Mikvaos 2:3-4

Mikvaos 2:3

Doubtful situations involving drawn water that the Sages ruled clean include: if there’s a doubt about whether or not it fell (into a mikvah); if it did fall in, there’s a doubt as to whether or not the mikvah contained 40 seah; if there were two mikvahs, only one of which contained 40 seah and drawn water fell into one of them but we don’t know which. In all of these of these cases, such doubts are ruled clean because there’s a basis on which we may rely. If both mikvahs contained less than 40 seah and drawn water fell into one of them but we don’t know which, the doubt is ruled ritually unclean because there’s no basis on which to rely.

Mikvaos 2:4

Rabbi Eliezer says that a reviis (about three ounces) of drawn water at the beginning (i.e., when the mikvah is still empty) renders a mikvah unfit, as does three log (about 42 ounces) on the surface of the water (i.e., after the mikvah has been filled). The Sages say that the volume is three log whether at the beginning or at the end.