Vayikra 11:36

עין ובור מקוה מים‎אך מ NEVERTHELESS A FOUNTAIN OR A PIT WHERE THERE IS A GATHERING OF WATERS which are attached to the ground are not receptive of uncleanness (not being included in וכל משקה וגו׳ בכל כלי mentioned in v. 34, since it states here יהיה טהור, it shall remain clean). But you may also give it the meaning: יהיה טהור, he shall be clean i.e. he who immerses himself in them to free himself from his uncleanness. ונגע בנבלתם יטמא BUT THAT WHICH TOUCHES THEIR CARRION SHALL BE UNCLEAN — even if one is in the fountain or pit and comes in contact with their uncleanness (their carcasses) he shall be unclean. This is specifically stated in order that you should not argue à fortiori as follows: since it cleanses the unclean from their uncleanness, it follows à fortiori that it will save the clean from becoming unclean, — it stales therefore, “but whoever touches their carrion shall be unclean” (Sifra, Shemini, Section 9 5; Nedarim 75b).

אַ֣ךְ מַעְיָ֥ן וּב֛וֹר מִקְוֵה־מַ֖יִם יִהְיֶ֣ה טָה֑וֹר וְנֹגֵ֥עַ בְּנִבְלָתָ֖ם יִטְמָֽא׃

Nevertheless a fountain or pit, wherein there is a collection of water, shall be clean: but that which touches their carcass shall be unclean.