The World Federation One Stop Fiqh

Ruling 18

The water of a fountain that is connected to kurr water purifies impure water. However, if it falls on impure water drop by drop, it does not purify it unless something is held over the fountain so that before the water begins to fall drop by drop, it connects to the impure water; and for the fountain water to purify the impure water, it must mix with the impure water.