The World Federation One Stop Fiqh

Ruling 21

With regard to water that had been equivalent to kurr, if one doubts (i.e. has a shakk) whether it has become less than kurr or not, it will be treated as kurr water, meaning that it can still purify an impure object and if an impurity makes contact with it, it does not become impure [as long as its smell, colour, or taste does not change]. As for water that had been less than kurr, if one doubts whether it has become equal to kurr or not, it is ruled as (i.e. it has the ḥukm of) being less than kurr.