Mishnah Mikvaos 4:3: If one carves out [furrows] in a [drain] pipe to collect stones, [a furrow] of any size in one of wood, and of a revi'it [a specific measure of volume, a quarter of a log] in one of earthenware [is sufficient for the pipe to be considered a vessel, and thus for its waters to render a mikveh invalid as drawn waters]. Rabbi Yose says: even for one of earthenware, [a furrow] of any size [is sufficient to invalidate waters flowing through a pipe]; they [the Sages] only said [that the minimum measure is] a revi'it regarding shards of earthenware vessels [that they are considered vessels as long as they can still contain that amount]. If there were stones rolling around inside it [the furrow], they [its waters] render a mikveh invalid. If dust descended into it and was compressed, it is valid. A duct which is narrow on either end and widens in the middle [built that way in order to increase water pressure] does not render [its waters] invalid, since it was not made to be a receptacle.

הַחוֹטֵט בְּצִנּוֹר לְקַבֵּל צְרוֹרוֹת, בְּשֶׁל עֵץ, כָּל שֶׁהוּא. וּבְשֶׁל חֶרֶס, רְבִיעִית. רַבִּי יוֹסֵי אוֹמֵר, אַף בְּשֶׁל חֶרֶס, כָּל שֶׁהוּא. לֹא אָמְרוּ רְבִיעִית אֶלָּא בְשִׁבְרֵי כְלֵי חֶרֶס. הָיוּ צְרוֹרוֹת מִתְחַלְחֲלִים בְּתוֹכוֹ, פּוֹסְלִים אֶת הַמִּקְוֶה. יָרַד לְתוֹכוֹ עָפָר וְנִכְבַּשׁ, כָּשֵׁר. סִלּוֹן שֶׁהוּא צַר מִכָּאן וּמִכָּאן וְרָחָב מִן הָאֶמְצַע, אֵינוֹ פוֹסֵל, מִפְּנֵי שֶׁלֹּא נַעֲשָׂה לְקַבָּלָה:

Mishnah Mikvaos 4:4: Drawn water and rainwater that mixed in a courtyard, or in a hole, or on the upper steps of a cavern [and from there flowed into a mikveh], if the majority [of the water in the mikveh] is from the valid [waters], it is valid; if the majority is from the invalid, it is invalid; if it is half and half, it is invalid. When is this the case? At a time when they mix before they arrive at the mikveh. If they are flowing into the waters [of the mikveh directly, rather than first mixing together], if it is known that forty se'ah [a specific unit of volume] of valid water fell into it before three log [a specific measure of volume] of drawn water, it is valid; and if not, it is invalid.

מַיִם שְׁאוּבִין וּמֵי גְשָׁמִים שֶׁנִּתְעָרְבוּ בֶחָצֵר, וּבָעֻקָּה, וְעַל מַעֲלוֹת הַמְּעָרָה, אִם רֹב מִן הַכָּשֵׁר, כָּשֵׁר. וְאִם רֹב מִן הַפָּסוּל, פָּסוּל. מֶחֱצָה לְמֶחֱצָה, פָּסוּל. אֵימָתַי, בִּזְמַן שֶׁמִּתְעָרְבִין עַד שֶׁלֹּא יַגִּיעוּ לַמִּקְוֶה. הָיוּ מְקַלְּחִין בְּתוֹךְ הַמַּיִם, אִם יָדוּעַ שֶׁנָּפַל לְתוֹכוֹ אַרְבָּעִים סְאָה מַיִם כְּשֵׁרִים, עַד שֶׁלֹּא יָרַד לְתוֹכוֹ שְׁלשָׁה לֻגִּין מַיִם שְׁאוּבִין, כָּשֵׁר. וְאִם לָאו, פָּסוּל: