A fountain or pit, etc. - This must either refer to running water, the stream of which soon carries off all impurities, or to large reservoirs where the water soon purifies itself; the water in either which touched the unclean thing, being considered as impure, the rest of the water being clean.

Nevertheless a fountain or pit, [wherein there is] plenty of water, shall be clean: but that which (k) toucheth their carcase shall be unclean.
(k) So much of the water as touched it.

Nevertheless, a fountain or pit, wherein there is plenty of water,.... Or, "a fountain or pit, a collection of waters", the copulative being wanting, as some observe, Aben Ezra takes notice of; or it may be by way of apposition, and so may explain what fountain or pit is meant, even such an one where there is a large continence of water, into which, if any carcass of a creeping thing fell, or any part of it, yet it
shall be clean: and fit for use, either because of the abundance of water in it, which could not be affected with the fall of such a creature into it as where there is but a small quantity; or rather this exception was made, because pools of water were of considerable value in these countries, and frequently in use for bathings, &c. and therefore for the good of men, and that they might not suffer so great a loss by such an accident, they are declared notwithstanding to be clean and free for use: hence you may learn, says Jarchi, that he that dips in them is pure from his uncleanness; that a man might lawfully make use of them for a bath on account of any uncleanness, notwithstanding the carcass of a creeping thing had fallen into it; as a mouse, or rat, or any such creature:
but that which toucheth their carcass shall be unclean; not the waters which touch the carcass, as Aben Ezra interprets it, for then the whole would be defiled, and unfit for use; but either the man that touched the carcass, laid hold upon it to pluck it out of the fountain or pit, or that which he made use of to get it out, or both these, were unclean in a ceremonial sense: the Targum of Jonathan is, "but he that toucheth their carcasses in the midst of these waters shall be unclean.''

Springs and wells were not defiled, because the uncleanness would be removed at once by the fresh supply of water. But whoever touched the body of the animal, to remove it, became unclean.

Of this no reason can be given, but the will of the law - giver and his merciful condescension to men's necessities, water being scarce in those countries; and for the same reason God would have the ceremonial law of sacrifices, give place to the law of mercy.