“Question: Can you explain why I’ve seen a bor hashoko with a divide in the middle (i.e. the bor is almost completely split into two sections horizontally)?
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 one mechanism for implementing hashoko; a bor hashoko:
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: Over the course of time it is inevitable that the rain water used to fill up the bor hashoko will slowly be replaced by mayim she’uvim – i.e. regular tap water used to fill up the bor tevila. This is a result of natural water diffusion, and also a result of water movement because of people immersing. According to the Ra’avad and Rambam (quoted in the Beis Yosef Yoreh De’ah Siman 201) this water movement poses a serious problem of nosan se’oh v’notal se’oh of mayim she’uvim and hence the mikva may become invalidated once there no longer remains in the bor hashoko the required minimum of 21 se’ah of natural rainwater.
Therefore, in order to delay this movement of water from occurring, several contemporary Poskim (the Minchas Yitzchak and Chelkas Ya’akov) instituted dividing the bor hashoko in two horizontally, with a shi’ur mikva (i.e. 40 se’ah) above and below the dividing barrier, and a small hole in the dividing barrier to connect both bodies of water. The “upper” bor hashoko is connected to the bor tevila, and the “lower” bor hashoko is only connected to the “upper” bor hashoko. This technique ensures that the rain water in the lower bor hashoko will be less affected by diffusion and water movement and the bor tevila will thus remain kosher also according to the Rambam and Ra’avad for a longer period of time.
Conclusion:
Dividing the bor hashoko into two sections is one method used to ensure that the natural rainwater that was used to fill up the bor hashoko will remain for a longer period of time, and consequently the connected mikva (bor tevila) will likewise remain kosher for a longer period of time, even according to the Rambam and Ra’avad. It is important to note that the Shulchan Aruch (Siman 201) does not rule in line with the opinion of the Rambam and Ra’avad, so according to the letter of the law a mikva that has a single (undivided) bor hashoko or bor zeriya is completely kosher even for tevilas niddah. Attempting to create a mikva that is kosher even according to the view of the Rambam and Ra’avad is considered a hiddur and if one is able to implement their opinion when building a mikva it is commendable, but not essential.
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