Purity: Ruling on Uncertain Water: Mixed or Unmixed

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 48

If it is not known whether some water is unmixed or mixed, or whether it was previously unmixed or mixed, then such water does not purify an impure object, and wuḍūʾ and ghusl performed with it are invalid. In the event that an impurity makes contact with it and the water is less than kurr, it becomes impure; and if it is equal to or more than kurr, then based on obligatory precaution, it also becomes impure.

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