr water, or flowing water must become mixed with it for it to be considered pure.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)