Reference for Mikvaot 3:1
רַבִּי יוֹסֵי אוֹמֵר, שְׁנֵי מִקְוָאוֹת שֶׁאֵין בָּהֶם אַרְבָּעִים סְאָה, וְנָפְלוּ לָזֶה לֹג וּמֶחֱצָה וְלָזֶה לֹג וּמֶחֱצָה וְנִתְעָרְבוּ, כְּשֵׁרִים, מִפְּנֵי שֶׁלֹּא נִקְרָא עֲלֵיהֶן שֵׁם פְּסוּל. אֲבָל מִקְוֶה שֶׁאֵין בּוֹ אַרְבָּעִים סְאָה וְנָפְלוּ בוֹ שְׁלשָׁה לֻגִּין וְנֶחֱלַק לִשְׁנַיִם, פָּסוּל, מִפְּנֵי שֶׁנִּקְרָא עָלָיו שֵׁם פְּסוּל. וְרַבִּי יְהוֹשֻׁעַ מַכְשִׁיר, שֶׁהָיָה רַבִּי יְהוֹשֻׁעַ אוֹמֵר, כָּל מִקְוֶה שֶׁאֵין בּוֹ אַרְבָּעִים סְאָה וְנָפְלוּ לוֹ שְׁלשָׁה לֻגִּין וְחָסַר אֲפִלּוּ קֹרְטוֹב, כָּשֵׁר, מִפְּנֵי שֶׁחָסְרוּ לוֹ שְׁלשָׁה לֻגִּין. וַחֲכָמִים אוֹמְרִים, לְעוֹלָם הוּא בִפְסוּלוֹ, עַד שֶׁיֵּצֵא מִמֶּנּוּ מְלוֹאוֹ וָעוֹד:
Rabbi Yose says: Two <i>mikvaot</i> [gatherings of water collected by natural means and immersed in for purification] that do not [each] contain forty <i>se'ah</i> [a specific measure of volume], and which a <i>log</i> [a specific measure of volume] and a half [of drawn waters] fell into this one, and a <i>log</i> and half into that one, and they [the two <i>mikvaot</i>] became mixed together, they are valid, since they were never given a title of invalidity [i.e. independently, each was considered valid by the law]. But a <i>mikveh</i> which does not contain forty <i>se'ah</i>, into which fell three <i>log</i> [of drawn waters], and was then split into two, it is invalid, since it was given a title of invalidity [since three <i>log</i> of drawn waters invalidate a <i>mikveh</i> containing fewer than forty <i>se'ah</i> of valid waters]. And Rabbi Yehoshua considers it valid, for Rabbi Yehoshua would say: Any <i>mikveh</i> which does not contain forty <i>se'ah</i> [of valid waters], if three <i>log</i> [of drawn waters] fell into it, and it [then] became lacking even a <i>kortov</i> [A minute volume measure], it is valid [when enough valid water is added], since it is lacking [part of the] three <i>log</i>. And the Sages say: it is forever in its invalid state, until its fullness plus some more depart from it [i.e. it would need to become lacking the amount of water it contained before the drawn waters were added, plus a little more, in order to be considered valid].