Mishnah Mikvaos 2:3: Cases of uncertainty regarding drawn waters are considered pure by the Sages. [For example,] if it is uncertain whether they [the drawn waters] fell or did not fall [into a mikveh]; even if they did fall, if there is an uncertainty as to whether they [the waters of the mikveh] contained forty se'ah or did not; or if there were two mikva'ot, one mikveh containing forty se'ah and one not, and [drawn waters] fell into one of them and one does not know which it fell into, [in all these cases] its uncertainty is pure, because one has something upon which to hang [a presumption of purity]. If both [mikvaot] contained fewer than forty se'ah, and [drawn waters] fell into one of them and one does not know which it fell into, its uncertainty is impure [i.e. such a case is uncertain and resolved as impure], since one has nothing upon which to hang [a presumption of purity].

סְפֵק מַיִם שְׁאוּבִין שֶׁטִּהֲרוּ חֲכָמִים, סָפֵק נָפְלוּ, סָפֵק לֹא נָפְלוּ, אֲפִלּוּ נָפְלוּ, סָפֵק יֶשׁ בָּהֶם אַרְבָּעִים סְאָה סָפֵק אֵין בָּהֶם, שְׁנֵי מִקְוָאוֹת, אֶחָד יֶשׁ בּוֹ אַרְבָּעִים סְאָה וְאֶחָד אֵין בּוֹ, נָפַל לְאַחַד מֵהֶן וְאֵינוֹ יוֹדֵעַ לְאֵיזֶה מֵהֶן נָפַל, סְפֵקוֹ טָהוֹר, מִפְּנֵי שֶׁיֶּשׁ לוֹ בַמֶּה יִתְלֶה. הָיוּ שְׁנֵיהֶם פְּחוּתִים מֵאַרְבָּעִים סְאָה, וְנָפַל לְאַחַד מֵהֶם וְאֵינוֹ יוֹדֵעַ לְאֵיזֶה מֵהֶן נָפַל, סְפֵקוֹ טָמֵא, מִפְּנֵי שֶׁאֵין לוֹ בַמֶּה יִתְלֶה:

Mishnah Mikvaos 2:4: Rabbi Eliezer says: A revi'it [a specific measure of volume, a quarter of a log] of drawn waters render a mikveh invalid at the outset [i.e. if the waters were in the pit before the valid mikveh waters were added, as do as three log [of drawn waters which were added] on the surface of the [valid] waters. And the Sages say: whether at the outset [before there were any waters in the pit] or at the end [after valid waters amounting to less than forty se'ah were added], the measure [for the minimum amount of drawn waters to invalidate a mikveh] is three log.

רַבִּי אֱלִיעֶזֶר אוֹמֵר, רְבִיעִית מַיִם שְׁאוּבִין בַּתְּחִלָּה, פּוֹסְלִין אֶת הַמִּקְוֶה, וּשְׁלשָׁה לֻגִּין עַל פְּנֵי הַמַּיִם. וַחֲכָמִים אוֹמְרִים, בֵּין בַּתְּחִלָּה בֵּין בַּסּוֹף, שִׁעוּרוֹ שְׁלשָׁה לֻגִּין: