Zochlin

Zochlin is one of the common flaws in mikvahs, mentioned in Masechet Mikvaot, which is caused by the mikvah water flowing, contrary to the rule stating that mikvah water only purifies when it is "ashboren" - meaning when the water is still and standing in place. This differs from a spring, which purifies even in "zochlin" state, when the spring water flows and moves from its place.

The Definition of Zochlin

In the Responsa of the Tzemach Tzedek, there are many questions asked of the Tzemach Tzedek on this topic. One of them is the following question:

"Regarding mikvahs that were made in our city, two adjacent to each other with a partition separating them. When one is heated with hot water, the water flows to the second one through cracks in the wall that are below the forty se'ah level. This is clearly visible, as the water flow isn't little by little, because when a large kettle of hot water is poured into one, within approximately 3 or 4 hours both become equal in height. I instructed not to immerse in it while it is flowing. Now the people of our city have protested that most of the congregation cannot adhere to this."

Dispute Among the Rishonim

In this question, there are actually several issues: First, at what level of "flowing" is water considered "zochlin."

1. It's possible that the definition of this law is that the water should not be like a spring, where all its water flows from place to place, as derived from the requirement in "Torat Kohanim" on the verse "But a spring or a mikvah, a gathering of water, shall be pure." From the word "but" we learn that only a spring purifies while flowing, but not a mikvah. According to this, perhaps only a mikvah whose entire water flows, similar to a spring, is invalid for immersion, but not a mikvah that has a small hole with only a little of its water escaping.
2. That the disqualification of the mikvah depends only on whether all forty se'ah will eventually leave the mikvah.

This is also the opinion of the Rosh, who cites the explanation of the R"Sh that a mikvah is disqualified even with a very small hole, and even though forty se'ah will remain in it, one is now also immersing in the upper waters which are flowing.

The Rosh himself tends to disagree with this opinion because "it is more correct to interpret that it refers to a case where forty se'ah would not remain in the mikvah unless the flow is stopped." However, in practice, it is brought in the Tur in the name of the Rosh that one should be concerned about the opinion of the R"Sh.

Similarly, the Rishonim disagreed whether the disqualification of "zochlin" applies only to those waters that are destined to exit, or to all the mikvah water. The source of the Rishonim's dispute is in understanding the words of Rabbeinu Shimshon about a mikvah whose upper edge was cracked, causing the upper waters to eventually exit. The Rosh holds that the disqualification applies only to the upper waters, but since the upper waters are destined to exit, and one immerses in them, the immersion is considered invalid. In contrast, the Rosh's son - the Tur - explains Rabbeinu Shimshon's reasoning: "since the upper waters are stirred to exit, all of it is considered zochlin." The Mordechai also wrote similarly - "even if there are a thousand se'ah in the mikvah, one must be careful that the water does not rise above the edge of the mikvah because it becomes zochlin."

The Ruling of the Tzemach Tzedek

The Rebbe Tzemach Tzedek brings proof for the Rosh's opinion from a Mishnah that states that a wave that detached and contained forty se'ah, and rested on vessels or on a person, purifies them. This Mishnah seems puzzling, as these waters are considered "zochlin." The Maharik, who already noticed this question, answered that since the wave comes from the sea, and the law follows Rabbi Yossi that a sea is considered like a mikvah, therefore the wave is considered like a spring that purifies while flowing. But the Tzemach Tzedek challenges this, asking how one can make such an argument after the wave has completely detached from the sea and rests on the ground. When it is not connected to the source of the spring, it is not considered a spring, as ruled in Masechet Mikvaot. Additionally, it is difficult to understand why the wave needs to be forty se'ah in size like a mikvah, which proves it is considered a mikvah, and how it would purify while flowing like a spring, when the law is always the opposite; it is easier to qualify a spring that does not have the required amount of water than a spring whose waters are flowing.

From here, the Tzemach Tzedek concludes that the disqualification of "zochlin" only applies when all the water is flowing and moving on its way, not just part of it. At the moment the wave falls on the person, not all of it is "flowing," but only part of it, while most of it is in a state of falling on the person or on the vessels and is not escaping to another place, and what happens a moment later does not concern us.

Additionally, we can also consider the opinion of the Rashba, who rules that the disqualification of "zochlin" only applies to a large and noticeable hole, but not to a small crack. He brings several compelling proofs. One of them is the Mishnah: "a mikvah that was found to be lacking, all purifications done in it are retroactively impure," implying that only when it is found to be less than forty se'ah are all purifications retroactively impure, but if there are forty se'ah, there is no concern that there might be a hole in the mikvah and therefore the water would be considered zochlin, despite the principle that in case of doubt regarding Torah law, we rule stringently.

Similarly, the Rebbe Tzemach Tzedek discusses a mikvah that has a hole in the bottom of its wall toward another mikvah, such that after several hours a large part of the mikvah water is destined to exit and enter the second mikvah. He questions whether all the water that is destined to exit should be considered "zochlin," and he determines that this depends on the aforementioned dispute among the Rishonim. According to Rabbeinu Shimshon, the mikvah would be invalid since the immersion also occurs in the invalid waters that are destined to exit, and according to the Rosh, as long as the waters have not exited - they are valid. But if most of the mikvah waters are destined to exit in such a way, the mikvah is invalid. Therefore, he recommends making a hole the size of "k'shfoferet hanod" (which has room for two fingers from each side of the hole) that will cause the two mikvahs to be considered as one, and consequently the flow between one and the other would not be considered "zochlin."