Lévitique 11:36

But fountains and cisterns, the place of the collection of running waters, shall be clean: but he who toucheth the carcase of any of those things (that may have fallen) into the water shall be unclean.

The Hebrew word for “well” is a feminine noun and denotes flowing water, as in “as a well keeps its water fresh” [Jeremiah 6:7]. A ditch is a cistern or a hole dug for collecting rainwater. Thus, a well is similar to a fountain . There was no need for the verse also to mention streams. The reasons Scripture speaks of a ditch of gathered water is that a ditch may also not have water. However, some say that the verse omits a connective, and that the verse should read “…a ditch, [or] gathered water…” (as in “Reuven [and] Shim‘on” [Exodus 1:2], “the sun [and] moon” [Habakkuk 3:11]).

that touches their remains the water that touches their remains will become ritually impure

NEVERTHELESS A FOUNTAIN OR A CISTERN WHEREIN IS A GATHERING OF WATER SHALL BE CLEAN. Scripture is stating that the waters in the immersion-pool whilst attached to the ground are not susceptible to impurity if any [source of] impurity should fall therein. BUT THAT WHICH TOUCHETH THEIR CARCASS SHALL BE UNCLEAN. The meaning thereof is as follows: “but waters which touch the carcass [of any of the creeping things] shall become impure, [meaning that waters which are separated [from a gathering of water] are rendered impure if they touch their carcass. [The phrase, and [waters] which touch their carcass shall be unclean, must be referring to detached waters], for when the waters are still attached to the pool, they cannot [be said to] ‘touch’ their carcass, but rather [Scripture should have said], their carcass ‘fell’ into it.” Scripture speaks of water in the singular [saying, v’nogeia — “and that which toucheth”], just as it says elsewhere, the water of sprinkling was not sprinkled upon him. (was not sprinkled) in referring to water, although the Hebrew word ‘mayim’ is in the plural form. Or it may be that Scripture is saying, “and whatsoever touches their carcass shall be impure,” and the intent is to include water and [the other] drinks mentioned. In general, this verse comes to teach that water is susceptible to impurity when it is detached [from a pool], but not when it is attached thereto, a principle that has not been mentioned heretofore.
Now in the Torath Kohanim we find that [the Rabbis gave this interpretation]: 9:6. “Rabbi Yosei the Galilean said: But that which toucheth their carcass shall be unclean. By touching they [i.e., the eight dead creeping things] convey impurity, but not by carrying them.” This too is feasible, that [the verse] is referring back to all the laws mentioned [in Verses 24-5] previously, and saying that creeping things convey impurity only by touch. Our Rabbis have further interpretations of the redundant expressions in this [whole] verse, all of them laws given to Moses on Sinai.
Now Scripture mentioned the purity of the immersion-pool and waters attached to the ground in speaking of [the impurity conveyed by] creeping things, but the same law applies to [that conveyed by] carrion [of animals forbidden as food, and of those permitted animals not slaughtered properly or which died by themselves]. It waited, however, to mention [the law concerning the purity of pools till now], until it had completed the enumeration of all objects that convey impurity. Or it may be that it is because creeping things frequently die in fountains or in cisterns. I have already mentioned my opinion [that this verse mentions the impurity of vessels conveyed by creeping things and their purification by immersion, and from it we learn the law of people and vessels in connection with the other forms of impurity mentioned above].

אך מעיין ובור, according to the plain meaning of the text, water connected to the earth, though in a container such as a pool, is not subject to ritual impurity as mentioned in verse 34, except water which is in a man made container, the container having already become subject to such purity before any water was poured into it.