Reference for Mikvaot 3:2
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:
How so? If there is a pit in the courtyard, and three <i>log</i> [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 <i>mikveh</i> containing] forty <i>se'ah</i> 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 <i>mikveh</i>].