Mishnah Mikvaos 1:3: If impure waters fell [into such a pool], and a pure person drank [from it], he is impure. If impure waters fell in, and one filled a pure vessel [from it], it is impure. If impure waters fell in, and then loaf of terumah fell in, if one rinsed [the loaf], it is impure; and if one did not rinse it, it is pure. Rabbi Shimon says: whether or not once rinsed it, it is impure.

נָפְלוּ מַיִם טְמֵאִים וְשָׁתָה טָהוֹר, טָמֵא. נָפְלוּ מַיִם טְמֵאִים וּמִלֵּא בִכְלִי טָהוֹר, טָמֵא. נָפְלוּ מַיִם טְמֵאִים וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אִם הֵדִיחַ, טָמֵא. וְאִם לֹא הֵדִיחַ, טָהוֹר. רַבִּי שִׁמְעוֹן אוֹמֵר, בֵּין שֶׁהֵדִיחַ בֵּין שֶׁלֹּא הֵדִיחַ, טָמֵא:

Mishnah Mikvaos 1:4: If a dead person fell into them [the waters of such a pool], or if someone impure walked through them, if a pure person then drank [from them], he is [still] pure. As with the waters of shallow pools, [similarly] the waters of pits, the waters of trenches, the waters of caves, runoff waters that stopped [flowing from the mountains], and mikvaot that do not contain forty se'ah [a specific unit of volume], they are all pure during the rain. Once the rain stops, those [waters] close to cities and to the roads are presumed are [presumed to be] impure; and those far away are [presumed to be] pure, but only until [the rain damage of the nearby paths is repaired such that] most people walk by them.

נָפַל לְתוֹכָן מֵת, אוֹ שֶׁהָלַךְ בָּהֶן הַטָּמֵא, וְשָׁתָה טָהוֹר, טָהוֹר. אֶחָד מֵי גְבָאִים, מֵי בוֹרוֹת, מֵי שִׁיחִים, מֵי מְעָרוֹת, מֵי תַמְצִיּוֹת שֶׁפָּסְקוּ, וּמִקְוָאוֹת שֶׁאֵין בָּהֶם אַרְבָּעִים סְאָה, בִּשְׁעַת הַגְּשָׁמִים הַכֹּל טָהוֹר. פָּסְקוּ הַגְּשָׁמִים, הַקְּרוֹבִים לָעִיר וְלַדֶּרֶךְ, טְמֵאִים. וְהָרְחוֹקִים, טְהוֹרִין, עַד שֶׁיְּהַלְּכוּ רֹב בְּנֵי אָדָם: