halacha - Mikvah with three additional chambers

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?

Answer

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