Mikvaot, Chapter Two, Mishnah Four
1) Rabbi Eliezer says: a quarter-log of drawn water in the beginning makes the mikveh invalid, and three logs on the surface of the water.
2) But the sages say: both in the beginning and at the end, the measure [which makes the mikveh invalid] is three logs.
Section one: According to Rabbi Eliezer there is a difference a case when drawn water falls into the mikveh before water is in the hole in the ground and after valid water has already been put in. If the drawn water falls in first, then all that has to fall in is a quarter of a log, which is a very small amount (about 125 grams). If the drawn water falls in at the end, on the surface of water that is already there, then there must be three logs to disqualify the mikveh.
Section two: The other sages rule more leniently. In either case there must be three logs to disqualify the mikveh.