Charles Ellicott Commentary Leviticus 11:36

"Nevertheless a fountain or a pit wherein is a gathering of water shall be clean: but that which toucheth their carcass shall be unclean." — Leviticus 11:36 (ASV)

Nevertheless a fountain or pit, wherein there is plenty of water. —Better, But wells and cisterns being gatherings together of water. But if the unclean carcass, or any portion of it, happens to fall or to be thrown into wells or cisterns, they are to be treated as large collections of water, such as pools, ponds, and lakes, and therefore are exempt from contracting defilement. The constant change of water that takes place in these reservoirs counteracts the effects of the polluting carcass.

When it is remembered how few wells and cisterns there are in the East, and how scarce water is, the merciful provision of this law will be apparent. According to the canon that prevailed during the Second Temple period, this immunity was only applicable to receptacles of water actually in the ground, but not to collections of water in vessels.

But that which toucheth. —Better, but he who toucheth. But though the water into which the carcass has fallen is mercifully exempted, he who comes in contact with the carcass in the water and removes it from the water is unclean, because the carcass itself remains a source of defilement.