“Question: Can you explain why some are particular to build the bor hashoko under the bor tevila?
Introduction: A mikva must consist of a minimum of 40 se’ah of valid water. Relevant to our discussion, valid water excludes tap water which cannot be used to fill up a mikva because of the invalidation of she’uvim. “She’uvim” denotes water that has entered a kli (either pipes, the tap itself or water meters) and is thus invalid to be used for a mikva. She’uvim is not a permanent invalidation, and the invalidation of mayim she’uvim can be rectified through the mechanism of hashoko (the precedent for the term hashoko can be found in the Mishna Beitzah 17a):
Hashoko is the process where invalid water comes into contact with kosher mikva water and the contact between these two bodies of water removes the invalidation; relevant to our discussion is the invalidation of she’uvim. We will look at two mechanisms for implementing hashoko; a bor zeriya and bor hashoko:
A bor zeriya is a body of valid mikva water through which invalid water runs and thus becomes “nizra” (see Rashi Beitzah ibid) and thus kosher for mikva use. A bor zeriya is implemented by adding a bor of rainwater near the bor tevila (i.e. the pool in which people or utensils immerse/are immersed), and regular tap water intended for the mikva is run through the bor zeriya, which overflows into the bor tevila. Once the invalid tap water has become “nizra” inside the bor zeriya it is now valid and kosher for tevila (according to most Rishonim).
A bor hashoko is a pool of water usually adjacent to a bor tevila and connected to the bor tevila. The function of a bor hashoko is to validate invalid water that is used to fill up the bor tevila and render the water kosher for tevila. The mechanism by which this validation occurs is referred to as hashoko, hence the name bor hashoko.
Discussion: The default and standard approach is to build the bor hashoko adjacent to the bor tevila; however some specifically prefer building the bor hashoko underneath the bor tevila. There are various reasons given for this:
The primary purpose of a bor hashoko in addition to a bor zeriya is to create a mikva that is kosher also according to the Ra’avad (in his sefer Ba’alei HaNefesh) and Rambam: The Ra’avad and Rambam posit that if there does not remain within the bor a minimum required volume of 21 se’ah of rainwater, then the bor is no longer valid, and thus cannot validate an adjacent bor. The addition of a bor hashoko appears to provide a viable way of ensuring that there constantly remains 21 se’ah of rainwater with which to validate the bor tevila.
However, it is questionable whether a conventional adjacent bor hashoko provides an adequate solution according to the Ra’avad. A bor hashoko underneath the mikva (ostensibly) provides a viable solution according to the Ra’avad. This is because regarding an adjacent bor hashoko there will inevitably be significant water movement between the two bodies of water (because of natural diffusion of water and as a result of the people immersing) and a real concern that there will not remain in the bor hashoko the required minimum 21 se’ah. A bor hashoko under the bor tevila will be subject to far less water movement; because although there will still be natural diffusion, people immersing will not cause significant water movement between the two bodies of water, and thus the original natural rainwater will remain for a longer period of time.
Another reason that many prefer to build the bor hashoko underneath the bor tevila is because an adjacent bor hashoko is connected to the bor tevila via a channel in the mikva wall (called the nekev hashoko), which, for practical reasons is usually located just under the waterline. Especially in areas where there is not adequate or reliable supervision, there is a danger that the bor tevila may inadvertently not be filled to the required level of the nekev hashoko and consequently the mikva (bor tevila) which has been filled up with tap water will remain she’uvim and completely invalid for tevila.
A bor hashoko underneath the mikva solves this concern because the nekev hashoko is in the dividing floor and by definition whenever the bor tevila is filled the nekev hashoko will inevitably be filled as well.
Conclusion: A bor hashoko underneath the mikva (bor tevila) appears to provide a more mehudar option than a conventional adjacent bor hashoko, and this is why many mikva’os are so built.
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