The most practical solution is to build a mikvah with two (or more) boros. One bor becomes the bor hatvilah (immersion pool), while the other contains natural rainwater. This technique enables us to clean the mikvah water without having to wait for rainfall.
There are three ways to build the bor that holds the natural
rainwater: Hashoko, Zriah and Bor al gabai bor.
Translation of zriah is "sowed", like grain, "sown" into the
ground. Forty se'ah of rainwater is gathered in a bor, to which
tap water is added.The tap water intermingles with the rainwater as one body
of water, and attains the qualification of the original rainwater.The water
then overflows through an outlet (hole) into the mikvah stopping at the
Hence, the tap waters are "sown", making them valid for immersion - equal to rainwater.The advantage of the zriah method is that the ordinary tap water becomes kosher immediately as it enters the bor and makes contact with the rainwater. Unlike the hashoko method, the zriah technique does not have to wait for a minimum level and for the waters to meet in order to be considered a united body of water, nor is there a stopper. Once the bor zriah is filled with rainwater the mikvah is valid and almost foolproof. The Rabbi supervising the mikvah has no reason to worry whether the mikvah waters are connected to the bor or if the attendant remembered to remove the stopper. Another advantage: The water in the bor zriah changes often and remains fresh. A typical mikvah using the zriah method has two adjoining boros with a hole connecting them at the top. The hole must be higher than the forty se'ah of rainwater in the bor zriah. In the bor hatvilah, it has to be higher than the water level (approx. 140 cm. (56 inches) from the floor). The waters do not mingle during immersion.
1. Begin by filling the bor zriah with a minimum of forty se'ah of rainwater.
2. Add tap water into the rainwater bor, preferably in a way that the waters enter at a level lower than the connecting hole (the hamshocho process is nonetheless above water level).
3. When the water level is high enough, the water will pour through the hole into the bor hatvilah.
4. The waters in the bor hatvilah are valid when the bor contains at least forty se'ah and the water level is high enough for immersing.
How can one determine whether a mikvah uses the zriah method? If upon investigation it is found that the city water flows into the rainwater bor and then into the bor hatvilah - it is identified as a bor zriah.Though more reliable than the hashoko method, the bor zriah is still prone to problems. It is common practice to drain the mikvah by using an electric pump that does not completely drain all the water. Occasionally, when the pump is turned off, some water may return into the mikvah from the pump, sheuvim (lit. "drawn waters", waters that are invalid for a mikvah via containment in a vessel).
Additionally, the mikvah maintenance attendant may sometimes choose to remove the remaining traces of water with a sponge or bucket. Water from the sponge or mop will drip back into the mikvah, sheuvim, as a matter of course.
Then also, the mikvah may be washed with water from a bucket (sheuvim) and some water may be left in the mikvah. In these cases, the remaining water is considered sheuvim - with qualified kosher waters from the bor zriah, the three initial luggin (a liquid measure used in the times of the Mishna) of remaining water - whether from the pump, mop or pail - preceded the "valid" water and invalidates the mikvah.
As explained in previous sections, a mikvah usually contains two boros - one filled with rainwater, the other with tap water. Generally, there are two ways the boros can be constructed:
1) Two boros are constructed side by side with a common wall. On
the common wall a hole is placed higher than the height of forty
se'ah. One bor is filled with forty se'ah of
rainwater. The other bor is filled with tap water until the waters make
contact and intermingle with each other, rendering the water in the bor
hatvilah kosher for immersing.
2) a mikvah be built bor al gabai bor, literally, "one bor on top of another bor". This mikvah is constructed in the following manner:
a) A single deep bor is built during construction.
b) A divider of cement is built, forming an upper bor and a lower bor. The walls below the divider can project into the bor to support the divider, or a keyway can be formed in the wall and the divider cast into it. The divider serves as a floor for the upper bor and a ceiling for the lower bor.
c) The upper bor becomes the bor hatvilah. Steps are built for the user to descent and immerse.
d) The lower bor contains [two times] forty se'ah.
e) An opening is left in the divider large enough for a person to pass through; then it is closed with a cover panel. There are two holes, each measuring a square tefach (handbreadth).
The holes unite both bodies of water; tap water in the upper bor unites with the lower rainwater, giving the ordinary water the needed validation for immersion.
Why two holes? Some suggest that
there be a second hole in case one of them becomes blocked. This can occur when
the person immersing places a foot in the hole, preventing the waters from
remaining united. Building a second hole guarantees constant contact between
the boros. The holes are therefore built apart from each other, to
ensure that the feet of the person immersing cannot block both holes
simultaneously.There are no holes or drains in the lower bor where
the rainwater is deposited.How can one determine whether a mikvah
uses the bor al gabai bor method? If upon investigation it is found
that there are two holes in the floor of the mikvah, leading to the
bottom bor, it is identified as having been built bor al gabai
bor.This mikvah is constructed preferably with the tap water
flowing directly above one of the holes. When filling the mikvah,
rainwater pours into the lower bor until it is full. Ordinary water is
then added through the hamshocho (a procedure where waters run across
an area of soft ground or cement) method into the upper bor hatvilah
directly above either hole, as in the zriah method.The procedure is
different if the tap water does not flow directly over the holes. First, fill
the lower bor with rainwater until it overflows and covers the floor of
the upper bor. Then, add ordinary water into the upper bor.
Since the rainwater preceded the tap water, this is also a form of
zriah. Now the mikvah is valid and ready to be used. When
the waters of the upper bor become dirty one need only to pump out the
upper bor and refill it.
The advantages of this unique method are
Unlike the side bor hashoko technique, there is no need to wait for the waters in both boros to come into contact with each other nor is there any need for a stopper. Once the lower bor is filled with rainwater the mikvah will be valid and foolproof. The Rabbi supervising the mikvah is secure in the knowledge that the mikvah is valid without worrying whether the waters met at the hole or whether the attendant may have forgotten to remove the stopper.
Similar to the zriah method, where tap water flows directly into the rainwater and is considered "sowed into the ground", this is also considered zriah in its best possible form.
One additional, but very significant advantage is the fact
that the upper section is a continuation of the rainwater bor. The
square tefach hole(s) dismiss, Halachically, the presence of
the divider. Therefore, when one is immersing in such a mikvah, they
are in actuality, immersing in the rainwater bor itself.
Therefore, if one has a mikvah constructed with the method of bor al gabai bor, there is no need for additional side hashoko or zriah boros.