Gemara

Liquids in the ground are Tehorim if there is a Revi'is, for it is a valid Mikveh for Tevilah of small Kelim.

Nazir 38a: One may be Tovel Kelim in a Mikvah containing a Revi'is of water.

Chachamim abolished this law.

Avodah Zarah 75b: Regarding the Kelim taken from Midyan it says "v'Taher" and "b'Mei Nidah Yischata";

Had it said only "v'Taher", one might have thought that Tevilah may be in any amount of water. "B'Mei Nidah" obligates water that a Nidah could immerse in, i.e. 40 Sa'im.

Yoma 31a (Beraisa): "V'Rachatz ba'Mayim Kol Besaro" teaches that the Mikveh must contain the entire body [of an average size person].

This is an Amah by an Amah by three Amos tall. Chachamim calculated that this is 40 Sa'im.

Chagigah (22a): If one divided a Mikveh with a basket, Tevilah there is invalid. [The two sides do not join through the holes in the basket, for] the ground is porous, yet we require 40 Sa'im in one place. [Underground water does not join to the Shi'ur.]

Mikva'os 5:1: A Mayan (spring) of any size is Metaher.

Rishonim

Rambam (Hilchos Mikva'os 9:8): A Mikveh requires 40 Sa'im. A Mayan of any size is Metaher.

Ra'avad (Ba'alei ha'Nefesh, Reish Sha'ar ha'Mayim): "Ach Mayan" teaches that a Mayan is Metaher b'Kolshehu, but a Mikveh requires 40 Sa'im.

Rosh (Hilchos Mikva'os Sof Siman 1): The Mishnah says that a Mayan of any size is Metaher, but a Mikveh requires 40 Sa'im. The Ri says that this is only for Tevilas Kelim, but 40 Sa'im are needed for Tevilah of a person, even if he is small. We learn from Chagigah 22a. We require 40 Sa'im even though land is porous. This applies only to a Mayan. "Sinews" in the land connect it to underground water. The Ra'avad said that any amount suffices even for people, as long as his entire body is inside at once. The Rambam agrees. I say that the Ri's proof is decisive. Also, it is reasonable that Chachamim's Shi'ur for a Mikveh (one by one by three Amos) is the same for everyone.

Tosfos (Avodah Zarah 75b DH Mayim): Mid'Oraisa a Revi'is suffices to immerse needles and small forks, i.e. for Tevilah from Tum'ah. Tevilah of Klei Midyan is a Chidush. It requires 40 Sa'im.

Semak (199, brought in Beis Yosef DH v'Chosav b'Sefer): A Mayan of any size is Metaher, i.e. from Tum'ah. To be Mekadesh Klei Nochrim for use by Yisrael, even a Mayan needs 40 Sa'im. We learn from "b'Mei Nidah" (Avodah Zarah 75b).

Suggestion: Perhaps this is only for a big Kli, but a Revi'is suffices for a small Kli before Chachamim abolished this, or a Mayan of any size, just like regarding Tum'ah.

Rebuttal (Semak ibid.): We expound another Taharah from "v'Taher", i.e. Tevilah. The Gemara says that we need also "b'Mei Nidah", lest one think that any amount suffices. Rather, we require 40 Sa'im. 'Any amount' does not mean less than a Revi'is. This is not enough even for Tum'ah! Do not say that it refers to a Mayan, for a big Kli. Also for Tum'ah this is not enough, since the Kli is not contained within it!

Note: If the Kli's shape matches the Mayan's shape almost perfectly, any amount of water could suffice!

Semak (ibid.): Rather, it refers to a small Kli. For Tum'ah, a Mayan of any size or a Mikveh of a Revi'is suffices, before Chachamim disqualified it. Even so, we require 40 Sa'im for a Kli bought from a Nochri.

Poskim

Shulchan Aruch (YD 120:1): If one bought Kelim for food from a Nochri, he must be Tovel them in a Mikveh or Mayan with 40 Sa'im.

Beis Yosef (DH ha'Loke'ach): The Yerushalmi says that this Tevilah is to leave Tum'as Nochri, and enter Kedushas Yisrael.

Beis Yosef (ibid.): Semak learned from Tosfos (75b). Also the Rashba (4:318) brings so from R. Tam (Sefer ha'Yashar Teshuvah 8:2).

Beis Yosef (DH u'Mah she'Chosav Rabeinu): The Tur disagrees, for we learn about this Tevilah from Tevilah of a Teme'ah (Nidah), so it is unreasonable to be more stringent about it than about Nidah!

Question (Beis Yosef, ibid.): The Tur connotes that a Nidah does not require 40 Sa'im in a Mayan. This is unlike the Ri. In Siman 201, the Tur says that the Rosh agrees with the Ri. According to them, Semak is no more stringent about Klei Nochrim than about Nidah!

Answer #1 (Darchei Moshe): The Tur questions SMaK, who is more stringent about this Tevilah than about Nidah.

Objection (Taz 2): The Tur's words connote unlike this. Also, surely, also SMaK requires 40 Sa'im for a Nidah even in a Mayan, but not for Tamei Kelim.

Answer #2 (Taz 2): The Tur asked that we learn Tevilas Klei Nochrim from "v'Taher." This does not connote that it is through water. Perhaps it is through something else! The Gemara answers that we learn from "Mei Nidah." The simple meaning of this refers to Mei Chatas (water on which ashes of the Parah Adumah were put). We explain that it is through [Tevilah in] water since it says regarding Tamei Kelim "b'Mayim Yuva v'Taher." A Mah Matzinu teaches that also "v'Taher" of Klei Nochrim is through water. Therefore, also Mei Nidah refers to water in which a Nidah immerses. It teaches that we require 40 Sa'im, like for a Nidah, like the Gemara says. The text of the Tur says "more than Tevilas Tum'ah", i.e. Tamei Kelim. Since we learn that [Tevilah in] water is needed [for Klei Nochrim] only from Tevilah of Tamei Kelim, we should not be more stringent about it regarding a Mayan, only for a Mikveh, for there the Torah explicitly taught that it must be like for a Nidah.

Gra (4): Toras Kohanim teaches that a Mayan of any size is Metaher, and a Mikveh of 40 Sa'im. Mikva'os 1:7, 5:1 and 5:3 say so. The Rambam and Ra'avad explain like this. The Ri says that this is only for Kelim, for which mid'Oraisa even for a Mikveh a Revi'is suffices (Pesachim 17b). Chachamim disqualified this, but a Mayan [of any size] still suffices, like Torah law. There (the Mishnayos in Mikva'os) discusses mid'Rabanan. Even though Toras Kohanim expounded a verse, it is a mere Asmachta. We require 40 Sa'im for a person mid'Oraisa, even in a Mayan (Chagigah 22a. It says that land is porous, and even so, we require 40 Sa'im in one place.) This applies only to a Mayan. "Sinews" in the land connect it to underground water. Semak says that this is only regarding Tum'ah, for Tevilas Klei Nochrim require 40 Sa'im even in a Mayan, like we learn from "Mei Nidah." This is like it says there (Avodah Zarah 75b) "had it said [only] v'Taher..." we would say that a Mayan or a Mikveh of a Revi'is suffices mid'Oraisa, for the verse discusses what is Metaher. A Mayan or a Mikveh of a Revi'is are the same mid'Oraisa.

Note: Tosfos Rid (22a DH d'HA) says that since a Mayan does not require 40 Sa'im, the Gemara must discuss joining two incomplete Mikva'os.

Gilyon Maharsha (3): Regarding the Mikveh, we are more stringent about Klei Nochrim than about Tum'ah. Regarding intent, we are more lenient (Taz 17).

Chachmas Adam (73:15, cited in Pischei Teshuvah 3): One must enter the entire Kli in the water at once. One cannot immerse part at a time, just like a Nidah cannot. This is unlike Kashering. There, each part of the Kli emits its absorptions when it is in the water. Here, the part outside the water remains Tamei, and this spreads to the entire Kli.

Chachmas Adam (73:19, cited in Pischei Teshuvah 4): A Mayan is Metaher even if it is not in one place, rather, it flows. However, a Mikveh is Metaher only if the water is in one place, in a cavity, but not if it flows. In pressed circumstances one may immerse glass Kelim in snow gathered in one place, since Tevilah of glass is mid'Rabanan. One may not immerse metal Kelim in snow, for many say that this is mid'Oraisa.

Shulchan Aruch (YD 201:1): A Nidah does not become Tehorah until she is Tovel her entire body at once in a Mikveh or Mayan with 40 Sa'im.

Gra (DH 1): Most explain that a Mayan of any size is Kosher for Tevilah even for people, unlike the Rosh. Even so, one should be stringent for a Torah law.