Albert Barnes Commentary Numbers 19:9

"And a man that is clean shall gather up the ashes of the heifer, and lay them up without the camp in a clean place; and it shall be kept for the congregation of the children of Israel for a water for impurity: it is a sin-offering." — Numbers 19:9 (ASV)

Water of separation - In Numbers 8:7, the water of purification from sin is the water of purifying. So that which was to remedy a state of legal separation is here called “water of separation.”