John Gill 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, [where 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 collection 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 by 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 used for bathing and therefore for the good of men, and so that they might not suffer such a great loss by such an accident, they are declared clean and free for use notwithstanding.

Hence you may learn, Jarchi says, that whoever dips in them is pure from his uncleanness; that a person might lawfully use them for a bath on account of any uncleanness, notwithstanding a carcass of a creeping thing had fallen into it; as a mouse, or rat, or any such creature.

but that which touches 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 person that touched the carcass, took hold of it to pull it out of the fountain or pit, or that which he used to get it out, or both these, were ceremonially unclean: the Targum of Jonathan is,

``but he that toucheth their carcasses in the midst of these waters shall be unclean.''