Rambam - 1 Chapter a Day

Whenever water passes over keilim that contain a receptacle or water falls into them, it is considered as drawn water and disqualifies a mikveh, provided that the receptacle was made to serve that purpose. Even containers that are not susceptible to ritual impurity, e.g., stone containers and containers made from earth, disqualify water.

Whenever a k'li was not made with the intent that it serve as a receptacle, even though it does serve as a receptacle, the water it contains does not disqualify a mikveh, for example, large pipes through which water flows. Even though they are wide in the middle and serve as receptacles, the water they contain does not disqualify a mikveh. This applies whether they were made of metal or of earthenware.

The water contained in a trough in a stone does not disqualify a mikveh, because the trough is not a k'li. If, however, one joins a k'li to a stone, water contained in it disqualifies a mikveh, even if it was joined with cement. If one made a hole from below or one as wide as the mouthpiece of a drinking pouch in the trough from the side, it is acceptable and water contained within it does not disqualify a mikveh.

When a person takes a large barrel or a large kneading trough and makes a hole large enough to purify it from susceptibility to impurity, and then permanently affixes it within the ground, making it into a mikveh, it is acceptable. Similarly, if one plugged the hole with lime and with building materials, this does not disqualify the barrel and the water collected within forms an acceptable mikveh. If one plugged it with lime or with gypsum, the water it contains makes a mikveh unacceptable, unless it was permanently affixed to the earth or made part of a building. If it was taken and placed on the surface of the earth or on lime and mud was smeared on its sides, it is acceptable to use as a mikveh.

The following laws apply when one places a tablet under a drainage pipe and water flows over it into a mikveh. If the tablet had borders on its sides, this water would disqualify the mikveh. If not, it does not disqualify it. If one stood the tablet upright on its point, at an angle under the drainage pipe to wash it, even though it has borders, it does not disqualify the water, because in this position, it was not intended to serve as a receptacle.

The following laws apply when one carves a place in a pipe for pebbles that are carried with the water to collect so that they will not descend together with the water. If it was a wooden pipe and he carved out even the slightest hollow, it disqualifies the water, because all of the water passes through a utensil that was made to serve as a receptacle. This applies even if one permanently affixed the pipe to the earth after he carved out the hollow. The rationale is that the pipe had already been considered as a k'li when it was unattached. If, by contrast, it was permanently affixed to the earth and then he carved out the receptacle, it does not disqualify the water. If the pipe was made from earthenware, it does not disqualify the water unless the hollow is large enough to contain a revi'it.

Even though the hollow in the pipe becomes filled with the pebbles that dribble into it, it remains a disqualifying factor. It is not considered as having been stopped up. If earth or pebbles descended into the hollow and stopped it up, because they were compressed there, the water is acceptable.

When either a sponge or a bucket that contains three lugim of drawn water falls into a mikveh, it does not disqualify it. For it was said only that three lugim of water that fall into it disqualify it, not a container into which drawn water had fallen.

When there is a closet or a chest in the sea, one may not immerse in them unless they have a hole the size of the mouthpiece of a drinking pouch. If there was a sack or a basket in the sea, one may immerse in them. Similarly, if one places a sack or a basket under a drainage pipe, the water that flows through them does not disqualify a mikveh.

When one immersed keilim over an impure base for a container that was placed inside a mikveh, even though the edge of the base extends above the water, the keilim are purified from their impurity. If, however, one lifts them up from the water into the inner space of the base, the water that is on the keilim contracts impurity because it is within the inner space of the base. The water in turn imparts impurity to the keilim.

Similarly, if a spring emerges from under an earthenware oven and a person descended and immersed in it, he is pure, but his hands contract impurity from the inner space of the oven unless the water extends above the oven for at least the height of his hands. Thus when he immersed, his hands will be above the oven. The difficulties arise, because earthenware keilim do not regain purity through immersion in a mikveh, as we explained.

When a barrel full of water falls into a sea, even into the Mediterranean Sea, one who immerses there is not considered to have immersed. The rationale is that it is impossible that there will not be three lugim of water from the barrel in one place. If a loaf of terumah falls there, it becomes impure. It contracts impurity due to contact with drawn water, for the water is standing there. If such a situation would take place in a river or the like, one would be able to immerse there, since it flows.

When there was a pool of drawn water next to a mikveh that contains less than 40 seah, even though it is touching the water of the mikveh, it does not disqualify it, because it is like a mikveh next to a mikveh. If the pool of the drawn water was in the middle of the mikveh, it disqualifies it.

The following rule applies when there are two pools of water, one above the other, they are separated by a wall, and the upper one is filled with acceptable water, but the lower one is filled with drawn water, and there is a hole in the wall between the upper pool and the lower one. If there are three lugim of drawn water opposite the hole, the upper pool is disqualified. The rationale is that it is considered as if the hole was in the center of the upper pool, not at its side.

How large must the hole be for there to be three lugim there? Everything depends on the quantity of water contained in the pool. If the lower pool contains 40 se'ah, the hole must be 1/320th of the pool. If the pool contains 20 se'ah, the hole must be 1/160th of the pool. Continue calculating according to this ratio for other amounts. A se'ah is six kabbin, a kab is four lugim, and a log is the size of six eggs.

The following laws apply when there are three mikveot, each containing exactly 20 se'ah, next to each other, and one [of those on the side contained drawn water. If three people descended and immersed themselves, causing all the water to rise and mix on the floor outside the mikveot, both the mikveot and the people who immersed themselves are pure. The rationale is that the entire amount totaled 60 se'ah, of which 40 se'ah of acceptable water came from two pools located next to each other. And drawn water does not disqualify a mikveh that contains 40 se'ah, as we explained.

If the pool containing drawn water was in the middle and they descended and immersed themselves, causing the water to rise and the mikveot thus to become joined, the status of the mikveot is the same as it was previously and those who immersed themselves are impure as they were previously. The rationale is that 40 se'ah of acceptable water did not mix together, because their pools were not located next to each other, for the pool of drawn water separates between them.