The Talmud (Pesachim 17b, Nazir 38a) indicates that on a biblical level, a mikveh used to purify impure utensils is sufficient if it contains a revi’it of water. This provokes a question as to the nature of the requirements for a mikveh of a person. The Talmud (Yoma 31a) teaches that a mikveh must cover the entire body of the individual, and that the Rabbis have understood the required amount of water to be forty sa’ah.

As R. Herschel Schachter observes (B’Ikvei HaTzon, 29), the requirements stated for immersing a human are apparently two separate requirements, thus mandating forty sa’ah even if the whole body can be covered with less, and mandating coverage of the whole body even if that demands more than forty sa’ah (the position of the Arukh HaShulchan, Y.D. 201, against that of Darkei Teshuvah, ibid, #2).

The Rogatchover Gaon (Resp. Tzofnat Pa’aneach, New York ed., #119) inquires as to the relationship between the first and second laws mentioned above. Perhaps the validity (on a biblical level) of a mikveh with just a revi’it, when used for utensils, teaches that a mikveh of that proportion is indeed a valid mikveh. The fact that the immersion of a human requires forty sa’ah is then an aspect of defining the act of immersion, rather than the validity of a mikveh itself. A mikveh containing less than forty sa’ah would then be a valid mikveh unsuited for the immersion of a person.

Alternatively, it might be understood that there are two types of mikva’aot, one for utensils and one for people. A mikveh that contains less than forty sa’ah, then, would not be considered a mikveh as far as people are concerned. Even though the act of immersion (assuming it covers the whole body) may be complete, it would have taken place in an invalid mikveh.

As noted by R. Chaim Soloveichik, a ramification of the two possibilities concerns the process of hashakah, by which a mikveh containing drawn water (sh’uvin), which is invalid, is made usable through contact with another mikveh filled with acceptable rainwater. The question can be asked as to whether the rainwater mikveh needs to contain forty sa’ah. If the first of the above possibilities is correct, than a mikveh can theoretically be valid with a smaller amount, despite being unfit for actual use by a person. Thus, this valid mikveh may be able to transfer the valid status to the sh’uvin mikveh, which will then possess both the qualities of being a valid mikveh and containing sufficient water for a legitimate act of immersion.

If, however, a mikveh for humans must contain forty sa’ah by definition, this would apply to the rainwater mikveh as well, even though it is not used for actual immersion. Thus, a sh’uvin mikveh connected to a rainwater mikveh of less than forty sa’ah can be valid for utensils but not for people.

As R. Schachter points out, this question also affects the issue of immersion in a ma’ayan (natural spring). Rishonim debate whether forty sa’ah is necessary for immersion in a ma’ayan (see Biur haGra, Y.D. 201:6; note that Rabbeinu Tam, Sefer haYashar, 620, relates a dispute between himself and Rabbeinu Meshulam as to whether or not ma’ayan immersion requires the whole body to be covered at once).

If forty sa’ah is a requirement defining the act of immersion, it would seem that even when a ma’ayan is being utilized, this measurement is necessary. If, however, it is an aspect of defining a valid mikveh, it might be concluded that when a ma’ayan is being used rather than a mikveh, forty sa’ah is not necessary. (See also the lengthy discussion on this point by R. Shmuel Rozofsky, Zikhron Shmuel, 80).