Leviticus 11:36 Commentary - Ellicott's Commentary for English Readers (Old and New Testaments)

(36) 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 carcase, 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 hence are exempt from contracting pollution. The constant change of water which takes place in these reservoirs counteracts the effects of the polluting carcase. When it is borne in mind how few are the wells and cisterns in the East, and how scarce water is, the merciful provision of this law will be apparent. According to the canon which obtained during the second Temple, 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 carcase has fallen is mercifully exempted, he who comes in contact with the carcase in the water and removes it from the water is unclean, because the carcase itself remains a source of defilement.