Rambam - 1 Chapter a Day

When three lugim of drawn water fall into a mikveh - whether from one k'li or from two or three keilim - they can be combined to reach a quantity that disqualifies the mikveh, provided the water begins descending from the second before it concludes descending from the first. If descends from four keilim, the water from them is not combined.

When does the above apply? When one did not intend to increase the amount of water in the mikveh. If, however, one intended to increase the amount of water in the mikveh, even if a dinar-size measure was added each year, they are all combined to reach the sum of three lugim, whether the drawn water was present there before the acceptable water, the acceptable water was present there before the drawn water, or they both fell into the mikveh at the same time. Since three lugim of water fell into 40 se'ah combining with the acceptable water to reach that amount or into less than 40 se'ah of water, the entire amount is invalidated and considered as drawn.

When two people each poured a log and a half into a mikveh, or one wrung out his garment and lifted it up, causing the water it contained to fall from several places, it invalidates a mikveh. A similar ruling applies when one pours from a distributor that causes water to pour from several places at the same time.

When one immerses a pillow or a cushion of leather into a mikveh that has exactly 40 se'ah, when he lifts their edges out of the water, the water inside of them is considered as drawn water.

What should he do? He should immerse them and lift them up by their ends. With regard to a basket and a sack, he should immerse them and lift them up in the ordinary manner without showing any concern.

The following law applies to a mikveh that had three pockets of drawn water with a log in each of the pockets and then acceptable water fell into it. If it is known that 40 se'ah of acceptable water fell into it before the water reached the third pocket, it is acceptable, If not, it is disqualified.

When there are two mikveot, neither containing 40 se'ah, a log and a half fell into each one of them, and then the mikveot became mixed together, they are acceptable. The rationale is that neither one of them had been designated as unacceptable.

If, by contrast, three lugim of drawn water fell into a mikveh that does not contain 40 se'ah [of acceptable water and afterwards, it was divided into two, even though enough acceptable water was added to each one to constitute an acceptable mikveh, they are invalid. The rationale is that whenever a mikveh is disqualified, all of its contents are considered as drawn water. It is as if all of the water had been drawn with a container.

When a cistern is filled with drawn water and a canal of rainwater flows into it and out of it, it is still considered as unacceptable until it can be calculated that not even three lugim of the drawn water that originally was in the cistern remain.

When three lugim of unacceptable water fall into a mikveh containing less than 40 se'ah of acceptable water, all of its contents are disqualified. Even if afterwards, he added enough acceptable water until the measure of 40 se'ah is reached, the mikveh remains invalid until all the water that was contained within it flows out and less than three lugim of the drawn water remain.

What is implied? A mikveh contains 20 se'ah of rainwater and a se'ah of drawn water fell into it. Afterwards, more acceptable water was added to it. It remains unacceptable until one knows that the 20 se'ah it originally contained and more than five and a quarter kabbin of the added water flowed out and less than three lugim of the entire quantity remain. Similarly, if one made a mikveh that contains 40 se'ah of acceptable water and joined it to this invalid mikveh, the acceptable water purifies the unacceptable water.

If one was moving mud from the bottom of the mikveh to the sides and, as a result, three lugim of water flowed into the mikveh, it remains acceptable. If one was removing the mud and lifted it up by hand, separating it from the mikveh and placing it on the mikveh's sides and three lugim flowed into the mikveh from it, they disqualify it.

When a legion is passing from one place to another - or similarly, an animal is passing from one place to another - and three lugim of water was splashed into a mikveh by their hands and feet, it is acceptable. Moreover, even if they made a mikveh in this manner initially, it is acceptable.

When a mikveh does not contain 40 se'ah and less than three lugim of impure, drawn water fell into it, the water is acceptable with regard to challah and terumah and one may use it for the ritual washing of hands. It is, however, invalid to be used as the base for the collection of water for an acceptable mikveh. If rainwater descended upon it to the extent that the rainwater constituted the majority of the mixture, the mixture is acceptable to be used as the base for the collection of water for an acceptable mikveh.

When three lugim of impure, drawn water fell into it, the water is unacceptable for challah and terumah. One may not use it for the ritual washing of hands, nor may it be used as the base for the collection of water for an acceptable mikveh. If rainwater descended upon it to the extent that the rainwater constituted the majority of the mixture, the mixture is acceptable with regard to challah and terumah and one may use it for the ritual washing of hands. It is, however, invalid to be used as the base for the collection of water for an acceptable mikveh until all of the original water that became considered as drawn flowed out and less than three lugim of it remained.

Similarly, if there was a mikveh that contained only a dinar-size measure less than 40 se'ah and three lugim of impure, drawn water fell into it, the water is unacceptable for challah and terumah. One may not use it for the ritual washing of hands, nor may it be used as the base for the collection of water for an acceptable mikveh. If less than three lugim of water fell into it - even if the water was all impure - and then a dinar-size measure of rainwater fell into it, causing it to comprise a complete measure of 40 se'ah, it is acceptable. Just as it is considered as pure with regard to immersion, it is considered pure in every respect.