Commentary for Eduyot 7:3
הֵעִיד רַבִּי צָדוֹק עַל זוֹחֲלִין שֶׁרַבּוּ עַל הַנּוֹטְפִים, שֶׁהֵם כְּשֵׁרִים. מַעֲשֶׂה הָיָה בְּבִירַת הַפִּלְיָא, וּבָא מַעֲשֶׂה לִפְנֵי חֲכָמִים וְהִכְשִׁירוּהוּ:
R. Tzaddok testified — that they are kosher — about running waters [zochalin viz. (Deuteronomy 32:24): "zochalei afar" ("the 'crawlers' in the dust"), which, halachically, are equivalent to a well, which purifies with any amount of running water, and which are kosher for the consecration of the waters of the sin-offering (the red heifer) and for the immersion of zavim], which (zochalin) (intermixed with and) "outnumbered" the notfim ["dripping waters," like rain water, which, halachically, is like a mikveh, which purifies, with forty sa'ah and in an eshboren (a water collection pit), and which are unfit for the consecration of the waters of the sin-offering, not being "living waters," and likewise, for the immersion of zavim — When the running waters have mixed with the dripping waters and "outnumbered" the dripping waters, they are halachically like running waters and are kosher for the consecration of the waters of purification (of the red heifer) and for the immersion of zavim, and they cleanse, as running waters do, with any amount.] It (such an "outnumbering") once occurred in Biryath Hapilya, and they brought it to the attention of the sages, who pronounced them (the waters) kosher.
Bartenura on Mishnah Eduyot
English Explanation of Mishnah Eduyot
There was such a case at Birath Hapilya, and when the case came before the Sages they declared it valid.
This mishnah discusses water which is used for a mikveh, the bath used by Jews to achieve ritual purification. This mishnah also appears in tractate Mikvaoth, where it is preceded by a distinction between running water and dripping water. Water which is flowing on the ground, such as a stream, creek or river, is valid for a mikveh even if there are less than 40 seahs in one place. However, dripping water, such as rain, must reach a minimum measure of 40 seahs in one defined place in order to be valid as a mikveh. [Note that water drawn to the mikveh is invalid in any case].
Our mishnah discusses a potential mikveh that has some flowing water and some dripping water. The question is, does this mikveh need to have 40 seahs?
According to Rabbi Zadok, as long as the quantity of flowing water exceeded the quantity of dripping water, the mikveh is valid, even if there are not 40 seahs in the place where the person immerses. If, however, there was more dripping water, then the mikveh would need to have 40 seahs of water in order to be valid. The mishnah then brings in an actual case where such a question arose and the Sages declared the mikveh to be valid.