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I was looking at documentation of a mikvah being constructed, and it confused me. In addition to the immersion pool, this mikvah had three additional chambers. From what I have gathered, they appear to be 1) a bor hashakah, 2) a bor zeriah, and 3) a bor tachton.

If I understand correctly, the purpose of each of these chambers is the same: to provide a way for the tap water to be considered kosher for the mikvah. What I don't understand is why three different kinds are present in one mikvah.

How common is it for a modern mikvah to have all three chambers as this one did? What is the most common scenario? Is it necessary to have all three, or what is the benefit in doing so?

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It's a three-way Chumra in case the other two methods don't work. –  Double AA Oct 21 '13 at 2:32
    
That's what I figured. Again, I'm curious if that's the norm or if this was just a "mehadrin min hamehadrin" mikvah. I'd also like to know what the problems are that make each one possibly insufficient, but perhaps that's beyond the scope of this forum. –  Aaron Oct 21 '13 at 2:37
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You expect all Jews to agree about something? –  Double AA Oct 21 '13 at 2:48
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I suppose not, but I am interested in the reasoning on all sides. –  Aaron Oct 21 '13 at 2:57
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1 Answer

up vote 4 down vote accepted

In short, there are two ways to make "drawn water" (as in tap water) kosher.

  1. Hashaka - If tap water touches "kosher" water, it becomes kosher. This is a classic "side by side" construction.
  2. Zeriya - If one "plants" "non-kosher" water into "kosher" water, it becomes kosher.

The problem with these two methods is that there are some opinions which say that the kosher water doesn't stay kosher forever (it doesn't become kosher, it just is considered attached to the main bucket). Therefore, if the "kosher" water looses it's status, the mikva becomes Pasul.

Since the water is side by side, the water mixes, causing sooner or later the rain-water reservoir to loose its rain water. According to some, the Mikva then becomes pasul.

The advantage of the "Bor Al Gabey Bor" is that since the bottom water (the rainwater reservoir) is cold (there is no heater there) and the top water is hot, the waters don't mix. In such a setup, the Mikva stays kosher for much longer.

Some don't approve of this construction, however, since there was one opinion (The Divrei Chaim of Zanz) who said that one cannot connect a not-kosher mikva to a kosher one through a "pipe" - in other words - any change in elevation. To allow people who worry about this Chumra, they build the other two "boros".

For more information, see http://chabadlibrary.org/books/chasidim/mkvh/7.htm

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Does the temperature difference really prevent any mixing? –  Double AA Oct 21 '13 at 3:07
    
@DoubleAA There's the famous Igros Moshe who says that Halachically, all two liquids touching are considered mixed. However, there is a Teshuvos VeHanhagos who says that there was an experiment and it showed that it doesn't, and he answered that it doesn't matter. –  Shmuel Brin Oct 21 '13 at 3:10
    
I think he said that the experiment involved colored water in the bottom bor. He answered that maybe the ink doesn't mix but the water does. –  Shmuel Brin Oct 21 '13 at 3:11
    
How else are we supposed to determine how much the water mixes (whether to a bor batzad or a bor tachton) except via experimentation? –  Double AA Oct 21 '13 at 3:13
    
@DoubleAA IM derives it from the Gemara. –  Shmuel Brin Oct 21 '13 at 3:20
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