And a pure man shall gather the ashes of the heifer and place them outside the camp in a pure place; and it shall be for the congregation of the children of Yisrael for a keeping, for water of sprinkling; it is a chatas.
Torah Temimah: Parah 7:10 (and Raavad) — One who gives his water to a tamei person to guard — the water is invalid. Why? It remains in the presumption of the guardian, and he is not particular to guard it, yet the Torah said “and it shall be for a keeping.”
English: One who gives his water to a tamei person to guard — the water is invalid. Why? It remains in the presumption of the guardian, and he is not particular to guard it, yet the Torah said “and it shall be for a keeping.”
Requirement of actual, intentional shemirah by a pure person.
Water set aside for the mei niddah must be guarded by a pure person who is particular about its purity; entrusting it to a tamei invalidates it.