Mikvaot, Chapter Two, Mishnah Six
1) If one scraped mud to the sides and then three logs [of water] were drawn out from it [from the mud], [the mikveh is still] valid.
2) But if removed the mud away and three logs were drawn from it [into the mikveh] it becomes invalid.
3) Rabbi Shimon pronounces it valid, since there was no intention to draw [the water].
Section one: If one scraped some mud out of the bottom of a mikveh that doesn’t have forty seahs of water in it, and then three logs of water dripped back into the mikveh from the scraped mud, the mikveh is still valid. The water that comes out of the mud is not considered to be “drawn water” such that it invalidates the mikveh.
Section two: In this case he removed the mud from the mikveh and didn’t just scrape it away to the sides. If the water again seeps out of the mud and goes back into the mikveh it does disqualify it. Such water is considered to be “drawn water.”
Section three: Water that one doesn’t intend to draw out from somewhere doesn’t count as drawn water in order to disqualify a mikveh. Rabbi Shimon considers the water that comes out of the mud to be unintended because his intention was to draw out the mud, not the water. However, the first opinion in the mishnah considers this water to be drawn because there was no way for him to draw out the mud without also drawing out the water.