Mishnah Mikvaos 3:1: Rabbi Yose says: Two mikvaot [gatherings of water collected by natural means and immersed in for purification] that do not [each] contain forty se'ah [a specific measure of volume], and which a log [a specific measure of volume] and a half [of drawn waters] fell into this one, and a log and half into that one, and they [the two mikvaot] 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 mikveh which does not contain forty se'ah, into which fell three log [of drawn waters], and was then split into two, it is invalid, since it was given a title of invalidity [since three log of drawn waters invalidate a mikveh containing fewer than forty se'ah of valid waters]. And Rabbi Yehoshua considers it valid, for Rabbi Yehoshua would say: Any mikveh which does not contain forty se'ah [of valid waters], if three log [of drawn waters] fell into it, and it [then] became lacking even a kortov [A minute volume measure], it is valid [when enough valid water is added], since it is lacking [part of the] three log. 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].

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:

Mishnah Mikvaos 3:2: How so? If there is a pit in the courtyard, and three log [of drawn waters] fell into it, it is forever in its invalid state until its fullness, plus some more, depart from it, or until one erects [another mikveh containing] forty se'ah in the courtyard [and makes an opening to connect between them], and the upper [waters] will be purified by the lower. Rabbi Elazar ben Azariah considers it invalid, except if one plugs up [all the sides of the pit containing the invalid waters, such that it is open only to the valid mikveh].

כֵּיצַד. הַבּוֹר שֶׁבְּחָצֵר וְנָפְלוּ לוֹ שְׁלשָׁה לֻגִּין, לְעוֹלָם הוּא בִפְסוּלוֹ, עַד שֶׁיֵּצֵא מִמֶּנּוּ מְלוֹאוֹ וָעוֹד, אוֹ עַד שֶׁיַּעֲמִיד בֶּחָצֵר אַרְבָּעִים סְאָה וְיִטְהֲרוּ הָעֶלְיוֹנִים מִן הַתַּחְתּוֹנִים. רַבִּי אֶלְעָזָר בֶּן עֲזַרְיָה פּוֹסֵל, אֶלָּא אִם כֵּן פָּקָק: