Requirements of A Mikveh
The traditional rules regarding the construction of a mikveh are based on those specified in classical rabbinical literature. According to these rules, a mikveh must be connected to a natural spring or well of naturally occurring water, and thus can be supplied by rivers and lakes which have natural springs as their source. A cistern filled by the rain is also permitted to act as a mikveh's water supply. Similarly snow, ice and hail are allowed to act as the supply of water to a mikveh, as long as it melts in a certain manner. A river that dries up on a regular basis cannot be used because it is presumed to be mainly rainwater, which cannot purify while flowing. Oceans for the most part have the status of natural springs.
A mikveh must, according to the classical regulations, contain enough water to cover the entire body of an average-sized person; based on a mikveh with the dimensions of 3 cubits long, 1 cubit wide, and 1 cubit deep, the necessary volume of water was estimated as being 40 seah of water. The exact volume referred to by a seah is debated, and classical rabbinical literature specifies only that it is enough to fit 144 eggs; most Orthodox Jews use the stringent ruling of the Avrohom Yeshaya Karelitz, according to which one seah is 14.3 litres, and therefore a mikveh must contain approximately 575 litres. This volume of water could be topped up with water from any source, but if there were less than 40 seahs of water in the mikveh, then the addition of 3 or more pints of water from an unnatural source would render the mikveh unfit for use, regardless of whether water from a natural source was then added to make up 40 seahs from a natural source; a mikveh rendered unfit for use in this way would need to be completely drained away and refilled from scratch.
There are also classical requirements for the manner in which the water can be stored and transported to the pool; the water must flow naturally to the mikveh from the source, which essentially means that it must be supplied by gravity or a natural pressure gradient, and the water cannot be pumped there by hand or carried. It was also forbidden for the water to pass through any vessel which could hold water within it (however pipes open to the air at both ends are fine) As a result, tap water could not be used as the primary water source for a mikveh, although it can be used to top the water up to a suitable level. To avoid issues with these rules in large cities, various methods are employed to establish a valid mikveh. One is that tap water is made to flow over the top of a kosher mikveh, and through a conduit into a larger pool. A second method is to create a mikveh in a deep pool, place a floor with holes over that and then fill the upper pool with tap water. In this way, the person dipping is actually "in" the pool of rain water.
Most contemporary mikvehs are indoor constructions, involving rain water collected from a cistern, and passed through a duct by gravity into an ordinary bathing pool; the mikveh can be heated, taking into account certain rules, often resulting in an environment not unlike a spa.
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