Mikvaos, Chapter 2, Mishnah 4. Our Sages ruled that mayim she'uvim, water that has been collected in a vessel, ruins a mikveh while it is still being formed. Should three lugin of drawn water fall in while the pit has yet to fill with its required forty se'ah from the rain, the mikveh cannot be used. The question raised here is whether three lugin is the measure in every case, or whether a smaller amount suffices when the drawn water lands in the pit at the outset, while it is entirely empty of rain water, as opposed to landing there once some rain water has already begun to gather, though the forty se'ah are not yet complete.
"Rebbi Eliezer omeir: revi'is mayim she'uvim batchilah poslin es hamikveh." Rebbi Eliezer holds that when the pit is empty and contains no rainwater whatsoever, the disqualifying measure is far smaller than three lugin. A mere revi'is of drawn water, a quarter of a log, falling into that empty pit before any rainwater has been collected, is enough to ruin the water there.
"U'shloshah lugin al pnei hamayim," the familiar measure of three lugin, according to Rebbi Eliezer, applies only when the drawn water falls upon water that is already there. That is, once kosher rainwater has begun to accumulate in the pit, though it has not yet reached forty se'ah, only three lugin of drawn water create a problem. But into a completely dry pit, at the very start, even a revi'is is enough to disqualify.
"Vachachomim omrim: bein batchilah bein basof, shiuran shloshah lugin." The chachomim disagree and hold that the measure is uniform. Whether the drawn water enters at the outset, before any rainwater has arrived, or later, once some rainwater has already gathered in the pit, the measure is the same in both situations: only three lugin of drawn water disqualify the mikveh.
In practice today, before a mikveh is filled, care is taken that the pit be completely dry, with not even the smallest trace of drawn water present in it, so that it will be filled with nothing but rainwater until it has reached its forty se'ah.