Commentary for Mikvaot 1:6
לְמַעְלָה מֵהֶן, מֵי תַמְצִיּוֹת שֶׁלֹּא פָסְקוּ. שָׁתָה טָמֵא וְשָׁתָה טָהוֹר, טָהוֹר. שָׁתָה טָמֵא וּמִלֵּא בִכְלִי טָהוֹר, טָהוֹר. שָׁתָה טָמֵא וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אַף עַל פִּי שֶׁהֵדִיחַ, טָהוֹר. מִלֵּא בִכְלִי טָמֵא וְשָׁתָה טָהוֹר, טָהוֹר. מִלֵּא בִכְלִי טָמֵא וּמִלֵּא בִכְלִי טָהוֹר, טָהוֹר. מִלֵּא בִכְלִי טָמֵא וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אַף עַל פִּי שֶׁהֵדִיחַ, טָהוֹר. נָפְלוּ מַיִם טְמֵאִים וְשָׁתָה טָהוֹר, טָהוֹר. נָפְלוּ מַיִם טְמֵאִים וּמִלֵּא בִכְלִי טָהוֹר, טָהוֹר. נָפְלוּ מַיִם טְמֵאִים וְנָפַל כִּכָּר שֶׁל תְּרוּמָה, אַף עַל פִּי שֶׁהֵדִיחַ, טָהוֹר. כְּשֵׁרִים לַתְּרוּמָה וְלִטֹּל מֵהֶם לַיָּדָיִם:
Above these [<i>mikvaot</i> described, and next in the hierarchy of <i>mikvaot</i>,] are runoff waters that have not stopped [flowing down from the mountains]. If an impure person drank [from them], and a pure person drank [afterwards], he is [still] pure. If an impure person drank, and someone filled up a pure vessel [from those waters afterwards], it is pure. If an impure person drank, and then a loaf of <i>terumah</i> fell in, even if one rinsed [the loaf], it is pure. If one filled an impure vessel [from the runoff waters], and a pure person drank [from it], he is pure. If one filled using an impure vessel, and then filled a pure vessel [from the first vessel], it is pure. If one filled an impure vessel, and then a loaf of <i>terumah</i> fell in [the runoff waters], even if one rinsed it, it is pure. If impure waters fell [into the runoff waters], and a pure person drank [from it], he is pure. If impure waters fell in, and one filled a pure vessel [from it], it is pure. If impure waters fell in, and then a loaf of <i>terumah</i> fell in, even if one rinsed it, it is pure. [And these waters are] valid for [use with] <i>terumah</i>, as well as for washing [literally: taking] one's hands with them.
Bartenura on Mishnah Mikvaot
English Explanation of Mishnah Mikvaot
If an unclean person drank of it and then a clean person drank of it, he is clean. If an unclean person drank of it and water was then drawn from it in a clean vessel, it is clean. If an unclean person drank of it and a loaf of terumah fell in, even if he washed his hand in it, it is clean.
If one drew water from it in an unclean vessel and then a clean person drank [out of the pool], he is clean. If one drew water from it in an unclean vessel and a loaf of terumah fell [into the pool], even if he washed his hands in it, it is clean.
If unclean water fell into it and a clean person drank of it, he is clean. If unclean water fell into it and one drew water from it in a clean vessel, it is clean. If unclean water fell into it and a loaf of terumah fell in, even if he washed his hands in it, it is clean.
[All such water] is valid for terumah and for the washing of the hands.
Today's mishnah is basically a repeat of mishnayot 1-3 except here we deal with rainwater that continues to flow into the pool. This rain will continuously nullify the water that has been defiled and put back into the pool. Thus, this water cannot become impure (although water taken out of it can be made impure).
The commentary to this mishnah would be the same as that found in mishnayot 1-3 so I have not repeated it from there. Section two is parallel to mishnah one, section three is parallel to mishnah two, and section four is parallel to mishnah three. Section five is similar to the end of yesterday's mishnah. It notes that this water is pure enough that it can be used in the preparation of terumah.