The water of purification:
Archimandrite Nikifor’s Biblical Encyclopedia
a) (Num. 8:7) used in the rite of the consecration of the Levites. It served as a symbol of the moral and spiritual purification with which the servants of the Most High God must be filled; b) (Num. 19:2) water used together with the ashes of a red heifer, for the ritual purification of persons who had defiled themselves by touching a dead body.