Numbers 19:9

9Now a man who is [ceremonially] clean shall collect the ashes of the heifer and deposit them outside the camp in a clean place, and the congregation of the Israelites shall keep it for 19:9 The purpose of the ash mixed with water was to remove the contamination from those defiled because of contact with a dead body.water to remove impurity; it is [to be used for] purification from sin.