Mikvaos 2:4: Rabbi Eliezer says: A reviis of mayim sh’uvim that enters the basin, before the rainwater is added, invalidates the mikveh, but if the mikveh is partially filled with rainwater, then only 3 lugin invalidate it. And the Sages say: whether at the beginning (before the rainwater) or at the end (after the mikveh is partially filled), the amount is 3 lugin.
מקואות פ”ב מ”ד: רבי אליעזר אומר רביעית מים שאובין בתחלה פוסלין את המקוה. ושלשה לוגין על פני המים. וחכמים אומרים. בין בתחלה בין בסוף. שעורו שלשה לוגין.
Rabbi Yonasan Steif used to travel far and wide to supervise the building of mikvaos. Even at the end of his life, when he was 80 years old, with whatever strength he had left, he would climb down into the basin of each mikvah holding a rag in his hand, and he would go over all four walls and the floor of the basin to remove every last drop of water, as the Shulchan Aruch dictates. He didn’t send anyone else to do this job for him.
Source: Otzros Mahari Steif, p. 341
[This is talking about a mikvah created using the zeriah method, where the water needed changing. The immersion basin is emptied and refilled with kosher water from the bor zeriah. The old water is typically sucked out of the basin with a pump. Often a little water remains in the pump, thus becoming mayim sh’uvim, and drips back into the mikveh. If there are 3 lugin of this old water, the new mikveh will never be kosher.
Even if one would argue that there is never as much as 3 lugin dripping out of the pump, there is reason to follow the stringent opinion of Rabbi Eliezer, who holds that even one reviis of mayim sh’uvim invalidates a mikveh, if it precedes all the kosher water. Although the halacha does not usually follow Rabbi Eliezer, in this case the Mishnah Acharona suggested that the underlying dispute between Rabbi Eliezer and the Sages is over whether an entire mikveh of mayim sh’uvim is invalid Mid’oraisa. Rabbi Eliezer holds it is invalid; therefore, since a reviis of rainwater is a kosher mikveh Mid’oraisa and can be used to tovel a pin, as soon as a reviis of mayim sh’uvim is in the basin, the mikveh is invalid forever, no matter how much rainwater comes in afterwards. The Sages hold that an entire mikveh of mayim sh’uvim is invalid only Mid’rabanan, and therefore the problem doesn’t apply to a reviis, since a reviis is never a kosher mikveh Mid’rabanan. Since the Rema (201:3) rules that an entire mikveh of mayim sh’uvim is invalid Mid’oraisa, the Chelkas Yaakov (3:54) argues that we should follow Rabbi Eliezer.
It’s likely that all the scattered droplets of water around the mikveh add up to a reviis; therefore it’s necessary to wipe down the basin thoroughly before refilling it from the bor zeriah.]
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